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 ext ended photoperiods of 14, 16, 20 and 24 h. Compared to natural light, extended photoperiod treatments increased shoot fresh weight and yield s 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 gr owth 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 obse rved that, for the first 5 to 7 weeks of treatments, tomato plants gro wn under continuous light had better growth and higher yields than pla nts receiving the 14 h photoperiod. During the second half of the expe riment, continuous lighting decreased the growth rate of tomato plants . At the end of the experiment, total growth and yields of tomato plan ts grown under continuous light were generally lower than plants expos ed to 14 h photoperiod. Reducing fruit load increased vegetative growt h 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 e xplain the leaf chloroses, reduced growth and yields observed under 24 h photoperiod. Continuous light increased leaf starch and sugar conte nts. Fruit pruning treatments did not alter plant response to photoper iod treatments regarding leaf starch and sugar contents and bad no inf luence on the date of appearance and the severity of leaf chloroses no ted under continuous light. Our data suggest that growth and yield red uctions as well as leaf chloroses of tomato plants grown under continu ous light are possibly explained by starch and sugar accumulation due to leaf limitations rather than a sink limitation. (C) 1998 Published by Elsevier Science B.V.