Shaozhong Kang1,2 and Jianhua Zhang3,*
1Center for Agricultural Water Research in China, China Agricultural University, East Campus, 100083 Beijing, China
2Key Lab for Agricultural Soil and Water Engineering in Arid and Semi-arid Areas of Ministry of Education, Northwest Sci-Tech University of Agriculture and Forestry, Yangling, Shaanxi 712100, China
3Department of Biology, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China
* To whom correspondence should be addressed. Fax: +852 3411 5995. E-mail: firstname.lastname@example.org
Received 13 July 2004; Accepted 20 July 2004
Controlled alternate partial root-zone irrigation (CAPRI), also called partial root-zone drying (PRD) in other literature, is a new irrigation technique and may improve the water use efficiency of crop production without significant yield reduction. It involves part of the root system being exposed to drying soil while the remaining part is irrigated normally. The wetted and dried sides of the root system are alternated with a frequency according to soil drying rate and crop water requirement. The irrigation system is developed on the basis of two theoretical backgrounds. (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. (ii) Part of the root system in drying soil can respond to the drying by sending a root-sourced signal to the shoots where stomata may be inhibited so that water loss is reduced. In the field, however, the prediction that reduced stomatal opening may reduce water consumption may not materialize because stomatal control only constitutes part of the total transpirational resistance. The boundary resistance from the leaf surface to the outside of the canopy may be so substantial that reduction in stomatal conductance is small and may be partially compensated by the increase in leaf temperature. It is likely that densely populated field crops, such as wheat and maize, may have a different stomatal control over transpiration from that of fruit trees which are more sparsely separated. It was discussed how long the stomata can keep ‘partially’ closed when a prolonged and repeated ‘partial’ soil drying is applied and what role the rewatering-stimulated new root growth may play in sensing the repeated soil drying. The physiological and morphological alternation of plants under partial root-zone irrigation may bring more benefits to crops than improved water use efficiency where carbon redistribution among organs is crucial to the determination of the quantity and quality of the products.
Key words: Abscisic acid, irrigation, soil drying, stomata, water use efficiency
Source: Journal of Experimental Botany 2004 55(407):2437-2446
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.
CAPRI can expose part of the root system to soil drying and the roots in the drying zone may produce a drying signal that restricts stomatal opening. Although this might be expected to increase the WUE as outlined above, the situation is complicated by the fact that, in many crop canopies, the stomatal control over transpiration is only minimal and depends on the degree of environmental or atmospheric ‘coupling’ (Jarvis, 1981, 1985; Jarvis and McNaughton, 1986). In a dense canopy, for example, the boundary resistance for vapour diffusion can be so high that stomatal resistance is only a small proportion of the whole diffusion resistance and evaporation from leaves is poorly coupled with the atmospheric condition. Canopy transpiration will largely be determined by the huge boundary resistance and the energy input that sets the leaf temperature difference. If the stomata are partially closed, the leaf will be heated up, the vapour gradient will be higher, and the transpiration will eventually reach an equilibrium rate where the energy input matches the energy used by evaporation.
Obviously, many situations need to be considered before it can be concluded whether CAPRI is practically useful in all situations. For example, what would happen in windy areas, or with sparse crops such as large-sized fruit trees? Their WUE may be improved by partial stomatal opening, because their stomatal resistance is the major transpirational resistance and the leaves are well-adapted to atmospheric conditions. Growing fruit trees is now encouraged in many parts of the world to prevent serious soil erosion. In arid and semi-arid areas they rely heavily on irrigation because of the scarce and unevenly distributed rainfall. Obviously CAPRI may provide a way of cutting down on the amount of water used for irrigation.
How long can the stomata keep ‘partially’ closed in a growing season? If it were established that stomatal response is due to a signal from roots that are exposed to drying soil, how long can such signal be expected to be continuously produced? ABA is produced in the drying roots and transported to shoots to regulate the physiology there (Davies and Zhang, 1991). More recent work suggests that a more alkaline pH in the xylem sap as a result of soil drying may enhance the effect of ABA or act in a synergistic way as a soil-drying signal (Wilkinson and Davies, 1997; Davies et al., 2001). Apparently, for the continuous export of such a root signal, the roots responsible should be kept ‘alive’ if a continuous stomatal regulation is needed. As discussed above, roots exposed to drying soil for a long time may lose their contact with the soil and therefore the sensibility of it. It is necessary to know how long these roots can survive and what effects will be brought on them if the wetting and drying cycle is shifted more frequently or less frequently.
