Augmentation of abscisic acid (ABA) levels by drought does not induce short-term stomatal sensitivity to CO2 in two divergent conifer species

The stomata of conifers display very little short-term response to changes in atmospheric CO2 concentration (C-a), whereas the stomatal responses of angiosperms to C-a increase in response to water stress. This behaviour of angiosperm stomata appears to be dependent on foliar levels of abscisic acid (ABA(f)). Here two alternative explanations for the stomatal insensitivity of conifers to C-a are tested: that conifers have either low ABA(f) or a higher or absent threshold for ABA-induced sensitivity. The responsiveness of stomatal conductance (g(s)) to a sequence of transitions in C-a (386, 100, and 600 mu mol mol(-1)) was recorded over a range of ABA(f) in an angiosperm and two divergent conifer species. The different ABA levels were induced by a mild drought cycle. Although the angiosperm and conifer species showed similar proportional increases in ABA(f) following drought, conifer stomata remained insensitive to changes in C-a whereas angiosperm stomata showed enhanced sensitivity with increasing ABA(f). The conifers, however, had much higher ABA(f) prior to drought than the angiosperm species, suggesting that non-sensitivity to C-a in these conifers was due to an absent or inactive response/signalling pathway rather than insufficient ABA(f).