well wat y'all think????!!! do plants REALLY need the dark photoperiod? I always tthought, ya it was a healthy idea but I've heard a lot of other ppl say its pointless and 24 of light will grow faster.
If you use the search function you will find that a very debated topic that stretches back to probably the creation of this site lol. Theres no proof that a plant requires "sleep". You will get alot more growth if you run it 24/0 vs 18/6. Every 4 days of 18/6 is 24 hours less of light. Being that growing Cannabis is not in anyway a "new" thing people all over the world have used either with great results and no negatives just about. That being said 24/0 works just fine and so does 18/6.
Agreed. I've been using 24/0 with my PC stealth grow and they seem to be doing just fine. My plants are also very bushy which is great for a stealth grow. Some people, me included, feel that 24/0 also helps produce shorty, stickier plants due to little or no stretch taking place.
Sorry in advance if this is a little long But for anyone curious about the details behind our dark phase when growing, this may be a good read 1) If you're using small enough lights, which do not put nearly the same amount of strain on a plants respiratory system as HIDs, or the sun, then you can sometimes get away with 24/0 and pull off a little extra growth, and ultimately a slightly larger yield. If a slightly less efficient or lower gram per kilowatt hour isn't an issue (and with small CFLs, it's probably just a matter of a few dollars) then this is a good thing, when it works out. Where low-watt rooms are smaller yielders anyhow, a few extra grams/ounces a year is a nice bonus. 2) However under normal (higher/natural) light conditions, or in other words under the sun or HID lighting, with no other species/type of plant, have they ever managed to prove anything besides the fact that 24/0 HINDERS growth and REDUCES yield, especially when compared to both the increased energy and nutrient consumption throughout the course of the grow. Unlike a low-wattage light system, when running their respiratory systems and photosynthesis on max 24 hours a day, with high wattage lights, every day for weeks on end, it damages the foliage that grows and exists during that period, and stunts its ability to regenerate cells, respire/transpire, and photosynthesize, ultimately limiting the energy it's capable of providing to the plant. Plants respire and have pores which function, open and close, depending on the light cycle.. if you keep them awake all the time, they become stressed and start losing vigor, and eventually (late-stage) they develop chlorosis. Light doesn't just control the flowering cycle, and especially for those plants whose fruit/flowers are controlled by light, it's important that they are given a short, dark resting period, even in veg. Prolonged light exposure induces a very specific kind of respiratory and hormonal stress which hinders, stunts and limits growth. By the time you begin blooming, the growth you've encouraged with all those extra hours of light may or may not even be greater in size, and what's worse, it will be damaged and will not have the same productivity as healthier growth produced under a normal light cycle. Every plant has a different 'preferred' photo-period, some require less light and some need more during veg, but nearly -all- species of plants prefer at least some amount of sleep, in order to achieve the most vegetative-growth/yield, with the least amount of stress. The ONLY REASON people assume at first glance that it's somehow safe or healthy for cannabis, is because the life-cycle we run them through is relatively short, and it's short enough that by the time we flip to 12/12 and allow them some rest, the damage has been done to their growth and vigor, while later-stage signs still may not be visible to the naked eye. These plants can take a lot of abuse and stunting without looking 'ugly', so often times it's only when examining the yield at the end, that we realize something was amiss. Yes, a few 'experts' in the cannabis industry, not really caring much for other plants, or for botany or horticulture in general, have over the years claimed that you can 'yield more' with a full day light cycle, but it's just not true. Many newer growers, when this thought occurs to them, will also sometimes chalk up a small increase in yield to the extension in light cycle... when in reality, their skills or even their supplies may have improved. Some growers are still dialing in their rooms on their decennial (ie. ten year, growing 'canna-versary' ). The idea that "More light may = Bigger yield" (cue the light-bulb feeling) hits almost every grower, eventually... And just like assuming that adding too much nutrient, or too hot or concentrated a compost to the soil too early, is going to