Validation of an Enzyme-Driven Model Explaining Photosynthetic Rate Responses to Limited Nitrogen in Crop Plants

The limited availability of nitrogen (N) is a fundamental challenge for many crop plants. We have hypothesized that the relative crop photosynthetic rate (P) is exponentially constrained by certain plant-specific enzyme activities, such as ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), NADP-glyceraldehyde-3-phosphate dehydrogenase (NADP-G3PDH), 3-phosphoglyceric acid (PGA) kinase, and chloroplast fructose-1,6-bisphosphatase (cpFBPase), in Triticum aestivum and Oryza sativa. We conducted a literature search to compile information from previous studies on C(3) and C(4) crop plants, to examine the photosynthetic rate responses to limited leaf [N] levels. We found that in Zea mays, NADP-malic enzyme (NADP-ME), PEP carboxykinase (PCK), and Rubisco activities were positively correlated with P. A positive correlation was also observed between both phosphoenolpyruvate carboxylase (PEPC) and Rubisco activity with leaf [N] in Sorghum bicolor. Key enzyme activities responded differently to P in C(3) and C(4) plants, suggesting that other factors, such as leaf [N] and the stage of leaf growth, also limited specific enzyme activities. The relationships followed the best fitting exponential relationships between key enzymes and the P rate in both C(3) and C(4) plants. It was found that C(4) species absorbed less leaf [N] but had higher [N] assimilation rates (A (rate)) and higher maximum photosynthesis rates (P(max)), i.e., they were able to utilize and invest more [N] to sustain higher carbon gains. All C(3) species studied herein had higher [N] storage (N(store)) and higher absorption of [N], when compared with the C(4) species. N(store) was the main [N] source used for maintaining photosynthetic capacity and leaf expansion. Of the nine C(3) species assessed, rice had the greatest P(max), thereby absorbing more leaf [N]. Elevated CO(2) (eCO(2)) was also found to reduce the leaf [N] and P(max) in rice but enhanced the leaf [N] and N use efficiency of photosynthesis in maize. We concluded that eCO(2) affects [N] allocation, which directly or indirectly affects P(max). These results highlight the need to further study these physiological and biochemical processes, to better predict how crops will respond to eCO(2) concentrations and limited [N].