Coordinated diffusional and biochemical limitations underlie age-related decline in photosynthetic capacity of rice...

The net photosynthetic rate (A) decreases with leaf aging and senescence, primarily due to reductions in stomatal conductance (g_s), mesophyll conductance (g_m), and the maximum carboxylation rate (V_cmax). However, the relative contributions of these factors to age-related declines in photosynthesis remain insufficiently understood. In this study, we investigated gas exchange parameters, chlorophyll fluorescence, and biochemical traits in mature and senescing rice leaves. The net photosynthetic rate (A) decreased with leaf age, from 22.2 µmol m⁻² s⁻¹ in mature leaves to 15.9 µmol m⁻² s⁻¹ in older leaves. The absolute limitations imposed by g_s (LS), g_m (LM), and V_cmax (LB) were 8.54%, 9.33%, and 11.2%, respectively. The observed reduction in V_cmax in senescing leaves was primarily attributed to a decline in Rubisco content, while the in vivo specific activity of Rubisco (V_cmax/Rubisco) remained comparable between mature and older leaves. Similarly, the apparent decrease in Rubisco activity was driven by reduced Rubisco content rather than limited CO₂ availability, as the ratio of chloroplast CO₂ concentration to Rubisco content (C_c/Rubisco) was even higher in older leaves, indicating that substrate supply was not a limiting factor for catalysis. Taken together, stomatal conductance, mesophyll conductance, and V_cmax imposed comparable limitations on photosynthesis during leaf aging, with the decline in V_cmax and Rubisco activity largely attributed to a reduction in Rubisco content.