High temperatures affect gas exchange, growth and ripening processes of 'Semillon' vines

High temperatures, above 40¢XC, are a regular occurrence during the growing season in many of the grape growing regions of Australia and elsewhere. Anecdotal evidence suggests these high temperatures affect flowering in spring and delay ripening in summer but a detailed understanding of the frequency and impact of such temperatures on the photosynthesis, growth processes as well as flowering and ripening has only started to be achieved in Australian conditions. In this paper, we describe the research conducted on 'Semillon' vines to quantify the temperature microclimate of the vines and assess photosynthesis, berry and vine growth and ripening during a major heat event. High temperature events in the Riverina grape growing region over the past decade occur most commonly over two days but a 6-day heat event occurred in midsummer of 2006 and a 14-day heat event in 2009. Radiometers located in the inter row and pointed at the canopy revealed that the 'Semillon' vines were usually 3-5¢XC below air temperature but during the 2009 heat event, canopy temperatures were 2-3¢XC higher than air temperature. This heat event caused a 30-50% reduction in leaf photosynthesis along the shoot but stomatal conductance and transpiration were increased, by as much as two-fold. Stem and leaf growth were unaffected by the high temperatures as growth had largely been completed but bunch growth (dry weight) was reduced by 60-70% in comparison with protected vines. Similarly, the rate of ripening was reduced about 50% by the high temperatures, resulting in berries 2-3 ¢XBrix behind in sugar accumulation at harvest. Berry damage, shrinking and sunburn, also increased as a consequence of bunch exposure to the high temperatures.