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Reducing Stretch in Indoor Hydroponic Crops


(excerpt from Integral Hydroponics Evolution – work in progress) 

 

Plant stretch is a major area of concern for many indoor gardeners. This has led numerous (too many) growers to purchase and use synthetic plant growth regulator (subclass growth retardant) additives that act to reduce stem elongation (stretch) and form closer internodes (i.e. ‘stack nodes’). For the purposes of this discussion I will refer to plant growth regulators (subclass growth retardants) as synthetic PGRs.

 

Examples of synthetic PGRs are (to name but a few), Dutchmaster Superbud/Phosphoload, Emerald Triangle Gravity, Emerald Triangle Bushmaster and Gravity, General Hydroponics Dr No (formerly Dr Nodes), Flower Dragon, Canadian Xpress U-Turn/Yield Masta, Cyco Nutrients Cyco Flower, Top Load, Boonta Bud, Rox and Rock Juice.

 

Testing in 2011 of just some of these products by the California Department of Food and Agriculture came up with these results.

 

Flower Dragon: 18,400-18,650ppm daminozide, 30-46.3ppm paclobutrazol
Phosphoload: 17,800ppm daminozide, 20.6ppm paclobutrazol
TopLoad: 3,467ppm daminozide
Emerald Triangle Bushmaster: 271ppm paclobutrazol
Emerald Triangle Gravity: 516ppm paclobutrazol

Tests Conducted on PGR Containing Products in March 2011 by the CDFA (FOI Data obtained from Californian Department of Food and Agriculture)

 

As a result of these findings the CDFA ‘quarantined’ and recalled these particular products. They can no longer be sold legally in California and Oregon[1]; however, due to lax enforcement of agricultural codes in other countries and States of the U.S. many of these products are still widely available through hydroponic stores across North America, Australia, the UK and elsewhere.

 

About Plant Stretch

 

There are several phases to the growth and bloom cycles in indoor crops.

 

The first phase, where using ‘clones’ (i.e. cuttings that have been rooted – a genetically identical copy from a another plant) and an 18/6 grow light cycle (18 hours of light and 6 hours of darkness), is what I call the settling phase where clones are placed into the growing system and where they must adapt to high intensity lighting, place roots down into the media (initiate adventurous root growth) and begin growing. This phase can be stressful to young plants as high intensity lighting and transplanting can shock the plant/s if things aren’t handled correctly. If handled incorrectly, the settling phase can take some time and plant health can be impacted. However, if handled correctly (e.g. clones are hardened before being placed in the growing system, cool day temperatures of no more that 25oC are initially maintained, MH lighting is implemented at the appropriate height above the young plants, and an auxin based root stimulant is used to encourage adventurous root growth), the settling phase should only take a few days, at which point the plants have adapted to their new environment and begin producing healthy leaves and growing, at first slowly and then far more vigorously. When the vigorous vegetative growth begins the plants are in what I refer to as the ‘rapid vegetative growth stage’.

 

The next important phase is when the plants have reached the desired height in the ‘rapid vegetative growth stage’ and are switched into the 12/12 light-cycle (12 hours of light and 12 hours of darkness) to induce flowering. This is when the stretch phase begins and rapid upward growth is observed, and the stems of the plants begin to elongate (or ‘stretch’ as it is commonly referred to). Various factors will influence the degree to which plants stretch, not the least of which is plant genetics. For example, to generalize somewhat, most long day equatorial dominant genetics will stretch far more than most short height early flowering dominant genetics.  For this reason, where indoor, under lights growing is concerned, cultivar choice becomes important when considering stretch. That is, some genetics are more suited to indoor growing than others and this means that consideration should be given to this when choosing seeds or clones to grow from. Additionally, other very important factors such as light colour spectrum (e.g. red spectrum high pressure sodium will promote stretch while blue spectrum metal halide will act to suppress stretch) and ‘shade avoidance syndrome’ (SAS) will influence proceedings greatly. We’ll talk more about these factors in moment. For now…

 

The stretch cycle typically lasts between 2 – 4 weeks, dependent on plant genetics. For example, long day equatorial genetics will typically have a longer stretch phase than short day, early finishing varieties.

 

During this period, if the plants are healthy, rapid upward growth and secondary branching is observed. As this rapid growth takes place, buds form at the node points of the plant on the lateral branches. These buds are called axillary buds. Where a plant is in vegetative mode the axillary buds would grow out into lateral branches. However, in flower mode the buds will eventually grow into the sort after floral clusters (colas) of the plant. Additionally, as the plant finishes it upwards growth it sets numerous buds at the top of the apical (main) and secondary lateral branches. These buds eventually form the top colas of the apical and lateral stems.  This stage of the plants lifecycle is called ‘bud set’ because this is where the all-important young buds form. This is also the point at which internode distance becomes very important because buds form at the nodes of the plant. The further apart the nodes, the further apart the young buds – the closer nodes, the closer the young buds. See following illustration that shows, buds, nodes, internodes etc.

 

plant_pic_site_ready

 

After the buds have set they begin to grow vigorously and swell. I refer to this phase of the plants lifecycle as the ‘swelling phase’. During this period the plants have high nutrient demand and are expending a great deal of energy into producing colas.

 

The last phase is what I call the hardening and ripening phase. This is where the swelling of colas slows and they begin to harden and ripen. At this point the colas don’t tend to grow/swell much more but instead put on a weight as the density of the formed colas increases and they harden. At this point nutrient demand is reduced and the plant is in its final phase of growth. At approximately 60% brown off (60% of the white hairs on the colas turn brown) the flowers/colas have ripened and are ready to harvest.

 

Where left to grow naturally (i.e. the plant is not tied/netted down) the largest single flower/cola on the plant will form at the top of the apical/main stem, with prolific secondary cola formation also occurring on the lateral branches, with the heaviest of these secondary colas forming at the top of these branches. I’ve oversimplified things somewhat because various factors will come into play, regarding node and bud formation, and branching, such as plant size and plant genetics. For example, if a plant is left in vegetative growth for a long period (i.e. where growing large plants) the key/primary lateral buds may set on a third set of lateral branches etc, and where genetics are concerned, e.g. long day equatorial genetics may act somewhat differently than described and the buds may not join resulting in loose, stringy or fluffy flower clusters that are spaced apart (as opposed to larger, dense flowers as is typically seen in the case of short height early flowering genetics).

 

I’ve covered the entire life cycle of the plant because each phase is interrelated to overall yield and one cannot consider one phase without considering all others. That is, if the vegetative phase is not handled correctly, this may impact on the stretch phase and if the stretch phase is not handled correctly this will impact on the swelling phase. Quite simply, achieving optimum yields is a holistic process where optimums (plant health and development) throughout all phases of the plant’s lifecycle will determine outcomes.

