Methods of mesophyll conductance estimation: its impact on key biochemical parameters and photosynthetic limitations in phosphorus-stressed soybean across CO2

Despite the development of various methods, the rapid estimation of mesophyll conductance (g(m)) for a large number of samples is still a daunting challenge. Although the accurate estimation of g(m) is critical to partition photosynthetic limitations by stomatal (L-s) and mesophyll (L-m) conductance and by photo-biochemical (L-b) processes, the impact of various g(m) estimation methods on this is ambiguous. As phosphorus (P) starvation and elevated CO2 (eCO(2)) strongly affect photosynthetic processes, their combined effect on the proportional changes in these limitations are not well understood. To investigate this, while also evaluating distinct recent methods of g(m) estimation sharing few common theories and assumptions, soybean was grown under a range of P nutrition at ambient and eCO(2). Methods significantly affected g(m) and carboxylation efficiency (V-Cmax) but not other photosynthetic parameters. In all the methods, all photosynthetic parameters responded similarly to treatments. However, the percentage difference between VCmax assuming finite and infinite g(m) was highly inconsistent among methods. The primary mechanism responsible for P limitation to soybean photosynthesis was not CO2 diffusion limitations but L-b comprised of reduced chlorophyll, photochemistry and biochemical processes. The eCO(2) decreased L-b but increased L-m without affecting L-s across leaf P concentration. Although each method explored advances of our understanding about g(m) variability, they all require assumptions of varying degrees, which lead to the discrepancy in the g(m) values. Among the methods, the oxygen sensitivity-based g(m) estimation appeared to be suitable for the quick assessment of a large number of samples or genotypes. Digital tools are provided for the easy estimation of g(m) for some methods.