improve your yield, it's just not logical.... These are living organisms, and interrupting something as integral as the daily dark phase that plants have required for thousands of years on an assumption that it will somehow improve their health and vigor, while it makes sense on the surface (ie. more = better ), it is not very logical thinking. It is wishful thinking. If you want the best of both worlds, try 20/4, or even 19/5. This adds a little more energy over the course of the vegetative stage, without totally removing the regular rest period. Most plants will yield the most fruit/flowers by their respective harvests, with a 14 - 20 hour per day light cycle. Depending on their individual, and unique genetic proximity to the hemisphere, so do most cannabis strains. They've proven time, and time again, that virtually all plants existing naturally under normal daylight-darkness conditions, require a regular, dark rest phase... cannabis is not all that unique in its requirements, compared to other light-sensitive plants that require a specific photo-period to function at their prime. After years of trial an error, on both cannabis, and a plethora of other flowering and fruit/veg-bearing plants, they've consistently determined that plants yield roughly the same or less when running 24/0. --- Yielding the same doesn't sound like such a big loss on the surface, but think about it this way.... During a month long veg cycle for instance, that's an extra 180 hours of bulb-life you're wasting, nutrient/water/evaporation, and electricity cost you're sacrificing, for no gain (and sometimes, at a loss!). So even if you're yielding more, which is the less-likely scenario, if you're not yielding enough more you would still be producing fewer grams per kilowatt hour. With a 3 week veg period at 24/0 you need to yield at least 20% more in order to break even with 18/6.. and the longer the veg period, and the more night-time hours used for light, the higher and less tangible that number becomes. In other words if you do go for it, short veg cycles are your best bet. Although, I will give it to you that the added costs for producing a small bump in yield, are still cheaper than buying herb off the streets And if money for the extra electricity and nutrient consumption isn't an issue, then there's no reason not to try shooting for that occasional larger yield every now and then.... except, that they haven't performed any long-running tests on how this 'early-life stress' effects the potency of the finished flowers. --- And a few articles, some on plant hormones regulated by the dark phase, some on several studies on a variety of plants discussing 24/0 light cycles, all of which displayed negative consequences when it came to yield and (particularly in long-term studies) overall visible plant health and vigor.... " One of the biggest contrasts between day and night for plants involves the concentrations of hormones called phytochromes. These are light-sensitive hormones found in all leaves that essentially wake up the plant's genetic mechanisms to face the new day when the lights come on. They also control the genetic switches that direct growth by plants, and determine all growth responses such as when plants grow vegetatively or whether they bloom. Phytochromes are the time-keeper hormones in plants, just as melatonin secreted from the pineal gland regulates the sense of time in animals. Although plants have not evolved endocrine glands and neurons, they do sense time in a remarkably similar way using their phytochromes. We can see by this similarity that at least on some level plants share with animals the ability to tell night from day and to measure time in a meaningful way—certainly both basic functions we recognize in "conscious" organisms. " - Maximum Yield - Indoor Gardening " OPTIMAL PHOTOPERIODS For tomato, best growth and yield were obtained under a photoperiod of 14 hours (Vézina et al., 1991; Demers et al., 1998b). Photoperiods longer than 14 h did not further increase yield. Photoperiods of 20 and 24 h can even decrease yield and caused leaf chlorosis after 6 to 8 weeks (Vézina et al., 1991; Demers et al., 1998b). Although long term use of a 17-h photoperiod does not increase growth and yield compared to 14 h, it might be interesting to extend the photoperiod to 17 h in order to increase total light provided to plants especially during the months with the lowest natural light levels (December-January). However, if a 17-h photoperiod is used, it is important that the dark period be uninterrupted, since splitting the dark period of 7 h in two short nights of 3.5 h (separated by a light period of 4 h) caused leaf chlorosis and decreased growth and yield (Vézina et al., 1991). For sweet pepper, a 20 h-photoperiod was optimal for plant growth and productivity (Demers et