Effects of supplemental light duration on greenhouse tomato (Lycopersicon esculentum Mill.) plants and fruit yields

In one experiment, tomato plants were grown under natural light or extended photoperiods of 14, 16, 20 and 24 h. Compared to natural light, extended photoperiod treatments increased shoot fresh weight and yields of tomato plants by, respectively, 40–57% (P<0.05) and 15–20% (P>0.05). However, photoperiods longer than 14 h did not further improve growth and yields. In the second experiment, tomato plants were exposed to a 14 or 24 h photoperiod and clusters were either pruned to 1 fruit or not. Tomato plants exposed to continuous light (24 h photoperiod) started developing leaf chloroses after 7 weeks of treatments. We observed that, for the first 5 to 7 weeks of treatments, tomato plants grown under continuous light had better growth and higher yields than plants receiving the 14 h photoperiod. During the second half of the experiment, continuous lighting decreased the growth rate of tomato plants. At the end of the experiment, total growth and yields of tomato plants grown under continuous light were generally lower than plants exposed to 14 h photoperiod. Reducing fruit load increased vegetative growth and decreased yields, but had no influence on the response of tomato plants to the photoperiod treatments. Photoperiods did not influence leaf mineral composition. Therefore, plant mineral nutrition did not explain the leaf chloroses, reduced growth and yields observed under 24 h photoperiod. Continuous light increased leaf starch and sugar contents. Fruit pruning treatments did not alter plant response to photoperiod treatments regarding leaf starch and sugar contents and had no influence on the date of appearance and the severity of leaf chloroses noted under continuous light. Our data suggest that growth and yield reductions as well as leaf chloroses of tomato plants grown under continuous light are possibly explained by starch and sugar accumulation due to leaf limitations rather than a sink limitation.