Resistance to Drought and Salinity: Avenues for Improvement

Aust. J . Plant Physiol., 1986, 13, 191-201

Resistance to Drought and Salinity: Avenues for Improvement

J. B. Passioura

Division of Plant Industry, CSIRO, G.P.O. Box 1600, Canberra, A.C.T. 2601.

Abstract

Various techniques for improving the resistance of plants to drought or salinity are discussed in relation to a notional spectrum ranging from rationality to random search. The rational end of the spectrum is exemplified by the control of phenology so that flowering occurs at an optimal time; the random end by wide crosses, such as that between wheat and Elytrigia, which give the remote hope that the high yield-potential of one will combine with the toughness of the other. Most of the techniques occupy the middle ground and involve selecting for characters whose protagonists have strong, but often poorly articulated, hunches that the characters will confer resistance to drought or salinity on the plants pos- sessing them. For drought resistance of grain crops, the possible worth of many of these characters is discussed in relation to their likely impact on improving a crop's ability to (a) extract water from the soil, (b) produce dry matter given a limited water supply, and (c) convert dry matter into grain.

Introduction

An underlying assumption throughout this volume has been that the processes that limit the growth of plants affected by drought or by salinity are often similar. That is why we have been studying these two environmental limitations in parallel. After all, both substan- tially lower the water potential in the soil and thence in the plant, and perhaps it is the low water potential in the leaves that induces the poor growth. We have seen several arguments in favour of this proposition, and several against. But although there is little doubt that a rapid and substantial fall in the water potential of the leaves will slow growth in the short term, it remains doubtful that it will do so in the long. As so often happens in biological research, a phenomenon that appears important at a given time-scale may become an unimportant transient at a larger one. When exploring ways of improving the yield of plants, which is the subject of this paper, we perforce must consider what is the longest physiological time-scale of all, namely that of a generation (at least for annual crops, although even for perennial horticultural crops a misadventure in one season may damage the crops of sub- sequent seasons). It is the integral of the whole season's or generation's growth, and its partitioning into usable forms, that determines the final yield. In this context our job as physiologists is to discover those processes that affect growth or partitioning in more than trivially transient ways. That is not easy to do, although it is somewhat easier for drought than for salinity because of the fairly robust empirical relations that occur between dry matter production and water transpired and to which I will return later.

Although there is this similarity in the water relations of plants affected by drought or by salinity, which may or may not be profound, there is also an interesting difference between the maladies that is conveniently discussed in terms of the demography of the leaves. Con-

J. €3. Passioura

sider two fairly typical examples of drought and salinity. In one the plant has a limited supply of water stored in the soil. If the plant slows the growth rate of its leaves, possibly diverting assimilate to the roots so that their growth rate is increased, its chances of survival will probably be better than those of a similarly situated plant that continues to grow rapidly with a concomitant rapid use of the limited water supply. In the other example the plant has a substantial supply of saline water in the soil. But if it diverts assimilate to its roots and slows its leaf growth, it may improve the water relations of its leaves, but it will probably decrease its chances of survival. The reason for this difference is that the steady accumulation of salts by the leaves of a salt-affected plant (see Munns and Termaat 1986) means that there is a distinctly limited transpirational life for a given leaf: for example, if a leaf dies when the concentration of salts in its cell sap exceeds 300 mol m-', and if the average concentration of these salts in the incoming xylem sap is 10 mol m-', then once the leaf has transpired 30 times its own volume it can transpire no more; (I am assuming that there is negligible translocation of salts out of the leaf - see Flowers and Yeo 1986). By contrast, there is no obvious reason why the leaves of a drought-affected plant should not remain alive as long as the plant does. Thus a major requirement for a salt-tolerant plant would seem to be that it keep the average age of its unit leaf area young. This conclusion may appear to be a trivial tautology: of course a fast-growing plant will have predominantly young leaves. But I believe that it does reflect an important underlying mechanism.

