Models for the physiological effects of short O3 exposures on plants

Some published effects of ozone on plant photosynthesis and evaporation are detailed, and attempts were made to develop explanatory models of increasing complexity. Stomatal regulation, which keeps the CO₂ concentration inside the lead constant, is assumed. The O₃ concentration inside the leaf is assumed to be neglible, so O₃ uptake, CO₂ uptake and stomatal conductance must be proportional. The suppression of photosynthesis is assumed to be proportional to the integrated effective O₃ uptake. For a first model, a differential equation is derived from these assumptions and a simple analytical solution is found. A second model includes a threshold O₃ flux. The repair process is discussed and three models are investigated with a constant repair rate, a repair rate dependent on the stage of injury, and with a repair rate dependent on the photosynthetic rate. A comparison of the analytical solutions of the appropriate differential equations with literature data shows that the model with repair dependent on photosytheesis is the most successful one, but a constant repair rate also gives a fair approximation. The model with repair dependent on photosynthesis implies the existence of a threshold level for the suppression of photosynthesis which separates reversible and irreversible O₃ effects. With the assumption of an inhomogeneous leaf a model to predict visible leaf injury is derived. The possible existence of a maximum leaf injury index for one exposure concentration is explained from the properties of the models. The restrictions and possible extensions of the models are discussed. The concept of a threshold O₃ flux and of a critical suppression of photosynthesis are shown to give possible explanations of antagonistic and synergistic effects of O₃, SO₂ and NO₂.