Co-ordination of physiological and morphological responses of stomata to elevated [CO] in vascular plants.

Plant stomata display a wide range of short-term behavioural and long-term morphological responses to atmospheric carbon dioxide concentration ([CO]). The diversity of responses suggests that plants may have different strategies for controlling gas exchange, yet it is not known whether these strategies are co-ordinated in some way. Here, we test the hypothesis that there is co-ordination of physiological (via aperture change) and morphological (via stomatal density change) control of gas exchange by plants. We examined the response of stomatal conductance ( G) to instantaneous changes in external [CO] ( C) in an evolutionary cross-section of vascular plants grown in atmospheres of elevated [CO] (1,500 ppm) and sub-ambient [O] (13.0 %) compared to control conditions (380 ppm CO, 20.9 % O). We found that active control of stomatal aperture to [CO] above current ambient levels was not restricted to angiosperms, occurring in the gymnosperms Lepidozamia peroffskyana and Nageia nagi. The angiosperm species analysed appeared to possess a greater respiratory demand for stomatal movement than gymnosperm species displaying active stomatal control. Those species with little or no control of stomatal aperture (termed passive) to C were more likely to exhibit a reduction in stomatal density than species with active stomatal control when grown in atmospheres of elevated [CO]. The relationship between the degree of stomatal aperture control to C above ambient and the extent of any reduction in stomatal density may suggest the co-ordination of physiological and morphological responses of stomata to [CO] in the optimisation of water use efficiency. This trade-off between stomatal control strategies may have developed due to selective pressures exerted by the costs associated with passive and active stomatal control. [ABSTRACT FROM AUTHOR]