The "one-point method" for estimating maximum carboxylation capacity of photosynthesis: A cautionary tale.

The maximum carboxylation capacity of Rubisco, Vc,max , is an important photosynthetic parameter that is key to accurate estimation of carbon assimilation. The gold-standard technique for determining Vc,max is to derive Vc,max from the initial slope of an A-Ci curve (the response of photosynthesis, A, to intercellular CO2 concentration, Ci ). Accurate estimates of Vc,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 Vc,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 Vc,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 Vc,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.