 

For now, let’s rein this in a bit and look at the importance of the stretch phase with regards to plant architecture and ultimately flower formation and yields.

 

Definitions

 

“Plant architecture” is defined as the three-dimensional organisation of the plant body. For the parts of the plant that are above ground, this includes the branching pattern, as well as the size, shape and position of leaves and flower organs.

 

“Bud”: a small part that grows on a plant and develops into a flower, leaf, or new branch/stem

 

The Relationship Between Nodes, Internode Distance, Buds and Stretch

 

The term internode is used to describe the distance between the nodes of the plant. The nodes are where the young buds set. Therefore, internode distance becomes very important in bud set and ultimately flower formation.

 

Stretched plants will have large internode distance, with buds spaced widely apart, while short squat plants will have closely formed nodes (i.e. small internode distance) with buds forming/setting closely together.

 

To generalize somewhat, because genetics will influence outcomes, the buds that form at the node points on the apical (main) and lateral (secondary) stems during the stretch cycle eventually grow, swell and join with bud growth above and below them. This means that multiple buds swell and join and become colas (i.e. clusters of swelled buds that have joined to create a single large cola/flower).

 

Keep in mind that the young buds grow at the node points of the plant and that nodes are where the leaf petiole (leaf stem) meets the stems/branches of the plant.

 

Therefore, the closer the nodes, the closer the buds and the better the result where flower formation is concerned. In other words, the ideal in indoor growing situations is to have a short squat, compact plant with small internode distance because this means buds set closely to one another, which promotes large colas. If the nodes are too far apart, because the plant has stretched, the buds at the node points can fail to join with other buds and this results in smaller, dispersed flower clusters (i.e. less than optimal flower formation). Therefore, the degree to which the stems elongate during the stretch cycle becomes important to final yield as this is the phase where internode distance is determined.

 

The quest for short squat plants with small internode distance has led many indoor growers towards the use of synthetic PGRs because these additives reduce stretch and, as a result, reduce internode distance which, in turn, promotes closely formed buds that grow into well formed, large, heavy colas.

 

Synthetic PGRs

 

I’ll avoid going into too much detail here and instead refer readers to the internet where they can find a lot of information about the synthetic PGRs used in indoor growing. A Google or Bing on the subject (e.g. “PGRs used in hydroponics”) will yield many pages on the subject. For now…

 

Defining Toxic Synthetic PGRs (subclass, “Growth Retardants”)

 

PGRs are without a doubt the hydro industries most controversial products. However, some misunderstanding surrounds them.

 

The term ‘plant growth regulator’ (PGR) covers a broad range of synthetic and natural (organic) PGRs. Just a few of these pose a risk to consumers while others are non-toxic.

 

In very simple terms synthetic PGRs in the subclass of “growth retardants” such as daminozide (Alar), paclobutrazol, and chlormequat chloride are potential toxins while other PGRs such as Jasmonates (subclass, “growth inhibitor”) and Triacontanol (naturally found in alfalfa meal and classed as a plant growth stimulator) pose no risk at all.

 

Many PGRs are hormones that are produced within growing plants themselves. Hormones are vital to plant growth and lacking them plants would be mostly a mass of undifferentiated cells. Because hormones stimulate or inhibit plant growth, many botanists also refer to them as plant growth regulators (PGRs). Botanists recognize eight major groups of hormones, auxins, gibberellins, ethylene, cytokinins, abscisic acid, jasmonic acid, strigolactones and brassinosteroids.

 

Other than naturally occurring plant hormones, other safe PGRs exist (e.g. Triacontanol) – just some of which can be found in various hydroponic industry products. It’s important to note this in order not to demonize all PGRs. Quite simply, when it comes to PGRs there are the good, the bad and the downright ugly. The latter (“downright ugly”) fall under the subclass of “Growth Retardants” and include daminozide, paclobutazol, chlormequat chloride, prohexadione, and uniconazole.

 

About Synthetic PGR Additives

 

Hydroponic bloom additives that dramatically reduce upward apical/main stem growth, stimulate axial branching and stack nodes typically contain systemic synthetic PGRs such as paclobutrazol (PBZ), chlormequat chloride (CCC), and/or daminozide/Alar. These chemicals, in all cases, are shown to have extremely long withholding periods (months in many cases) and, therefore, remain residual in the harvested product. Further, many of these chemicals are shown to be harmful to human health. Additionally, many synthetic PGRs  are either not licensed for use, or in the case of daminozide/Alar are banned for use, worldwide, on ANY consumable crop and registered for use only on ornamental flowering plants.

 

Daminozide/Alar is listed as a probable human carcinogen. Its breakdown product UDMH (unsymmetrical dimethylhydrazine) is highly toxic and a potent human carcinogen.

 

Paclobutrazol is known to be toxic to the liver, with its carcinogenic potential unknown due to a lack of adequate data. i.e.

 

(Quote)

 

“Substance Name — Paclobutrazol

This substance/agent has not undergone a complete evaluation and determination under US EPA’s IRIS program for evidence of human carcinogenic potential. “ (US EPA)

 

(End Quote)

 

Chlormequat chloride is a known immune toxin. That is, exposure to chlormequat chloride can suppress the human immune system. This makes it less than ideal for use on medical crops where it is possible the end user has immune deficiencies.

 

Other compounds that can be used as synthetic PGRs (e.g. triazole fungicides) also have serious question marks over their safety/suitability for use on consumable crops.

 

Synthetic PGRs Reduce Essential Oil Production

 

Putting aside the health risks linked to synthetic PGRs, another problem associated to their use is that they can greatly reduce essential oil production by antagonizing the gibberellic pathway in plants. Put simply, the gibberellic pathway plays an important role in resin production and, therefore, because the GA pathway is inhibited by many synthetic PGRs there is less essential oil produced by the plant. This means that while synthetic PGRs will reduce stretch and, as a result, potentially increase yields, essential oils such as terpenes and phenolics are greatly reduced.

 

Withholding Periods

 

As noted, synthetic PGRs usually have extremely long withholding periods and, therefore, are not suitable for use on short-term deciduous crops.

 

While some manufacturers of synthetic PGR containing products have made claims to the contrary, stating, among other things, that the actives in their products are at such low levels they pose no threat to human health, and/or that because their product is used during early flower etc no residues would exist in the harvested product, such claims have been categorically disproven in numerous studies that have shown PBZ, CCC and daminozide/Alar residues can persist for many months (regardless of crop type) after application.

 

This is also shown to be true in the case of indoor crops. For example, while research is limited surrounding PGR use by indoor hydroponic growers, the California based Werc Shop (medical marijuana testing laboratory) informs me (November 2014) that 15% of the medical marijuana they test fails the ‘pesticide’ screen, of which approximately 50% fails on the basis of testing positive for paclobutrazol. See lab results following.

