Integrating Curve-Shape Traits to Assess Photosynthetic Thermal Responses in Rice

Abstract

We examined whether conventional peak-based traits or curve-shape characteristics better explain photosynthetic performance under realistic warming conditions that combine elevated mean temperatures with pronounced diurnal fluctuations and recurrent heat extremes. Photosynthetic temperature-response curves of 14 rice genotypes, including wild relatives and cultivated varieties, were quantified using a unified modified Arrhenius model, and thermal breadth and sensitivity traits were extracted and used to simulate daily carbon gain under midsummer diurnal temperature patterns. All 14 genotypes exhibited substantial divergence in their photosynthetic thermal responses. While the optimum temperature for photosynthetic rate (T_opt) was highly conserved (the phenotypic coefficient of variation, PCV = 5.7%), shape-based metrics showed far greater variation, especially the high-temperature sensitivity (Slope_higher), with PCV exceeding 70%. Thermal breadth (Breadth80) ranged from 14.2°C to 29.1°C, and A_opt varied by more than 60% across genotypes. Then the genotypes were divied into distinct thermal response groups, which are broad-and-stable types (e.g., SY63, N22), high-capacity but fragile types (e.g., O. glumaepatala (E8-2)), and heat-sensitive types (e.g., LYPJ). Simulations revealed that daily carbon gain corresponded more strongly to Breadth80 and high-temperature sensitivity than to peak traits. Our findings demonstrate that thermostability, not peak performance, is the key determinant of carbon assimilation under hot environments with substantial daytime temperature fluctuations. Incorporating curve-shape traits into breeding and phenotyping efforts will be essential for developing climate-resilient rice.