al., 1998a). Yield under continuous light (24-h photoperiod) was equivalent to yield under photoperiods of 15 or 16 h (Costes et al., 1970; Demers et al., 1998a). (However) extension of the photoperiod from 15 or 16 h to 24 h decreased the average size of pepper fruits (Costes et al., 1970; Demers et al., 1998a). Continuous light caused some leaf deformities (wrinkles) but no chlorosis in sweet pepper grown in greenhouses. Although long term use of continuous light is detrimental to tomato and pepper plants, tomato and sweet pepper plants can take advantage of the extra light energy provided by continuous lighting for a short period of time. Early vegetative growth and fruit production of tomato and pepper plants were generally improved under continuous light compared the 14-h photoperiod (Demers et al., 1998a, 1998b). However, after that initial period, plants under continuous light grew more slowly than plants exposed to 14-h photoperiod; so that tomato and pepper plant growth and yield under 14-h photoperiod were then equal to or higher than under continuous light at the end of the experiment. Costes et al. (1970) also observed that continuous light improved the early performance (hastening of flowering and fruit set, increased early yield) of sweet pepper plants compared to a 15-h photoperiod. Therefore, it might be possible to use continuous light for a short period of time (5 to 7 weeks) to improve growth of tomato and sweet pepper, especially during the months with the lowest natural light levels (December and January). However, such a practice should be investigated in order to determine if short term use of continuous light might have residual negative effects on tomato and sweet pepper plants. NEGATIVE EFFECTS OF LONG PHOTOPERIODS AND THE FACTORS INVOLVED IN THEIR DEVELOPMENT Tomato and sweet pepper plants do not take advantage (no increase in yield) when grown under photoperiods longer than 14 h (tomato) or 20 h (pepper). Tomato plants, but not sweet pepper, develop leaf chlorosis under continuous light. In the next sections, we will examine the role of the carbon metabolism, pigments, light spectral quality and day/night temperature differential in the development of these negative effects of long photoperiods. Carbon Metabolism High starch and soluble sugar accumulations were observed in leaves of tomato plants grown under long photoperiods, and it was suggested that these accumulations could be related to the development of the leaf chlorosis (Bradley et al., 1985; Logendra et al., 1990; Dorais, 1992). Studies on other species support the hypothesis of a relationship between leaf chlorosis development and starch and sugar accumulations. For example, continuous light caused increased leaf starch and hexose accumulations and leaf chlorosis of eggplants (Solanum melongena L.) (Murage et al., 1996). However, eggplants growing under continuous light but in a CO2-free atmosphere for 12 h per day accumulated less starch and hexoses, and did not develop leaf chlorosis. Exposure of tomato and sweet pepper plants to continuous light resulted in increased foliar contents in starch in tomato and sweet pepper, in hexoses (glucose and fructose) in tomato and sucrose in sweet pepper (Dorais et al., 1996; Demers et al., 1998a, 1998b). However, the reduction of the number of fruits on the plants did not modify the pattern of accumulation of starch and sugars in leaves of tomato and sweet pepper plants exposed to photoperiods of 14 and 24 h (Demers et al., 1998a, 1998b). Moreover, the reduction of the number of fruits on the plants did not influence the severity nor the date of appearance of the foliar chlorosis in tomato plants grown under continuous light. This indicates that accumulations of starch and soluble sugars are not caused by a limiting sink capacity. If there is a relationship between the excessive starch and soluble sugar accumulations and the development of the negative effects (leaf chlorosis, decreased growth and productivity) of the long photoperiods on tomato and sweet pepper, it is most likely a limitation of the carbon metabolism at the leaf level which is responsible for these accumulations. In tomato, the use of continuous light caused, in addition to the foliar chlorosis and increased foliar contents in starch and hexoses, a reduction of the photosynthesis rate and of the activity of the sucrose phosphate synthase (SPS) enzyme (Demers, 1998). These reductions in photosynthesis and of SPS activity occurred between 6th and 8th week under continuous light, i.e. about at the same time as the foliar chlorosis appeared, while starch and hexoses contents in leaves increased during the first 4 weeks of the experiment. Since the reduction of the SPS activity occurred after the increase in starch and hexoses, it is thus impossible that the reduction of