Responses of CO2 assimilation to changes in irradiance: Laboratory and field data and a model for beans (Phaseolus vulgaris L.)

Abstract

The responses of net CO2 assimilation to sudden changes in irradiance were studied in Phaseolus vulgaris L. in the laboratory and the field. For irradiance changes between 50 μmol m-2 s-1 to 350 μmol m-2 s-1 in the laboratory, assimilation rate increased with half-times of 2.7 and 4.1 min in well-watered and water-stressed plants, respectively. In a field experiment with a change in irradiance from 400 to 1200 μmol m-2 s-1 the response was faster (half-time=c. 1.2 min). In all cases when irradiance was returned to a low value, assimilation declined rapidly with a halftime of approximately 1 min, which approached the time resolution of the gas-exchange system. The corresponding changes in stomatal conductance in response to both increasing and decreasing irradiance were much slower than the assimilation responses, indicating that biochemical processes, rather than CO2 supply, primarily determined the actual rate of assimilation in these experiments. The concept of stomatal limitation to photosynthesis is discussed in relation to these results. A simple model for assimilation in a fluctuating light environment is proposed that depends on a steadystate light response curve, an 'induction lag' on increasing irradiance, and an induction-state memory. The likely importance of taking account of such induction lags in natural canopy microclimates is considered.