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. 1984 Nov;76(3):723–729. doi: 10.1104/pp.76.3.723

Assimilatory Power (Postillumination CO2 Uptake) in Leaves

Measurement, Environmental Dependencies, and Kinetic Properties

Agu Laisk 1, Olavi Kiirats 1, Vello Oja 1
PMCID: PMC1064362  PMID: 16663913

Abstract

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 Km (CO2) ranging from 25 to 35 micromolar in the liquid phase of mesophyll cells (extremes 23 and 100 micromolar).

It is concluded that the CO2− 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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Selected References

These references are in PubMed. This may not be the complete list of references from this article.

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