RETRANSLOCATION OF CARBON RESERVES FROM THE WOODY STORAGE TISSUES INTO THE FRUIT AS A RESPONSE TO DEFOLIATION STRESS DURING THE RIPENING PERIOD IN VITIS-VINIFERA L

A technique for reliable labelling of the carbon reserves of the trunk and roots without labelling the current year's growth of grapevines w as developed in order to study retranslocation of carbon from the pere nnial storage tissues into the fruit in response to defoliation stress during the ripening period. A special training system with two shoots was used: the lower one (feeding shoot) was cut back and defoliated t o one single leaf ((CO2)-C-14-feeding leaf) while the other (main shoo t) was topped to 12 leaves. The potted plants were placed in a water b ath at 30 degrees C to increase root temperature and therefore their s ink activity. Additionally, a cold barrier (2-4 degrees C) was install ed at the base of the main shoot to inhibit acropetal C-14 translocati on. Using this method, we were able to direct labelled assimilates to trunk and roots in preference to the current year's growth. On vines w ith root and shoot at ambient temperature, 44% of the C-14 activity wa s found in the main shoot 16 h after feeding whereas only 2% was found in the temperature-treated vines. At the onset of fruit ripening, and at three-week intervals thereafter until harvest, potted grapevines w ere fed with (CO2)-C-14 using the temperature treatment described abov e. Sixteen hours after feeding, half of the vines of each group were d efoliated by removing all except the two uppermost main leaves. Three weeks after each treatment, vines were destructively harvested and the dry weight and C-14 incorporation determined for all plant parts. Und er non-stressing conditions, there was no retranslocation of carbon re serves to support fruit maturation. Vines responded to defoliation str ess by altering the natural translocation pattern and directing carbon stored in the lower parts to the fruit. In the three weeks following veraison (the inception of ripening in the grape berry), 12% of the la belled carbon reserves was translocated to the fruit of defoliated pla nts compared to 1.6% found in the clusters of control vines. Retranslo cation from trunk and roots was highest during the middle of the ripen ing period, when 32% of the labelled carbon was found in the fruit com pared to 0.7% in control plants. Defoliation during this period also c aused major changes in dry-matter partitioning: the fruit represented 31% of total plant biomass compared to 21% measured in the control vin es. Root growth was reduced by defoliation at veraison and during the ripening period. Defoliation three weeks before harvest did not affect dry matter or C-14 partitioning.