DIFFERENTIAL-EFFECTS OF CANOPY MANIPULATION AND SHADING OF VITIS-VINIFERA L CV SAUVIGNON,CABERNET - LEAF GAS-EXCHANGE, PHOTOSYNTHETIC ELECTRON-TRANSPORT RATE AND SUGAR ACCUMULATION IN BERRIES

Partial cluster and leaf removals were performed on Vitis vinifera L. cv. Cabernet Sauvignon at veraison during two years to give 4 treatmen ts: control (C), cluster thinning (CT), leaf removal (LR) and cluster thinning + leaf removal (CT+LR). A half of each plot was shaded by a 5 0 % shading net at veraison (40 % berries coloured). Shading significa ntly reduced stomatal conductance but not CO2 assimilation rate (P-n) and carboxylation efficiency. P-n was decreased by cluster thinning an d enhanced by leaf removal. Leaves of CT vines showed a photosynthetic decay 2 days after the treatment while LR leaves presented an afterno on photosynthetic enhancement 3 days after the treatment probably due to a higher photoassimilate requirement of the bunches (sink). Stomata l conductance did not totally contribute to the P-n depression or enha ncement. Electron transport rate and maximum Rubisco activity were str ongly affected by CT and LR suggesting that photochemical and biochemi cal processes were affected to a greater extent than physical processe s. Juice sugar concentration was reduced by shading, an effect explain ed straightforwardly by the lessening of stomatal conductance. CT and LR had large effects on biochemical and photochemical processes but th ese were in the opposite direction to the expected effects on juice su gar concentration. It is suggested that CT increased sugar concentrati on because of the low fruit sink load per vine, and that LR decreased sugar concentration because of the low leaf area per vine. If cluster thinning is to be used by growers to gain increased sugar concentratio n it is recommended to do it at veraison; the sugar accumulation rate is then high and the adaptation of photosynthetic processes to thinnin g takes several days.