Grape ripening: why berry transpiration and xylem backflow matter
Abstract
At the beginning of ripening, grape berries switch their water import pathway from the xylem to the phloem. However, the cause of the decline in xylem inflow remains a matter of debate. Applying xylem-mobile dye in conjunction with root or shoot pressurization demonstrated that the direction of xylem flow was reversible and responded to pressure manipulation. Using a fruit growth model, grape berry vascular flows were estimated based on measurements of pedicel phloem sap sugar concentration and rates of berry growth, sugar accumulation, and transpiration. In order to meet the sugar demand of ripening berries, phloem inflow exceeded the combined water demand for berry growth and transpiration. The surplus phloem water was discharged as xylem backflow. This means that the decline in xylem inflow at veraison occurs because phloem inflow increases rather than the other way around. When the xylem pathway was disrupted, berry sugar accumulation and color change were impaired. Similar results were found when berry transpiration was reduced. Collectively, these results indicate that both xylem backflow and berry transpiration are important in discharging surplus phloem water during ripening. Both pathways may serve as ‘overflow valves’ that facilitate normal grape ripening. Xylem backflow may be especially important under humid conditions that limit berry transpiration.
Issue: GiESCO 2017
Type: Extended abstract
Format: Oral
Authors
1 Washington State University, Irrigated Agriculture Research and Extension Center, 24106 N. Bunn Road, Prosser, WA 99350, USA
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Keywords
cracking, grape berry, thin shell theory, skin, cuticle, softening, Vitis