Ozone (O3) deleteriously affects organisms ranging from humans to crop plants, yet little is understood regarding the underlying mechanisms. In plants, O3 decreases CO2 assimilation, but whether this could result from direct O 3 action on guard cells remained unknown. Potassium flux causes osmotically driven changes in guard cell volume that regulate apertures of associated microscopic pores through which CO 2 is supplied to the photosynthetic mesophyll tissue. We show in Vicia faba that O3 inhibits (i) guard cell K ؉ channels that mediate K ؉ uptake that drives stomatal opening; (ii) stomatal opening in isolated epidermes; and (iii) stomatal opening in leaves, such that CO2 assimilation is reduced without direct effects of O 3 on photosynthetic capacity. Direct O3 effects on guard cells may have ecological and agronomic implications for plant productivity and for response to other environmental stressors including drought.

O, is a phytotoxic air pollutant and one form of oxidative stress to which plants may be exposed. It is known to induce visible injury, reduce growth, reduce the rate of net photosynthesis, and accelerate foliar senescence in a large number of plant species (Reich and Amundson, 1985;Darrall, 1989;Pell et al., 1994a). A reduction in the quantity of Rubisco has been associated with a decline in net photosynthesis and the induction of accelerated foliar senescence in many species, including potato, poplar, aspen, and radish (Pell et al., 1994a). This reduction in Rubisco quantity could be the result of direct degradation of the proteins in the chloroplast due to increased oxidative stress (Landry and Pell, 1993). The reduction could also be caused by an effect on Rubisco synthesis, since a rapid decrease in levels

The essential role of brassinosteroids (BRs) in normal plant growth, development and physiology has been established by the analysis of biosynthesis and signal transduction mutants. Some of the BR-related mutants also display altered sensitivity to the phytohormone abscisic acid (ABA) suggesting that BRs normally counteract the effects of ABA on root growth, seed germination, and possibly stomatal movement. In this study, the effect of a specific BR, brassinolide (BL), on guard cell function of Vicia faba was examined alone and in conjunction with ABA. Unlike other described plant responses, BL did not oppose the effect of ABA in regulation of stomatal movement. On the contrary, BL modulated stomatal aperture by promoting stomatal closure and inhibiting stomatal opening, functions of this hormone that were previously undescribed. This study also demonstrated a role for plant steroidal hormones in ion channel regulation: BL inhibited inwardly rectifying K þ currents of V. faba guard cell protoplasts in a manner similar to ABA. In both stomatal movement assays and whole-cell patch clamp experiments, the effects of BL and ABA applied together were not additive, suggesting that these two hormones may function in interacting pathways to regulate stomatal apertures and guard cell physiology.