Estimation of carboxylation efficiency from net CO2 assimilation rate as a function of chloroplastic CO2 concentration in strawberry (Fragaria × ananassa cv. Maehyang) leaves

Abstract: Carboxylation efficiency in fully expanded leaves of strawberry (Fragaria × ananassa cv. Maehyang) was estimated based on net CO₂ assimilation rate (A _n) as a function of chloroplastic CO₂ concentration (C _c). To estimate the mesophyll conductance (g _m) and then construct A _n-C _c curves, A _n and electron transport rate as a function of intercellular CO₂ concentration (C _i) were simultaneously determined at a range of 50 to 2,200 μmol CO₂/mol air at a saturating photosynthetic photon flux (PPF), 1,200 μmol·m⁻²·s⁻¹. Mitochondrial respiration rate (R _d) and CO₂ compensation point in the absence of R _d required for calculating g _m were found to be 0.15 μmol·m⁻²·s⁻¹ and 44.1 μmol CO₂/mol air, respectively, as determined from A _n-C _i curves below 200 μmol CO₂/mol air at three different PPFs. Both stomatal conductance (g _s) and g _m decreased with increasing C _i. However, the g _m responded more sensitively to various C _i than the g _s. The g _m was significantly lower than the g _s at C _i beyond 600 μmol·mol⁻¹ air examined. Maximum carboxylation efficiency (α _cmax) derived from A _n-C _c curves was 0.28 μmol·m⁻²·s⁻¹ and 2.2 times higher than that from A _n-C _i curve. Since the A _n-C _i curve was based on infinite g _m, the α _cmax derived from the A _n-C _i curve might be underestimated. Actually, g _m was rather dynamic with changing C _i and thus a crucial component of the diffusional limitation of A _n. For estimating photosynthetic characteristics in strawberry leaves more accurately, A _n-C _c curve should be constructed in consideration of g _m, especially for closed plant production systems.