The point of all this is that it provides us with an example of a link between a short-term process, the expansion of the newly divided cells in the leaf, and the long-term growth of the plant, as elucidated by Munns and Termaat (1986). Such links are useful in helping us assess possible avenues for improvement, but they are rare.

One of the difficulties that bedevils attempts to improve the yield of plants exposed to drought and salinity is that both the timing and the intensity of exposure can influence the yield in various ways. Empirical plant breeding in this area progresses very slowly for two reasons. First, it is notoriously difficult to select for such vague qualities as resistance to drought or salinity in controlled environments, for the correlation with performance in the field is very poor. Second, because of the variability of the weather, it is very difficult to get consistent results by selecting in the field; the rankings of one's lines will change from year to year as different characters become more or less important with different types of exposure. There have been many attempts by physiologists to discover individual traits that will confer resistance to drought or salinity on a plant. Very few have been even slightly successful. I think that one of the main reasons for the lack of success is that the target is so very vague. We are inclined to look at what happens in stressed plants: a buildup of proline, or of abscisic acid, stomata1 closure, osmotic adjustment, leaf rolling, an increase in the root-shoot ratio, and assume or hope that by either intensifying or moderating these effects we might influence the resistance of the plants. There is often very little rationale for such attempts, i.e. there is no cogent reason why these mostly short-term effects should integrate over the whole growing season to give an increased yield. That is very understand- able for as I mentioned before such integrations are extremely difficult to do. But I believe that they can be made much easier if we improve the focus of our target by dissecting yield into approximately independent components, each being a separate target.

There are two such dissections that have proven useful in relation to drought resistance. Both are identities, i.e. they are always true no matter what the values of the components are, but this does not make them trivial provided that the components are largely independent. They are (assuming that we are considering grain crops, although the principles will apply to many other crops):

Yield = Grain number per unit area x Weight per grain and

Yield = Water transpired x Efficiency of water use x Harvest index These identities are discussed in some detail in Fischer and Turner (1978), Fischer (1979), and Passioura (1977, 1983). There is little point in covering the same ground here, but I

Resistance to Drought and Salinity

would like, with their help, to examine some of the prospects for improving drought resistance, then to move on to salinity resistance, and finally to comment on various lottery techniques for tackling the same problems.

Drought Resistance

Drought occurs in many different ways, but there are a few fairly distinct and useful patterns around which to organize our thoughts. In one, the crop relies predominantly on current rainfall, i.e. its water supply depends largely on the weather during the growing season. In a second, the crop relies predominantly on a store of water accumulated in the soil during a fallow. In fact these two patterns are towards opposite ends of a spectrum, with the extremes being total reliance on either current rainfall or stored water. As an illustration of this spectrum, Fig. 1 shows the average proportion of a wheat crop's water

Fig. 1. The Australian wheat belt classified according to the proportion of a crop's water supply that is contained in the soil, on average, at the time of sowing (H. A. Nix, personal communication).

supply that is contained in the soil at the time of sowing for various parts of the Australian wheat belt. These are averages, and it is not uncommon for particular crops to experience either of the extremes. Over any given pattern we can overlay the severity of the drought, ranging from mild, in which the crop is never in danger of completely running out of water providing its roots can keep extracting water from a moist subsoil, to severe, in which there is the danger that the crop may fail through completely running out of available water in the soil. In the context of such patterns it is clear that a given trait of a crop may range from beneficial to disastrous in its effect on yield. For example, a plant with a deep and

J. B. Passioura

vigorous root system is likely to yield well where there is no likelihood of the subsoil running dry. But it may fail rapidly when there is a meagre supply of water. It is little wonder that selection for drought resistance in the field makes such slow progress. But let us take the components of yield in the above identities to discuss where the best opportunities for improvements may be.

