Báo cáo khoa học: "Current focuses in woody and drought resistance"

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Current focuses in woody water relations plant and drought resistance R. Ceulemans T.M. 2 1 Hinckley ’College of Forest Resources, University of Washington, Seattle, WA 98195, U.S.A., and Departmental 2 of Biology, University of Antwerp, Universiteitsplein 1, B-2610 Wilrijk, Belgium Since the advent of the pressure cham- Introduction ber, the porometer and the pressure-vol- ume technique in the mid to late 1960s, Stress, such as drought, affects physio- there has been a dramatic increase in the processes and is the result of one logical number of studies on drought resistance or a combination of environmental and of plants. Much of this work has been biological factors.The degree of stress is comparative in nature and has had a related both to the degree of change in the single organ focus (e.g., leaf level). More process as well as the amount of energy recently, there has been an increased expended by the plant to resist and re- emphasis on scaling from the organ level cover from the stress. Although zero either to the whole plant or stand level or stress seldom, if ever, occurs in plants, to the molecular/biophysical level. and, in particular, plants growing in the In this paper, we will examine 3 aspects field, it has theoretical and experimental of the water relations and drought resis- relevance. Drought stress may be induced tance of forest trees: 1) the movement of environmental (e.g., low precipitation, by water in plants and its regulation; 2) the low humidity, cold temperature, etc.) or interaction between stomatal responses biotic (e.g., root decaying fungus, xylem and water movement; and 3) allometric borers, etc.) factors which cause plant relationships or the expression of func- water potential to decrease below levels tional relationships at the structural level. which maintain optimal growth and devel- We will examine both the historical foun- opment. Plants resist drought stress by dation as well as the current status of postponing dehydration and/or by toler- these 3 aspects. Finally, we will present a ating dehydration. The degree to which a number of research topics which have plant utilizes these mechanisms will be resulted as a consequence of a broader species and tissue dependent. The level examination of these 3 aspects. Because of drought resistance achieved by using of the presence of a large number of fairly such mechanisms will be species, tissue, recent, excellent reviews on drought resis- developmental stage and life history al., 1986; Koz- (e.g. Hennessey tance et dependent.
alizations. First, most, if not all, current lowski, 1968-1983; Kramer, 1983; Levitt, 1980; Meidner, 1983; Paleg and Aspinall, observations and concepts not only have 1981; Schulze, 1986; Stone and Willis, their roots in the past, but they are largely 1983; Teare and Peet, 1983; Turner and repetitive of past observations and conclu- Kramer, 1980; Turner, 1986), this paper sions. Second, elegant research does not will not be a review of this literature. In- by necessity equate itself with elegant stead, we will assume that it is at the inter- equipment. Finally, many of the scientists listed in Fig. 1 were either physicists or face of a number of areas (e.g., hydraulic very well trained in physics. These obser- architecture and stomatal function) and under the effort of scaling up or down from vations would probably hold whether one the leaf that exciting new ideas about how did this examination today or 100 years from today. Although it seems that articles plants resist stress will be forthcoming. Our paper will deal with a number of these published in the 1960s and 1970s are interfaces as well as with scaling, particu- already dated, we would strongly suggest larly to the whole plant level. that the historical literature not be neglect- ed. Based upon this examination as well contention that studies with It is also our as our appreciation of current research, singular focusat the leaf level lack inno- a we have identified for areas further discus- vation and that, unless scaled either up or sion (Fig. 1 ). down, will not significantly contribute to our understanding of either the mecha- nisms of response or the pattern and inte- Stomatal activity, gration at the whole plant level of re- sponse. For these reasons, we will try to assume a whole plant focus. Key to a vastly improved understanding of the role of storriatal activity in plants has been the acceptance that properties of the water potential equation measured at the Discussion bulk leaf level are at best correlated with stomatal aperture and that the entire plant has an impact on the response of a given leaf’s stomata (Davies et al., 1988; Individuals responsible for key observa- tions or important developments in 3 Frensch and Schulze, 1988; Kuppers et areas of plant water relations (i.e., stoma- al., 1988; Masle and Passioura, 1987; tal control, movement of water in plants Munns and King, 1988; Richter, 1973; Schulte and Hinckley, 1987; Teskey et al., and allometry) have been identified in Fig. 1 (sources: Aloni, 1987; Huber, 1956; Jar- 1983; Tyree and Sperry, 1988). A summa- ry of the above work includes the following vis, 1975; Kramer, 1983; Meidner, 1987; Reed 1942; Zimmermann, 1983; as well points: 1) the importance of isolating the as original literature: e.g., Askenasy, 1895; water potential of the guard cell complex from that of the bulk leaf; 2) the biochemi- Bode, 1923; B6hm, 1893; Darwin, 1898; Dixon and Joly, 1895; Ewart, 1905; Grad- cal and biophysical roles that roots have in sensing the soil environment; and 3) the mann, 1928; Hales, 1727; Hartig, 1878; biophysical and perhaps biochemical role Huber, 1924; Jost, 1913; Sachs, 1882). Although it might be that shoots play in sensing their environ- most appropriate to examine in detail much of this early work, ment. This subject is covered in greater it suffices here to summarize with 3 gener- detail by Dr. Goll,an in these proceedings.
