Abstract
Climate change projects an increase in the occurrence of extreme weather events whether occurring individually or in combination. Among these events, heatwaves and periods of drought pose the greatest threat to viticulture on a global scale. In the face of such conditions, vineyard managers seek to implement mitigating measures to guarantee the minimum impact of these stresses on plant performance. Therefore, we studied the effects of irrigation on mitigating the effects of heatwaves on grapevine physiology, both under controlled environmental conditions and under field conditions. Under heat stress, irrigation water helped the plants to better withstand elevated temperatures. This was primarily achieved by enhancing plant transpiration and reducing leaf temperature, thereby positively influencing leaf carbon assimilation and overall growth. In addition, the sufficient water availability increased the resistance of the photosystem under heat stress and helped the plants to resume their photochemical activities upon the removal of the heat stress. Water-stressed plants were mainly affected by water stress, while a minor impact of heat stress was observed on the leaf gas exchange when both stresses were combined. However, PSII functionality was significantly affected by the combined heat and water stress and water stressed plants were unable to recover their photosynthetic activities after removing the heat stress. Despite the demonstrated positive impact of the irrigation on the plant performance during the heatwaves, the scarcity of water resources in grapevine-growing areas necessitates a sustainable approach to water use for irrigation. Our findings suggest that a reduction in soil water potential to -0.2 MPa resulting from compensating 70 % instead of 100 % of plant transpiration during heatwaves did not negatively affect the leaf carbon assimilation, photochemical efficiency, and plant growth. Further reduction in soil water potential (below -0.4 MPa), however, restricted physiological performance and impeded plants to recover from heat stress. Translating these findings from controlled environments to field conditions should be done with caution as further environmental factors should be considered. When heat and drought stress interacted with the heterogeneous radiation exposure, grapevine leaves showed a different response to the combined stresses. High radiation intensified the negative impact of heat stress on the maximum efficiency of PSII, and when coupled with water stress, further damage was observed on the PSII efficiency. Under low light conditions, the impact of heat and water stress on PSII efficiency was less evident compared to leaves with higher radiation exposure. These findings imply that adjustment of the irrigation practices in the vineyard during heatwaves can minimize the heat-induced stress impacts on the plants as a short time strategy. While for longer term measures, reducing the plant exposure to radiation by using sunscreens, such as shading nets, could be implemented to provide protection against excessive sunlight and heat.