 

paclobutrazol_fail

 

 

Synthetic PGRs – For and Against 

 

For:

 

1.     PGRs reduce stretch, stack nodes and encourage well-formed, dense flowers.

2.     PGRs can increase yields where optimum growing practices aren’t in check

 

Against:

 

These chemicals:

  1. Are classed as systemic pesticides and/or fungicides
  2. Are scheduled poisons
  3. Have extremely long withholding periods and therefore remain residual in the harvested product
  4. Are in many cases and many countries not licensed for use, or banned for use, on consumable crops
  5. Are subject to strict regulations, pertaining to registration, labelling and use (crop type, application rates and times, handling procedures etc)
  6. Are potentially harmful to human health
  7. Greatly reduce essential oil production
  8. Are unnecessary because stretch can be reduced through some simple to implement growing practices

 

I’ll leave the subject of synthetic PGRs there for now (enough said).

 

However, given that stem elongation (stretch) is a major area of concern for many indoor growers, let’s take a look at safe and effective alternatives to synthetic PGRs for reducing stretch.

 

Reducing Stretch Naturally

 

Various factors such as plant hormones, light intensity and colour, thermoperiod (the difference between night and day temperatures), carbon acquisition, and nutrients influence the amount of stretch a plant will exhibit during its life cycle. Because of this, plant stretch can be greatly reduced if the right growing practices are implemented in the grow room. For instance, due to the toxic nature of synthetic PGRs, much research surrounding reducing stem elongation in various crops has been conducted in the past several years. This research has demonstrated several things:

 

  1. Light colour spectrum and intensity play an important role in reducing stretch
  2. Ensuring adequate plant spacing is critical to reducing stretch. i.e. plants that are placed too close to one another are forced to compete for light and this results in shade avoidance syndrome (SAS). As a result of SAS the plants stretch
  3. Thermoperiod (the difference between night and day temperatures, termed ‘DIF’) can be manipulated in such a way as to greatly reduce stretch. For example, studies show that stretch can be reduced by approximately 30% by reducing the morning temperature below that of the night temperature for approximately the first 3 hours after the lights turn on.
  4.  Mechanically induced stress (MIS) such as pruning, pinching, repeated brushing, shaking, or bending caused by air movement or contact with animate or inanimate objects can significantly reduce stretch. A study conducted by Dr. J Latimer demonstrated the commercial potential of this technique for controlling the height of vegetable transplants, particularly tomato. This work was initiated, in part, by the fact that the synthetic PGR, B- Nine  (85% daminozide) was deregistered for use on consumable crops. One system of mechanical conditioning adapted to commercial greenhouses involves drawing a bar across the tops of the plants once or twice a day. The bar is set low enough to contact the plants, but not so low that the plants are injured or uprooted. 30 to 40% reductions in height have been reported with this system.
  5. Fertilization influences stretch. For example, low phosphorus levels during the stretch cycle will promote compact plants while higher phosphorus levels will promote stretch
  6. Plant hormones (phytohormones) such as cytokinins and jasmonic acid (methyl jasmonate) reduce stretch

 

 

Light Colour Spectrum and Stretch  

 

The light energy required by plants is confined almost entirely to the visible spectrum of light (400nm – 700 nanometers). While there are key points within this spectrum, growth is optimized under the entire range of the spectrum. This is because different color wavelengths stimulate different biochemical reactions within the plant. As a result, different physiological functions are activated and energized, which – in turn – determine plant growth rates and formation (architectural) characteristics.

 

Growers who have experimented with different lighting combinations can/will tell you that different lighting configurations can produce very different results. For instance, plants that are flowered under a combination of mostly yellow, orange and red spectrum ‘high pressure sodium’ (HPS) and predominantly blue spectrum ‘metal halide’ (MH) lighting form very differently than plants that are flowered under HPS light alone. In simple terms, blue light (400 to 500 nm) has been shown to have an inhibitory effect on stretch,[2] whereas red light, particularly red/far red light at 660:730 nm[3], has been shown to increase stretch.

 

This means that lamps which possess higher degrees of blue spectrum light and lower levels of red spectrum light are more ideal for use during the stretch cycle.  For example, in one study with tomato (2013) it was shown that plant height and internode length was highest in plants that were grown under HPS (52.2cm height and 5.1cm internode length) while the most compact plants were grown under lights that had the highest blue fraction (21.4cm height and 2.8cm internode length). [4]  In another study (1997) where soy bean plant height and artificial lighting were compared (HPS v. MH) in controlled environments, including hydroponics, plant height was reduced from 46 to 33 cm when plants were grown under metal halide lamps compared to high pressure sodium lamps at the same photosynthetic photon flux.[5] In yet another study (2002), the effectiveness of two types of far red light absorbing greenhouse films in reducing stem elongation of cucumber, tomato, and bell pepper seedlings was investigated. Both FR light absorbing films were effective in reducing stem elongation in cucumber, bell pepper, and tomato seedlings.[6]

 

See following images of light MH and HPS colour spectrums in ‘nanometers’ (nm).

 

 

 

   

 

 

Compare the colour spectrum differences of MH and HPS lamps and you will note far higher relative degrees of blue light in MH and relative to blue light, high yellow, orange and red light in HPS.

 

Quality (Essential Oil) and Light Colour Spectrum

 

An important point to consider also, when discussing light types that are most suitable for indoor growing, is that essential oil production is stimulated by a wide range of light colour spectrums and, therefore, the combination of HPS and MH lighting better promotes essential oil production than HPS alone. For example, research shows that a combination of blue light and red light promotes more flavonoid synthesis than where predominantly blue light or predominantly red light are used alone; however, other wavelengths and possibly ratios between wavelengths also have an effect. For example, one study compared multiple artificial light types and found that maximum flavonoid synthesis occurred under a continuous lighting spectrum of 400  – 750 nm.[7]  In other words flavonoid synthesis is best stimulated by a diverse and wide band and ratio of colours. Both metal halide and HPS only provide limited colours at different ratios to each other. Therefore, by combining both MH and HPS we can increase the range of the colour spectrum and better promote essential oil production.

 

What this really means is that the all too common cultural practice of using high pressure sodium (HPS) lighting alone in indoor growing has played a significant role in promoting stretch and reducing quality.

 

What this also means is that plants are ideally grown under MH lights (or other predominantly blue spectrum lighting) during the settling, vegetative and stretch phases and thereafter a combination of MH and HPS lighting will provide the best results where yields and essential oil production are concerned.

 

LED Lighting

 

While I have found that, at least for now, a combination of MH and HPS produces far higher yields than LED lighting, where LED (particularly, predominantly blue spectrum LED lighting) may be of benefit is for use in the grow and stretch phases of the crop cycle, either as stand alone or supplemental lighting with HPS.

