Assimilate transport in grapevines - effect of phloem disruption

Assimilate translocation in mature grapevines (cv. Gewurztraminer and cv. H arslevelu) under field conditions was investigated during the growth season by quantifying individual Sugars and organic acids in mature leaves, shoot bark and berries, as affected by girdling the Shoot just above the bunches . Tissue was sampled at berry set, pea size, veraison and ripeness stages o f the vine. Invertase activity was determined in the shoot bark at ripeness . in the leaves, malic acid concentrations reached lowest levels at pea siz e, but increased thereafter. Tartaric acid decreased after peaking at pea s ize stage. Tartaric acid concentrations increased with girdling. Despite th e increase in leaf age, sucrose concentrations remained virtually stable du ring the season, emphasising the importance of mature leaves for nourishing bunches. Girdling resulted in a build-LIP of sucrose in the leaves. in the bark, malic and tartaric acid stayed more or less the same during the grow th period, but increased above the girdle. As a result of phloem disruption , sucrose also increased. The increase in glucose and tartaric acid is beli eved to result from catabolic cleavage of sucrose by invertase. invertase a ctivity was evident in the bark (of mature Harslevelu vines) at ripeness, w hich may indicate involvement in osmotic adjustments and gradients in the b ark/phloem structure. in the berries, malic and tartaric acids reached peak concentrations at pea size. The Volume increase during the ripening period , and in the case of malic acid also respiratory loss, resulted in a decrea se in organic acid concentration. Malic acid continued to decrease after th e initial decline, whereas tartaric acid stayed virtually stable. Girdling had no marked effect on organic acid accumulation in the berries. Sucrose c oncentrations were low during the first part of the season, but increased t hereafter. Sucrose concentrations during ripening increased with girdling, which may represent a concentration effect and/or import from the rest of t he vine. Sucrose concentrations (in Mature Harslevelu vines) were indeed lo wer below than above the girdle. Comparison Of Sucrose concentrations in th e leaves, bark and berries showed the existence of a decreasing concentrati on gradient, in line with the source:sink transport concept. An equally pro minent decrease in sucrose:glucose ratio in the berries from the start of t he ripening period indicates that vacuolar integrity (compartmentation) was affected in the ripening berry, most probably allowing hydrolysis of sucro se by invertase and decreasing osmotic potential within the berry. The resu lts provide further evidence for the hypothesis of an osmotic gradient driv en transport to the berry.