INTERACTION OF NITROGEN AVAILABILITY DURING BLOOM AND LIGHT-INTENSITYDURING VERAISON - I - EFFECTS ON GRAPEVINE GROWTH, FRUIT-DEVELOPMENT,AND RIPENING

A variety of environmental factors affects both vegetative and reprodu ctive growth in grapevines. The effect of nitrogen (N) availability at bloom (0.34, 1.7, or 3.4 g N per plant as NH4NO3) and light intensity at veraison (three weeks at 100%, 20%, or 2% sunlight, using shade cl oth) on growth and fruit ripening of pot-grown Cabernet Sauvignon (Vit is vinifera L,) vines were investigated. Low N supply during bloom red uced fruit set due to inflorescence necrosis. Both high N availability and low-light conditions stimulated shoot growth and leaf area expans ion, and delayed leaf senescence, but decreased the weight of grape sk ins and the skin to berry ratio. Abundant N nutrition also reduced pul p sugar and acid levels by the beginning of the ripening period. Howev er, the N induced differences became gradually smaller towards fruit m aturity. Limited sunlight during veraison also delayed grape ripening, and this was exacerbated by high rates of applied N, Low light inhibi ted the degradation of malate during fruit maturation. However, ripeni ng was not stopped entirely by light intensities as low as the light c ompensation point of photosynthesis. Upon restoring full sunlight the vines compensated partially for the previous photosynthate deficit by maintaining relatively high rates of sugar accumulation in the berries . Under full sunlight sugar accumulation reached a plateau five weeks after veraison. No relationship between crop yield and fruit quality c ould be established at harvest, although there was an almost sixfold r ange in yield. Yield was determined primarily by N availability at blo om, while grape quality was determined predominantly by light conditio ns during veraison.