Amount of Water Transpired

The total water supply for a dryland crop is the sum of the rainfall during the growing season and the store of usable water in the soil at sowing. To a good approximation we can assume that the more of that water that passes through the plants the greater will be the production of total dry matter by the crop. Weeds aside, there are two main losses from this supply: direct evaporation from the soil surface, and deep drainage beyond the effective reach of the roots.

There is substantial evidence that losses by direct evaporation from the soil can often be very large (Fischer and Turner 1978). Two recent papers have estimated it for two different regions (but both conforming to the pattern of the crop's water supply coming predominantly from current rainfall) to be 30-40% in South Australia (French and Schultz 1984) or as much as 60% in Syria (Cooper et al. 1983). Is there any way that these enormous losses could be reduced? The problem looks at first sight to be intractable, although small gains may be possible through agronomic improvements such as the use of the mulches that are available using direct-drilling techniques. But most of the loss occurs while the crop is building up its leaf area, and the losses become small once the leaf area index exceeds about 2.5 (Ritchie 1983). The best prospect, so far as changing the plant is concerned, is to have a vigorous establishment of plants with a prostrate, rather than erect, habit. That is, it is important to establish a leaf area index approaching 2 as fast as possible, with the leaves well spread out. In practice, the limitation to growth during the early part of the crop's life is often temperature. It may be that the best route to improving drought resistance in this context is to select plants for a trait that on the face of it has no bearing on the plant's water rela~ions, namely the ability to establish rapidly in cool temperatures.

The losses of water by drainage beyond the effective reach of the roots has long been recognized as an important problem, but it is very difficult to d o anything about it. Jordan et al. (1983) have analysed the problem and have calculated for a few different environments the expected increases in yield that would result from having an effectively deeper rooting system in sorghum. However, the projected gains have to be offset by the cost, in terms of the balance of trade between carbon and water a t the leaf surface, of producing the more effective root system. The cost in carbon, and by implication in water transpired to produce that fixed carbon, may offset the extra water gained (Passioura 1983). This is a difficult issue that can only be resolved by breeding deeper-rooted plants, an undertaking which is very time-consuming. There have been some promising attempts to d o so (Hurd 1974, with wheat; Jordan and Miller 1980, with sorghum) and continued progress may be possib1.e.

These losses due to drainage beyond the effective reach of the roots do not, however, mean that the water is beyond the vertical reach of the roots. There are several examples of water being left behind in the subsoil by a water-limited crop despite the fact that the crop's roots had seemingly penetrated the subsoil (e.g. Schultz 1971; Jordan and Miller 1980). A good recent example comes from some unpublished work by P. M. Bremner (personal communication) who found, in a comparison of sorghum and sunflower, that the sunflower was much more effective at extracting water from deep in the subsoil (> 1 a 5 m) than was sorghum, despite having no obviously greater rooting density there and despite having higher leaf water potentials, so that it was not simply a matter of the sunflower leaves sucking harder. It is hard to see why there should be such a large difference in the abilities of the two root systems to extract the water. It may be due to there being a much larger axial hydraulic resistance in the roots of sorghum than in sunflower, for sorghum being a monocotyledon has no secondary radial growth of the roots so that it must rely on having


