Assimilatory Power (Postillumination CO2 Uptake) in Leaves

Assimilatory power was measured in ten C(3) species by means of a rapid-response gas exchange device as the total amount of CO(2) fixed in N(2)-CO(2) atmosphere after switching the light off. Different steady-state levels of the assimilatory power were obtained by varying light intensity and O(2) and CO(2) concentrations during the preexposition periods in the leaf chamber.Within the limits of the linear part of the CO(2) curve of photosynthesis in N(2), the assimilatory power is constant, being sufficient for the assimilation of about 20 nanomoles CO(2) per square centimeter leaf. The pool starts to decrease with the onset of the CO(2) saturation of photosynthesis. Increase in O(2) concentration from 0 to 100% at 350 microliters CO(2) per liter produces a considerable decrease in the assimilatory power.THE MESOPHYLL CONDUCTANCE (M) WAS FOUND TO BE PROPORTIONAL TO THE ASSIMILATORY POWER (A): M = mA. The most frequently occurring values of the proportionality constant (m) (called the specific efficiency of carboxylation) were concentrated between 0.03 and 0.04 centimeter per second per nanomole A per square centimeter but the measured extreme values were 0.01 and 0.06 centimeter per second per nanomole A per square centimeter. The specific rate of carboxylation (the rate per unit A) showed a hyperbolic dependence on CO(2) conentration with the most frequent values of K(m) (CO(2)) ranging from 25 to 35 micromolar in the liquid phase of mesophyll cells (extremes 23 and 100 micromolar).It is concluded that the CO(2(-) ) and light-saturated rate of photosynthesis is limited by the reactions of the formation of the assimilatory power and not by ribulose-1,5-bisphosphate carboxylase. O(2) is a competitive consumer of the assimilatory power, and the inhibitory effect of O(2) on photosynthesis is caused mainly by a decrease in the pool of the assimilatory power at high O(2) concentrations. In intact leaves, the kinetic properties of ribulose-1,5-bisphosphate carboxylase seem to be variable.