the SPS activity is responsible for these accumulations. However, it is possible that the SPS activity in vivo is limiting, which would explain the hexose increase. This suggests the limiting step of the export of photosynthates is the synthesis of sucrose in tomato and would explain the absence of growth and the productivity increase under continuous light. Furthermore, the increased hexose levels in the cytoplasm, by a feedback effect, would limit the export of the triosephosphate (photosynthesis products) out of the chloroplast, which would then be redirected towards starch synthesis, thus explaining the increased starch contents. Moreover, the increased accumulation of starch would generate, by a feedback effect, an overload of the Calvin cycle, which would gradually cause the observed decrease of the CO2 fixation rate. Are the starch accumulations responsible for the leaf chlorosis in tomato? It is possible that the overload imposed on the Calvin cycle (decreased photosynthesis) could limit the use of the reducing potential (ATP, NADPH) produced by the luminous phase of photosynthesis, thus causing an overload on the electron transport chain and the photo-oxidation of the chlorophylls (decrease in the leaf chlorophyll contents), and thus explaining the observed leaf foliar chlorosis. Transgenic tomato plants (in which a gene coding for the SPS enzyme was incorporated and overexpress this enzyme) could be used in future studies to test if accumulations of starch in leaves are responsible for the development of chlorosis observed in tomato plants exposed to continuous light. Transgenic tomato plants (overexpressing SPS) have higher photosynthesis rates and accumulate less starch and more sucrose than non-transformed plants, especially under conditions of saturating light and CO2 (Galtier et al., 1993, 1995; Micallef et al., 1995). One can put forth the assumption that, under continuous light, leaf starch contents would be lower in transgenic plants than in normal plants. If this is the case, the reduction of the leaf starch content in transgenic plants should thus prevent the development of the leaf chlorosis, or at least decrease its severity. In sweet pepper, the use of continuous light caused an increase in the leaf starch and sucrose contents, but did not affect leaf hexose contents, photosynthesis rates and SPS activity (Demers, 1998). The increased foliar contents in sucrose indicate that SPS activity in sweet pepper is not limiting as in tomato. Increased accumulation of starch in sweet pepper plants exposed to continuous light would be explained by the fact that continuous light results in a longer period of time over which starch synthesis occur, but without overloading the starch synthesis pathway. Thus, starch accumulation in sweet pepper under continuous light would not be important enough to cause a reduction in CO2 fixation (no overload of the Calvin cycle). Increased leaf contents in sucrose suggest that sucrose export would be possibly limiting. In sweet pepper plants, the export rate of carbon (as sucrose) out of the leaf is constant, and the export rate would be limited at the level of the loading of sucrose in the phloem (Grange, 1985, 1987). This would explain why the growth and the productivity of the sweet pepper plants do not increase under continuous light. Pigments In growth chambers, continuous light caused leaf chlorosis, decreased photosynthesis rates, and reductions in leaf contents in pigments (chlorophyll a and b, carotene, xanthophylls) in both tomato and sweet pepper plants (Demers, 1998). Leaf chlorosis, decreased photosynthesis rates and loss of pigments were more important and occurred earlier in tomato plants than in sweet pepper. Compared to sweet pepper plants, EPS ratio (epoxidation state of the pigments of the xanthophyll cycle) was lower in tomato, indicating a greater need for energy dissipation and a more important state of stress (caused by excessive light). Pigments such as carotene and xanthophylls (violaxanthin, antheraxanthin, zeaxanthin) play a significant role in the protection of the photosynthetic apparatus against damage that could be caused by an excess of light. Carotene and xanthophyll levels were higher in sweet pepper plants than in tomato. Thus, sweet pepper has a better protection against the degradation of chlorophylls, which would explain why leaf chlorosis appeared later and were less severe in sweet pepper." What are the Consequences of Leaving Lights on 24/7 - Growing under Lights Forum - GardenWeb ----- Anyhow, I know this is a lot for most people! But it should be an interesting read for anyone actually curious about 24/0 vs. a more commonly/natural light cycle. Hope this helps!