many main root axes penetrating the soil to provide enough carrying capacity for the water (Passioura 1981; Klepper 1983). There is little evidence on this point, for no one has attempted the huge task of tracing the distribution of sorghum roots in the field. Alter- natively, it may be due to differences in the horizontal reach of the roots. One of the features of the distribution of roots that has become widely recognized in the last decade is that, outside the plough-layer, the distribution is far from uniform horizontally. It is common for the roots to be constrained to continuous large pores in the soil such as those created by earthworms or by the slight movements of the soil that produce fracture planes. The characteristic length separating such large pores may be many centimetres, so that the ability of the roots to extract the water from soil distant from them is much less than if the roots were distributed uniformly. In fairly crude quantitative terms the characteristic time for the extraction of available water by roots restricted to worm holes is 2B2/D, where B is the effective mean radius of the catchment area for those roots (i.e. roughly half the mean distance between the worm holes) and D is the diffusivity for soil water (Passioura 1985). Putting D = 2 x m2 s-I (or about 2 cm2 day-'), which is a common value during the extraction of about the last third of the usable water in the soil, we get a characteristic time of B2 days if B is measured in centimetres. If B exceeds about 4 cm, which it may often do (M. J. Goss and R. A. Fischer, personal communication), the characteristic time becomes a large proportion of the life of an annual crop and it is not then surprising that the roots fail to extract water from places deep in the subsoil that they may have reached quite late in the life of the crop. What is surprising is that some crops are so much more effective than others in extracting such water even when the apparent depth of penetration of their roots is the same, as in Bremner's example of the sunflower and the sorghum mentioned above. Is it that the sunflower roots can escape the confines of the large pores and send fine roots into the surrounding soil? Or is it that the problem is as much one of hydraulic continuity between the roots and the soil? A root growing in a hole wider than itself may be poorly appressed t o the surface of the soil. Perhaps sunflower roots are better able to achieve hydraulic continuity with the soil than are sorghum roots. Whatever the answer turns out to be the difference makes me feel optimistic that we may eventually be able to improve the performance of these deep roots.

Eficiency of Water Use

The efficiency of water use, as commonly defined, is the ratio of the total above-ground dry matter at final harvest to the total amount of water transpired. Tanner and Sinclair (1983) term it the T-efficiency (in contrast to the ET-efficiency, which includes evaporation from the soil in the denominator), and have thoroughly discussed the influences on it. These include: a t the level of the leaf, the balance of trade between the uptake of CO, and the loss of water; a t the level of the cell, or tissue, respiratory losses of the fixed carbon; and at the level of the whole plant, the partitioning between root and shoot, for roots are usually excluded from the numerator of the definition.

Of these three the first is currently providing the most exciting prospects for making improvements. Farquhar and Richards (1984) have recently shown promising correlations between T-efficiency and the ratio of the carbon isotopes I3C and I2C in a plant. The rationale for their work centres on the discrimination against "C during carboxylation. The lower is the partial pressure of CO, within the leaf, the higher is the T-efficiency and the less is the discrimination against I3C. The most exciting features of this work are first, that it has uncovered what looks like substantial genetic variation in T-efficiency where previously there appeared to be little, and second, that the isotopic analysis offers a powerful selection pro- cedure for field-grown plants.

Wilson's (1984) work on improving the yield of ryegrass by selecting for low rates of respiration is also exciting and, although few have tried to extend his work to other plants or to see if the improvements persist in plants affected by environmental problems, his approach is one that should be explored further.

J . B. Passioura

Finally, in this bracket, there is the issue of rootlshoot ratio. We have very little idea how large a root system a domesticated plant needs to maximize its yield. If it is a member of a community of plants of identical genotype, it presumably needs rather less root than a plant growing in competition with others of different genotype for a limiting supply of water or nutrients. I have argued elsewhere (Passioura 1983) that the root systems of many crop plants may be unnecessarily large and that if they were smaller more assimilate may be available for the shoot, with a concomitant increase in T-efficiency. At first sight the possible savings would appear to be slight, given that the roots of a mature crop are often only about 10% the weight of the shoots. But in droughted crops the proportion is typically much larger than this, and may exceed 30%; furthermore there is mounting evidence that roots consume much more, perhaps twice as much, assimilate in producing unit dry matter as does the shoot (summarized in Passioura 1983). Thus the savings could be considerable. However, such speculations are likely to remain untested for a long time because of the difficulty of selecting for root/shoot ratio. It would be interesting to know if there are differences in this ratio between old and modern cultivars.