 

Note that the quality of LED grow lights can vary greatly; therefore, speak to you hydroponic retailer for further information about product/brand choice, light colour spectrum options, recommended wattage etc.

 

Additionally, there are all sorts of suspect claims made about the efficiency of LEDs with limited research data, if any, to support them. For example, just some LED suppliers assert that LED grow lights, among other things, result in higher growth rates because their colour spectrum better promotes photosynthesis more so than high intensity discharge (HID) lighting (i.e. HPS and MH lighting). This is, in fact, misleading. Studies show that photons within the photosynthetic spectrum of 400 to 700 nm are essentially equally capable of driving photosynthesis, and therefore plant growth rates. Light intensity has been shown to have a much larger effect on plant growth than light colour spectrum.[8] Thus, be somewhat wary of the claims made by various suppliers of LEDs and read the material on page … before rushing out to your local hydroponic store to purchase LED lighting.

 

Shade-Avoidance Syndrome – Plant Spacing

 

One of the most common mistakes made by indoor growers is that they fail to appreciate that fewer plants can mean more yield. Too many plants crowded into a small space will compete for available light and as a result stretch as they compete for light. This is called shade avoidance syndrome (SAS) where plants respond to competition signals generated by neighbors by evoking SAS, which results in stem elongation, increased internode distance, altered flowering time and reduced shoot branching. [9] Basically, what happens with SAS is a plant grown under canopies of other plants perceives the reduction in the ratio of red (R) to far-red (FR) light as a warning of competition, and enhances elongation growth in an attempt to overgrow its neighbors. As a result, plants stretch in order to compete with one another for light. [10] As Morelli et al (2000) put it: “The most dramatic shade avoidance response is the stimulation of elongation growth.” [11]

 

This results in stretch and less than optimal yields.

 

Therefore, it is imperative that plants aren’t overcrowded and competing for light. This means that appropriate plant spacing needs to be considered as an optimal growing practice.

Read more about the role of light colour in reducing stretch here….

 

SCROG (Screen of Green)

 

Scrog is a term used by many indoor growers and stands for ‘SCReen Of Green’. The plants grow up to the screen and then are “trained” or tied to the screen, resulting in a flat table of plant growth. Through this means plants can be trained/tied out to remain shorter than they normally would grow. Another advantage to scrogging is because all the flowers/fruit are growing at about the same height, it is possible to get all the growth within the effective circle of light from the lamp, resulting in increasing yields within a given space. See following illustration of a SCROG setup.

 

 

 

Tipping/Topping 

 

Apical dominance (stretch) is caused by the apical bud (top shoot of the plant) producing IAA (auxin) in abundance.

 

When the apical bud is removed, the source of IAA is removed. Since the auxin concentration is much lower, the lateral buds (side shoots) are stimulated to grow. Thus, decapitating (pruning) the top of the plant will cause it to branch, reducing upward growth.

 

Due to this tipping/topping is a good technique, along with scrogging, to control plant height.

 

Genetics! Genetics! Genetics!

 

Genetics play a major role in plant characteristics/traits. In very simple terms, sativas are a much ‘leggier’ plant than indicas and hence indicas, or indica dominant indica/sativa crosses are better suited to indoor growing environments.

 

Flipping Times and Understanding the Genetics You Work With

 

This is a tricky one as different people will use different techniques of growing. Some people will grow small plants in numbers while others will grow just one large plant etc. Also, different plants (genetics) will do different things. One type of plant may be very explosive and another type may not have the same vigour. One strain may be far leggier (prone to elongation) than another. Getting to know your strain will help you fine-tune the finishing height.

 

As a rule of thumb – the plant does 80% of its growing during the 12 hr light cycle. So be wary. Don’t think that you have to grow a plant in the 18-hour light cycle for too long. As a general rule if you switch down an 8 – 10inch plant you will finish with a 2 1/2 to 3 1/2-foot plant.

 

Know your plant and flip at the appropriate time!

 

Thermoperiod – Manipulating Night and Day Temperatures to Reduce Stretch

 

The difference in temperature during the day/night (light/dark) period, known as “thermoperiod”, has a major effect on plant growth and architecture.

 

Through manipulating these temperatures in the right manner stretch can be greatly reduced.

 

Gibberellins, Stretch and Thermoperiod

 

Gibberellins (GA) are the plant hormone most associated to stretch. For example, many synthetic PGRs (e.g. paclobutrazol, chlormequat chloride and daminozide/Alar) reduce the bioactive GA levels in plants and through this mode of action reduce stretch.

 

However, with gibberellins in mind, studies have shown that where day temperatures are lower than night temperatures bioactive GA levels are reduced in plants and, as a result, stretch is reduced.[12]

 

DIF

 

DIF stands for the difference between ‘day temperature’ (DT) and ‘night temperature’ (NT). This can be expressed as either an equal, positive or negative number. For example night 22.40C (72.40F), day 280C (82.40F), equates to positive DIF (DIF = + 5.60C or +100F). Conversely, night 280C (82.40F), day 22.40C (72.40F) equates to negative DIF (DIF = – 5.60C or -100F).  Where night and day temperatures are the same (e.g. 28oC DT and 28oC NT) this is equal DIF, also referred to as “zero DIF”.

 

Stem elongation is promoted by warmer days than nights (+ DIF) and inhibited by warmer nights than days (- DIF). Plants become taller as DIF becomes more positive and plants become shorter as DIF becomes more negative. Therefore, plants grown under a positive DIF are taller than plants grown at an equal or zero DIF, and plants grown under an equal or zero DIF are taller and have larger internode distance than plants grown under a negative DIF.

 

As a result, a negative DIF treatment (low DT and high NT) is a tool to produce compact plants with short internodes without a delay in production time. For example, in a study by Maas et al (1996) it was shown that stem elongation in Fuchsia x hybrida was influenced greatly by cultivation at different day and night temperatures. Internode elongation of plants grown at a DT (25°C), NT (15°C) difference (DIF+10°C) was almost twofold of that of plants grown at the opposite temperature regime (DIF-10°C).[13]

 

The negative DIF technique is so effective it has largely replaced the use of chemical PGRs in a number of commercially produced crops.[14]

 

‘Morning Temperature Dip’ or ‘Cool Morning Pulse’ Technique 

 

Negative DIF, like chemical PGRs, has its greatest effect on height during the phase of most rapid stem elongation. Therefore, negative DIF is most effective when applied during the stretch phase of the crop cycle (e.g. first 2 – 3 weeks or so of the 12/12 light cycle). Further, studies show that the most active phase of stem elongation in plants occurs at the end of the dark period and during approximately the first 2-3 hours after the sun rises. Therefore, stretch can be reduced through reducing the morning temperature (when lights first come on) below that of the nighttime temperature for approximately 2 -3 hours. This is called the “morning temperature dip” or “cool morning pulse” technique.

