Torregrosa, L. J.-M. | bioRxiv Science

bioRxiv2023

Impact of water deficit on single grapevine berry ripening

The effect of water deficit on grapevine fruit ripening has most often been addressed under the assumption that individual berries behave identically to their blend in the future harvest, both kinetically and metabolically. However, mixing unsynchronized berries, whose water and sucrose import pathways critically change according to their own developmental stages, intrinsically blurs the physiological and phenological effects of stress. We investigated the consequences of water deprivation on berry growth and primary metabolites content (glucose, fructose, tartaric and malic acids) on sixteen genetically distant genotypes of Vitis vinifera and fungus-tolerant hybrids submitted to 10 watering regimes, from well-watered to partial leaf shedding. Then, six genotypes were selected for comprehensive single berry analyses. Own-rooted potted plants bearing berries at the late herbaceous plateau stage were subjected to the different water treatments for four weeks in a greenhouse with automated regulation of soil water content. Berry and cluster growth were monitored by image analysis, before performing a final destructive sampling to determine berry weight and composition. Grape phenology was highly dependent on water availability. In some cultivars, ripening was considerably delayed or even prevented under well-watered conditions. These cultivars required an intermediate water deficit to trigger the second berry growth period along with sugar accumulation and malate breakdown, typical of the ripening process. Ripening still occurred in all genotypes upon severe water deprivation, although sugar accumulation and concentration were dramatically impaired, and the second growth period was annihilated or even replaced by shrivelling. Water deficit increased malate breakdown, uncoupling it from sugar accumulation. Single berry analyses suggest that although asynchronicity in berry ripening was reduced upon stress, individual fruits within the same cluster may undergo heterogeneous water budgets, expansion or shrivelling.

physiology↗

bioRxiv2022

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.

plant biology↗