Effects of fruit load on partitioning of 15N and 13C, respiration, and growth of grapevine roots at different fruit stages

The effects of different fruit loads on the root activity and growth of roots and shoots in large potted grapevines (Vitis labruscana, var. Aki-queen) were investigated, heavy fruit load (HFL, two bunches per new bearing shoot), medium fruit load (MFL, one bunch per new bearing shoot, which was the standard fruit load), and no fruit load (NFL), in an unheated greenhouse at three growth stages: berries enlarging (June), veraison (July), and pre-harvest (August) were compared. A liquid oxygen electrode method was used to determine root activity from the absorption of N-15 (as ammonium sulfate), the partitioning percentage of N-15 in the roots, and the respiration of fine roots. We also examined the C-13-photoassimilate partitioning and absorption percentages in whole plants. HIFL and MFL grapevines had a lighter dry mass of fine roots (17-19%) and shoots at all stages than NFL grapevines. NFL grapevines had the highest dry mass percentage in fine roots (24%). HFL reduced lateral shoot growth and reduced rooting production of new fine roots. Although the total sugar accumulation per vine was the highest in the HFL grapevines, sugar accumulation per berry was lower in the HFL vines than in the MFL vines. The fresh weight of both bunches and berries and the sugar content of berries were significantly decreased by heavier fruit loads at harvest in late August. The respiration rate of the fine roots of NFL grapevines (about 8 mumol O-2 g FW-1 h(-1)) was higher than those of HFL and MFL grapevines (between 4 and 6 mumol O-2 g FW-1 h(-1)) at all stages of fruit growth. From roots, N-15 partitioning was highest in the fine roots. Among treatments, the partitioning percentage in the fine roots was highest in the NFL grapevines (between 20 and 40%) compared with MFL and HFL vines (about 15%) at all three stages. N-15 assimilated per unit dry mass of the NFL fine roots was the highest in July and August. During veraison, the fruit of the HFL and MFL grapevines received the highest partitioning percentage of C-13-photoassimilates, and the fine roots and trunks of the NFL grapevines received the highest percentages. Among roots, the fine roots received the highest percentage in all treatments at all stages. The fine roots of the NFL grapevines received a higher C-13-photoassimilate percentage (13-14%) than those of the HFL (4-8%) and MFL (6-7%) grapevines throughout the experiment. These results demonstrate that root activity and vegetative growth were strongly affected by fruit load and that a heavy crop load forced vines to accumulate less sugar in the roots and shoots. This means that fruit has priority in the accumulation of photoassimilates and partitioning nitrogen. (C) 2002 Elsevier Science B.V. All rights reserved.