 

Although research findings are somewhat variable, there is a general consensus that many plants are sensitive to a temperature dip during the first 2 to 4 h of the photoperiod.[15] For example, a temperature dip given during the first 2 hours of the day reduced internode length of cucumber and tomato seedlings.[16] Plant height in this study was reduced in direct response to the degree of the morning temperature dip (between -2 and -10oC).

 

In research by Ueber and Hendriks (1992) it was shown as a response to a 2 hour temperature drop from 24 °C to 8 °C (DIF = – 16°C) in the morning, stem elongation was reduced by more than 50% in poinsettia. Conversely, a moderate temperature drop from 24 to 16°C (DIF = – 8°C) reduced plant height by between 5 and 25% depending on duration.[17]

 

Various studies have shown that lowering the temperature for a two hour period starting 30 minutes prior to dawn is almost as effective as maintaining negative DIF throughout the entire day.[18]

 

It is important to note, however, that optimum negative DIF treatment is species dependent. For example, ester lilies’ show the greatest effect at a DIF of -15°C, poinsettias at -16°C, and fuchsia at -20°C.   Additionally, tomato, corn, and cucumber have strong responses to DIF, while squash, watermelon, pea, and bean are less responsive.[19]

 

Other than this, negative DIF can elicit other not sort after responses if handled incorrectly. For instance, leaf chlorophyll content is reduced in plants grown with a negative DIF.[20]  Reduced leaf chlorophyll results in reduced rates of photosynthesis (chlorophyll is the key molecule responsible for light absorption in plants and therefore pivotal in photosynthesis) and can result in visibly chlorotic plants (yellowing and/or dying leaves). However, negative DIF-induced leaf chlorosis is typically reversible, with plants greening rapidly after removal from the negative DIF environment.[21] Therefore, reducing the day temperature to accommodate negative DIF also reduces the growth rate in heat-loving plants and too excessive a negative DIF (temperature and/or duration) has been demonstrated to reduce relative flower number and size in several species. Additionally, leaves on plants grown with a negative DIF tend to point downwards while those grown with positive DIF point upward.

 

Therefore, some cautious experimentation is advised where working with the morning temperature dip/cool morning pulse (in commercial growing applications maximum negative DIF is usually no more than – 6°C).

 

My Experiences with the Morning Temperature Dip

 

Firstly, ideals when using the morning temperature dip technique seem to be somewhat genetic dependent. That is, some strains respond far more to the technique than others.

 

However, I find when growing under metal halide lamps, during stretch, that by maintaining equal DIF (same night and day temperature) and then a 3-5°C drop one hour before and two hours after the lights turn on (total 3 hours negative DIF) does the job nicely with most genetics. This said, your genetics may differ and want more or less. Therefore, begin conservatively with equal night and day temperatures and a moderate temperature dip (e.g. 3°C) and see where this takes you. If the reduction in stretch isn’t enough increase the DIF in small increments until you achieve the desired result. i.e. to increase the inhibitory effect of a temperature drop on stem elongation, you can either extend the period of the temperature drop or increase the degree of the temperature drop.[22]  Conversely, to decrease the inhibitory effect of temperature drop on stem elongation decrease the period of the temperature drop or reduce the degree of the temperature drop.

 

Most importantly, if choosing to use the morning temperature dip technique, watch the plants closely for early signs of chlorosis (leaves – particularly older leaves – will lose their dark green hue and begin paling and going yellow). If signs of chlorosis do appear then restore normal night and day temperatures (i.e. cooler nights than days).

 

By the way, I have refined the equal/zero DIF and negative morning temperature dip technique to achieve about a 30% reduction in stretch in several strains. Therefore, this technique is an extremely effective way of reducing stretch and results in outcomes similar to that of the use of synthetic PGRs.

 

Author’s note: some growers report that they get the desirable reduction in stretch by running equal DIF alone (i.e. equal DIF with no morning temperature dip). Again. I have found the reduction in stretch elicited by equal DIF and/or the morning temperature dip does tend to be genetic dependent and different strains may require different approaches. However, you may find that by running equal DIF alone this provides you with the sort after or, at least, acceptable growth responses.

 

Use Negative and/or Equal DIF During the Stretch Phase Only

 

It is commonly asserted by authors on the subject of plant growth (myself included) that plants require a higher daytime than nighttime temperature (positive DIF) to achieve optimum yields. This stands true! However, this assertion is perhaps overly simplistic when considering stem elongation in certain flowering/fruiting crops. For example, having covered thermoperiod and DIF we can see that equal DIF or negative DIF (or combinations thereof) during the stretch phase can be beneficial to plant architecture and yields.

 

This said, the scientific community has long understood that growth and yields in many plant species are best stimulated by higher day and lower night temperatures.

 

This comes down to photosynthesis, respiration and thermoperiod DIF.

 

Most thermoperiodic plants (e.g. tomato, cucumber, capsicum, chilli) produce maximum growth when exposed to a day temperature that is about 5 to 10°C higher than the night temperature. This allows the plant to photosynthesize (build up sugars) and respire (break down sugars) during an optimum daytime temperature, and to curtail the rate of respiration during a cooler night. Too high night temperatures cause increased respiration, sometimes above the rate of photosynthesis. This impacts on growth and yields. Put simply, thermoperiodic plants typically produce the highest yields when day temperatures are higher than night temperatures.  For example, optimum flowering of tomato plants occurs under the conditions of a positive DIF at 25oC DT and 15oC NT (DIF = +10oC).[23]

 

Conversely, negative growth responses can be elicited through diverging from thermoperiod ideals. For example, we have seen that a negative DIF results in reduced chlorophyll content in the leaves of plants, and reduced leaf chlorophyll results in reduced rates of photosynthesis.  In turn, reduced photosynthesis results in reduced growth. This is just one reason that negative DIF works to reduce stem elongation. Technically, by manipulating the thermoperiod to negative DIF we slow the plant down (i.e. reduce its growth rates during a period of rapid upward growth and stem elongation), which results in a more compact plant with smaller internode distance. However, while slowing growth may be desirable during stretch we don’t want to slow growth down during the swelling phase when the flowers are forming and growing. Therefore, once negative DIF has done its job to reduce stretch we want to encourage growth by increasing the photosynthetic capacity/potential of the plant as much as possible. This is done by, among other things, having a positive DIF where day temperatures are approximately 8 -10oC higher than night temperatures.

 

Therefore, while an equal or negative DIF, or combinations thereof, may be beneficial during the stretch phase (for inhibiting stem elongation and reducing internode distance) equal or negative DIF will prove detrimental to yields thereafter. For this reason, only use equal or negative DIF during stretch (i.e. the first 2 – 2 1/2 weeks or so of the 12/12 light cycle). After this, run a positive DIF with days (lights on) approximately 8 – 10oC warmer than nights (lights off).

