Introduction

Effect of elevated CO₂ on photosynthesis and Rubisco

1. Increase photosynthesis

The effect of elevated CO₂ on photosynthesis have been well studied. In a recent review Drake et al 1997 have surveyed the results of 60 studies and found that the evidence supporting the stimulation of photosynthesis by elevated CO2 is overwhelming, averaging 58% higher photosynthetic rates in elevated CO2 relative to ambient CO2 levels. The kinetics of rubisco can explain both the short-term and several long term response of photosynthesis to elevated CO2. Most important is the reduction in photorespiratory carbon oxidation which is catalysed by rubisco and for which CO2 is a competitive inhibitor. A doubling in atmospheric CO2 will be approximately halved the rate of photorespiration. In elevated CO2 this reduction in photorespiration occurs regardless of whether the plant is under light, water or nutrient stress. A second reason for the enhancement of photosynthesis is that rubisco, is not CO2- saturated such that an increase in CO2 increases the carboxylation rate of rubisco.

2. Acclimation of photosynthesis at elevated CO2.

There is abundant evidence that long-term photosynthesis in elevated CO2 leads to an acclimation of the photosynthetic properties of the leaves. Acclimation photosynthesis is generally recognised by higher carbohydrate concentrations, lower concentration of rubisco enzymes and decrease in maximum carboxylation rates (Vcmax). There are two apparent reasons for the acclimation: 1) the accumulation of excess carbohydrates results in a decrease in source activity. 2) Less rubisco is required at elevated CO2. Calculations show that 35% of the rubisco present at ambient Ca can be lost at double CO2 before it co-limits photosynthesis. The molecular mechanism of down-regulation of rubisco is believed to be related to the accumulation of leaf non-structural carbohydrates, and most likely operates at the transcriptional level. The correlation between acclimation and carbohydrate accumulation is not always found in nature, and other mechanisms of regulation may be present.

Proposed work will 1). Produce parameterisation for WIMOVAC suitable for aspen, poplar and pine that will allow both short and long term effects of CO2 on photosynthesis and productivity to be predicted. 2). Generate response curve of steady state mRNA levels, protein levels, enzyme activity? Activation state and kinetic measurements of gas exchange. For both poplar and pine as a function of developmental stage.

B. Interaction of CO2 X ozone on photosynthesis and rubisco

A response common to many plants is the induction of accelerated foliar senescence which is accompanied by a decrease in photosynthesis rate and rubisco protein levels. Decreases in rubisco have been attributed to enhanced degradation of the protein. The exact mechanism of ozone action on the photosynthetic apparatus is not well understood. However, the effective dose of ozone causing damage to plants is a function of stomatal opening as well as atmospheric ozone concentration. The concentration of ozone inside the cell is very low and probably never reach the stroma. It has been suggested that reaction with cell and organellar membrane will generated secondary antioxidant which will then react with rubisco. Pell has suggested that the tolerance of rubisco to ozone may be related to longevity of the foliage.

The regulation transcription and translation of rbc mRNA are highly regulated, the mechanism for the induction of degradation is not well understood, especially in the context of leaf senescence (Pell, 1993). Pell assumes that the time of active synthesis of rubisco is predetermined for a given type of leaf . In the case of annuals and trees like poplar which are indeterminate perennial, the life span of an individual leaf is a matter of weeks or a few months at most. In leaves with short life span the capacity to synthesise rubisco must be limited to until shortly after the completion of leaf expansion (Pell). Pine neddles in contrast have a long life time lasting sometimes several years and the synthesis and turnover of rubisco is expected to last longer. 

Proposed work will 1). Produce and parameterise a model of ozone effects on the photosynthetic model of aspen. 2). Correlate this with the results being derived from the Aspen/FACE experiment at Rhinelander.3) Produce a long term growth model incorporating these effects.

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Last modified: August 19, 1997