Influence of extended photoperiod on photosynthate partitioning and export in tomato and pepper plants

Changes in photosynthetic efficiency and carbon partitioning under extended photoperiods (from 8 to 24 h) was studied using a photoperiod-sensitive species (tomato-Lycopersicon esculentum Mill.) and a non-photoperiod-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 photoperiod principally favored shoot development. Dry weight of tomato plants increased by c. 30% when light periods were increased from 12 to 18 h although no significant differences we,re observed in fruit yields. In contrast, extended photoperiod did not increase shoot dry weight of pepper plants but significantly increased its fruit yields. Furthermore, studies on CH2O translocation supported these data. In pepper plants, translocation efficiency varied from 71 to 90%, whereas it varied from 54 to 69% in tomato plants. On a daily basis, tomato plants exposed to extended light periods accumulated more carbohydrates than pepper plants. Tomato plants had 4.4-4.9 times more hexoses than sucrose whereas this ratio was only 1.1-1.5 in pepper plants. Such results lead 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 phosphate levels in the cytoplasm and stroma. This could explain the significant accumulation of starch in chloroplasts of tomato as well as the absence of fruit yield increase in plants of this species submitted to extended light periods.