Some like it hot: differential photosynthetic performance and recovery of english walnut accessions under emerging California heat waves

Authors: MOMAYYEZI, MINA - University Of California, Davis; WILLIAMS, TROY - University Of California, Davis; TOLENTINO, PETER - University Of California, Davis; HAMMERMEISTER, ABBY - University Of California, Davis; Kluepfel, Daniel; FORRESTEL, ELISABETH - University Of California, Davis; McElrone, Andrew

Submitted to: Plant Cell and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/26/2024
Publication Date: 2/3/2025
Citation: Momayyezi, M., Williams, T., Tolentino, P., Hammermeister, A., Kluepfel, D.A., Forrestel, E.J., McElrone, A.J. 2025. Some like it hot: differential photosynthetic performance and recovery of english walnut accessions under emerging California heat waves. Plant Cell and Environment. 48:2099-2110. https://doi.org/10.1111/pce.15273.
DOI: https://doi.org/10.1111/pce.15273

Interpretive Summary: Heat waves (HWs) pose a significant threat to California agriculture, with potential adverse effects on crop photosynthetic capacity, quality and yield leading to significant economic loss. Lack of heat resilient cultivars puts perennial crop production under severe threat with 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 9 English walnut accessions (Juglans regia) for physiological heat stress resilience and recovery in the USDA-ARS National Clonal Germplasm over two-years, and identified accessions with superior resilience to heat stress. Photosynthetic capacity, assessed as net and maximum assimilation rates (An and Amax, respectively), quantum efficiency of PSII and stomatal conductance (gs), was consistently reduced by heat stress for all accessions, but remained higher and/or recovered completely for a few accessions after HW events. Irrigation delivered during HWs, which is commonly used to mitigate detrimental effects of heat stress, improved photosynthetic recovery for several accessions, especially for A3 & A5. These same accessions exhibited the highest An under non-stressed conditions and at ambient temperatures of 35° to 45°C. Higher performance for A3 and A5 under HWs was associated with greater carboxylation rates, electron transport rates, and carbonic anhydrase activity and changes in leaf reflectance spectral indices. All accessions suffered significant declines at 45°C; ambient leaf temperatures approached this level for some accessions during record setting heat waves in September 2022, suggesting severe threat to the economically important walnut growing regions in California under future climatic conditions.

Technical Abstract: Heat waves (HWs) pose a significant threat to California agriculture, with potential adverse effects on crop photosynthetic capacity, quality and yield leading to significant economic loss. Lack of heat resilient cultivars puts perennial crop production under severe threat with 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 9 English walnut accessions (Juglans regia) for physiological heat stress resilience and recovery in the USDA-ARS National Clonal Germplasm over two-years, and identified accessions with superior resilience to heat stress. Photosynthetic capacity, assessed as net and maximum assimilation rates (An and Amax, respectively), quantum efficiency of PSII and stomatal conductance (gs), was consistently reduced by heat stress for all accessions, but remained higher and/or recovered completely for a few accessions after HW events. Irrigation delivered during HWs, which is commonly used to mitigate detrimental effects of heat stress, improved photosynthetic recovery for several accessions, especially for A3 & A5. These same accessions exhibited the highest An under non-stressed conditions and at ambient temperatures of 35° to 45°C. Higher performance for A3 and A5 under HWs was associated with greater carboxylation rates, electron transport rates, and carbonic anhydrase activity and changes in leaf reflectance spectral indices. All accessions suffered significant declines at 45°C; ambient leaf temperatures approached this level for some accessions during record setting heat waves in September 2022, suggesting severe threat to the economically important walnut growing regions in California under future climatic conditions.