Physiological responses of grapevine leaves to lightflecks

The gas-exchange response of sun and shade grapevine leaves to various lightfleck regimes was investigated under laboratory conditions. Intermittent lighting varied as to the duration of the intervening dark/light intervals (3 s/3 s to 4.5 s/l .5 s within a 6 s cycle) and frequency of dark/light periods of equal length (0.04-l 0.33 Hz). Carbon Exchange rate (CER) decreased in sun leaves as the duration of the dark phase increased; maximum reduction (38Oh less than continuous light of similar photon flux density (PFD) ) was registered at 4.5 s/ 1.5 s dark/light. CER and transpiration ( T) were not significantly affected in shade leaves by lightflecks over varying light/dark intervals. CER increased both in sun and shade leaves with lightfleck frequency. Frequencies higher than 5 Hz resulted in CER rates very close to those found under continuous light of high intensity. Sun and shade leaves exposed to increasing areas of spot light revealed a more than proportional CER gain of up to 5% of lighted leaf area and then a steady linear increment. The results show that grape leaves grown under different light intensities may respond differently to regimes of alternating light.