Hydraulic architecture scribe flow through the soil-plant-atmo- sphere continuum: 1) unbranched (e.g., Elfving et al., 1972) and 2) branched The important role that xylem anatomy catena models (e.g., Richter, 1973; Tyree, and hydraulic architecture at the crown 1988). Most typically the latter model level play on the water relations of trees includes considerations of both the con- has been described in these proceedings sequences of branching structure and tis- by Tyree and Sperry as well as extensively sue capacitance. Although the former in the literature (Dickson and lsebrands, model represents a gross over-simplifica- 1988; Schulte et al., 1987; Sperry and tion of the nature of flow through a tree, it Tyree, 1988; Tyree, 1988; Tyree and Sper- has useful interpretative functions (e.g., ry, 1988; Zimmermann, 1978, 1983). Two Kaufmann, 1975; Kjelgren, 1988). From important conclusions are derived from these 2 models, a consideration of the this work: 1) all species may operate near factors controlling water movement within the brink of catastrophic xylem dysfunction the SPAC has been forthcoming. As due to dynamic water stress (where sto- pointed out by van den Honert (1948) and mata play a key role; and 2) the branches Jarvis (1975), water loss from the plant is of a tree might be regarded as a collection controlled at the liquid-air interface and, of small independent plantlets, each ’root- therefore, is only affected through ed’ in the bole. This latter observation can changes in leaf conductance. However, be nicely integrated into the concept of the relative importance of this point in the autonomous branches based upon a car- pathway has been argued both by those bon budget (Sprugel and Hinckley, 1988). examining flow through the components of The former observation is interestingly a single individual (e.g., Kaufmann, 1975; similar to conclusions reached by Richter Running, 1980; Passioura, 1988; Teskey (1976) and others that many species op- ef al., 1984; Tyree, 1988; Tyree and Sper- erate near the osmotic potential when tur- ry, 1988) and by those scaling from the gor will be zero (e.g., Hinckley et al., 1983; leaf to thelandscape (e.g., Jarvis and Fig. 2). An interesting research topic McNaughton, 1986). would be a study of the interaction be- tween the point of catastrophic xylem dys- function and osmotic potential especially Allometry as periods of diurnal or seasonal osmotic adjustment are noted. The presence of xylem-tapping mistletoes in which stoma- As illustrated in Fig. 1, from as early as tal opening has been observed, while the Leonardo da Vinci, scientists have been stomata of the host’s foliage is closed and interested in how various parts of an or- its impact on hydraulic architecture would ganism are related both functionally and be another topic (Glatzel, 1983; Schulze, structurally and how changes in develop- 1986). ment and stress affect these relationships. Although the fields of mensuration and forest measurements are based upon allo- Flow through the soil-plant-atmosphere metric relationships, it was not until the continuum (SPAC) publication of 2 papers in 1964 by Shino- zaki et aL, that an interest in allometric Currently, 2 models, based upon the cate- relationships amongst physiological ecolo- nary theory of water flow (Huber, 1924; gists developed (e.g., Waring et al., 1982; van den Honert, 1948), are used to de- Schulze, 1986). Such studies have ele-
gantly shown that there is a functional matal closure occurred in Heliannthus equilibrium between the various parts of a annus, not as a consequence of changes tree. In very young material or within a in foliar water potential, but because 50% given branch or root system, this equili- of the root system was in a dry soil, was brium may be quite dynamic; however, growing and, not as a was consequence, when one scales to the whole tree, the sending biochemical messages to the response time is increased. As will be dis- foliage. More recent studies (Davies et cussed later, when interest in allometry is al., 1988; Kuppers et al., 1988; Masle and combined with interest in one or more of Passioura, 1987; Munns and King, 1988; the other aspects just discussed, some Passioura, 1988) have increased our very fruitful observations can be made. understanding of the importance of the rapid biochemical interaction between the root and the foliage. Table I represents our Two areas which represent combina- sense of the relative importance of bio- tions of the 4 subjects just discussed appear to hold promise for improving our chemical and biophysical communications between the root and shoot in a variety of understanding of how tissues within a tree different types of trees. For example, rela- function both at the tissue and at the tively little is known about the importance whole tree level. First, the area of root-to- of biochemical communication in the shoot (or foliage) communication, in a short-term in conifers. The clarification of sense a combination of all 4 subjects, is the role that biochemical, nutritional and/or extremely exciting. The biophysical inter- biophysical messages play in root-to-foli- action between the root and the shoot has age communication will clearly be an long been recognized; however, the na- important topic of the next decade (Kuiper ture of how a change in water potential or and Kuiper, 1988). In our effort to discover water flow is sensed are still not well a or the biochemical messenger, Moss et understood (e.g., Teskey et aL, 1983). In al. (1988) caution: &dquo;... (that there is) the the mid-1970s, Dr. Rolf Borchert con- danger of proposing a causal role for hor- ducted a number of very elegant experi- mones in developmental (or physiological) ments from which he concluded that there phenomena on the basis of correlative evi- was an intimate feedback system between of joint occurrence between dence root and foliage expansion (Borchert, changes in the titre of hormone and the 1975). Using a split-root design, Blackman physiological process of interest.&dquo; and Davies (1985) demonstrated that sto- columns a under biophysical or biochemical should only be compared vertically.
