Insights into the Photosynthetic Efficiency and Chloroplast Ultrastructure of Heat-Stressed Edamame Cultivars During the Reproductive Stages

Abstract

High temperatures have adverse impacts on the photosynthetic efficiency and yield of many crop plants. This study investigated how high temperatures affect the photosynthetic efficiency parameters and chloroplast ultrastructure of three edamame cultivars (AGS354, UVE17, and UVE14) at the reproductive stages (flowering and pod-filling). Heat stress (HS) treatments were performed under controlled conditions in climate chambers set at 25/18 °C (control), 30/23 °C (HS-I), and 35/28 °C (HS-II). The AGS354 cultivar exhibited the greatest susceptibility under HS-II treatment, characterised by a reduction in the photochemical reactions, decreased chlorophyll-a (chl-a) and carotenoid accumulation, the highest increase in the starch grain traits, and reduced plastoglobule and grana area traits. In UVE 14 and UVE17, the HS-II treatment enhanced chl-a and chl-b accumulation. Elevated carotenoid levels in UVE14 and UVE17 likely protected chlorophyll from degradation and mitigated photooxidative damage. The HS-II treatment also enhanced the grana traits, supporting improved light-harvesting capacity during heat stress in UVE14 and 17. However, heat stress disrupted the photochemical reactions (quantum efficiency of photosystem II, performance index absorbance, and performance index), indicating that elevated carotenoids alone do not exhibit complete tolerance to heat stress. Since plastoglobules play an essential in carotenoid biosynthesis, increased or stabilised plastoglobule traits in UVE14 and UVE17 under HS-II treatment strongly indicate improved heat stress tolerance. Overall, UVE14 and UVE17 emerged as the most heat-tolerant cultivars, with AGS354 being the most susceptible. These findings provide valuable insights into heat stress adaptation mechanisms and suggest the UVE14 and UVE17 cultivars as potential candidates for breeding heat-tolerant edamame cultivars.