CO2 DIFFUSIONAL AND STOMATAL LIMITATIONS OF PHOTOSYNTHESIS IN AMOMUM VILLOSUM

Abstract

Photosynthetic rate is a function of not only the CO_2 concentration gradient between the outside and inside of the leaf, but also the CO_2 diffusional resistance. It is accepted that stomatal resistance is the greatest factor that controls CO_2 diffusional resistance and is thus a crucial factor influencing photosynthesis. Hence, adequate analysis of CO_2 diffusional resistance is necessary for understanding photosynthesis. Intercellular CO_2 concentrations (C_i) are often utilized to calculate stomatal limitation (L_s). This traditional analysis is unreliable when plants are under stress, because C_i cannot be accurately measured under such conditions. Here we introduced the concept of diffusional limitation and introduced a new method to calculate the diffusional limitation value (L_d) without using C_i. In addition, C_i, estimated indirectly through chlorophyll fluorescence parameters (C_i′), was used to calculate a new stomatal limitation value (L_s′) for plants grown under 40% relative soil moisture (RSM). We compared the L_s′, L_s and L_d for Amomum villosum grown under both 100% and 40% RSM. Photosynthetic rates (P_n) and stomatal conductance (G_s) decreased after noon in both RSM treatments, and P_n and G_s were both higher in 100% RSM than in 40% RSM. Under 100% RSM, L_s did not increase after noontime in A. villosum, indicating stomatal limitation of photosynthesis did not increase, whereas L_d increased indicating the diffusional limitation of photosynthesis increased due to the relatively high mesophyll resistance. Under 40% RSM, L_s′ increased sooner than L_d after noon, indicating stomatal resistance was the dominant factor controlling diffusional resistance. In contrast, L_s, calculated using the traditional stomatal limitation method, did not increase under 40% RSM, which might lead us to the wrong conclusion that stomatal limitation was not a factor. The estimated value, C_i′, was lower than C_i, indicating that gas exchange system was overestimated using C_i under conditions of water stress. Our results suggest that the traditional method is unreliable under soil water stress conditions, and the two new methods presented are more reliable. Furthermore, mesophyll resistance can be estimated indirectly through the joint analysis of diffusional and stomatal limitation.