SEASONAL AND DIURNAL CHANGES IN PHOTOSYNTHESIS AND CARBON PARTITIONING IN VITIS-VINIFERA LEAVES IN VINES WITH AND WITHOUT FRUIT

In Vitis vinifera L. cv. Chardonnay maintained in a greenhouse, the ma ximum rate of photosynthesis, the measured rates of de novo sucrose an d starch synthesis and the total leaf sucrose and starch contents were relatively constant throughout the period from April to July although the partitioning of newly fixed carbon was modified in favour of sucr ose synthesis half-way through the growing period. In these experiment al conditions, no significant differences in these parameters were obs erved in plants from which the fruit had been removed in comparison to the controls. In field-grown vines, photosynthesis rose to a maximum in the early morning consistent with the increase in ambient irradianc e and then subsequently progressively decreased. This occurred every d ay. On clear days the mid-morning depression in the rate of CO2 assimi lation was closely linked to decrease in stomatal conductance, but the re was no correlation between these parameters on days when the sun wa s overcast. There was no correlation between leaf sucrose content and the depression in photosynthesis. The calculated rate of non-cyclic el ectron flow did not decline in parallel with the mid-morning depressio n and the quantum efficiency of photosystem II was constant for the wh ole of the period when the CO2 assimilation was decreasing. The mid-mo rning depression of photosynthetic CO2 assimilation was related to bot h stomatal and non-stomatal effects. In neither situation did it have any measurable feedback effect on the electron transport rate or on th e carbohydrate contents of the leaves.