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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 may s, 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].

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