Why controlled alternate partial root-zone irrigation (CAPRI)?
The effective use of irrigation water has become a key component in the production of field crops and high-quality fruit crops in arid and semi-arid areas. Irrigation has been the major driving force for agricultural development in these areas for some time. Efficient water use has become an important issue in recent years because the lack of available water resources in some areas is increasingly becoming a serious problem. Much effort has been spent on developing techniques such as RDI (regulated deficit irrigation) and CAPRI [controlled alternate partial root-zone irrigation or partial root-zone drying (PRD in the literature)] to improve field and fruit crop water use efficiency (WUE) (Goodwin et al., 1992; Boland et al., 1993; Kang et al., 1997, 1998, 2000a, b, 2001a, b, 2002a, b, 2003; ZS Liang et al., 1997, 1998; Loveys et al., 1997, 1998; Dry and Loveys, 1998, 1999, 2000a, b; Gu et al., 2000; Shi and Kang, 2000; Stoll et al., 2000; Han and Kang, 2002; Kang and Cai, 2002).
CAPRI is a new irrigation technique which requires that approximately half of the root system is always exposed to drying soil while the remaining half is irrigated as in full irrigation. The wetted and dried sides of the root system are alternated in a frequency according to crops, growing stages and soil water balance. This technique has the potential to reduce field-crop and fruit-tree water use significantly, increase canopy vigour, and maintain yields when compared with normal irrigation methods. This technique has two theoretical assumptions. (i) Fully irrigated plants usually have widely opened stomata. A small narrowing of the stomatal opening may reduce water loss substantially with little effect on photosynthesis (Jones, 1992). (ii) Part of the root system in drying soil can respond to drying by sending a root-sourced signal to the shoots where stomata may be closed to reduce water loss (Davies and Zhang, 1991).
Plants open their stomata for CO2 uptake and at the same time lose their internal water. Mathematical modelling of these two opposite diffusion processes has predicted that plants generally should have the capability to optimize their water use in the short term and to maximize their chance of survival over a drought in the longer term. In the short term, for example, during a day, their carbon gained is maximized with only a limited amount of water lost (Cowan, 1977, 1982; Farquhar and Sharkey, 1982). In the longer term, for example, over a season, their water loss should be regulated according to the amount of water available in the soil (Jones, 1980; Cowan, 1982, 1986). In a world where rainfall is unpredictable, the long-term regulation means that plants must be able to ‘detect’ the soil drying and then ‘respond’ to it by regulating their water consumption. Such a mechanism may be termed as a feed-forward mechanism, since such regulation, for example, reduced stomatal opening, may take place well before all the water available in the soil is completely depleted.
Early work from this laboratory (Zhang et al., 1987; Zhang and Davies, 1989a, b, 1990a, b; Davies and Zhang, 1991; J Liang et al., 1996a, 1997) has revealed that such a feed-forward mechanism may work through abscisic acid (ABA), and could act as a plant growth substance, as a signal of soil drying. ABA can be produced in the roots in the drying soil and transported through the transpiration stream to the shoots where the shoot physiology (mainly leaf expansion rate and stomatal opening) is regulated. This root-sourced signal may substantially reduce the water loss through stomata at a time when no water deficit is detectable in the shoots, and may be described as the first line of defence against a possible drought. With prolonged soil drying, a second line of defence may operate through a progressive wilting starting from the lower and older leaves. Massive amounts of ABA will be produced in these wilted leaves and be sent to the upper younger leaves and buds where water loss will be cut down further (Zhang and Davies, 1989b). Understandably, it is essential for the plants to maintain some turgor in their growing points for survival, even under a prolonged drought. With the decreasing availability of water in the soil, signals from the first and second lines of defence will increase in strength and stomata will close further. Therefore, it may be concluded that stomata do work in a responsive pattern.
