Whole plant respiration and photosynthesis of wheat under increased CO2 concentration and temperature: Long-term vs short-term distinctions for modelling

Short- and long-term effects of elevated CO2 concentration and temperature on whole plant respiratory relationships are examined for wheat grown at four constant temperatures and at two CO2 concentrations. Whole plant CO2 exchange was measured on a 24 h basis and measurement conditions varied both to observe short-term effects and to determine the growth respiration coefficient (r(g))r dry weight maintenance coefficient (r(g)), basal (i.e. dark acclimated) respiration coefficient (r(b)), and 24 h respiration:photosynthesis ratio (R:P). There was no response of r(b) to short-term variation in CO2 concentration. For plants with 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 concentration relative to ambient. Nevertheless, R:P was not affected by growth in elevated CO2. Whole plant respiration responded to short-term variation of + 5 degrees C around the growth temperature with low sensitivity (Q(10) = 1.8 at 15 degrees C, 1.3 at 30 degrees C). The shape of the response of whole plant respiration to growth temperature was different from that of the short term response, being a slanted S-shape 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) increased with growth temperature. Despite these complex component responses, 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 temperature caused a transient increase in R:P which recovered to the pre-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 more simple and accurate way to model the responses of wheat crop respiration to 'climate change' than the more complex and mechanistically dubious functional analysis into growth and maintenance components.