Another question concerning the CAPRI is its possible effect on new root growth, induced by alternating the wetting and drying, and the relevant functions of these new roots. Rewatering the soil-dried roots can cause a flush of secondary roots to grow out (Liang et al., 1996b). When examining root growth and distribution with every furrow or every-other-furrow irrigation, i.e. a partial root-zone irrigation (PRI), Skinner et al. (1998) found that, as the non-irrigated furrow began to dry, root biomass increased as much as 126% compared with the irrigated furrow and the greatest increase was at lower depths where moisture was still plentiful. Some other reports also showed that roots tend to proliferate in regions of high water availability in a root zone where water is unevenly distributed (Ben-Asher and Silberbush, 1992; Gallardo et al., 1994). Dry and Loveys (2000b) showed that the pattern of grape root distribution changed when they received PRI. More roots were developed in deeper layers of soil and a larger root system was observed under this irrigation.
It is possible that the ‘newly’ produced roots may recover the capability to respond to drying soil when they are is exposed to it again. Further experiments are needed to investigate just how complete such a recovery could be. Are these new roots able to sense and send the soil-drying signal? What are their contributions to the recovery of root hydraulic conductivity? North and Nobel (1991) observed that hydraulic conductivity of Agave deserti roots increased significantly after drying and rewetting cycles. Kang et al. (1999) found similar results for maize, sunflower, Acacia confusa, and Leucaena glauca. The hydraulic conductivity of apple, grape, peach, and pear tree roots also increased noticeably under a locally restricted water supply (Poni et al., 1992). Such compensation may be partly attributed to the newly formed roots and also partly to the old roots which may undergo some changes when exposed to rewetting again.
One extra benefit from the CAPRI-induced new roots may be related to their function in nutrient uptake. It is well known that nutrients are taken up by roots only when there is some available water. The drying and rewetting cycle, plus its induced new roots, may make the nutrients in this soil zone available to plants. Apparently, such an hypothesis needs to be tested further.
Can CAPRI improve the quality of products, as suggested by Loveys et al. (2004) with grapes, or the WUE for economic yield in crops such as cotton where control of vegetative growth is always a problem? In crops where vegetative growth could be properly controlled, adequate carbohydrate should be supplied to reproductive organs once the reproductive growth stage has started. Several research groups have shown that, using CAPRI, the quality of fruits such as grapes in terms of sugar content can be increased (Fuller, 1997; Loveys et al., 1997, 1998; Dry and Loveys, 1998, 1999, 2000a, b; Gu et al., 2000; Stoll et al., 2000). They have shown that this is largely a result of better control of vegetative growth on the grapevine. Biomass production may be reduced as a result of CAPRI, if that means a substantial reduction of irrigation, but the harvest index may be improved such that economic yield may not necessarily be reduced. Experience with soil drying has shown that, in many situations, the harvest index can be improved by mild soil drying during the grain-filling stage (Zhang et al., 1998; Yang et al., 2000, 2001; Kang et al., 2000c, 2002c).
It is also important to assess how much water CAPRI can save in a growing season. Many results have shown that water is preferentially extracted from wet zones in a root system, and any effects of partial drying may be compensated by enhanced water uptake from roots in the wet zone (Green and Clothier, 1995, 1999; Green et al., 1997). Clearly this hypothesis needs to be tested.
Many other questions can be asked in relation to the practical application of CAPRI in different cropping situations. For example, can cropping benefit from the effect of CAPRI in an area with a shallow ground water-table? The irrigation water may be reduced, but how much water may be contributed to the root zone from groundwater? If the soil salinity content is high, is CAPRI still practical or not? What patterns of CAPRI are suitable for different crops or soil types?