Harvest Index

Perhaps the most remarkable feature of the improvement in yield potential of the major field crops this century is that it has been very largely in harvest index, that is, in the proportion of the above-ground dry matter that is in the grain. There has been little improvement in dry matter production for cultivars with growing seasons of similar length, with the possible exception of barley (summarized by Gifford 1986). This improvement in harvest index is due largely to the plant's ability to produce a large number of grains per unit ground area, an ability that diminishes with increasingly severe drought, not only absolutely, but relative to older varieties with much lower yield potentials (e.g. Fischer and Wood 1979, on wheat).

In water-limited crops that rely predominantly on stored water there is evidence that the harvest index is related to the proportion of the crop's water supply that is available for use after ear emergence or anthesis (Fischer and Turner 1978; Passioura 1982), i.e. that the grain yield is roughly proportional to the amount of water available for use after ear emergence or anthesis (Nix and Fitzpatrick 1968). It is clear enough why this should be in the extreme when there is no water available at anthesis, for the plants would be on the verge of death, and no matter how good their pre-anthesis growth had been there would be no grain produced.

Apart from this extreme, however, the reasons for such correlations are not so clear. Given the usual tight relation between water transpired and carbon fixed (Tanner and Sinclair 1983), the correlation between harvest index and the pattern of water use does suggest that grain yield depends predominantly on photosynthesis during grain filling. But there are two other effects that may be as, or even more, important here than photosynthesis during grain- filling. These are first, that the water available in the soil for use after anthesis provides time for the plant to mobilize pre-anthesis assimilate and transfer it to the grain; in droughted crops such assimilate can be a large proportion of the final grain yield (Biscoe et al. 1975; Bidinger et al. 1977) and there may be useful genetic variation in mobilization that offers scope for improvement (Blum et al. 1983). The second, and perhaps most important reason of all, is that the water supply in the soil a t about the time of ear emergence or anthesis influences the well-being of the plants during the time that they are setting their final grain number, and hence strongly influences that grain number. But whatever the reasons for the correlation it is clear that drought per se need not damage harvest index, which can be as high as 0 .5 even in a severely droughted crop (e.g. Angus et al. 1980).

On the supposition that wheat crops relying predominantly on stored water are prone to use too much of that store before anthesis, R. A. Richards (personal communication) has been pursuing a back-cross (BC) breeding program that aims at increasing the hydraulic resistance of the seminal roots (which are the ones primarily concerned with exploiting the


subsoil in spring wheats) by decreasing the diameter of the main xylem vessel in these roots (see Richards and Passioura 1981a, 1981b). His results so far are encouraging. In a comparison of near-isogenic BC3 lines in the field during a drought he found that the lines with small vessels outyielded those with large vessels by 5-10% depending on the recurrent parent. Furthermore, the lines with small vessels suffer no disadvantage during a good season, presumably because during a good season the nodal roots of the plants develop well and the properties of the seminal root system become unimportant. He is currently testing BC5 lines in the field at several locations.

In crops that rely predominantly on current rainfall the harvest index is not related to the proportion of the water supply that is used after ear emergence or anthesis, but it does seem to depend on the conditions between floral initiation and anthesis that determine grain number (French and Schultz 1984). In practice grain number is much more influential than grain weight in determining yield, and hence the processes controlling barrenness are obvious targets in our attempts to improve yield under drought, as in the very encouraging work of Fischer et al. (1983) on maize.

In droughted wheat, if competition for assimilate between the elongating stem and the ear during the formation of florets were the most important process in determining grain number, as it may well be in well-watered plants (Brooking and Kirby 1981), then there may not be much scope for improvement. But given that droughted plants typically have a high carbohydrate status (e.g. Ackerson 1981; McCree 1986) the effect of drought on grain number is probably mediated in other ways. Low turgor in the shoot is a possibility, through its effects on ABA and the induction of male sterility (Morgan 1980). Another possibility is that grain number may be influenced by signals (or the lack of them) from the roots that are experiencing the dry soil, in much the same way that stomata1 conductance seems to be (Davies et a(. 1986).

