Grape cultivars adapted to hotter, drier growing regions exhibit greater photosynthesis under hot conditions despite less drought-resistant leaves.

BACKGROUND AND AIMS: Many agricultural areas are expected to face hotter, drier conditions from climate change. Understanding the mechanisms crops use to mitigate these stresses can guide breeding for more tolerant plant material. We tested relationships between traits, physiological function under hot conditions, and historical climate associations to evaluate these mechanisms for winegrapes. We expected a more negative leaf osmotic potential at full hydration (πo), which reduces leaf turgor loss during drought, and either a metabolically cheaper or more osmoprotectant leaf chemical composition, to allow cultivars associated with hot, dry regions to maintain greater gas exchange under hot growing conditions.

METHODS: We measured πo, gas exchange, and leaf chemistry for 7 commercially important winegrape cultivars that vary widely in historical climate associations. Vines were grown under common garden field conditions in a hot wine-growing region (Davis, California) and measured over the hottest period of the growing season (July - September).

KEY RESULTS: □o varied significantly between cultivars, and all cultivars significantly reduced □o (osmotically adjusted) over the study period, though osmotic adjustment did not vary across cultivars. πo was correlated with gas exchange and climate associations, but in the opposite directions than expected. Photosynthesis and πo were higher in the cultivars associated with hotter, less humid regions. Leaf chemical composition varied between cultivars, but was not related to climate associations.

CONCLUSIONS: These findings suggest that leaf turgor maintenance is not a primary limitation on grapevine adaptation to hot or atmospherically dry growing conditions. Thus, selecting for a more negative πo or greater osmotic adjustment is not a promising strategy to develop more climate-resilient grape varieties, contrary to findings for other crops. Future work is needed to identify the mechanisms increasing photosynthesis in the cultivars associated with hot, dry regions.