Carbon partitioning to berries in water stressed grapevines: The role of active transport in leaves and fruits

• Grape berries reach equal sugar concentration despite the water status of plants.
• Water stress accelerates sugar intake, around veraison, increasing VvSUC27, VvHT3 and VvHT5.
• VvSUC11, VvSUC12, VvHT5 and VvCWI increase their abundance even after berries are full of sugars.
• Leaves transiently reduces sugar export, accumulating starch, as berries are full in sugar.
• Water stress increased the gene expression for mesophyll cell sucrose transporters in leaves.

Although imposed water stress is a common agricultural practice worldwide, the physiological and molecular responses of grapevine leaves and fruits, and their interactions, in relation to carbon partitioning remain unknown. We have assessed, in field grown grapevines, the effect of four deficit irrigation regimes, from veraison through to the end of the season, on daily and seasonal non-structural carbon stocks and assimilation in leaves and sugar content in berries, along with the transcript profile for sugar transport proteins in leaves and berries. Average midday xylem water potentials along the season ranged from mild to severe water stress, i.e., −0.7 MPa to −1.05 MPa, respectively. In all the treatments, berries reached equal sugar concentration 20–35 days after veraison because of a proportional effect on berry volume and sugar content per berry. In berries, mild water stress accelerated the sugar accumulation increasing the abundance of VvSUC27, VvHT3 and VvHT5, only strictly around veraison. Transcripts abundance in berries did not match sugar uptake rate since, VvSUC11, VvSUC12, VvHT5, as well as the cell wall invertase VvCWI, kept rising after berries were filled. In leaves, when berries reached maximal sugar content, export was transiently reduced resulting in starch accumulation. Water stress increased the gene expression for sucrose transporters known to code for mesophyll cell proteins in leaves, without affecting the transcript abundance for the phloem loading protein. The latter suggests that mild water stress triggers active sugar transport in the source tissues as a means for supporting the sugar accumulation in berries under depressed carbon assimilation by leaves.