Were we to succeed in decreasing the barrenness of droughted crops, we would increase yield but also increase the risk of producing shrivelled grain. It is hard t o say where the optimum may lie, but it does seem safe to say that for most droughted cereal crops the returns would be greater if grain number were greater.

Salinity Resistance

If our understanding of the behaviour of droughted plants is meagre so far as providing guidelines for improvement is concerned, then that of salt-affected plants is very much more so. The review by Greenway and Munns (1980) leaves us perplexed as to what distinguishes the tolerant from the sensitive, a t least among the non-halophytes. Yeo and Flowers (1986) suggest several physiological attributes that might correlate with salinity resistance in rice and in which they have unearthed spectra of genotypic variation. They argue that each of these attributes is unlikely t o be selected for in a conventional breeding program, for each on its own will have little impact. But they d o expect that when several such attributes are com- bined in one genotype there will be a substantial effect. A breeding program based on their ideas is 'under way. Apart from this example, almost all serious attempts to improve salinity resistance by breeding have been quite empirical, and are discussed below in the section on Lottery Techniques.

As McWilliam (1986) points out, successful breeding for salinity resistance may d o little more than buy time unless there is some way of removing the salt from the soil, for once a salinity problem has emerged it usually steadily worsens as more and more salt accumulates in the topsoil. An exception from this gloomy prognosis is the low-lying coastal land in the humid tropics that he has depicted in his Fig. 3, for there the salinity is mostly due to temporary incursions by the sea, and the high rainfall and easy drainage mean that recla- mation is easy. It is in such areas that salinity-resistant rice would be very valuable.

One of the features of salinity is its patchiness. Richards (1983) has argued persuasively that, for land that is patchily affected by salt, most of the yield comes from the least affected

J . B. Passioura

patches, so that even a spectacular improvement in salinity resistance is not likely to improve the contribution from the most-affected patches to the overall yield. His argument is that in such circumstances it is more effective to breed for higher yield potential than for salinity resistance.

Lottery Techniques

My discussion so far has concentrated on the analysis of the behaviour of plants affected by drought or salinity with the aim of finding mechanistically based physiological or morphological attributes that could be used in breeding programs. Hitherto, apart from substantial and deliberate improvements in the timing of flowering (Pugsley 1983), almost all of the useful genetic improvements in resistance to drought or salinity have been quite empirical, as in traditional breeding programs, or marvellously fortuitous (as with the introduction of dwarfing genes in wheat and rice). Some have derived from deliberate searches based on hunches about what might be worthwhile characters, that have been successful for unexpec- ted reasons - for example, Morgan and Condon's (1986) work on osmotic adjustment in wheat that has unearthed a remarkable and so far unexplained correlation between the propensity of a variety to adjust osmotically during drought and the ability of its roots to extract water from deep in the soil. A quote from Schmidt (1983) sums this up: "Most of my planned 'can't miss' crosses were nonproductive in providing the expected superior lines while unique segregates from afterthought crosses made it interesting."

Can we do no better than a random search in our attempts to improve the performance of droughted and salinity-affected crops? I believe we can, but I also believe that lottery techniques will continue to be profitable. There are several such techniques, which can conveniently be classified into three groups, elementary, intermediate, and advanced, although the distinctions among the three are blurred.

The elementary techniques are those of the traditional plant breeder who restricts himself to juggling genes from a familiar pool. Success is inevitable, but the rate of progress is extremely slow.

The intermediate techniques are those inspired by hunches about the efficacy of physiological or morphological characters. These hunches often derive from:

(1) Observed responses to drought or salinity (e.g. leaf-rolling, the accumulation of proline, osmotic adjustment, an increase in ABA) that conceivably could enhance the performance of the affected plants if they were intensified (or the reverse, depending on one's frame of mind!);

(2) Associations between a character and the observed resistance of a given genotype to drought or salinity (e.g. glaucousness, presence of awns, the ability to maintain chloride low in the leaves when exposed to salinity); or

(3) Contrived characters that, on the basis of our physiological perceptions, might be expected to improve performance (e.g, dehydration tolerance of leaf tissue as discussed by Sullivan and Ross (1979)), but for which there is no well-articulated connection with yield.

