Photosynthetic recovery in drought‐rehydrated grapevines is associated with high demand from the sinks, maximizing the fruit‐oriented performance

To understand how grapevine sinks compete with each other during water stress and subsequent rehydration, carbon (C) allocation patterns in drought-rehydrated vines (REC) at the beginning of fruit ripening were compared with control vines maintained under drought (WS) or fully irrigated (WW). In the 30 days following rehydration, the quantity and distribution of newly fixed C between leaves, roots and fruits was evaluated through 13CO2 pulse-labelling and stable isotope ratio mass spectrometry. REC plants diverted the same percentage of fixed C towards the berries as the WS plants, though higher than that of WW plants. Net photosynthesis (measured simultaneously with root respiration in a multi-chamber system for analysis of gas exchange above- and below-ground) was about twice in REC compared to WS treatment, and comparable or even higher than in WW plants. Maximizing C assimilation and delivery in REC plants led to a significantly higher amount of newly fixed C than in both control treatments, already two days after rehydration in root, and two days later in the berries, in line with the expression of genes responsible for sugar metabolism. In REC plants, the increase in C assimilation was able to support the requests of the sinks during fruit ripening, without affecting the reserves, as was the case in WS. These mechanisms clarify what is experienced in fruit crops, when occasional rain or irrigation events are more effective in determining sugar delivery toward fruits, rather than constant and satisfactory water availabilities.