Stomatal and Non-Stomatal Limitation to Photosynthesis in Pinus tabulaeformis Seedling under Different Soil Water Conditions:Experimental and Simulation Results

Abstract

Midday depression in photosynthesis is common in plants, but the relative importance of stomatal and non-stomatal limitation to photosynthesis is variable among species. Pinus tabulaeformis is a conifer and distributed widely over China, which suggests its adaptability to a wide range of climate and soil water conditions. The aims of this study were to (a) understand the leaf gas exchange characteristics and stomatal and non-stomatal limitation of photosynthesis in P. tabulaeformis seedlings under different soil water conditions; (b) to compare the relationships between photosynthesis and stomatal conductance through simulation with three stomatal models; and (c) to determine the impact of the soil water content and the CO2 supply and demand on the simulation results. We measured diurnal variations in leaf-scale gas exchange of P. tabulaeformis seedlings grown under four soil water conditions:8% (W0), 12% (W1), 16% (W2) and 20% (W3) soil water content. There was a clear midday depression of net CO2 assimilation rate (A) and stomatal conductance (gs) in the all four soil water conditions. In the low and moderate soil water conditions (W0, W1 and W2), the midday depression in A and gs was accompanied with the decrease in intercellular CO2 concentration (Ci). However, in the high soil water condition (W3), the decreases in A and gs at midday were coupled with an increase in Ci. In addition, reduction of gs was bigger than the inhibition of A in the W0, W1 and W2. By contrast, in the W3 treatment, change in gs was in a smaller magnitude than the inhibition of A. Moreover, a high correlation between A and gs was observed across all treatments, and decrease in Ci paralleled to reductions in gs in W0, W1 and W2 treatments. However, in the W3 treatment, A and gs correlated positively with each other, and Ci remained unchanged at high gs and either increased or decreased at low gs. These results suggested that midday depression of A in W0, W1 and W2 soil water conditions was caused by closure of stomata rather than a decreased photosynthetic capacity of mesophyll cells. In contrast, midday depression in A in wet soil was controlled by non-stomatal decreases in the photosynthetic capacity of mesophyll cells. The comparison of stomatal models showed that the Medlyn model performed best in all of the four soil water conditions whether or not morning and afternoon were analysed separately. However the relationships between photosynthesis and stomatal conductance simulated by the Medlyn model differed significantly in different soil water treatments, and with morning and afternoon data separated, indicating that the Medlyn model can be improved by incorporating a function which can reflect the influence of soil moisture on the stomatal behaviour in different soil water conditions. Furthermore, it is suggested that the model simulations should be run separately in morning and afternoon for the plants that have remarkable midday depression in photosynthesis. Finally, we suggest that when both stomatal and non-stomatal limitations exist, the simulation of the relationships between photosynthesis and stomatal conductance should be operated separately, and a function, which assumes that stomatal conductance is regulated by rates of electron translation and by rates of Rubisco activity, or by the balance between the two processes, should be incorporated into the model to improve the simulation.