Plant Science

The responses of CO₂ assimilation to [CO₂] (A/C_i) were investigated at two developmental stages (R5 and R6) and in several soybean cultivars grown under two levels of CO₂, the ambient level of 370 μbar versus the elevated level of 550 μbar. The A/C_i data were analyzed and compared by either the combined iterations or the separated iterations of the Rubisco-limited photosynthesis (A_c) and/or the RuBP-limited photosynthesis (A_j) using various curve-fitting methods: the linear 2-segment model; the non-rectangular hyperbola model; the rectangular hyperbola model; the constant rate of electron transport (J) method and the variable J method. Inconsistency was found among the various methods for the estimation of the maximum rate of carboxylation (V_cmax), the mitochondrial respiration rate in the light (Rd) and mesophyll conductance (g_m). The analysis showed that the inconsistency was due to inconsistent estimates of g_m values that decreased with an instantaneous increase in [CO₂], and varied with the transition C_i cut-off between Rubisco-limited photosynthesis and RuBP-regeneration-limited photosynthesis, and due to over-parameters for non-linear curve-fitting with g_m included. We proposed an alternate solution to A/C_i curve-fitting for estimates of V_cmax, Rd, J_max and g_m with the various A/C_i curve-fitting methods. The study indicated that down-regulation of photosynthetic capacity by elevated [CO₂] and leaf aging was due to partially the decrease in the maximum rates of carboxylation and partially the decrease in g_m. Mesophyll conductance lowered photosynthetic capacity by 18% on average for the case of soybean plants.