[quote name='"BadKittySmiles"']Sorry in advance if this is a little long But for anyone curious about the details behind our dark phase when growing, this may be a good read 1) If you're using small enough lights, which do not put nearly the same amount of strain on a plants respiratory system as HIDs, or the sun, then you can sometimes get away with 24/0 and pull off a little extra growth, and ultimately a slightly larger yield. If a slightly less efficient or lower gram per kilowatt hour isn't an issue (and with small CFLs, it's probably just a matter of a few dollars) then this is a good thing, when it works out. Where low-watt rooms are smaller yielders anyhow, a few extra grams/ounces a year is a nice bonus. 2) However under normal (higher/natural) light conditions, or in other words under the sun or HID lighting, with no other species/type of plant, have they ever managed to prove anything besides the fact that 24/0 HINDERS growth and REDUCES yield, especially when compared to both the increased energy and nutrient consumption throughout the course of the grow. Unlike a low-wattage light system, when running their respiratory systems and photosynthesis on max 24 hours a day, with high wattage lights, every day for weeks on end, it damages the foliage that grows and exists during that period, and stunts its ability to regenerate cells, respire/transpire, and photosynthesize, ultimately limiting the energy it's capable of providing to the plant. Plants respire and have pores which function, open and close, depending on the light cycle.. if you keep them awake all the time, they become stressed and start losing vigor, and eventually (late-stage) they develop chlorosis. Light doesn't just control the flowering cycle, and especially for those plants whose fruit/flowers are controlled by light, it's important that they are given a short, dark resting period, even in veg. Prolonged light exposure induces a very specific kind of respiratory and hormonal stress which hinders, stunts and limits growth. By the time you begin blooming, the growth you've encouraged with all those extra hours of light may or may not even be greater in size, and what's worse, it will be damaged and will not have the same productivity as healthier growth produced under a normal light cycle. Every plant has a different 'preferred' photo-period, some require less light and some need more during veg, but nearly -all- species of plants prefer at least some amount of sleep, in order to achieve the most vegetative-growth/yield, with the least amount of stress. The ONLY REASON people assume at first glance that it's somehow safe or healthy for cannabis, is because the life-cycle we run them through is relatively short, and it's short enough that by the time we flip to 12/12 and allow them some rest, the damage has been done to their growth and vigor, while later-stage signs still may not be visible to the naked eye. These plants can take a lot of abuse and stunting without looking 'ugly', so often times it's only when examining the yield at the end, that we realize something was amiss. Yes, a few 'experts' in the cannabis industry, not really caring much for other plants, or for botany or horticulture in general, have over the years claimed that you can 'yield more' with a full day light cycle, but it's just not true. Many newer growers, when this thought occurs to them, will also sometimes chalk up a small increase in yield to the extension in light cycle... when in reality, their skills or even their supplies may have improved. Some growers are still dialing in their rooms on their decennial (ie. ten year, growing 'canna-versary' ). The idea that "More light may = Bigger yield" (cue the light-bulb feeling) hits almost every grower, eventually... And just like assuming that adding too much nutrient, or too hot or concentrated a compost to the soil too early, is going to improve your yield, it's just not logical.... These are living organisms, and interrupting something as integral as the daily dark phase that plants have required for thousands of years on an assumption that it will somehow improve their health and vigor, while it makes sense on the surface (ie. more = better ), it is not very logical thinking. It is wishful thinking. If you want the best of both worlds, try 20/4, or even 19/5. This adds a little more energy over the course of the vegetative stage, without totally removing the regular rest period. Most plants will yield the most fruit/flowers by their respective harvests, with a 14 - 20 hour per day light cycle. Depending on their individual, and unique genetic proximity to the hemisphere, so do most cannabis strains. They've proven time, and time again, that virtually all plants existing naturally under normal daylight-darkness conditions, require a regular, dark rest phase... cannabis is not all that unique in its requirements, compared to other light-sensitive plants that require a specific