William J. Davies
5.1 Introduction: a changing climate and its effects on plant growth and functioning
As rainfall patterns become more unpredictable as climate changes, plants will be subjected to increasing fluctuations in soil moisture availability. These fluctuations are likely to have substantial impacts on plants in natural communities and on crop plants in agriculture (Davies & Gowing, 1999). For example, Silvertown etal. (1999) have shown how sensitive plant community composition can be to small changes in soil moisture status. The mechanisms of such changes in composition are likely to be a combination of the responses discussed below. These may be perturbations in plant hydraulics or in plant chemistry, with the driving variable for change being a direct or an indirect result of soil drying or a combination of the two, e.g. reduced soil water availability will reduce water uptake by plants but can also restrict nutrient uptake by roots and transport to the shoots. Changes in N deposition and the resulting nutrient status of ecosystems may also be a direct consequence of environmental change, and other recent work by Gowing and co-workers (Stevens et al., 2004) has shown how changes in N deposition of only 2.5 kg ha-1 year-1 can result in the addition or removal of a plant species from a 4 m2 quadrat of an acid grassland community. Other environmental variation as a result of human activities, such as continuing increases in concentrations of ozone in the atmosphere, will also impact significantly on plant water relations and interact with the other important climatic variation highlighted above, but the specific action of this variable is outside the scope of this review.
Results such as those of Stevens et al. (2004) show clearly that reductions in plant growth can be brought about by only very small reductions in water and nutrient availability. Similarly, Boyer (1982) has made an important point that when operating under conditions where irrigation, fertiliser and other management aids are in plentiful supply, US farmers achieve yields that are only around 20% of record yields. This again argues for highly tuned sensitivity of plant growth and development to changes in soil and atmospheric water status.
Excessive precipitation resulting in inundation of soil will reduce the partial pressure of oxygen around the roots of plants, which usually reduces their hydraulic conductivity, thereby reducing water uptake. Therefore, rather counter-intuitively, plant water deficits can result, even when there is plenty of water available in the soil (Jackson et al., 1995). Such changes in plant water status will reduce plant growth, as will any additional flood-induced chemical perturbations, including modifications in hormone content of the soil and the plants (e.g. Else & Jackson, 1998) and the accumulation of toxic metabolites.
This brief introduction should be enough to highlight the fact that even subtle changes in the environment are likely to have significant effects on composition and functioning of natural plant communities and on the productivity of agriculture in even the most productive areas of the world. As the climate changes, it is important that we understand the basis of stress-induced changes in plant growth and functioning and if possible intervene, through plant improvement or management programmes, to sustain biodiversity of natural communities where desirable and maintain food production, particularly in some of the most water scarce, populous regions of our planet. This review highlights some of the most sensitive limitations on plant growth and functioning that are imposed by water scarcity. We also focus on the possible exploitation of some of this knowledge to help sustain the production of food under increasingly challenging environmental conditions for farmers.
5.2 Growth of plants in drying soil
As soil water availability is reduced, water uptake by roots is reduced (see below) and the water potential of the expanding cells will be reduced. Invariably this will limit growth, with the impact on the growth rate of the shoots greater than that on the growth of the root (see, e.g. Sharp et al., 2004). Growth of other plant parts that contribute to crop yield is differentially sensitive to reduced water potential (Westgate & Boyer, 1985) and it may be that reduced sensitivity of growth of some organs to low water potential is explained by solute accumulation in expanding plant parts (Sharp & Davies, 1979). While solute accumulation in roots seems to sustain some growth at low water potential, albeit at a reduced rate, turgor maintenance in shoots does not always sustain growth, and there can even be an inverse relationship between the extent of solute accumulation in plant cells and growth, as carbohydrates accumulate in plant cells as expansion is limited at low water potential. Despite this, the selection of wheat lines for capacity to accumulate solutes has resulted in yield enhancement in water scarce environments (Morgan, 2000). This may not necessarily be a result of continued expansion of vegetative plant parts at low water potential since solute regulation can have other beneficial effects on functioning of plants, such as a delay in the accumulation of potentially damaging concentrations of ABA (abscisic acid) in developing reproductive plant parts.
The beneficial effects of solute regulation on crop yield in certain circumstances, even though turgor maintenance is not sufficient to sustain shoot growth at low water potential, illustrate effectively the complexity of the processes leading to reproductive yielding. Of course, this is something that is well known to plant breeders, and Richards (2004) has recently highlighted the fact that sustained yield in water scarce environments can often be ensured by manipulations of processes that have no direct relationship with drought tolerance or even with plant water relations.
Transfer of solutes between organs to sustain seed yield can be promoted by soil drying, even under circumstances where the effects of reductions in soil water availability are so subtle that no changes in plant water status are obvious. In certain circumstances these changes in allometric relations can even increase seed yield. In a recent paper by Yang et al. (2000), high soil nitrogen in the late growth stages of a wheat crop reduced seed yield compared to that of a crop grown with slightly less nitrogen available. This was because high soil N delayed senescence and a high proportion of carbohydrate in the plant was trapped in the stem of the non-senescing plants. Deficit irrigation mobilised this reserve from the stems to developing grains such that seed yield of the plants grown with high N was significantly enhanced compared to that of the well-watered, high-N plants. The deficit irrigation treatment alone had no impact on seed yield of low-N plants (Yang et al., 2000).
Many environmental stresses will impact on the growth of plant cells via an effect on the hydraulic relations of the cell. These stresses can therefore affect plant growth directly since cell turgor is a motive force for growth, and positive turgors are required to stretch cell walls irreversibly. Changes in cell water relations can also indirectly limit growth by an effect on cell metabolism, which can be altered by changes in the spatial relationship between cell organelles and macromolecules or by changes in the concentration of solutes in the cell (Kaiser, 1987). Stress-induced change in cell wall properties will also affect plant growth rates, and these properties may be altered by the impact of chemical signalling or by a change in the solutes concentrating in the cell wall. Chemical signalling effects are discussed in detail below.
The impact of changing cellular hydraulic relations on growth of cells is commonly visualised via the Lockhart equation. This treatment suggests that growth rate is linearly related to cellular turgor above a threshold value, with the slope of the relationship being a function of cell wall extensibility. Both threshold turgor and cell wall extensibility are defined by this model as being under metabolic control (e.g. Pritchard & Tomos, 1993). An alternative model has turgor acting as a switch rather than a proportional controller (e.g. Zhu & Boyer, 1992), with the rate of growth determined by another variable such as the cell wall properties.
It has not proved easy to collect data to support the Lockhart model of growth control, as both growth and cell water relations must be measured in the same population of cells. Even in a single growing organ such as a root tip that might be accessible to water relations assessment, it is clear that cells in different regions of the growing zone are growing at different rates (Figure 5.1) and are differentially sensitive to stresses such as a reduction in cell water potential (see, e.g. Sharp et al., 2004). If this population of cells is treated as responding identically to reduced water potential then an apparent Lockhartian relationship between growth rate and turgor will be observed. This will arise because there is cessation of growth of cells distal to the tip, slowing of growth of cells in another zone only a few millimetres towards the tip and maintenance of growth in the cells closest to the tip (see Spollen
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