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 maximum rate of photosynthesis, the measured rates of de novo sucrose and 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 sucrose synthesis half-way through the growing period. In these experimental conditions, no significant differences in these parameters were observed 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 irradiance and then subsequently progressively decreased. This occurred every day. On clear days the mid-morning depression in the rate of CO2 assimilation was closely linked to decrease in stomatal conductance, but there was no correlation between these parameters on days when the sun was overcast. There was no correlation between leaf sucrose content and the depression in photosynthesis. The calculated rate of non-cyclic electron flow did not decline in parallel with the mid-morning depression and the quantum efficiency of photosystem II was constant for the whole of the period when the CO2 assimilation was decreasing. The mid-morning depression of photosynthetic CO2 assimilation was related to both stomatal and non-stomatal effects. In neither situation did it have any measurable feedback effect on the electron transport rate or on the carbohydrate contents of the leaves.