photo-period to function at their prime. After years of trial an error, on both cannabis, and a plethora of other flowering and fruit/veg-bearing plants, they've consistently determined that plants yield roughly the same or less when running 24/0. --- Yielding the same doesn't sound like such a big loss on the surface, but think about it this way.... During a month long veg cycle for instance, that's an extra 180 hours of bulb-life you're wasting, nutrient/water/evaporation, and electricity cost you're sacrificing, for no gain (and sometimes, at a loss!). So even if you're yielding more, which is the less-likely scenario, if you're not yielding enough more you would still be producing fewer grams per kilowatt hour. With a 3 week veg period at 24/0 you need to yield at least 20% more in order to break even with 18/6.. and the longer the veg period, and the more night-time hours used for light, the higher and less tangible that number becomes. In other words if you do go for it, short veg cycles are your best bet. Although, I will give it to you that the added costs for producing a small bump in yield, are still cheaper than buying herb off the streets And if money for the extra electricity and nutrient consumption isn't an issue, then there's no reason not to try shooting for that occasional larger yield every now and then.... except, that they haven't performed any long-running tests on how this 'early-life stress' effects the potency of the finished flowers. --- And a few articles, some on plant hormones regulated by the dark phase, some on several studies on a variety of plants discussing 24/0 light cycles, all of which displayed negative consequences when it came to yield and (particularly in long-term studies) overall visible plant health and vigor.... " One of the biggest contrasts between day and night for plants involves the concentrations of hormones called phytochromes. These are light-sensitive hormones found in all leaves that essentially wake up the plant's genetic mechanisms to face the new day when the lights come on. They also control the genetic switches that direct growth by plants, and determine all growth responses such as when plants grow vegetatively or whether they bloom. Phytochromes are the time-keeper hormones in plants, just as melatonin secreted from the pineal gland regulates the sense of time in animals. Although plants have not evolved endocrine glands and neurons, they do sense time in a remarkably similar way using their phytochromes. We can see by this similarity that at least on some level plants share with animals the ability to tell night from day and to measure time in a meaningful way—certainly both basic functions we recognize in "conscious" organisms. " - Maximum Yield - Indoor Gardening " OPTIMAL PHOTOPERIODS For tomato, best growth and yield were obtained under a photoperiod of 14 hours (Vézina et al., 1991; Demers et al., 1998b). Photoperiods longer than 14 h did not further increase yield. Photoperiods of 20 and 24 h can even decrease yield and caused leaf chlorosis after 6 to 8 weeks (Vézina et al., 1991; Demers et al., 1998b). Although long term use of a 17-h photoperiod does not increase growth and yield compared to 14 h, it might be interesting to extend the photoperiod to 17 h in order to increase total light provided to plants especially during the months with the lowest natural light levels (December-January). However, if a 17-h photoperiod is used, it is important that the dark period be uninterrupted, since splitting the dark period of 7 h in two short nights of 3.5 h (separated by a light period of 4 h) caused leaf chlorosis and decreased growth and yield (Vézina et al., 1991). For sweet pepper, a 20 h-photoperiod was optimal for plant growth and productivity (Demers et al., 1998a). Yield under continuous light (24-h photoperiod) was equivalent to yield under photoperiods of 15 or 16 h (Costes et al., 1970; Demers et al., 1998a). (However) extension of the photoperiod from 15 or 16 h to 24 h decreased the average size of pepper fruits (Costes et al., 1970; Demers et al., 1998a). Continuous light caused some leaf deformities (wrinkles) but no chlorosis in sweet pepper grown in greenhouses. Although long term use of continuous light is detrimental to tomato and pepper plants, tomato and sweet pepper plants can take advantage of the extra light energy provided by continuous lighting for a short period of time. Early vegetative growth and fruit production of tomato and pepper plants were generally improved under continuous light compared the 14-h photoperiod (Demers et al., 1998a, 1998b). However, after that initial period, plants under continuous light grew more slowly than plants exposed to 14-h photoperiod; so that tomato and pepper plant growth and yield under 14-h photoperiod were then equal to or higher than