Response of spring wheat to elevated CO2:: The relationship between assimilation and stomatal resistance

Spring wheat (Triticum aestivum L. cv. Minaret) was exposed to 363 and 628 mu mol mol(-l) CO2 in open top chambers. Gas exchange was measured on flag leaves directly under the variable environmental conditions in the chambers, using a closed loop photosynthesis system. Under elevated CO2 strong stomatal closing response was found, causing reduction of mean transpiration rates by 50% and causing c(i)/c(a) ratio to be 0.51. The stomatal response probably was a result of low air humidity and drought stress during the measurements under elevated CO2. However, a photosynthetic stimulation due to CO2 was lacking and therefore instantaneous water use efficiency was nearly doubled. Boundary lines were used to show the reversed responses of assimilation (A) and stomatal resistance (R-s) to the environmental variables vapour pressure deficit and temperature and their differing responses to photon flux density. When plotting A against R-s a variable assimilation was found with low stomatal resistance. This behaviour was explained by rapid decreases of light intensity after full sunshine and faster response time of assimilation than that of stomata, when shading occured. Elevated CO2 increased the relation of maximum A and R-s and consequently increased intercellular CO2 concentration for these combinations. Under conditions of prolonged high light intensity and medium temperature photosynthesis was enhanced maximally in absolute terms, leading to an increase of 20.9%. It is discussed, that frequency and duration of conditions leading to the largest photosynthetic increases determine the growth response to CO2 in a variable environment.