It is almost certain that breeding for high yield over the years has increased stomatal opening (Jones, 1992). This may have increased the transpiration in some areas where the cooling effect of transpiration may have helped the leaves to avoid the damaging effect of high temperature on photosynthesis. Certainly this suggests an advantage for high transpiration and reducing it may have an adverse effect on carbon assimilation. Results of CAPRI research may test to what extent this is true and give a better understanding of the relationship between transpiration and carbon assimilation.
The conventional view holds that plant biomass production is linearly coupled with the amount of water used and WUE is generally a conservative parameter (Jones, 1992). In a normal irrigation system, stomata operate for most of the time to their full extent (a luxury state?) and assimilation while the stomata are closed or partially closed is only a small proportion of the total assimilation. This may explain the conservative nature of WUE usually seen. However, the continuous production and application of stress signals may change this normal pattern of stomatal rhythm and a different WUE may be generated without much loss of biomass production. Certainly if that is the case, knowledge of plant photosynthesis in relation to water use will be enriched.
Drip irrigation, an advanced method in which water in pipes is introduced into soil slowly, may not only save water through reduced evaporation from the soil surface, but may also have an improved physiological WUE through reduced stomatal opening. It can be expected that under this type of irrigation some of the roots are always exposed to drying soil. Results from CAPRI research will give new ideas in the design of irrigation strategy.
When will be the right time to irrigate the crops? Does a partial wilting in the shoots signal the severity of a drought? CAPRI research may find out when irrigation is required. It is expected that under CAPRI whole shoot wilting can be avoided as long as the wet root zone can supply water. An early sign of progressive wilting in the shoots, i.e. the early wilting of older leaves, may indicate the right time for irrigation (Zhang and Davies, 1989b). It is well known that precise and reliable indicators will certainly cut down much unnecessary irrigation.
Other irrigation methods, including deficit irrigation, have been reported to improve WUE (Goodwin et al., 1992; Boland et al., 1993; Kang et al., 2000c, 2002c). These were often explained either in terms of reduced soil surface evaporation and/or a trade-off of better water use for a lesser yield. CAPRI should produce more benefits than these. Early studies involving partially drying part of the root system investigated effects on the ASA contents of the roots, xylem sap, and leaves in either a horizontally split root system in Helianthus annuus (Neales et al., 1989), or a vertically split root system with upper soil drying (Li and Zhang, 1994). These experiments were conducted using fixed partial root-zone drying, and the roots in the dried part experienced anatomical changes. Based on this earlier work, Kang et al. (1997) conducted an experiment with pot-grown maize plants where the plant root system was divided into two or three parts and only the partial root zone was watered (ZS Liang et al., 1997; Kang et al., 1998). Compared with conventional watering, alternate irrigation on half the root zone reduced water consumption by 35% with a total biomass reduction of only 6–11%. Another experiment with hot peppers and drip irrigation also showed that CAPRI reduced water used for irrigation by about 40% (Kang et al., 2001a).
Another experiment, where CAPRI was applied to a vertical soil profile (Kang et al., 2002b) showed that water consumption was reduced by 20% (moderate soil drying) and 40% (severe soil drying) through extending the watering intervals. The alternate surface and subsurface irrigation, or drying, on either part of the soil column largely maintained its biomass production under moderate soil drying. In addition, alternate vertical irrigation outperformed the sole subsurface irrigation or sole surface irrigation in the biomass production when the same amount of water was consumed. Root development, in both root length and dried mass, was significantly enhanced. Significant increases in WUE and root-to-shoot ratio were observed as a result of the alternate vertical irrigation treatment. Leaf resistance for vapour diffusion was increased substantially while the rate of photosynthesis and leaf water content were not significantly altered. The results also showed that nutrient uptake, the K and N, and shoot biomass production were enhanced by the alternate drying and rewatering at the two parts in the vertical soil profile (Shi and Kang, 2000; Kang et al., 2002b). It was concluded that controlled alternate watering in the vertical soil profile is an effective and water-saving method of irrigation and may have the potential to be used in the field.
