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Assimilatory Power (Postillumination CO₂ Uptake) in Leaves

Measurement, Environmental Dependencies, and Kinetic Properties

Institute of Astrophysics and Atmospheric Physics, Estonian Academy of Sciences, 202444 Tôravere, Tartu, Estonia, U.S.S.R

Assimilatory power was measured in ten C₃ species by means of a rapid-response gas exchange device as the total amount of CO₂ fixed in N₂-CO₂ atmosphere after switching the light off. Different steady-state levels of the assimilatory power were obtained by varying light intensity and O₂ and CO₂ concentrations during the preexposition periods in the leaf chamber.

Within the limits of the linear part of the CO₂ curve of photosynthesis in N₂, the assimilatory power is constant, being sufficient for the assimilation of about 20 nanomoles CO₂ per square centimeter leaf. The pool starts to decrease with the onset of the CO₂ saturation of photosynthesis. Increase in O₂ concentration from 0 to 100% at 350 microliters CO₂ 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 CO₂ conentration with the most frequent values of K_m (CO₂) 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. O₂ is a competitive consumer of the assimilatory power, and the inhibitory effect of O₂ on photosynthesis is caused mainly by a decrease in the pool of the assimilatory power at high O₂ concentrations. In intact leaves, the kinetic properties of ribulose-1,5-bisphosphate carboxylase seem to be variable.

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