Báo cáo lâm nghiệp: "Control of gas exchange: evidence for root-shoot communication on drying soil"

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Control of gas exchange: evidence for root-shoot communication on drying soil T. Gollan W.J. Davies 2 1 2 J. Zhang U. Schurr 1 Lehrstuhl Pflanzen6kologie, POB 10 f2 Universitit Bayreuth, 51, 8580 Bayreuth, F.R.G., and 2 University of Lancaster, Department of Biological Sciences, Bailrigg,Lancaster LA I 4YQ, U. K. Decrease in leaf conductance (stomatal maintained until a critical soil water with drying soil is a common phe- closure) content was reached. With the introduc- nomenon and has been reported in tion of thermodynamics in plant water rela- myriads of publications. Stomatal closure tions and the development of more with soil drying generally occurs in parallel sophisticated measurement techniques, with a deterioration of plant water status. leaf water potential became the controlling With a decrease in relative water content, factor in most experimental hypotheses. It leaf turgor and water potential in general was an obvious thought, because stoma- decline. Since both leaf conductance and tal movements operate via changes in tur- leaf water potential decrease more or less gor of the guard cells and the surrounding at the same time during a drying cycle, the epidermal cells (e.g., Raschke, 1979). decrease in leaf conductance is often Also, in most experiments under normal explained as a function of the decrease in conditions, we are unable to uncouple the leaf water potential. During the last few decrease in leaf conductance and the years, increasing evidence has been decrease in water potential; both are com- accumulated that stomatal closure at mon plant responses to drying soil. Leaf drying soil is not only related to a deterio- water relation parameters, however, failed ration in shoot water potential but also to to explain the stomatal response due to changes in soil conditions. In this paper, drought. Often there is no unique relation- we summarize the experimental evidence ship between leaf conductance and leaf that led us to hypothesize a communi- water potential for different species (e.g., cation between root and shoot on drying Schulze and Hall, 1982). Some species soil. show a more linear relationship between Changes in plant performance with the two, others an expressed threshold drying soil have been widely discussed response, which means that, during a soil during the last 50 years. Martin (1940), drying cycle, leaf conductance was main- Veihmeyer and Hendrickson (1950), and tained at a high value until a critical leaf Veihmeyer (1956) had previously con- water potential was reached (Turner, cluded that the rate of transpiration was 1974; Ludlow, 1980). However, Bates and
surprising to see that the leaf conduc- that leaf conductance Hall (1981) showed, was tance of the single leaf was independent decrease without any detectable can of its leaf water potential related to the soil changes in bulk leaf water potential. Turn- er et al. (1985) and Gollan et aL (1985) water content (Fig. 1 ). showed for a herbaceous and a woody The conclusion of their experiments was species, that within one species there was that the stomatal aperture is under the unique relationship between leaf no control of signals from the root system that conductance and leaf water potential with experiences the drying soil and is medi- drying soil. In their studies, leaf conduc- ated to the shoot via the transpiration tance of a single leaf was measured at stream. constant high humidity with the remainder The problem in working out controlling of the plant being either at high or low air factors on stornatal conductance at drying humidity (Fig. 1Depending upon the soil is to uncouple soil and leaf water rela- humidity treatment, transpiration of the tions. Since there is a hydraulic link be- shrub was high at low humidity and vice tween water in the soil and in the leaf, leaf versa. High rates of transpiration caused a water potential will always decrease when decrease in leaf water potential of the the soil becomes dry and soil water poten- whole shrub, and also in the single leaf. tial decreases (pathway 1, Fig. 2). Besides Leaf conductance, however, did not possible reactions to leaf water potential decrease, as would have been expected if or turgor, stomiata might react to changes a simple decrease in leaf water potential is in leaf metabolism with decreasing leaf a controlling factor for stomatal aperture. It
