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 of g(s) were measured for plants grown in field plots in open-topped chambers at the current ambient [CO2], which averaged 350 mu mol mol(-1) in the daytime, and at ambient + 350 mu mol mol(-1) [CO2] for winter wheat, winter barley, potato and sorghum. The acclimatory response was determined by comparing 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) [CO2]. The reduction in g(s) caused by growth at the higher [CO,] was larger for all species on days with low leaf to air water vapour pressure difference for a given temperature, which coincided with highest conductances and also the smallest direct effects of increased [CO2] on conductance. For barley, 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 interaction, the magnitude of stomatal acclimation varied directly in proportion to the magnitude of down-regulation of photosynthetic rate through the season. In sorghum, g(s) consistently exhibited acclimation, but there was no down-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) when averaged over measurement dates. The net effect of growth at elevated [CO2] on mid-day g(s) resulted from unique combinations of direct and acclimatory responses in the various species.