Direct and acclimatory responses of stomatal conductance to elevated carbon dioxide in four herbaceous crop species in the field

In order to separate the net effect of growth at elevated [CO2] on stomatal conductance (g(s)) into direct and acclimatory responses, mid-day values o f g(s) were measured for plants grown in field plots in open-topped chamber s at the current ambient [CO2], which averaged 350 mu mol mol(-1) in the da ytime, and at ambient + 350 mu mol mol(-1) [CO2] for winter wheat, winter b arley, potato and sorghum. The acclimatory response was determined by compa ring g(s) measured at 700 mu mol mol(-1) [CO2] for plants grown at the two [CO2]. The direct effect of increasing [CO2] from 350 to 700 mu mol mol(-1) was determined for plants grown at the lower concentration. Photosynthetic rates were measured concurrently with g(s). For all species, growth at the higher [CO2] significantly reduced g(s) measured at 700 mu mol mol(-1) [CO 2]. The reduction in g(s) caused by growth at the higher [CO,] was larger f or all species on days with low leaf to air water vapour pressure differenc e for a given temperature, which coincided with highest conductances and al so the smallest direct effects of increased [CO2] on conductance. For barle y, there was no other evidence for stomatal acclimation, despite consistent downregulation of photosynthetic rate in plants grown at the higher [CO2]. In wheat and potato, in addition to the vapour pressure difference interac tion, the magnitude of stomatal acclimation varied directly in proportion t o the magnitude of down-regulation of photosynthetic rate through the seaso n. In sorghum, g(s) consistently exhibited acclimation, but there was no do wn-regulation of photosynthetic rate. In none of the species except barley was the direct effect the larger component of the net reduction in g(s) whe n averaged over measurement dates. The net effect of growth at elevated [CO 2] on mid-day g(s) resulted from unique combinations of direct and acclimat ory responses in the various species.