Photosynthetic light-response curves. I. The influence of CO2 partial pressure and leaf inversion

The shapes of photosynthetic light-response curves for leaves of Eucalyptus maculata (Hook) and E. pauciflora (Sieber ex Sprengel) were examined. Three different methods were used to measure photosynthesis: CO2 and H2O-vapour exchange, O2 evolution at a 5-kPa CO2 partial pressure, and chlorophyll fluorescence. The three methods were compared and gave good agreement when measured under equivalent conditions. However, O2 evolution was inhibited by high CO2 partial pressures. A non-rectangular hyperbolic curve has been used widely to describe photosynthetic light-response curves. It has three variables which define the maximum quantum yield (photosynthetic rate divided by absorbed irradiance at very low irradiances), the maximum capacity and the curvature factor. We found that the curvature factor was affected by the CO2 partial pressure, declining to a minimum of about 0.6 as CO2 partial pressure increased to 100 Pa. Further increases in the CO2 partial pressure began to inhibit the rate of O2 evolution at 2000 micromoles quanta m-2.s-1 and the curvature factor increased back to 0.95 by 5 kPa CO2 partial pressure. At low irradiances, photosynthesis is limited by the rate of electron transport while at high irradiances, photosynthesis is frequently limited by the activity of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco). The dependence of the curvature factor on CO2 partial pressure arises because the transition between limitations changes as a function of the CO2 partial pressure. The light-response curve is truncated by the transition to a Rubisco limitation and the lower the irradiance at the transition, the higher the value of the curvature factor. There is a gradient in light absorption through the leaf which influences the photosynthetic capacity of of different layers within the leaf. The gradient in photosynthetic capacity can be demonstrated by the fact that the shape of the light-response curve changes when the leaf is illuminated unilaterally onto either the adaxial or abaxial surface. We compared two Eucalyptus species which had either isolateral or dorsiventral leaf anatomy. Leaves were able to reverse completely the gradients in photosynthetic capacity following inversion of the leaves for a week, irrespective of their anatomy.