Coordinated Diffusional and Biochemical Limitations Underlie Age-Related Decline in Photosynthetic Capacity of Rice Leaves

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 CO ₂ availability, as the ratio of chloroplast CO ₂ 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.