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). 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.

Longer periods of light are not better, at least after a point!

Mike

Supplemental UV(380nm) Light, greatly enhances tomato plant growth; while supplemental Orange (622nm), Yellow (595nm) and Green (520nm) Light produces negative effects over the life of the plant.

As seen here:
http://www.lzi.lt/tomai/972tomas/97_2_tomas_str10.pdf