Walker and H. Richter for numerous discus- Another area that is clearly interesting is sions about the historical foundations of the cur- the interface between hydraulic architec- rent thinking in plant-water relations. ture and allometric relationships. As re- ported in this conference by Pothier, Margolis and Waring, when saturated sap- References wood permeability (i.e., relative conduc- tivity; Jarvis, 1975) at the base of the live crown rather than sapwood area was Aloni R. (1987) Differentiation of vascular tis- sues. Annu. Rev. Plant Physiol. 38, 179-204 measured, the effects of age and site Askenasy E. (1895) Ober das saftsteigen. Bot. quality could be nicely isolated. They Zentralbl. 62, 237-238 hypothesized that age-related increases in Blackman P.G. & Davies W.J. (1985) Root to saturated sapwood permeability could shoot communication in maize plants of the explain how trees can maintain similar effects of soil drying. J. Exp. Bot. 36, 39-48 daytime leaf water potentials at different Bode H.R. (192!3) Beitrdge zur dynamik der stages of development. However, Carter wasserbewegun in den gefasspfianzen. Jahr. 9 and Smith (1988) have noted that, lNiss. Bot. 62, 9.-127 2 although water potentials may be quite B6hm J. (18931 Capillaritat und saftsteigen. Ber. Deutsch Bo Ges. 11, 203-212 similar in different conifer species at dif- A 2 ferent stages of development, leaf con- Borchert R. (1975) Endogenous shoot growth under constant conditions. Physiol. Plant. 35, ductances are not. Differences in leaf 152-157 conductance may reflect differences in Busgen M. & Munch B. (1929) In: The Struo- higher relative photosynthetic potential or ture and Life of Forest Trees. (Thomson T., conductivity or both. translator). Chapman & Hall, Ltd., London, pp. 436 When studies of water relations are Carter G.A. & Smith W.K. (1988) Microhabitat related to other whole plant studies of car- comparisons of transpiration and photosynthe- bon and nutrient relations, a vastly im- sis in three subal’pine conifers. Can. J. Bot. 66, proved understanding of how trees func- 963-969 tion under both optimal and stress Darwin F. (1898) Observations on stomata. conditions should be forthcoming. This Philos. Trans. R. Soc. London B. 190, 531-621 conference has provided an excellent Davies W.J., Metcalfe, J.C., Schurr U., Taylor C. intellectual framework from which such & Zhang J. (1988) Hormones as chemical studies may continue and be forthcoming. signals involved in root to shoot communication of effects of changes in the soil environment. In: Hormone Action in Plant Development - A Critical Appraiss!l (Hoad G.V., Jackson M.B., Lenton J.R. & Aitken R., eds.), Butterworths, Acknowledgments London, (in press) Dickson R.E. & Isebrands J.G. (1989) Role of leaves in regulating structure-functional de- Partial funding provided under subcontract no. velopment in pla.nt shoots. In: Integrated Re- 19X-43382C with the Oak Ridge National La- sponse of Plants to Stress. (Mooney H.A., Win- boratory under Martin Marietta Energy Sys- Pell E.J., eds.), Academic Press, ner W.E. & tems, Inc. contract DE-AC05-840R21400 with London, (in press.) the U.S. Department of Energy. Title of Project: Dixon H.H. & Joly J. (1895) On the ascent of ’Genetic Improvement and Evaluation of Black sap. Philos. Trans. R. Soc. London B. 186, 563- Cottonwood for Short-Rotation Culture’, R.F. 576 Stettler, P.E. Heilman and T.M. Hinckley, princi- pal investigators. A special thanks to Drs. G. Kaufmann M.R. & Hall A.E. (1972) Elfving D.C., Goldstein, D. Pothier, H. Margolis, R. Waring, J. Interpreting leaf potential measurements water Sperry and M. Tyree for making unpublished with a model of the SPAC. PhysioL Plant. 27, data available. A special thanks to Drs. R.B. 161-168
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