Grape berry water relations and their roles in sugar accumulation

Yun Zhang

Grape (Vitis spp.) berries accumulate high amounts of sugar, which is unloaded apoplastically starting from the onset of ripening. Using berries of diverse grape genotypes as a model system, the physiological significance and quantification of berry transpiration and xylem backflow, as well as the coupling of phloem and xylem flows were demonstrated. A distal-proximal gradient of osmotic potential within individual berries was observed. Additionally, xylem inflow (indicated by apoplastic dye movement) into ripening berries was restored when cluster xylem was pressurized. Together, these results suggest that the general reversal from xylem inflow to backflow at the onset of ripening may be due to changes in hydrostatic pressure of the berry apoplast. Obstructing water flow through the cluster xylem (backflow) and/or berry transpiration before ripening slowed down solute accumulation and increased berry cracking incidence. Therefore, it is proposed that both xylem backflow and berry transpiration are important in disposing surplus phloem water during ripening, to release internal pressure caused by phloem-derived water, thus facilitating sugar accumulation and limiting berry cracking. Moreover, it was demonstrated that xylem backflow and berry transpiration underwent developmental changes during berry ripening. The former correlated with phloem inflow, and the latter was determined by both cuticular conductance and vapor pressure deficit. Using a mathematical model of berry growth, it was revealed that changes in xylem backflow mirrored changes in phloem inflow. This implies that xylem backflow is not only physiologically important to dissipate phloem pressure, but also quantitatively coupled with phloem inflow. Further, a mechanism that may reduce the loss of apoplastic sugar unloaded in the berries via xylem backflow was investigated. The expression of genes encoding ten sugar transporters in the pedicels indicates the molecular machinery was present for active sugar retrieval from the apoplast to the symplast. A functional symplastic connection between parenchyma cells in the vascular tissues provides a possible pathway with low resistance for the movement of retrieved sugar. These two lines of evidence from both molecular and anatomical aspects support the proposed sugar retrieval mechanism in the pedicels of grape berries.

Grape berry water relations and their roles in sugar accumulation

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