Apparent hydraulic strategies of diverse winegrape cultivars depend on the choice of driving and response variables.

backgroundCurrent and projected increases in the frequency and intensity of drought episodes require adaptation of irrigation management for crop plants. Such adaptation is complicated by incomplete knowledge of intra-species differences in physiological responses to drought stress. For instance, winegrapes have been variously classified as behaving isohydrically or anisohydrically depending on the cultivar and the environment. Aiming to determine whether cultivars should be irrigated differently depending on their hydraulic strategy, this study investigated the behavior of 30 winegrape cultivars with diverse geographic origins in a common garden experiment. Using controlled irrigation in an arid climate, we imposed two successive soil drydown cycles in each of two years and measured changes in canopy size, soil moisture, meteorological conditions, and plant water status.

resultsThough the canopy size varied fivefold across cultivars, their hydraulic strategy did not differ but changed depending on the extent of drought stress and the choice of driving and response variables. When soil moisture was expressed as relative extractable soil water (θ

conclusionAt least in winegrapes, the choice of driving and response variables, and the range over which they are measured, strongly influences conclusions about plant hydraulic strategies and may alter modeling outcomes. This offers a benchmark for identifying water deficit and has implications for field data interpretation and irrigation management. Irrigation scheduling may be guided by common soil- or plant-based thresholds across cultivars, while the time to reach such thresholds depends on differences in canopy size among cultivars and on weather patterns and soil properties. KEY

findingsVariable choice matters: Conclusions about plant hydraulic strategy depend on whether soil moisture is expressed as volumetric water content or water potential, and on the degree of drought stress. Shared threshold versus gradual change: 30 winegrape cultivars differing fivefold in canopy size transitioned from isohydric to anisohydric behavior at a common threshold of volumetric soil water content, but changed gradually from anisohydric to isohydric behavior as water potential declined. Similar hydraulic strategy: All 30 cultivars showed similar changes in leaf water status in response to declining soil moisture, indicating limited within-species variation in adaptation to drought stress. No isohydric-anisohydric categorization: The 30 cultivars did not differ in hydraulic strategy; responses were modulated by drought severity and measurement definitions, undermining strict classification of hydraulic strategies. PRACTICAL IMPLICATIONS: Irrigation management: Irrigation scheduling may use shared physiological thresholds across cultivars but should consider differences in vigor and hence canopy size that affect water use. Modeling: Physiological models must account for differences in variable choice and measurement range to avoid misleading outcomes. Breeding and adaptation: Winegrape cultivars may lack diversity in hydraulic strategies, suggesting limited potential for genetic adaptation to intensifying drought stress.Breeding and adaptation: Winegrape cultivars may lack diversity in hydraulic strategies, suggesting limited potential for genetic adaptation to intensifying drought stress.