Examination of factors contributing to onset and degree of pre-harvest weight loss in shiraz berries

Abstract

Shiraz berries can undergo shrivelling during late ripening. Shrivelling can begin at around 85 days after flowering in the South-west slopes region of NSW, and more than 85% of the weight loss is in the form of water. The physiological cause for this berry weight loss was examined, and the impacts of irrigation, rootstock and nutrition on onset and severity of weight loss was assessed. A combination of element accumulation patterns, dye tracer infiltration, anatomical observation, gas exchange measurements, berry volume and sap flow sensors were used to deduce rates of water movement into and out of berries. Water flow into the bunch through the xylem stream was diminished after veraison in three out of four seasons. After maximum berry size, water flow through the xylem increased again. The phloem’s contribution to the berry’s water budget increased after veraison, then diminished after maximum berry size. Water loss from the berry through transpiration diminished after veraison, but was ongoing. Water flow through the xylem from the berry to adjacent berries in the bunch did occur, however flow back to the vine was very restricted, while backflow through the symplasm was not detected. These results indicate that shrivelling in Shiraz was likely due to ongoing slow transpiration combined with decreased water flow into the berry through the phloem. The trigger for the onset of shrivelling was not regulated by day length or temperature. Cultural management of the rate in weight loss from berries was possible through irrigation and rootstocks. Standard irrigation ameliorated the rate of post-weight maximum volume loss as compared to deficit irrigation, and the onset of the weight loss was delayed by one week when Ramsey was used as a rootstock as compared to 101-14 Mgt.

Summary

Vascular transport studies under controlled conditions. Influx and transpiration determine water status  in grapes near harvest. Cumulatively, the data indicate that shrivelling in Shiraz is not likely caused by a sudden spike in transpiration rates, but rather reduced vascular flow into the berry along with ongoing transpiration resulting in shrinkage.