Abstract: | In Vitis vinifera L. cv. Chardonnay maintained in a greenhouse,the maximum rate of photosynthesis, the measured rates of denovo sucrose and starch synthesis and the total leaf sucroseand starch contents were relatively constant throughout theperiod from April to July although the partitioning of newlyfixed carbon was modified in favour of sucrose synthesis half-waythrough the growing period. In these experimental conditions,no significant differences in these parameters were observedin plants from which the fruit had been removed in comparisonto the controls. In field-grown vines, photosynthesis rose toa maximum in the early morning consistent with the increasein ambient irradiance and then subsequently progressively decreased.This occurred every day. On clear days the mid-morning depressionin the rate of CO2 assimilation was closely linked to decreasein stomatal conductance, but there was no correlation betweenthese parameters on days when the sun was overcast. There wasno correlation between leaf sucrose content and the depressionin photosynthesis. The calculated rate of non-cyclic electronflow did not decline in parallel with the mid-morning depression and the quantum efficiency of photosystem II was constantfor the whole of the period when the CO2 assimilation was decreasing.The mid-morning depression of photosynthetic CO2 assimilationwas related to both stomatal and non-stomatal effects. In neithersituation did it have any measurable feedback effect on theelectron transport rate or on the carbo hydrate contents ofthe leaves. Key words: Vitis vinifera L., source-sink interactions, sucrose, starch, photosynthesis |