Impact of the water deficit on grapevine single-berry ripening

Addressing single fruit is essential to disentangle the physiological consequences of environmental constraints on water and primary metabolite accumulation pathways, both of which undergo major developmental transitions from anthesis to ripening and terminal berry wilting. Here, we examined how water deficit affects berry growth and metabolite dynamics, specifically glucose, fructose, tartaric acid, and malic acid, across sixteen genetically diverse Vitis vinifera genotypes and fungus-tolerant hybrids. Plants were subjected to three water-supply regimes: well-watered (WW), moderate water stress (MWS), and severe water stress (SWS), ranging from comfortable water balance to levels that induced partial leaf shedding. We then selected six representative genotypes for in-depth single-berry analyses. Ripening phenology and berry asynchrony depended strongly on water availability. Unexpectedly, in some cultivars, WW significantly delayed or even entirely suppressed ripening, whereas MWS was required to trigger the second phase of berry growth, typically accompanied by sugar accumulation and malate breakdown. Under SWS (Ψstem < -1.2 MPa), ripening still occurred in all genotypes but was associated with impaired growth and sugar accumulation, with the second growth period often replaced by shriveling at low sugar concentration and accumulation levels. Water deficit also reduced asynchrony in sugar accumulation, although individual berries within clusters exhibited heterogeneous water budgets and dilution effects. Notably, water stress enhanced malate degradation, decoupling malate breakdown from sugar accumulation.