The “one‐point method” for estimating maximum carboxylation capacity of photosynthesis: A cautionary tale

The maximum carboxylation capacity of Rubisco, V_c,max, is an important photosynthetic parameter that is key to accurate estimation of carbon assimilation. The gold‐standard technique for determining V_c,max is to derive V_c,max from the initial slope of an A–C_i curve (the response of photosynthesis, A, to intercellular CO₂ concentration, C_i). Accurate estimates of V_c,max derived from an alternative and rapid “one‐point” measurement of photosynthesis could greatly accelerate data collection and model parameterization. We evaluated the practical application of the one‐point method in six species measured under standard conditions (saturating irradiance and 400 μmol CO₂ mol^?1) and under conditions that would increase the likelihood for successful estimation of V_c,max: (a) ensuring Rubisco‐limited A by measuring at 300 μmol CO₂ mol^?1 and (b) allowing time for acclimation to saturating irradiance prior to measurement. The one‐point method significantly underestimated V_c,max in four of the six species, providing estimates 21%–32% below fitted values. We identified ribulose‐1,5‐bisphosphate‐limited A, light acclimation, and the use of an assumed respiration rate as factors that limited the effective use of the one‐point method to accurately estimate V_c,max. We conclude that the one‐point method requires a species‐specific understanding of its application, is often unsuccessful, and must be used with caution.