under continuous light at the end of the experiment. Costes et al. (1970) also observed that continuous light improved the early performance (hastening of flowering and fruit set, increased early yield) of sweet pepper plants compared to a 15-h photoperiod. Therefore, it might be possible to use continuous light for a short period of time (5 to 7 weeks) to improve growth of tomato and sweet pepper, especially during the months with the lowest natural light levels (December and January). However, such a practice should be investigated in order to determine if short term use of continuous light might have residual negative effects on tomato and sweet pepper plants. NEGATIVE EFFECTS OF LONG PHOTOPERIODS AND THE FACTORS INVOLVED IN THEIR DEVELOPMENT Tomato and sweet pepper plants do not take advantage (no increase in yield) when grown under photoperiods longer than 14 h (tomato) or 20 h (pepper). Tomato plants, but not sweet pepper, develop leaf chlorosis under continuous light. In the next sections, we will examine the role of the carbon metabolism, pigments, light spectral quality and day/night temperature differential in the development of these negative effects of long photoperiods. Carbon Metabolism High starch and soluble sugar accumulations were observed in leaves of tomato plants grown under long photoperiods, and it was suggested that these accumulations could be related to the development of the leaf chlorosis (Bradley et al., 1985; Logendra et al., 1990; Dorais, 1992). Studies on other species support the hypothesis of a relationship between leaf chlorosis development and starch and sugar accumulations. For example, continuous light caused increased leaf starch and hexose accumulations and leaf chlorosis of eggplants (Solanum melongena L.) (Murage et al., 1996). However, eggplants growing under continuous light but in a CO2-free atmosphere for 12 h per day accumulated less starch and hexoses, and did not develop leaf chlorosis. Exposure of tomato and sweet pepper plants to continuous light resulted in increased foliar contents in starch in tomato and sweet pepper, in hexoses (glucose and fructose) in tomato and sucrose in sweet pepper (Dorais et al., 1996; Demers et al., 1998a, 1998b). However, the reduction of the number of fruits on the plants did not modify the pattern of accumulation of starch and sugars in leaves of tomato and sweet pepper plants exposed to photoperiods of 14 and 24 h (Demers et al., 1998a, 1998b). Moreover, the reduction of the number of fruits on the plants did not influence the severity nor the date of appearance of the foliar chlorosis in tomato plants grown under continuous light. This indicates that accumulations of starch and soluble sugars are not caused by a limiting sink capacity. If there is a relationship between the excessive starch and soluble sugar accumulations and the development of the negative effects (leaf chlorosis, decreased growth and productivity) of the long photoperiods on tomato and sweet pepper, it is most likely a limitation of the carbon metabolism at the leaf level which is responsible for these accumulations. In tomato, the use of continuous light caused, in addition to the foliar chlorosis and increased foliar contents in starch and hexoses, a reduction of the photosynthesis rate and of the activity of the sucrose phosphate synthase (SPS) enzyme (Demers, 1998). These reductions in photosynthesis and of SPS activity occurred between 6th and 8th week under continuous light, i.e. about at the same time as the foliar chlorosis appeared, while starch and hexoses contents in leaves increased during the first 4 weeks of the experiment. Since the reduction of the SPS activity occurred after the increase in starch and hexoses, it is thus impossible that the reduction of the SPS activity is responsible for these accumulations. However, it is possible that the SPS activity in vivo is limiting, which would explain the hexose increase. This suggests the limiting step of the export of photosynthates is the synthesis of sucrose in tomato and would explain the absence of growth and the productivity increase under continuous light. Furthermore, the increased hexose levels in the cytoplasm, by a feedback effect, would limit the export of the triosephosphate (photosynthesis products) out of the chloroplast, which would then be redirected towards starch synthesis, thus explaining the increased starch contents. Moreover, the increased accumulation of starch would generate, by a feedback effect, an overload of the Calvin cycle, which would gradually cause the observed decrease of the CO2 fixation rate. Are the starch accumulations responsible for the leaf chlorosis in tomato? It is possible that the overload imposed on the Calvin cycle (decreased photosynthesis) could limit the use of the reducing potential (ATP, NADPH) produced