The cellular basis of guard cell sensing of rising CO2
Numerous studies conducted on both whole plants and isolated epidermes have documented stomatal sensitivity to CO 2 . In general, CO 2 concentrations below ambient stimulate stomatal opening, or an inhibition of stomatal closure, while CO 2 concentrations above ambient have the opposite effect. The rise in atmospheric CO 2 concentrations which has occurred since the industrial revolution, and which is predicted to continue, will therefore alter rates of transpirational water loss and CO 2 uptake in terrestrial plants. An understanding of the cellular basis for guard cell CO 2 sensing could allow us to better predict, and perhaps ultimately to manipulate, such vegetation responses to climate change. However, the mechanisms by which guard cells sense and respond to the CO 2 signal remain unknown. It has been hypothesized that cytosolic pH and malate levels, cytosolic Ca 2+ levels, chloroplastic zeaxanthin levels, or plasma-membrane anion channel regulation by apoplastic malate are involved in guard cell perception and response to CO 2 . In this review, these hypotheses are discussed, and the evidence for guard cell acclimation to prevailing CO 2 concentrations is also considered.