Commelina communis stomata closed within 1 h of transferring intact plants from 27°C to 7°C, whereas tobacco (Nicotiana rustica) stomata did not until the leaves wilted. Abscisic acid (ABA) did not mediate cold-induced C. communis stomatal closure: At low temperatures, bulk leaf ABA did not increase; ABA did not preferentially accumulate in the epidermis; its flux into detached leaves was lower; its release from isolated epidermis was not greater; and stomata in epidermal strips were less sensitive to exogenous ABA. Stomata of both species in epidermal strips on large volumes of cold KCl failed to close unless calcium was supplied. Therefore, the following cannot be triggers for cold-induced stomatal closure in C. communis: direct effects of temperature on guard or epidermal cells, long-distance signals, and effects of temperature on photosynthesis. Low temperature increased stomatal sensitivity to external CaCl 2 by 50% in C. communis but only by 20% in tobacco. C. communis stomata were 300-to 1,000-fold more sensitive to calcium at low temperature than tobacco stomata, but tobacco epidermis only released 13.6-fold more calcium into bathing solutions than C. communis. Stomata in C. communis epidermis incubated on ever-decreasing volumes of cold calcium-free KCl closed on the lowest volume (0.2 cm 3 ) because the epidermal apoplast contained enough calcium to mediate closure if this was not over diluted. We propose that the basis of cold-induced stomatal closure exhibited by intact C. communis leaves is increased apoplastic calcium uptake by guard cells. Such responses do not occur in chill-sensitive tobacco leaves.
When abscisic acid (ABA) was fed to isolated epidermis of Commelina communis L., stomata showed marked sensitivity to concentrations of ABA lower than those commonly found in the xylem sap of well-watered plants. Stomata were also sensitive to the flux of hormone molecules across the epidermal strip. Stomata in intact leaves of Phaseolus acufifolius were much less sensitive to ABA delivered through the petiole than were stomata in isolated epidermis, suggesting that mesophyll tissue and/or xylem must substantially reduce the dose or activity of ABA received by guard cells. Delivery of the hormone to the leaf was varied by changing transpiration flux and/or concentration. Varying delivery by up to 7-fold by changing transpiration rate had little effect on conductance. At a given delivery rate, variation in concentration by 1 order of magnitude significantly affected conductance at all but the highest concentration fed. The results are discussed in terms of the control of stomatal behavior in the field, where the delivery of ABA to the leaf will vary greatly as a function of both the concentration of hormone in the xylem and the transpiration rate of the plant.During the last 15 years, evidence has accumulated to support the view that in the early stages of soil drying ABA produced in the roots and transported in the transpiration stream can function as a physiological signal in the regulation of gas exchange (Davies and Zhang, 1991). Laboratory experiments have shown a clear correlation between xylem ABA concentration and the degree of stomatal opening (Zhang and Davies, 1989,1990; Khalil and Grace, 1993). Field results have supported this conclusion and have also shown a tight and even more sensitive correlation between xylem ABA concentration and stomatal conductance (Wartinger et al., 1990; Tardieu et al., 1992). This is perhaps surprising, since during its passage through the plant xylem sap is exposed to several factors that can alter the amount of ABA arriving at the sites of ABA action on the guard cells (Hartung and Slovik, 1991). Much ABA will move out of the xylem into alkaline compartments, and Trejo et al. (1993) have shown that the mesophyll and its capacity to metabolize ABA can play an important role in determining the amount of the growth regulator arriving in the epidermis and the final extent of stomatal closure. In a recent paper, Gowing et al. (1993) described an experiment designed to address the question of whether the guard cells perceive the concentration of ABA in the xylem or respond to the flux of the growth regulator into the leaf. The results led the authors to conclude that both variables are important in the determination of a local ABA concentration (around the guard cells) to which the guard cells react but that this concentration is not necessarily similar to the xylem ABA concentration.In the experiments performed by Gowing et al. (1993), the amount of ABA arriving at the guard cells was varied by feeding short pulses of the hormone, preceded and followed by hormone-free artif...
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