by the luminous phase of photosynthesis, thus causing an overload on the electron transport chain and the photo-oxidation of the chlorophylls (decrease in the leaf chlorophyll contents), and thus explaining the observed leaf foliar chlorosis. Transgenic tomato plants (in which a gene coding for the SPS enzyme was incorporated and overexpress this enzyme) could be used in future studies to test if accumulations of starch in leaves are responsible for the development of chlorosis observed in tomato plants exposed to continuous light. Transgenic tomato plants (overexpressing SPS) have higher photosynthesis rates and accumulate less starch and more sucrose than non-transformed plants, especially under conditions of saturating light and CO2 (Galtier et al., 1993, 1995; Micallef et al., 1995). One can put forth the assumption that, under continuous light, leaf starch contents would be lower in transgenic plants than in normal plants. If this is the case, the reduction of the leaf starch content in transgenic plants should thus prevent the development of the leaf chlorosis, or at least decrease its severity. In sweet pepper, the use of continuous light caused an increase in the leaf starch and sucrose contents, but did not affect leaf hexose contents, photosynthesis rates and SPS activity (Demers, 1998). The increased foliar contents in sucrose indicate that SPS activity in sweet pepper is not limiting as in tomato. Increased accumulation of starch in sweet pepper plants exposed to continuous light would be explained by the fact that continuous light results in a longer period of time over which starch synthesis occur, but without overloading the starch synthesis pathway. Thus, starch accumulation in sweet pepper under continuous light would not be important enough to cause a reduction in CO2 fixation (no overload of the Calvin cycle). Increased leaf contents in sucrose suggest that sucrose export would be possibly limiting. In sweet pepper plants, the export rate of carbon (as sucrose) out of the leaf is constant, and the export rate would be limited at the level of the loading of sucrose in the phloem (Grange, 1985, 1987). This would explain why the growth and the productivity of the sweet pepper plants do not increase under continuous light. Pigments In growth chambers, continuous light caused leaf chlorosis, decreased photosynthesis rates, and reductions in leaf contents in pigments (chlorophyll a and b, carotene, xanthophylls) in both tomato and sweet pepper plants (Demers, 1998). Leaf chlorosis, decreased photosynthesis rates and loss of pigments were more important and occurred earlier in tomato plants than in sweet pepper. Compared to sweet pepper plants, EPS ratio (epoxidation state of the pigments of the xanthophyll cycle) was lower in tomato, indicating a greater need for energy dissipation and a more important state of stress (caused by excessive light). Pigments such as carotene and xanthophylls (violaxanthin, antheraxanthin, zeaxanthin) play a significant role in the protection of the photosynthetic apparatus against damage that could be caused by an excess of light. Carotene and xanthophyll levels were higher in sweet pepper plants than in tomato. Thus, sweet pepper has a better protection against the degradation of chlorophylls, which would explain why leaf chlorosis appeared later and were less severe in sweet pepper." What are the Consequences of Leaving Lights on 24/7 - Growing under Lights Forum - GardenWeb ----- Anyhow, I know this is a lot for most people! But it should be an interesting read for anyone actually curious about 24/0 vs. a more commonly/natural light cycle. Hope this helps! [/quote] Kitty I only had time to read half of that because I have to leave in a minute, but I've been wondering if 24/0 is a viable method for SOG grows and micro grows where there is only a very short veg time, and therefor, minimal time to stress out the leaves? I don't think it's a good idea for large grow rooms with large plants because weeks of veg at 24/0 is stressful on the plant, like you mentioned. But I wonder how they would have 1-2 weeks of it and if it would increase yield in that scenario.
After reading this im concerned about my veg lighting. Im running 24/0 on 2 150w cfls and after reading im thinking about switching to a 20/4 or 18/6 just for the plants to use up their "batteries". My concern however is that i just transplanted a little over an hour ago so would all the stress from the transplant and the change in lighting cause too much stress on my younglings?
Well thanks Imagine, appreciate the feedback. Gonna give the lil ladies sometime to bounce back then perhaps give some naptime so to speak...feel free to hop over to my journal, atleast i hope its a journal, and take a look at what i got going on. Its kind of "ghetto" but im making due