 

 

Maintaining Low Phosphorus (P) in Solution to Reduce Stretch

 

This one may not be viable for recycling growers, nor perhaps hobby growers who purchase off the shelf hydroponic nutrients in general. However, we’ll cover low phosphorus in solution as a means for reducing stretch for several reasons; 1) this is shown to be an extremely effective strategy for reducing stretch – in my own experiences I have found that by breaking away from traditional hydroponic nutrient norms and running low ppm of phosphorus in solution (20ppm) during stretch has reduced stem elongation by approximately 10 – 15% and resulted in increased and stacked nodes with no loss of quality; 2) providing low phosphorus to the plants is viable for those who formulate their own hydroponic nutrients and; 3) at some point a hydroponic nutrient manufacturer may release a liquid fertilizer, geared towards use during the stretch cycle, with the aim of reducing stem elongation. Any such fertilizer will, no doubt, contain low phosphorous.

 

The Science of Low P and Stretch Reduction     

 

Many commercial agricultural growers have traditionally used fertilization to promote stem elongation (promote taller plants) and believed that the elongation was due to high ammonium nitrogen. However, Dr. Paul Nelson from the Department of Horticultural Science, North Carolina State University demonstrated in 2002 that phosphorus, not ammonium nitrogen, was responsible for the promotion in stem elongation. To quote: “low phosphate levels result in compact plants, while high phosphate levels result in tall plants.” [24]

 

This finding has been supported in previous research that shows one of the characteristics of plants grown with low phosphorus is that root growth gains at the expense of shoot growth. Shoot growth is therefore restrained and root activity strengthened. In a study by Danish researchers (2000) with chrysanthemum and potted miniature roses, low phosphorus was found to have a strong growth-retarding effect on stems. Plant height was reduced, with little or no influence on flowering, and there was no impairment of plant quality. The preliminary results of this study indicated a growth-retarding effect in all the tested plant species when the phosphorus levels were at least 20 times reduced compared to a traditionally high phosphorus level. In some species, growth regulation is so effective that it might be possible to replace synthetic PGRs completely with low phosphorus application.”[25]

 

How Much Phosphorus in Solution to Reduce Stretch?

 

Running 20ppm of elemental phosphorus (P) in solution with run-to-waste (RTW/DTW) growing systems during the stretch cycle has been shown to reduce stretch and stack nodes when compared to higher amounts of P (>20ppm). Keep in mind that this is elemental P and not P as P2O5. That is, P2O5 is only 43% elemental P; therefore, 20ppm of elemental P is 46.ppm P as P2O5.

 

Further, it is important to note (a word of warning) that where recycling systems are concerned optimal phosphorus ppm in solution may/will differ due to the preferential uptake of phosphorous by plants at relatively high levels.

 

That is, in RTW/DTW growing systems fresh nutrient is delivered to the plants at each and every feed. Therefore, if we were to have 20 ppm of P in solution each plant in the RTW/DTW system would receive 20ppm of P at each and every feed.

 

This situation does not apply to recycling systems where nutrient tank/reservoir practices differ amongst growers. That is, phosphorus (P) is preferentially taken up by plants at relatively high levels. If the phosphorus removed from the nutrient by the plants is not replaced daily in solution at the same levels a P deficiency may occur over several days. For example, to dumb this down a bit, let’s say that we start with 20ppm of phosphorus in solution in a recycling system and the plants in total remove 10 ppm of P per day from this solution. Let’s also say, for this example, the nutrient tank is just topped up with water (no additional nutrients) daily to replace (top up) the volume of solution that the plants remove from the nutrient tank/reservoir every day. Therefore, on day one we would begin with 20ppm of P in solution; on day two we would begin with 10ppm of P in solution; on day 3 we would begin with 0ppm of P in solution and so on. Within three days, under these conditions, the plants are being starved of phosphorus. As such, while reducing phosphorus levels to reduce stretch is an ideal tool for RTW/DTW growers it may be best avoided by recycling growers.

 

One Other Issue Also Presents  

 

A problem presents with achieving 20ppm of P in solution in RTW/DTW systems and that is standard off the shelf hydroponic nutrients in many instances contain too much phosphorus to achieve the sort after value, while maintaining optimal EC. For example, based on lab analysis of a U.S. Canna Coco nutrient sample diluted at 2ml/L in RO water to achieve 1.21EC there is 39. 85ppm P – or nearly two times that of the sort after phosphorus in solution. See following lab analysis.

 

canna_coco_test

 

Looking at the Canna Coco nutrient lab analysis above you can see 39.85ppm of elemental phosphorus is in solution at an EC of 1.12. What this really means is that in order to achieve 20ppm of phosphorus in solution one would need to dilute Canna Coco at 1ml/L, which would then give you an EC of about 0.5 – 0.6 in RO water. This presents as a problem because substrate salinity (EC) is critical where stem elongation is concerned and reducing EC below optimal is shown to increase stretch, while increased EC (salinity) is shown to reduce stretch.[26]  Therefore, an EC of 0.5 – 0.6 is too low for the stretch cycle and ideally your EC, during stretch, should be at about 1.2 – 1.4 (in coco coir dependent on fertigation frequency).

 

What this means is that while maintaining low phosphorus in solution is a means to reduce stretch, this system/technique may only be practical for those that formulate nutrients themselves.

 

Mechanically-induced stress (MIS) to Reduce Stretch

 

‘Mechanically-induced stress’ (MIS), also termed ‘mechanical handling’, occurs in nature as the above ground parts of plants are moved, usually by wind, but also by such things as rain, people and animals. It can be induced indoors by various actions such as directing oscillating fan air directly at the plants, rubbing or bending the stems or shaking or brushing or rubbing the entire plant. In other cases, MIS can be applied by pruning or pinching parts of the plant at the appropriate times.[27] The most noticeable effect of MIS is a reduction in stem, leaf or petiole length, invariably resulting in plants that are more compact with closer internode distance than unstressed controls. [28]

 

The Growth, Stress Response and MIS 

 

The effects MIS elicits in plants basically comes down to the growth, stress response where plants under stress sacrifice growth and instead direct energy towards dealing with biotic or abiotic stressors. This growth, stress response can be used to our advantage to reduce stretch and encourage closer internode distance.

 

I’ve covered some material on the growth, stress response when discussing the phytohormone jasmonic acid on page…

 

To repeat and abbreviate this material, growth and defense tradeoffs are thought to occur in plants due to ‘biotic’ (i.e. living disturbances such as fungi and pests) and ‘abiotic’ (i.e. factors that occur in nature such as high temperatures, drought, extreme sunlight and high UV) stresses, which demand prioritization towards either growth or defense, depending on external and internal factors. These tradeoffs have significant implications to growth because both processes are vital for plant survival, reproduction, and, ultimately, plant health.

