WHOLE-PLANT RESPIRATION AND PHOTOSYNTHESIS OF WHEAT UNDER INCREASED CO2 CONCENTRATION AND TEMPERATURE - LONG-TERM VS SHORT-TERM DISTINCTIONS FOR MODELING

Short- and long-term effects of elevated CO2 concentration and tempera ture on whole plant respiratory relationships are examined for wheat g rown at four constant temperatures and at two CO2 concentrations. Whol e plant CO2 exchange was measured on a 24 h basis and measurement cond itions varied both to observe short-term effects and to determine the growth respiration coefficient (r(g))r dry weight maintenance coeffici ent (r(g)), basal (i.e. dark acclimated) respiration coefficient (r(b) ), and 24 h respiration:photosynthesis ratio (R:P). There was no respo nse of r(b) to short-term variation in CO2 concentration. For plants w ith adequate N-supply, r(g) was unaffected by the growth-CO2 despite a 10% reduction in the plant's N concentration (%N). However, r(m) was decreased 13%, and r(b) was decreased 20% by growth in elevated CO2 co ncentration relative to ambient. Nevertheless, R:P was not affected by growth in elevated CO2. Whole plant respiration responded to short-te rm variation of + 5 degrees C around the growth temperature with low s ensitivity (Q(10) = 1.8 at 15 degrees C, 1.3 at 30 degrees C). The sha pe of the response of whole plant respiration to growth temperature wa s different from that of the short term response, being a slanted S-sh ape declining between 25 and 30 degrees C. While r(m) increased, r(g) decreased when growth temperature increased between 15 and 20 degrees C. Above 20 degrees C r(m) became temperature insensitive while r(g) i ncreased with growth temperature. Despite these complex component resp onses, R:P increased only from 0.40 to 0.43 between 15 degrees and 30 degrees C growth temperatures. Giving the plants a step increase in te mperature caused a transient increase in R:P which recovered to the pr e-transient value in 3 days. It is concluded that use of a constant R: P with respect to average temperature and CO2 concentration may be a m ore simple and accurate way to model the responses of wheat crop respi ration to 'climate change' than the more complex and mechanistically d ubious functional analysis into growth and maintenance components.