Differences between the flag leaf and the ear of a spring wheat cultivar (Triticum aestivum cv. Arkas) with respect to the CO2 response of assimilation, respiration and stomatal conductance

The CO₂- and H₂O-exchanges in the flag leaf and the ear of a spring wheat cultivar (Triticum aestivum L. cv. Arkas) were measured at CO₂ partial pressures, p_i(CO₂), between 8 and 400 Pa under high photosynthetic photon flux densities (2000 μmol m^?2 s^?1). The experiments were carried out on each organ separately while attached to the intact plant, from the time of ear emergence through senescence. To study the contribution of the kernels to the gas exchange of ears, experiments were also carried out on sterilized ears (treatment A), and on ears from which the kernels were removed (treatment B). Flag leaves and ears differed considerably with regard to CO₂-dependence of assimilation, response of stomata to varying p_a(CO₂), CO₂ compensation point (and its temperature dependence), dark respiration, and dissimilation in the light (i.e. CO₂ production which is not due to oxygenation of ribulose 1,5-bisphosphate). The higher dark respiration of the ear originated mainly from the kernels and continued to some extent in the light. Thus, the CO₂ compensation point was attained at higher CO₂ partial pressures for the ear than for the flag leaf. The CO₂ uptake of the ear was not saturated at intercellular CO₂ partial pressures below 180 Pa CO₂, while that of the flag leaf reached saturation at about 80 Pa CO₂. CO₂-saturated rates of CO₂ uptake were 2.5 and 1.5 times the rates at natural CO₂ partial pressure for ear and flag leaf, respectively. The stomatal conductance decreased with rising CO₂ partial pressure above 35 Pa, in a more pronounced manner for the flag leaf than for the ear.