Whole plant respiration and photosynthesis of wheat under increased CO₂ concentration and temperature: long‐term vs. short‐term distinctions for modelling

Abstract

Abstract Short‐ and long‐term effects of elevated CO 2 concentration and temperature on whole plant respiratory relationships are examined for wheat grown at four constant temperatures and at two CO 2 concentrations. Whole plant CO 2 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 ), dry weight maintenance coefficient (r m ), basal (i.e. dark acclimated) respiration coefficient (r g ), and 24 h respiration:photosynthesis ratio (R:P). There was no response of r g to short‐term variation in CO2 concentration. For plants with adequate N supply, r g was unaffected by the growth‐CO 2 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 CO 2 concentration relative to ambient. Nevertheless, R:P was not affected by growth in elevated CO 2 . Whole plant respiration responded to short‐term variation of ± 5 °C around the growth temperature with low sensitivity (Q 10 = 1.8 at 15 °C, 1.3 at 30 °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 °C. While r m , increased, r g decreased when growth temperature increased between 15 and 20 °C. Above 20 °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° and 30 °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 CO 2 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.