Some Like It Hot: Differential Photosynthetic Performance and Recovery of English Walnut Accessions Under Emerging California Heat Waves.

Heat waves (HWs) pose a significant threat to California agriculture, with potential adverse effects on crop photosynthetic capacity, quality and yield, all of which contribute to significant economic loss. Lack of heat-resilient cultivars puts perennial crop production under severe threat due to increasing HW frequency, duration and intensity. Currently, available walnut cultivars are highly sensitive to abiotic stress, and germplasm collections provide potential solutions via genotypes native to varied climates. We screened nine English walnut accessions (Juglans regia) for physiological heat stress resilience and recovery in the USDA-ARS National Clonal Germplasm over 2-years, and identified accessions with superior resilience to heat stress. Heat stress impacted photosynthetic capacity in most accessions, as evidenced by reductions in net (A) and maximum (A) assimilation rates, quantum efficiency of PSII, and changes in stomatal conductance (g). However, two accessions exhibited either higher or complete recovery post-irrigation. This aligns with the established practice of using irrigation to mitigate heat stress, as it improved recovery for several accessions, with A3 and A5 demonstrating the most resilience. One of these two superior accessions is native to one of the hottest and driest habitats of all studied accessions. These same accessions exhibited the highest A under non-stressed conditions and at higher temperatures of 35° to 45°C. Higher performance for A3 and A5 under HWs was associated with greater carboxylation rates, electron transport rates, and A. All accessions suffered significant declines in photosynthetic performance at 45°C, which were the ambient leaf temperatures approached during record-setting heat waves in California during September 2022.