The water relations of Semillon vines

Abstract

This comprehensive study of Semillon has yielded information which will assist selection and management to ensure the effects of water stress are minimised.

Summary

Semillon is an important white wine variety grown in Australia and is particularly important to the regional economies of the Riverina and Hunter Valley. Unfortunately it is more sensitive than other common varieties to dehydration. Related problems are wilting and leaf burn, which are most likely to occur when there is low leaf water status.

To provide a better understanding of the cause of Semillon’s sensitivity to dehydration and leaf burn, several paths were taken. Vineyard trial sites were established in the Riverina and Hunter Valley and characteristics of each site were investigated and documented, including soil properties, water availability and demand, and canopy temperature and humidity. A replicated variety block at Charles Sturt University, Wagga Wagga, was also used in the project for varietal comparisons. Growth cabinet experiments and studies under controlled environments enabled closer examination of how stomatal conductance and other features of an efficient variety like Grenache differ from Semillon. There were also studies to explore any differences between Semillon clones and various rootstocks.

The project confirmed the sensitivity of Semillon. Field trials involving nine other varieties, for example, showed Semillon canopies were relatively cool on hot days due to high transpiration rates but had leaf burning at margins while the other varieties exhibited no burning. In pot comparisons with Grenache where soil moisture levels were allowed to dry out and leaf gas exchange and water potential were monitored, Semillon vines had higher transpiration, photosynthesis and stomatal conductance. The differences could not be put down to less of the hormone responsible for stomatal behavior as concentrations in the sap were similar. While results have pointed to Semillion as a variety which has stomatal response to drying soil at a naturally lower rate than other varieties, the work demonstrated that several practices including row orientation and canopy management could reduce effects of high temperatures and low water status. It was also found from pot experiments that clonal differences existed and that symptoms could be aggravated or alleviated by different rootstocks. Although this gives some positive messages for grape growers the pot studies need to be followed up by field trials before recommendations can be made.