The sugarless grape trait characterized by single berry phenotyping
For grape production, an important driver for the selection of varieties better adapted to climate fluctuations, especially warming, is the balance between fruit sugars and acidity. Since the past decades, temperature during ripening has constantly raised causing excessive sugars concentrations and insufficient acidity of the wine grapes in warmest regions. There is thus an increasing interest in breeding new cultivars, able to ripen at lower sugar concentration while preserving fruit acidity. However, the phenotyping of berry composition challenges both methodological and conceptual issues. Indeed, most authors predetermine either average harvest date, ripening duration, thermal time or even hexoses concentration threshold itself, to compare accessions at an hopefully similar ripe stage. Here, we have phenotyped the fruit development and composition of 6 genotypes, including 3 new disease-tolerant varieties known to produce wines with low alcoholic contents. The study was performed at single berry level from the end of green growth stage to the arrest of phloem unloading, when water and solute contents reach a maximum per berry. The results confirmed that sugarless genotypes achieve fruit ripening with 20-30% less hexoses than classical varieties, Grenache N and Merlot N, without impacting berry growth, total acidity or cations accumulation. Sugarless genotypes displayed a higher malic acid/tartaric acid balance than other genotypes with similar sucrose/H+ exchanges at the onset of ripening. Data suggest that sugarless phenotype results from a specific plasticity in the relationship between growth and the turgor imposed by organic acid accumulation and sugar loading. This opens interesting perspectives to understand the mechanism of grapevine berry growth and to breed varieties better coping with climate warming.