No doubt the advocates of par'ticular characters would dispute my apparently pejorative use of the word 'hunch', and would argue that they do have in mind (if not in print) well- articulated connections between their pet characters and yield. Indeed there is no clear boundary between these characters and those that I discussed earlier, such as Farquhar and Richard's (1984) use of isotopic discrimination or the technique of Blum et al. (1983) for assessing the ability of a genotype to retranslocate pre-anthesis assimilate to the grain. It is just that these latter examples have very much stronger rationales. We all rely on serendipity, but it does seem to me that we should give serendipity as big a chance as possible. So far as drought resistance is concerned, not being much of a gambler, I can see little point in pursuing a character unless there are good reasons for believing that it can increase the plant's water supply, or increase the efficiency of water use, or increase the harvest index.


Without such good reasons we are just gambling wildly. Nevertheless, gamblers sometimes win, even in the long term.

Advanced lottery techniques are those that involve in vitro techniques such as tissue culture and genetic engineering to generate genetic variation that is entirely novel, such as the injection of rye genes into the wheat genome that has produced the outstanding new lines discussed by Rajaram et al. (1983), or the hybridization of wheat and Elytrigia (Dvorak et al. 1985). Such work is best viewed as an exciting adventure, and the claims made for it rank with Columbus's that he was going to bring back the goods from Asia, or possibly with Burke and Wills's that they were going to discover Australia's verdant heart. I think that the analogy with Columbus is the better, but it is hard to cast from one's mind the traditional breeders' sardonic expression 'yield resistance' which they use in relation to crosses with undomesticated plants such as Elytrigia, or the wild tomato, Lycopersicon cheesmanii.

If the manufacture of genetic novelty is a great adventure akin to Columbus's, in vitro selection for resistance to drought or salinity is an adventure akin to Burke and Wills's. Downton (1984) has, with some kindness, reviewed such work on salinity without giving us much ground for optimism, but anyone who has assimilated the papers by Flowers and Yeo (1986), Munns and Termaat (1986), and Yeo and Flowers (1986), in which the integral behaviour of the plant in relation to salt is repeatedly emphasized, will find it difficult to believe that selection at a cellular level is likely to be worthwhile.

Conclusion

I am conscious of the fact that very little of what I have written has been closely related t o the previous papers in this volume. The reason for this is the difficulty of time-scale that I referred to in the Introduction. Most of the matters we have discussed occur at time-scales of hours to days, yet yield is usually the integral over a time-scale of weeks to months and sometimes even years. The integral does of course depend on the short-time processes, but we d o not yet know how to integrate them properly. Simulation modellers try to carry out this integration but their efforts continue to founder because of their inability to predict how today's photosynthate is used to produce tomorrow's new leaves. Many well-developed models that try to describe water-limited growth assume, implicitly or explicitly, that fixed carbon is the limiting ingredient for growth, yet the papers by Kriedemann (1986), McCree (1986) and Munns and Termaat (1986) all contain strong arguments that fixed carbon is not limiting the growth of plants affected by drought or salinity except perhaps when the plants are close to death. The papers by Davies et al. (1986), Munns and Termaat (1986), and Turner (1986) all emphasized the influence of the roots on the behaviour of the shoot, and if the burgeoning work that they discussed continues to uphold their views, then drastic changes will be needed in the way that we currently view the behaviour of plants affected by drought or salinity, and in the ways in which we might hope to improve their performance.

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Manuscript received 13 September 1985

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Resistance to Drought and Salinity: Avenues for Improvement