 

While many of the molecular mechanisms underlying growth and defense tradeoffs remain to be fully understood, phytohormone cross-talk has emerged as a major player in regulating the balance between growth and plant defense. Just two of the phytohormones that are shown to be effected by stress are gibberellins and ethylene.

 

Studies have demonstrated that MIS techniques such as brushing or shaking plants stimulates ethylene production[29] and reduces endogenous gibberellin levels.[30]

 

Higher ethylene levels result in decreased stretch and closer internode distance. For example, when discussing the synthetic PGRs that reduce stretch, an example of one such PGR is ethephon. Unlike other PGRs, ethephon does not inhibit gibberellin or brassinosteroid biosynthesis. Instead, plants take up ethephon where it is converted to ethylene in plant cells. The increased ethylene causes cells to limit elongation and increase in width. Besides this, the release of ethylene can also reduce apical dominance, which promotes axillary branching.  Theoretically, plants release a small amount of the plant hormone ethylene when they are touched or moved (by people, the wind, etc.). With repeated and frequent plant movement, plants release enough ethylene to inhibit elongation. Research has shown that plants generally respond in a quantitative manner to the number of times they are brushed or shaken: The more frequent the brushing or shaking, the more suppression of stem elongation. This suggests that brushing or shaking plants is similar to repeatedly providing a very low concentration of ethephon to plants.

 

One of the most dramatic correlations between MIS and phytohormones involves the gibberellins. When only the shoot tips of sunflower (Helianthus annuus L.) plants were subjected to MIS, all extractablen gibberellin (GA)-like activity disappeared from those tissues compared to undisturbed controls, which contained substantial amounts of GA activity.[31]

 

Gibberellins (GAs) are the hormone most associated to promoting plant stretch. Research has shown that MIS reduces endogenous GA levels and, thus, acts to reduce stretch. For example, in research by Zeng et al (2006) it was concluded;

 

(Quote)

 

“…plants treated 30 days after planting with mechanical stress by brushing for 30 days, produced more ethylene on the third day and maintained high ethylene production, while control plants had low ethylene levels throughout the experiments. Gibberellic acid (GA)-like substances in the control plants were separated into 4 bands on silica gel TLC plates. At the same Rf of authentic GA1, GA3, and GA7, GA1- and GA3-like substance contents decreased markedly in brushed plants compared with the control plants. This result suggests that mechanical stress reduced stem elongation of chrysanthemum is affected by the activity of GA and ethylene production.” [32]

 

(End Quote)

 

MIS has been found to be a very effective way of controlling plant height (30% to 50% reductions have been recorded) of many vegetable transplants and herbs. The MIS brushing technique involves the movement of a PVC pipe, wooden dowel rod, aluminium or steel bar, or burlap bags over the top third of the plant. Research at the University of Georgia suggests that plants should be brushed daily for about 40 strokes to obtain the greatest effect. The foliage should be dry to avoid damage to the leaves. The effects of brushing on plant growth dissipate within three to four days after you stop applying the treatment.

 

Other than plant brushing, MIS can also be effectively applied by shaking plants. The general effect of shaking plants is retardation of internode elongation and inhibition of leaf expansion, which dwarfs plants in size and mass, depending on the dose of stress received.[33]

 

In addition to growth control, MIS affects other plant characteristics. Specific chlorophyll content is higher in MIS treated tomato,[34] eggplant,[35] lettuce and celery.[36] In addition, MIS increases specific leaf weight of tomato, eggplant, lettuce, celery, and cauliflower. Increased chlorophyll levels results in darker green leaves and healthier looking plants.

 

MIS also increases stem and petiole strength. For example, analysis of stem structural components showed increased percent of cellulose in the fiber component of shaken tomato stems.[37]

 

Studies have shown extremely good results in MIS treated crops. Brushing several cultivars of tomato transplants daily inhibited stem elongation by 30% to 37% compared to nontreated plants and either decreased or did not affect stem diameter.[38] However, stems of the treated plants were tougher and more elastic than those of the controls. Liptay (1985) noted that vibration reduced tomato seedling stem length by 40% and decreased stem diameter by 14%.[39] In research by Samimy (1993), where plexiglas was used to impede tomato seedling vertical growth, impedance reduced stem length of 29-day old tomato seedlings by 21% and increased stem diameter by 20%.[40] In a study by Hidayatullah (2013), where MIS was introduced through pruning cucumber plants, yield increase in relation to control was up to 61% higher. This was attributed to a decrease in endogenous GA levels at blooming and fruiting stages and increase in IAA levels at flower initiation, blooming and fruiting stages.[41]

 

 

References


[1] G.Low, 2010 – 2011, The Curious Case of the Flower Dragon and PGRs and Medical Marijuana available at www.mainbotanix.com

[2] Shimizu H. Ma Z. Tazawa S. Douzono M. Runkle E. S. Heins R. D. (2005) Blue Light Inhibits Stem Elongation of Chrysanthemum

[3] Pausch, R.C., S.J. Britz, and CL. Mulchi, Growth and Photosynthesis of Soybean (Glycine mux (L.) Merr.) in Simulated Vegetation Shade: Influence of the Ratio of Red to Far-Red Radiation, Plant, Ceil and Env.,

[4] Kotriana, S. (2013) The effect of light quality on tomato (Solanum lycopersicum L. cv ‘Efialto’) growth and drought tolerance

[5] Dougher T. A. 0. and Bugbee B. (1997)  EFFECT OF LAMP TYPE AND TEMPERATURE ON

DEVELOPMENT, CARBON PARTITIONING AND YIELD OF SOYBEAN, Adv. Space Res. Vol. 20, No. 10, pp. 1895 -1899.1997

[6] Rajapakse N. C. and Li S (2002) Exclusion of Far Red Light by Photoselective Greenhouse Films Reduces Height of Vegetable Seedlings

[7] Kotriana, S. (2013) The effect of light quality on tomato (Solanum lycopersicum L. cv ‘Efialto’) growth and drought tolerance

[8] Runkle E. Nelson J. and Bugbee B.  LEDs vs. HPS Lamps: A Reality Check, published in GPN Magazine June 2014, www.gpnmag.com

[9] Krishna Reddy S, Finlayson SA (2014) Phytochrome B promotes branching in Arabidopsis by suppressing auxin signaling. Plant Physiol. 2014 Mar;164(3):1542-50. doi: 10.1104/pp.113.234021. Epub 2014 Feb 3. See also Ronald Pierik and Mieke de Wit (2013) Shade avoidance: phytochrome signalling and other aboveground neighbour detection cues

[10] Carabelli M, Possenti M, Sessa G, Ciolfi A, Sassi M, Morelli G, Ruberti I. 2007. Canopy shade causes a rapid and transient arrest in leaf development through auxin-induced cytokinin oxidase activity. Genes and Development 21, 1863–1868.

