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24hr light ? don't they need at least a few hrs. of dark time ?
Yep in short-day photoperiod sensitive plants its not advisable to run 24 hour lighting. Another myth in the pot scene that has remained with a few. Its also the reason he is finding it is better at low temperatures
A 2010 literature review covering numerous studies surrounding continuous (24-hour) lighting highlights several things:
1) Research findings surrounding continuous (24-hour) lighting are highly variable and often contradictory. In some instances, growth gains are seen while in other studies no gains are seen, while in yet other studies growth rates and plant health is negatively impacted.
2) In most long-day plants continuous light accelerated growth while in most short-day plants no benefits were seen. Some long-day plants include carnation, oat, pea and barley while the short-day plants include cotton, rice and cannabis.
3) The use of continuous light caused a reduction in the photosynthesis rate of photoperiod-sensitive tomato, but did not affect photosynthesis rates in less photoperiod-sensitive sweet pepper. The hypothesis here is that the continuous lighting of tomato increased the accumulation of starch, by feedback effect, causing an overload of the Calvin Cycle. The end result was that photosynthesis and plant health was negatively impacted.
4) Under extended photoperiods, many sensitive species tend to develop physiological disorders. Light injury symptoms caused by exposing plants to continuous light were reported for several species including tomato.
5) Light quantity and quality play important roles in determining outcomes pertaining to leaf injury. For example, the extent of the leaf injury under continuous light was heavily influenced by the intensity of light received. Additionally, the severity of leaf damage caused by continuous light varied with the type of lamps (high pressure sodium versus metal halide) used to provide supplemental light.
6) Daytime temperatures and thermoperiod DIF also appear to play an important role in leaf injury under continuous light. [1]
Technicalities aside, while some might tell you that using a 24-hour light cycle in ‘veg’ is the go, things are more complex than this. Many indoor growers produce a short-day, photoperiod-sensitive plant under lights and what studies are available show that short-day photoperiod-sensitive plants 1) do not gain much, if any, growth benefits from 24 hours of light and 2) under certain conditions they can develop physiological disorders (i.e. 24-hour lighting causes plant stress).
[1] Plants under Continuous Light: A Review Sysoeva M.L, Markouskaya E. F & Shibaeva T.G. Plant Stress, Global Sciences Book 2010