PLANT-RESPONSES TO TEMPERATURE UNDER CONDITIONS OF ELEVATED CO2

A literature survey of the interactive effects of CO2 enrichment and temperature on plant development and growth, indicated that the responses cannot be interpreted within a simple framework. For example, although plant development is generally accelerated by increased temperature, CO2 enrichment can accelerate it even further in some instances, or CO2 enrichment may have neutral or even retarding effects in other cases. Where the temperature and CO2 effects are additive, it is argued that CO2 is operating in the same way as radiation to reduce a carbon limitation. If this were true, CO2 enrichment would be most likely to accelerate development in tropical regions during the low-radiation monsoon season. Similarly, while it would be expected that CO2-enrichment would have increasingly enhancing effects with increasing temperature on phytomass growth, this is not invariably the case. In extreme examples which followed the expected trend, plants grown in twice-normal CO2-enriched atmospheres performed progressively better than those grown at current levels of CO2 by 8.7% for every 1-degrees-C rise in temperature. However, the difference between the two CO2 treatments more commonly increased by only around 2% for every 1-degrees-C rise in temperature. Of examples examined, both sunflower and nodulated cowpea showed the reverse response to temperature, while non-nodulated cowpea, supplied with luxuriant levels of nutrition, showed no interaction with temperature but a strong interaction between CO2 and radiation. Other aspects of the environment such as nutrition and radiation strongly modify the responses to temperature. It is also clear that plant factors such as stage of development can alter the response to CO2. Long-term studies with several species are required which will take into account many environmental variables within a realistic envelope. One methodology for doing this is presented. There was no evidence among species that responses to CO2 arise through any consistent change in morphology such as via increased branching or increased leaf number. Plant plasticity is such that responses can be expressed in a variety of ways determined by other environmental variables.