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).
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