and Davies potential (pathway 2, Fig. 2), like the Blackman water water content. reduction in photosynthetic rate or the (1985), Zhang et al. (1987) and Zhang synthesis or accumulation of chemical and Davies (1987) using such a system substances like abscisic acid (e.g., Pierce showed that leaf conductance decreased and Raschke, dramatically in such a situation even 1980). though leaf water potential did not change To study effects of drying soil on leaf or may even have increased. This situa- behavior independent of leaf water status tion is similar to a plant living in soil with (pathway 3, Fig. 2) it is necessary to different water contents. Although the uncouple leaf and soil/root water relations. shoot does not experience changes in leaf There are two experimental tools available water status, it reacts to reduced supply of that enable us to do this. Using the split water to part of the root system. root technique, the root system is divided Using the split root technique, one might and grown in two pots. Whereas the soil in find slight changes in leaf water potential one pot is permanently watered and thus and therefore metabolic effects within the supplying the shoot with enough water to leaf cannot be completely excluded (path- keep leaf water potential high, the soil in the second pot is allowed to decrease in way 2, Fig. 2). In subsequent experiments, Zhang and Davies (1989) showed that the concentra- tion of abscisic acid (ABA) increased in roots that experienced dry soil (Fig. 3). The increase in root ABA content in this experiment was correlated with the water content of the surrounding soil (Fig. 4). The ABA that accumulates in the root sys- tem could then be transported with the transpiration stream to the shoot. During the day, abscisic acid accumulates in the epidermal cells, whereas there is no detectable change in the abscisic acid concentration of the bulk leaf (Zhang et al., 1987). The second approach to separate shoot and root/soil water relations is an experi- mental design introduced by Passioura (1980). A plant is grown in special pots that can be placed in a pressure chamber with the root and soil inside and the shoot outside the chamber facing atmospheric pressure (Fig. 5). Applying pneumatic pressure inside the chamber to the soil and root system increases the xylem water potential in the shoot but does not alter water potential gradients in the root and the soil (Passioura and Munns, 1984). A cut through the xylem at any given posi- tion of the shoot is used to control the
balancing pressure, i.e., the pressure that is necessary to bring the hydrostatic pres- sure in the xylem of the shoot to atmo- spheric pressure. When balancing pressu- re is applied, a drop of water attached to the cut in the xylem will neither increase decrease in size. If the pressure is too nor sap will bleed out of the cut, if high, xylem it is too low, water will be sucked into the xylem. This feature is used by an elec- tronic device to control the pressure in the pressure chamber within 0.005 MPa of the balancing pressure (Passioura and Tan- 1985). ner, Fin- 4 Ralatinnshin h ARA rontant nf maize n pp tw p -
When soil water potential decreases, soil water content as control plants same allowed to decrease in leaf the balancing pressure applied will in- that were crease and thus keep the xylem sap of the water potential (Fig. 6; Gollan et al.,1986). shoot at atmospheric pressure (about The pressure chamber system can be 0 MPa xylem water potential). used to collect xylem sap from intact plants (Passioura and Munns, 1984; Gol- By applying the balancing pressure per- lan, 1987). This enables us to measure manently throughout a drying cycle, the several components in the xylem sap of a shoot never experiences any change in shoot water potential due to the drying plant throughout a drying cycle which soil. Even under such condition. with the might affect stomata, such as abscisic a potential of the shoot being acid, inorganic ions or pH (reviewed by xylem water zero, leaf conductance decreased at the Schulze, 1986).
xylem sap of sunflower plants taken from As would expect from the results of one the midrib of a leaf, and the decrease in and Davies (1989, Figs. 4 and 5) Zhang the increase in ABA content with drying leaf conductance was often linearly re- soil appears not only in the root, but also lated to the increase in ABA concentration in the xylem sap of individual plants (Fig. in the xylem sap of the plant (Fig. 7). Ab- scisic acid increased several fold in the 7). However, not only the ABA concentra-
tion changed with drying soil, but many References other components in the xylem sap did as Bates L.M. & Hall A.E. (1981) Stomatal closure well (Gollan, 1987; Gollan et aL, submit- with soil water depletion not associated with ted; Schurr et al., submitted). While the changes in bulk leaf water status. Oecologia change in the concentration of abscisic (Berlin) 50, 62-65 acid in the sap was the most evident, the Blackman P. & Davies W.J. (1985) Root to effect of abscisic acid on stomatal aper- shoot communication in maize plants of the ture might be, e.g., synergistically altered effects of drying soil. J. Exp. Bot. 36, 39-48 by the presence of cations like calcium De Silva D.L.R., Hetherington A.M. & Mansfield TA. (1985) Synergism between calcium ions (De Silva et al., 1985). There is additional and abscisic acid in preventing stomatal open- information from Munns and King (1988), ing. New PhytoL 100, 473-482 who concluded that abscisic acid is not Gollan T. (1987) Wechselbeziehungen zwi- the inhibitor of stomatal opening in the schen abscisinsaure, n und pH hrstoffhaushalt d xylem sap. In their experiments, they im xylemsaft und ihre bedeutung fur die sto- sampled xylem sap from plants in wet and matare regulation bei bodenaustrocknung. Doc- toral thesis, University of Bayreuth, F R.G. in drying soils. Xylem sap of plants in dry Gollan T., Passioura J.B. & Munns R. (1986) soil had a higher abscisic acid content Soil water status affects the stomatal conduc- than that of plants in wet soil. Feeding tance of fully turgid wheat and sunflower plants. xylem sap from ’dry’ plants to detached Aust. J. Plant Physiol. 