Papyrus Grasslands Ozone Sorghum Wheat Isoprene Emission Current Objectives


Can the stomatal changes caused by acute ozone exposure be predicted by changes occurring in the mesophyll?

Marion J. Martin, Peter K. Farage, Steven W. Humphries and Steven P. Long.

Keywords:- Ozone; acute; photosynthesis; Vcmax; stomatal conductance; wheat.


The development of a mechanistic model of acute ozone exposure effects on wheat leaf photosynthesis is described. Based on the experimental data of Farage et al. (1991) the magnitude of ozone-induced decline in Vcmax, was found to be linearly related to increasing "Effective Ozone Dose", that is, the ozone dose to the leaf above a threshold flux. Plants produce protective mechanisms against the damaging oxygen radicals formed by the breakdown of ozone within the leaf. Therefore, the threshold flux provides a mechanism of acute ozone effect on photosynthetic metabolism, assuming that damage occurs only once the maximum rate of metabolism of oxygen radical protection within the leaf has been exceeded.

Thus, it is the maximum rate of metabolism of the oxygen radical protection and rate of removal of harmful radicals that determines the magnitude of the threshold ozone flux into the leaf. The effective ozone dose model is thus a mechanistic approach to modeling acute ozone effects on photosynthesis.

The new ozone model is combined with both a mechanistic biochemical model of leaf photosynthesis and a phenomenological model of stomatal conductance to investigate the degree of dependency of ozone-induced stomatal closure on changes in the mesophyll, via the reduction in Vcmax. The model was integrated into WIMOVAC, and the predicted stomatal conductance of the model simulation was compared with the magnitude of stomatal closure measured by Farage et al. (1991). The results indicate that, at least under these experimental conditions, the stomatal changes caused by acute ozone exposure occurred as a result of changes in the mesophyll.

Operational applications and limitations of the ozone model within WIMOVAC to predict leaf assimilation and plant productivity responses to future climate change are briefly discussed.

The ozone model in Wimovac uses the standard user friendly dialog interface for communicating with the user and allows you to examine the effects of ambient ozone concentration and exposure time upon leaf photosynthetic assimilation rate and other key photosynthetic parameters. The dialogs also allow the user to monitor the indirect effect of ozone through assimilation on stomatal conductance.

The model dialog above is for the ozone effects model with 3D plots. The typical output of this model dialog is shown below.

See also:

Marion Martins Thesis (ozone/CO2 interactive effects in wheat)

Ozone/CO2 Interactions in ASPEC - A Face Study

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