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

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