Temperature increases from global warming will occur simultaneously with [CO2] increases, and the interaction has been suggested by a number of researchers (e.g. Idso et al., 1987) to be beneficial. In a number of species, the response of vegetative growth to CO2 enrichment increases with temperature

(Boote et al., 1997 and references cited therein), presumably because the plants are better able to utilize additional carbohydrate when growth rates are more rapid and this minimizes downward acclimation to [CO2] (Wolfe et al., 1998).

The upper range for the stimulation of [CO2] response with elevated temperature response has been reported as high as 34°C and as low as 26/20°C (Boote et al., 1997 and references cited therein). In a study of four crops in the UK - wheat (Triticum aestivum L.), onion, carrot and cauliflower -Wheeler et al. (1996) found that wheat yield gains from an increase in [CO2] to about 700 |mmol mol-1 were offset by mean seasonal temperatures of only 1.0 and 1.8°C warmer in each of 2 years. Temperature increases of 2.8 and 6.9°C in two onion cultivars was required to offset the yield gains at [CO2] of 560 |mmol mol-1. Of the four crops studied, larger yield increases in high [CO2] were observed in crop species with higher harvest indices in current climates. Warmer temperatures reduced the duration of crop growth, and hence yield, of determinate crops such as winter wheat and onion. However, yield of carrot, which is an indeterminate crop, increased progressively with temperature over the range used in the study (Wheeler et al., 1996).

In two onion cultivars (also grown in the UK), Daymond et al. (1996) found that an increase in bulb yield due to a rise in [CO2] from 374 to 532 |mmol mol-1 was offset by a temperature warming of 8.5-10.9°C in one cultivar but only a 4.0-5.8°C increase in another cultivar. They suggested that the difference in cultivar responses was due to the temperature-sensitive cultivar (i.e. requiring only a 4.0-5.8°C temperature increase to offset [CO2] increases) being a short-season type. Thus, global climatic change may affect relative cultivar performance, and with altered [CO2] and temperatures additional cultivar trials may be required.

Stomatal closure from high [CO2] may reduce water losses, thereby decreasing the adverse effects of increased transpiration at higher temperatures. In addition, this closure protected photosynthesis from the effects of ozone (Reid and Fiscus, 1998) and possibly other pollutants (Allen, 1990). On the other hand, transpiration also has a cooling effect on leaf temperature. If transpirational cooling is reduced, leaves could become more vulnerable to heat stress.

Beneficial effects of elevated [CO2] may not carry over to yields in all crops. Mitchell et al. (1993) found that a 4°C increase in temperature decreased wheat yield significantly, whether grown in elevated [CO2] or not. Similar results were seen by Nair and Peet in tomato (unpublished data) and in potatoes (see section 10.3.1). In cauliflower (Wheeler et al., 1995), no interaction between [CO2] and temperature on total biomass was detected in final harvests. The total dry weight of plants grown at 531 |mmol CO2 mol-1 was 34% greater than from those grown at 328 |mmol mol-1, whereas a 1°C rise reduced dry weight by 6%. Boote et al. (1997) concluded that in a number of crops (including rice, wheat and soybean), high temperature damage on reproductive growth is not offset by CO2 enrichment. This was despite predictions based on the beneficial effects of CO2 enrichment on vegetative growth at high temperatures.

To the extent that global warming increases the frost-free period, more warm-season crops could presumably be grown in northern areas. If springtime temperatures increase, farmers may plant earlier, and/or at higher altitudes and latitudes. Such plantings will be exposed to similar risks of chilling and frost as in their current production areas. Effects of elevated [CO2] on low-temperature tolerance are poorly understood: some reports indicate that high [CO2] mitigates (Boese et al., 1997) while others report that it increases low-temperature sensitivity (Lutze et al., 1998).

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