Abstract
The increased intensity and frequency of heatwaves, coupled with prolonged periods of drought, poses a significant threat to viticulture worldwide. Under these conditions, greater damage may occur when the leaf is also exposed to high radiation intensity. To better understand the impact of drought and high radiation exposure during heatwaves on grapevine physiology, we established a factorial experiment using well-watered or water-deficit Shiraz grapevines with different radiation exposure due to the row orientation. Given the East-West row orientation, the two sides faced directly north or south, receiving different radiation intensities. To monitor the impact of irrigation and radiation exposure on PSII functionality, leaf chlorophyll fluorescence was continuously monitored over a 20-day period on leaves on the north side and on the south side of the canopy of each plant, while leaf gas exchange measurements were performed on adjacent leaves. Water-deficit vines were maintained at a midday stem water potential (SWP) of –1.4 MPa, while well-watered plants had SWP of –0.8 MPa during the experiment. High radiation exposure was the dominant factor impairing PSII performance rather than heat or water stress alone. The north side leaves (N) showed lower maximum efficiency of PSII than the south side leaves (S) on hot days, especially when plants were water-stressed. S leaves had higher photochemical (Y(II)) and lower non-photochemical yields (Y(NPQ)) than N leaves, predominantly at midday. Water stress further decreased Y(II) and increased Y(NPQ) in N leaves, but not in S leaves. Leaf net assimilation, stomatal conductance, and transpiration were higher in N leaves compared to S leaves, notably pronounced in the well-watered plants than in the water-deficit ones. The coupling between stomatal conductance and assimilation showed similar pattern in water-deficit and well-watered vines in N leaves, while in S leaves, waterdeficit plants showed lower changes in stomatal conductance compared to well-watered ones for the same increase in assimilation. These findings suggest that despite the positive impact of irrigation to sustain the canopy during heatwaves, additional management strategies (such as shade netting) may be required to reduce radiation exposure and to maintain leaf function during and following heatwaves