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 CO 2 ‐ and H 2 O‐exchanges in the flag leaf and the ear of a spring wheat cultivar ( Triticum aestivum L. cv. Arkas) were measured at CO 2 partial pressures, p i (CO 2 ), 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 CO 2 ‐dependence of assimilation, response of stomata to varying p a (CO 2 ), CO 2 compensation point (and its temperature dependence), dark respiration, and dissimilation in the light (i.e. CO 2 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 CO 2 compensation point was attained at higher CO 2 partial pressures for the ear than for the flag leaf. The CO 2 uptake of the ear was not saturated at intercellular CO 2 partial pressures below 180 Pa CO 2 , while that of the flag leaf reached saturation at about 80 Pa CO 2 . CO 2 ‐saturated rates of CO 2 uptake were 2.5 and 1.5 times the rates at natural CO 2 partial pressure for ear and flag leaf, respectively. The stomatal conductance decreased with rising CO 2 partial pressure above 35 Pa, in a more pronounced manner for the flag leaf than for the ear.