Maximizing leaf carbon gain in varying saline conditions: An optimization model with dynamic mesophyll conductance

While the adverse effects of elevated salinity levels on leaf gas exchange in many crops are not in dispute, representing such effects on leaf photosynthetic rates (A) continues to draw research attention. Here, an optimization model for stomatal conductance (g_c ) that maximizes A while accounting for mesophyll conductance (g_m ) was used to interpret new leaf gas exchange measurements collected for five irrigation water salinity levels. A function between chloroplastic CO₂ concentration (c_c ) and intercellular CO₂ concentration (c_i ) modified by salinity stress to estimate g_m was proposed. Results showed that with increased salinity, the estimated g_m and maximum photosynthetic capacity were both reduced, whereas the marginal water use efficiency λ increased linearly. Adjustments of g_m , λ and photosynthetic capacity were shown to be consistent with a large corpus of drought-stress experiments. The inferred model parameters were then used to evaluate the combined effects of elevated salinity and atmospheric CO₂ concentration (c_a ) on leaf gas exchange. For a given salinity level, increasing c_a increased A linearly, but these increases were accompanied by mild reductions in g_c and transpiration. The c_a level needed to ameliorate A reductions due to increased salinity is also discussed using the aforementioned model calculations.