There is now strong evidence that the plant hormone abscisic acid (ABA) plays an important role in the regulation of stomatal behaviour and gas exchange of droughted plants.

Recent reports show ethylene-dependent reductions in stomatal sensitivity to abscisic acid (ABA) under ozone stress. These changes reduce stomatal control of plant water loss in drying soil. Here we review evidence that ABA and ethylene, and interactions between these two stress-induced hormones, control many of the responses of intact plants to drought and ozone stress, with emphasis on effects on stomata and shoot growth. We draw attention to convergent signalling and response pathways induced by ozone and drought that can increase production of hydrogen peroxide (H2O2) and nitric oxide (NO). Stomatal responses to a wider range of stresses and developmental cues may also be controlled via the same sets of signalling pathways. Other hormones, or effectors such as xylem/apoplastic pH or changes in plant water status, also play a role in signalling within and between organs. We discuss the implications, for crops, natural ecosystems and water catchment processes, of ethylene's antagonism of the stomatal response to ABA, against a back-drop of predictions for reduced precipitation and increasing ozone pollution, as part of global climate change and increasing urbanization and industrial development.

SummaryStomatal behaviour of plants in drying soil can be regulated by (long distance) chemical signals that provide the shoot with some measure of water availability. Although much emphasis has been placed on the plant hormone abscisic acid (ABA) as a central component of the signalling process, soil drying will modify the delivery to the shoot of a range of potential chemical signals. Here we consider the role that changes in the xylem sap pH might play in determining the access that ABA has to sites of action on the guard cells. We also show how redistribution of inorganic ions between different compartments in the leaf (localized chemical signalling) can provide sensitive control of stomata and water loss in response to potentially damaging changes in leaf temperature. Partial root zone drying is an irrigation technique that has been developed to allow exploitation of the plant's long distance signalling system. When the system is optimized, stomatal behaviour, shoot water status and leaf growth can be regulated such that water use efficiency (fruit yield/water used) can be significantly increased. We show how an understanding of the drought stress physiology of the whole plant can lead to substantial saving of irrigation water in agriculture.

Drought increased the p H of Commelina communis xylem sap from 6.1 t o 6.7. Conductances of transpiring leaves were 50% lower i n p H 7.0 than in pH 6.0 buffers, but bulk leaf abscisic acid (ABA) concentration and shoot water status were unaffected by pH. Stomatal apertures of isolated abaxial epidermis incubated on simple buffers increased with external pH, so in vivo this must be overridden by alternative p H effects. Reductions in leaf transpiration rate at p H 7.0 were dependent on the presence of 1OP8 mo1 dm-3 ABA in the xylem stream. We inferred that at p H 7.0 leaf apoplastic ABA concentrations increased: p H did not affect distributions of ABA among leaf tissues, but isolated epidermis and mesophyll tissue took up more 3H-ABA from p H 6.0 than from p H 7.0 buffers. The apoplastic ABA increase at p H 7.0 may result from reduced symplastic sequestration. A portion of 3H-ABA uptake by the epidermis was saturable at p H 6.0 but not at p H 7.0. An ABA uptake carrier may contribute t o ABA sequestration by the leaf symplast of well-watered plants, and its inactivity at p H 7.0 may favor apoplastic ABA accumulation i n droughted plants. Effects of external p H on stomatal apertures in the isolated epidermis indicate that published data supporting a role for interna1 guard cell ABA receptors should be reassessed.In droughted plants an increase in the ABA concentration of the apoplastic compartment of the leaf (Vanrensburg et al., 1996) is an important determinant of stomatal behavior (Gowing et al., 1993), and there is some evidence that a group of ABA receptors is located at the external surface of the plasmalemma of the guard cell (Hartung 1983;Hornberg and Weiler, 1984). When bound, the receptors induce changes in membrane ion transport and reduce the osmolarity of the guard cell such that it loses turgor pressure, which leads to stomatal closure (see Assmann, 1993).ABA carried by the transpiration stream from the xylem vessels arrives in the leaf apoplast as a dominant source (Weyers and Hillman, 1979), giving rise to a concentration sufficient to close stomata (Loveys, 1984). There is much evidence that some of the ABA carried by the xylem stream (1993,1995) have shown that nmol dmP3 concentrations of ABA supplied directly to isolated epidermal strips of the same species were sufficient to close stomata. These results demonstrate that the leaf itself influences the response of the stomata in the epidermis to the constituents of the transpiration stream. Trejo et al. (1993) concluded that, in whole leaves, the accumulation of ABA by the epidermis was limited by the presence of the mesophyll. Cells of this tissue rapidly metabolized ABA (see also Gowing et al., 1993), thus maintaining a concentration gradient for continuous ABA sequestration into the symplast. Daeter and Hartung (1995) demonstrated that the epidermal symplast of barley leaves is an even better sink for apoplastic ABA than the mesophyll, because it has a catabolic rate that is 5-fold greater.A physicochemical basis for ABA distribution betw...

Here we highlight how both the root and shoot environment impact on whole plant hormone balance, particularly under stresses such as soil drying, and relate hormone ratios and relative abundances to processes influencing plant performance and yield under both mild and more severe stress. We discuss evidence (i) that abscisic acid (ABA) and ethylene act antagonistically on grain-filling rate amongst other yield-impacting processes; (ii) that ABA's effectiveness as an agent of stomatal closure can be modulated by coincident ethylene or cytokinin accumulation; and (iii) that enhanced cytokinin production can increase growth and yield by improving foliar stay-green indices under stress, and by improving processes that impact grain-filling and number, and that this can be the result of altered relative abundances of cytokinin and ABA (and other hormones). We describe evidence and novel processes whereby these phenomena are/could be amenable to manipulation through genetic and management routes, such that plant performance and yield can be improved. We explore the possibility that a range of ABA-ethylene and ABA-cytokinin relative abundances could represent targets for breeding/managing for yield resilience under a spectrum of stress levels between severe and mild, and could circumvent some of the pitfalls so far encountered in the massive research effort towards breeding for increases in the complex trait of yield.