[11] Morelli G, Ruberti I. 2000. Shade avoidance responses. Driving auxin along lateral routes. Plant Physiology 122, 621–626.

[12] Jon Anders Stavang, Bente Lindgård, Arild Erntsen, Stein Erik Lid, Roar Moe, and Jorunn E. Olsen:

Thermoperiodic Stem Elongation Involves Transcriptional Regulation of Gibberellin Deactivation in Pea, Plant Physiol. Aug 2005; 138(4): 2344–2353. See also Jensen E.  Eilertsen S.  Ernsten A. Juntilla O and Moe R: Thermoperiodic control of stem elongation and endogenous gibberellins in Campanula isophylla, Journal of Plant Growth Regulatation October 1996, Volume 15, Issue 4, pp 167 – 171

[13] Frank M. Maas, J. Hattum (1996) The Role of Gibberellins in the Thermo- and Photocontrol of Stem Elongation in Fuchsia

[14] Jon Anders Stavang, Bente Lindgård, Arild Erntsen, Stein Erik Lid, Roar Moe, and Jorunn E. Olsen:

Thermoperiodic Stem Elongation Involves Transcriptional Regulation of Gibberellin Deactivation in Pea, Plant Physiol. Aug 2005; 138(4): 2344–2353.

[15] Myster, J. and R. Moe. 1995. Effect of diurnal temperature alternations on plant morphology in some greenhouse crops a mini-review. Scientia Hort. 62:205-215.

[16] Grimstad, S.O. 1993. The effect of a daily low temperature pulse on growth and development of greenhouse cucumber and tomato plants during propagation. Scientia Hort. 53:53-62.

[17] Ueber E and Hendriks L. (1992)  Effects of intensity, duration and timing of a temperature drop on the growth and flowering of Euphorbium pulcherrima Willd. ex Klotzsch. Acta Horticulturae 1992;327:33-40.

[18] Douglas A. Bailey, Professor and Brian E. Whipker, Height Control of Commercial Greenhouse Flowers, NC State University, Department of Horticultural Science retrieved 12/14 at http://www.ces.ncsu.edu/hil/hil-528.html

[19] Erwin, J.E. and R.D. Heins. 1995. Thermomorphogenic responses in stem and leaf development. HortScience 30(5):940-949.

[20] Berghage, R.D., J.E. Erwin, and R.D. Heins. 1991. Photoperiod influences leaf chlorophyll content in chrysanthemum grown with a negative DIF temperature regime. HortScience 26:92.

[21] Erwin, J.E. and R.D. Heins. 1995. Thermomorphogenic responses in stem and leaf development. HortScience 30(5):940-949.

[22] Ueber E, and Hendriks L (1992) Effects of intensity, duration and timing of a temperature drop on the growth and flowering of Euphorbium pulcherrima Willd. ex Klotzsch. Acta Horticulturae 1992;327:33-40.

[23] Downs R. (1975) Environment and the Experimental Control of Plant Growth pp. 25, Academic Press

Paul V. Nelson, Chen-Young Song, and Jin-Sheng Huang (2002) What Really Causes Stretch? Retrieved from http://www.gpnmag.com/what-really-causes-stretch: see also, Plaster, E. J (2008) Soil Science and Management pp. 266

[25] Hansen C. W. and Kai N. L, Non-Chemical Growth Regulation of Ornamental Plants. Department of Ornamentals, Aarslev, Denmark, found in Gron Viden Special Issue published in English No. 121 April 2000

[26] Oki, L.R. and Lieth, J.H. Effect of changes in substrate salinity on the elongation of Rosa hybrida L. ‘Kardinal’ stems. Scientia horticulturae; 2004 May 3, v. 101, issues 1-2

[27] Hidayatullah, Asghari  Bano and Mansab Ali Khokhar (2013) Phytohormones Content in Cucumber Leaves by Using Pruning as a Mechanical Stress

[28] Biddington N. L. (1986) The effects of mechanically-induced stress in plants — a review, Plant Growth Regulation, 1986, Volume 4, Issue 2, pp 103-123

[29] Mensuali-Sodi, A., Serra, G., Veiga de vincenzo, M.C., Tognoni, F. and Ferrante, A. 2006. ETHYLENE RESPONSE TO MECHANICAL STRESS PERTURBATION OF SALVIA SPLENDENS L. POTTED PLANTS . Acta Hort. (ISHS) 723:421-426 See also Biddington N. L.The effects of mechanically-induced stress in plants — a review, Plant Growth Regulation, 1986, Volume 4, Issue 2, pp 103-123

[30] Zheng C. Wang W. and Hara T. (2006) Mechanical Stress Modifies Endogenous Ethylene and Gibberellin Production in Chrysanthemum

[31] Beyl, C. and C. Mitchell. 1983. Alteration of growth, exudation rate, and endogenous hormone profiles in mechanically dwarfed sunflower. J. Amer. Soc. Hort. Sci. 108:257–262.

[32] Zheng C. Wang W. and Hara T. (2006) Mechanical Stress Modifies Endogenous Ethylene and Gibberellin Production in Chrysanthemum

[33] Mitchell C. A. (1996) Recent Advances in Plant Response to Mechanical Stress: Theory and Application

[34] Mitchell, C.A., C.J. Severson, J.A. Wott, and P.A. Hammer. 1975. Seismomorphogenic regulation of plant growth. J. Amer, Soc. Hort. Sci. 100:161-165

[35] Latimer, J.G. and C.A. Mitchell. 1988 Effects of mechanical stress or abscisic acid on growth, water status, and leaf abscisic acid content on eggplant seedlings. Scientia Hort. 36:37-46.

[36] Biddington, N,L. and A.S. Dearman 1985. The effect of mechanically induced stress on the growth of cauliflower, lettuce and celery seedlings. Ann. Bot. 55:109-119.

[37] Latimer J. G. Mechanical Conditioning to Control Height, HortTechnology October – December 1998 8(4)

[38] Johjima, T., J.G. Latimer, and H. Wakita. 1992. Brushing influences transplant growth and subsequent yield of four cultivars of tomato and their hybrid lines. J. Amer. Soc. Hort. Sci. 117:384-388. See also Latimer, J.G. and P.A. Thomas. 1991. Application of brushing for growth control of tomato transplants in a commercial setting. HortTechnology 1:109-110.

[39] Liptay, A. 1985. Reduction of spindliness of tomato transplants growth at high density. Can. J. Plant Sci. 65:797-801.

[40] Samimy C. Physical Impedance Retards Top Growth of Tomato Transplants. HORTSCIENCE 28(9):883-885. 1993.

[41] Hidayatullah, Asghari  Bano and Mansab Ali Khokhar (2013) Phytohormones Content in Cucumber Leaves by Using Pruning as a Mechanical Stress