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Pakistan Journal of Biological Sciences
Year: 2003  |  Volume: 6  |  Issue: 13  |  Page No.: 1096 - 1107

Elevated CO2 and Stress Tolerance in Crop Plants with Particular Reference to Agro-climatic Conditions of Pakistan

F. Azam and S. Farooq    

Abstract: One of the dramatic observations on response of plants to elevated CO2 is their enhanced tolerance to stresses like light, temperature, salinity and nutrients. High temperatures and light intensity (like that in Pakistan) have generally a depressing effect on growth of plants, especially those having C3 photosynthetic system. These plants have a lower optimum temperature for photosynthesis that is raised at elevated CO2 thereby protecting the plants from being over-heated. In addition, the photorespiratory activities of C3 plants at higher temperatures are curtailed in the presence of high CO2 concentrations, thereby helping the plants conserve C and energy. Low as well as high light intensities are reported to limit photosynthesis at ambient concentrations of CO2, while elevated CO2 levels have a mitigating effect. Elevated CO2 levels allow plants to live under light conditions insufficient to meet photosynthetic requirements, while under high light intensities photorespiratory activities are curtailed. Similarly, positive effect of elevated CO2 on plant performance under conditions of limited water availability (e.g., because of soil salinity, drought) has consistently been reported. It is believed that the on-going rise in air`s CO2 content will protect trees from debilitating water stress. The water stress may not necessarily be that of drought, excessive irrigation may have negative effect as well and is reported to be mitigated by elevated CO2. Plants growing under soil salinity stress have also been reported to benefit from elevated CO2. The benefit is reported to be derived from the availability of more solutes for osmoregulation by reducing the transpirational intake of salts or by improving RUBISCO activity. In C4 plants, elevated CO2 helps overcome the leakiness induced by salt stress. In addition, positive effect of elevated CO2 on root proliferation and root-induced microbiological and biochemical changes may help plants withstand salinity stress. One of the reasons for a more positive effect of elevated CO2 on plants under growth limiting conditions is the enhanced root proliferation. Thus at low level of N availability that would generally limit plant growth, elevated CO2 helps plants extract more N from soil by exploring a greater soil volume and stimulation of photosynthetic rates. This paper gives an overview of the available information on the subject vis-à-vis agroclimatic conditions prevailing in Pakistan.

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