Can this kind of responding mechanism be used to increase plant WUE? Typically, plant photosynthetic rate shows a saturation response as stomata open, while transpiration rate shows a more linear response. It would be expected that narrowing fully opened stomatal apertures would substantially reduce water loss but would have little effect on the rate of photosynthesis. If this can be achieved, then WUE, if calculated as the carbon gained per unit of water lost, will be improved at little expense of CO2 uptake.
Are stomata opened at maximum most of the day? Even well supplied with water, stomatal opening is also a function of the atmospheric conditions, i.e. the evaporation demand. Cowan (1977) and Farquhar and Sharkey (1982) suggested that stomata might tend to function in such a way that the diurnal course of conductance allows maximal carbon gain for a given amount of water lost. Their models suggested that this would be achieved if stomatal movement is such that it would hold the gain ratio G (G = E/A, where E is the rate of transpiration and A is the rate of carbon assimilation) constant over the course of the day. Cowan (1982) attributed this stomatal movement as a short-term optimal stomatal regulation. From this modelling, it can be seen that a ‘midday depression’ becomes more substantial when evaporation demand gets higher at midday, i.e. the increase of temperature that leads to a higher vapour pressure gradient between the inside and the outside of the leaves.
It has long been known that plants growing in dry land with periodic soil drying have a higher WUE (Bacon, 2004). Apparently the increased WUE should be an integrated result of both short-term, as a function of atmosphere condition, and long-term, as a function of soil water availability, regulation of water loss. Improved WUE with a responsive stomatal behaviour is indeed predicted by Cowan (1982) from an analysis of the optimization pattern of water use by plants. Jones (1992) concluded that such a responsive pattern of stomatal opening whould be the best pattern for both plant survival and carbohydrate production, i.e. the WUE.
Many research results show that some plants have the ability rapidly to resume water uptake after drought, and the water uptake rate would be enhanced after rewatering compared with a full water supply treatment (North and Nobel, 1991; Huang and Nobel, 1992; Wraith et al., 1995). Earlier research by the present authors also showed that hydraulic conductivity of root systems could be improved greatly when restoring wetting after drought (Kang and Zhang, 1997, Kang et al., 1999). Liang et al. (1996b) also reported that rewatering can greatly encourage the initiation and growth of secondary roots. Moreover, Shi and Kang (2000) and Han and Kang (2002) reported that the ability of roots to absorb nutrients was also improved when the root zone was partially watered and the partial watering was shifted alternately in a horizontal direction or along the vertical soil profile.
CAPRI is therefore designed to expose part of the root system to drying soil and to produce the root signal of drying, while the remaining roots in wet soil can maintain the water supply so that leaves are kept hydrated (Kang et al., 1997; Loveys et al., 1997, 1998; Dry and Loveys, 1998, 1999, 2000a, b; Gu et al., 2000; Stoll et al., 2000). Why is it necessary to alternate sides and not to keep a fixed part of the root system in drying soil? It is thought that prolonged exposure of roots to drying soil may cause anatomical changes in the roots, such as suberization of the epidermis, collapse of the cortex, and loss of succulent secondary roots (North and Nobel, 1991). These changes are such that the roots under prolonged soil drying may function simply as transportation ‘pipes’ with a very low radial permeability to water. Such hydraulically isolated roots in soil would have reduced ability to sense soil drying. Alternate watering or rewatering, after a long period of soil drying, may improve this situation by inducing new secondary roots (Liang et al., 1996b). Apparently such new roots are succulent enough to sense further soil drying and may also enhance the nutrient uptake from this soil zone.
CAPRI is a general technique which may be applied in different ways in the field (Kang et al., 1997, 2001b, 2002a; Kang and Cai, 2002). These include controlled alternate drip (or subsurface drip) irrigation on part of the root zone, CAPRI applied in a vertical soil profile, controlled alternate border irrigation, controlled alternate furrow irrigation (AFI), and controlled alternate surface irrigation and subsurface irrigation and so on. In this paper, some results and conclusions of experiments conducted in pots and the field are reviewed, and questions requiring further study are discussed.