INFLUENCE OF EXTENDED PHOTOPERIOD ON PHOTOSYNTHATE PARTITIONING AND EXPORT IN TOMATO AND PEPPER PLANTS

Changes in photosynthetic efficiency and carbon partitioning under ext ended photoperiods (from 8 to 24 h) was studied using a photoperiod-se nsitive species (tomato-Lycopersicon esculentum Mill.) and a non-photo period-sensitive species (sweet pepper-Capsicum annuum L.). Extending photoperiod by supplemental lighting resulted in an increase in total carbohydrates (CH2O) produced in both species. In tomato, extended pho toperiod principally favored shoot development. Dry weight of tomato p lants increased by c. 30% when light periods were increased from 12 to 18 h although no significant differences we,re observed in fruit yiel ds. In contrast, extended photoperiod did not increase shoot dry weigh t of pepper plants but significantly increased its fruit yields. Furth ermore, studies on CH2O translocation supported these data. In pepper plants, translocation efficiency varied from 71 to 90%, whereas it var ied from 54 to 69% in tomato plants. On a daily basis, tomato plants e xposed to extended light periods accumulated more carbohydrates than p epper plants. Tomato plants had 4.4-4.9 times more hexoses than sucros e whereas this ratio was only 1.1-1.5 in pepper plants. Such results l ead to the hypothesis that low sucrose phosphate synthase activity in tomato plants might have resulted in the accumulation of carbohydrates and phosphorylated intermediates, which in turn decreased inorganic p hosphate levels in the cytoplasm and stroma. This could explain the si gnificant accumulation of starch in chloroplasts of tomato as well as the absence of fruit yield increase in plants of this species submitte d to extended light periods.