Application of shade treatments during Shiraz berry ripening to reduce the impact of high temperature

Background and Aims: The impact of high temperature on grapevine function is predicted to be a serious problem in our warming climate. We studied the impact of shading on some aspects of vine physiology, berry cell death and other components of berry development and on the resultant wines in order to understand grapevine physiology and the ripening process in shaded conditions and to assess shading as a mitigation strategy for high temperature. Methods and Results: Several shading treatments on Shiraz vines were imposed from veraison to harvest during three seasons at two locations. An overhead shade ( OS) treatment ( 62% absorption) was the most consistent in high temperature mitigation. Compared with controls ( no shading) or to soil shade, the leaves of OS vines had higher chlorophyll concentration, higher net CO2 assimilation at saturating light, and higher midday stem water potential but similar sap flow and leaf transpiration rates. Overhead shade berries accumulated sugars per berry at a similar rate but had lower TSS due to higher water content. Berry cell vitality measured with impedance or vital stains was higher in OS vines, and mass loss was delayed. Wines had lower alcohol with no significant difference in the concentration of anthocyanins; however, phenolic substances and some of the colour properties were affected. Conclusion: Overhead shade can ameliorate the impact of heat stress through a significant reduction in berry cell death and in loss of berry mass. This results in reduced alcohol concentration with relatively minor effects on other wine components. Significance of Study: Shading from veraison to harvest may be a viable mitigation strategy against extreme high temperature during ripening.