EFFECTS OF SINK DEMAND ON PHOTOSYNTHESIS IN CUCUMBER

In Cucumis sativus L. net photosynthesis of leaf number 12 increased until 12 d after its unfolding and then decreased gradually. Sink demand was varied by imposing the number of fruits per plant without changing the number of leaves per plant. Net leaf photosynthesis did not respond to sink demand during the first 16 d of fruit growth. Subsequently, a substantial reduction in photosynthesis was found only when all fruits were removed. This reduction was accompanied by a decrease in the rate of transpiration, indicating a higher stomatal resistance. Neither starch nor mono- and disaccharides accumulated noticeably at reduced sink demand, suggesting that there was no end-product inhibition of photosynthesis. Dry matter content of the leaves increased and specific leaf area decreased when sink demand was reduced. When modelling cucumber production, effects of sink demand on leaf photosynthesis can be ignored.