Modulation of grape cuticular wax composition following multiple heatwaves influences grape transpiration

Heat stress events (heatwaves) significantly impair grapevine leaf and berry physiology. The prevalence and frequency of heatwaves are predicted to increase in the coming decades, even in moderate climates. Despite this, many facets of heatwave effects on grapevine physiology remain unstudied. In this work, we subjected potted Gewürztraminer grapevines to a single heatwave event at mid-ripening or a double heatwave event at both the onset of ripening and mid-ripening and compared them to respective untreated control vines. The recovery of leaf and grape physiology from both events were analyzed on the day following the event and at harvest maturity through measurements of leaf gas exchange and chlorophyll fluorescence, and grape size, total soluble solids, and cuticular wax composition. Both single heatwave (S-HW) and double heatwave (D-HW) treatments reduced chlorophyll fluorescence (Fv/Fm) by 12.8 % and 6.1 % on the day following the heatwave, respectively. However, values recovered at harvest maturity only by S-HW. Photosynthetic rate (Pn) was not affected by treatments on the day following the heatwave, but was reduced in D-HW by 48 % at harvest maturity. Total soluble solids were also decreased only by D-HW at harvest maturity. Following the D-HW, total berry wax (by 11.8 %), as well as the total contents of triterpenoids (by 11.7 %) and aldehydes (by 14.3 %) were increased significantly at harvest maturity. Fruit cuticular transpiration was reduced by D-HW. Interestingly, this decrease in transpiration was correlated with an increase in the ratio between the two most abundant wax compounds, oleanolic acid and ursolic acid, which are both triterpenoids. This data corroborates to suggest that grapevines are resilient to a single but not a double heatwave event. Future work will further explore the effects of heatwave frequency and recovery on additional facets of leaf physiology and grape quality.