13, 459-464 leaves induced stomatal closure. How- Gollan T, Turner N.C. & Schulze E.D. (1985) ever, the same sap also affected stomatal The responses of stomata and leaf gas ex- conductance, when abscisic acid was change to vapour pressure deficits and soil water content. 111. In the scierophyllous species removed by passing the sap through an Nerium oleander. Oecologia (Berlin) 65, 356- immunoaffinity-column before feeding. 362 The xylem sap of drying plants had an Kramer P. (1988) Changing concepts regarding inhibiting effect regardless of its abscisic plant water relations. Plant Cell Environ. 11, acid content. 573-576 Ludlow M.M. (1980) Adaptive significance of There is controversy in the literature stomatal responses to water stress. In: Adap- about the more general aspects of tation of Plants to Water and High Temperature root/shoot interaction on drying soil, e.g., Stress. (Turner N.C. & Kramer P.J., eds.), J. in volume 11 (1988) of Plant Cell Environ- Wiley and Sons, New York, pp. 123-138 ment. In different opinions on the subject, Martin E.V. (1940) Effect of soil moisture on Kramer (1988) is worried about the shift in growth and transpiration in Helianthus annuus. Plant Physiol. 15, 449-466 emphasis from traditional water relations to the idea of (bio-)chemical signaling in Munns R. & King R.W. (1988) Abscisic acid is not the only stomatal inhibitor in the transpira- plants and increasing interest in root tion stream of wheat plants. Plant Physiol. 88, metabolism. The idea of root/shoot inter- 703-708 action and communication on drying soil Passioura J.B. (1980) The transport of water does not exclude direct effects of a from soil to shoot in wheat seedlings. J. Exp. decrease in water potential on stomatal Bot 31, 333-345 aperture, but rather includes an additional Passioura J.B. (1988) Response to Dr. P.J. Kra- biochemical effect on the stomatal aper- mer’s article, ’Changing concepts regarding plant water relations’. Plant Cell Environ. 11, ture independent of changes in leaf water 569-571 relations (Schulze et al., 1988). ’The Passioura J.B. & Munns R. (1984) Hydraulic return (to emphasis on conditions in the resistance of plants 11. Effects of root medium soil) is not a circle. It is a helix.’ (Passiou- and time of day in barley and lupin. Aust J. ra, 1988). Plant Physiol.11, 341-350
Passioura J.B. & Tanner C.B. (1985) Oscilla- nisms of Regulation of Plant Growth, (Bieleski tions in apparent hydraulic conductance of cot- R.L., Ferguson A.R. & Cresswell M.M., eds.), ton plants. Aust. J. Plant Physiol. 12, 455-461 R. Soc. N.Z. Bul’. 12, 423-432 Pierce M. & Raschke K. (1980) Correlation be- Turner N.C., Schulze E.D. & Gollan T. (1985) tween loss of turgor and accumulation of absci- The responses of stomata and leaf gas ex- sic acid in detached leaves. Planta 148, 174- change to vapour pressure deficits and soil 182 water content. II, In the mesophytic herbaceous species Helianthus annuus. Oecologia (Berlin) Raschke K. (1979) Movements of stomata. In: 65, 348-355 Physiology of Movements. Encyclopedia of Plant Physiology, new ser. vol. Vil. (Haupt W. & F.J. (1956) Soil moisture. In: Water Veihmeyer Feinlieb M.E., eds.), Springer, Berlin, pp. 383- Relations of Plants. Encyclopedia of Plant Phy 441 siotogy, vol. 111. (Rukland U., ed.), Springer, Berlin, pp. 64-123 Schulze E.D. (1986) Carbon dioxide and water vapor exchange in response to drought in the F.J. F3! Hendrickson A.H. (1950) Soil Veihmeyer atmosphere and in the soil. Annu. Rev. Plant moisture in relation to plant growth. Annu. Rev. Physiol. 37, 247-274 Plant Physiol. 1, 285-304 Schulze E.D. & Hall A.E. (1982) Stomatal re- J. & Davies W.J. (1987) Increased syn- Zhang sponses, water loss and C0 assimilation rates 2 thesis of ABA in partially dehydrated root tips of plants in contrasting environments. In: Phy- and ABA transport from roots to leaves. J. Exp. siological Plant Ecology II, Encyclopedia of Bot. 38, 2015-2023 Plant Physiofogy, New ser. Vol. 12B. (L.ange Zhang J. & Davies W.J. (1989) Abscisic acid O.L. et al., eds.), Springer, Berlin produced in dehydrating roots may enable the Schulze E.D., Steudle E., Gollan T. & Schurr U. plant to measure the water status of the soil. (1988) Response to Dr P.J. Kramer’s article, Plant Cell Environ. 12, 73-81 ’Changing concepts regarding plant water rela- Zhang J., Schurr U. & Davies W.J. (1987) tions’. Plant Cell Environ. 11, 573-576 Control of stomatal behaviour by abscisic acid which apparently originates in the roots. J. Turner N.C. (1974) Stomatal response to light and water under field conditions. In: Mecha- Exp. Bot 38, 11; 4-1181 7