In the field, photosynthesis rate, transpiration rate, yield production, WUE, soil water distribution, irrigation water advance and uniformity with CAPRI were tested for irrigated maize in an arid area with seasonal rainfall of 77.5–88.0 mm over 4 years (1997–2000) (Kang et al., 2000a). The size of the trial area was about 33.3 ha. Irrigation was applied through furrows in three ways: alternate furrow irrigation (AFI), fixed furrow irrigation (FFI) and conventional furrow irrigation (CFI). AFI means that one of the two neighbouring furrows was alternately irrigated during consecutive waterings. FFI means that irrigation was fixed to one of the two neighbouring furrows. CFI was the conventional way where every furrow was irrigated during each watering. Each irrigation method was further divided into three treatments with different irrigation amounts, i.e. 45, 30, and 22.5 mm water for each watering. Results showed that there was no significant difference for photosynthesis rate between the three irrigation treatments when irrigated with the same amounts. Transpiration rate in CFI was larger than that in FFI and AFI after each irrigation (Table 1). The results also indicated that luxury water consumption can be reduced, without necessarily reducing the rate of photosynthesis substantially when CAPRI is applied under the field conditions. The results also showed that root development was significantly enhanced by AFI treatment. Primary root numbers, total root dry weight, and root density were all higher in AFI than in FFI and CFI treatments. Less irrigation significantly reduced the total root dry weight and plant height in both FFI and CFI treatments but this was not so substantial with AFI treatments. The most surprising result was that AFI maintained high grain yield with up to 50% reduction in the amount of irrigation, while FFI and CFI both showed a substantial decrease in yield with reduced irrigation. As a result, WUE for applied water was substantially increased. It was also found that, in all cases, there was satisfactory separation of wetted and dried zones in a range of irrigation water use under field conditions, even if the water applied to one side infiltrated the other supposedly dry side, and infiltration in CFI was deeper than that in AFI and FFI. The time of water advance did not differ between AFI, FFI, and CFI at all the distances monitored, and water advanced at a similar rate in all the treatments. The Christiansen uniformity coefficient of water content in the soil (CUs) was used to evaluate the uniformity of irrigated water distribution and showed no decrease in AFI and FFI, although irrigation water use was smaller than in CFI (Kang et al., 2000b). The technique was extended to 4133 ha in this region in the following year.The new technique was also tested in peach and apple orchards at Yangling, Shaanxi, China by using a drip irrigation system (Gong et al., 2001), and in a pear orchard in Victoria, Australia by using a flood irrigation system (Kang et al., 2002a). The aim was to compare CAPRI with the fixed partial root-zone irrigation (FPRI) and the whole root-zone irrigation (WRI) on fruit trees in terms of root water uptake, fruit yield and size, water use, and WUE. Results showed that water use declined in FPRI and CAPRI by 28% and 12%, respectively, compared with WRI in the 0–110 cm soil layer. Therefore 52% and 23% less irrigation water was applied in the FPRI and CAPRI treatments. The ratio of water uptake in the irrigated wet side to that of the same side in the WRI was larger than 1.0 in both CAPRI and FPRI, a compensatory effect in the wet part of the root zone. When less irrigation was introduced in the FPRI and CAPRI, the fruit number, yield per tree, and the total yield in unit area were not decreased, and the pear tree WUE and the production efficiency of the irrigation water were substantially improved. The results of measured root and trunk sap flows using heat-pulse sap flow meters showed that the root sap flow on the wet side was substantially enhanced as a result of CAPRI, and was greater than that from the same side in WRI. The trunk sap flow in FPRI and CAPRI was smaller than that in WRI. On average, both CAPRI and FPRI reduced plant daily water consumption by about 10% and 18%, respectively, when compared with WRI during the partial root-zone drying period (Table 2). Daily root water flow was a significant function of the reference evapotranspiration and such a relationship also indicated that the wetted side contributed more water flow than the dried side. The daily trunk water flow was also related to the reference evapotranspiration but the WRI carried more water than CAPRI and FPRI under the same evaporation demand, suggesting a restriction of transpirational water loss in the CAPRI and FPRI trees. WRI needed a higher soil water content to carry the same amount of trunk flow than the CAPRI and FPRI trees (Fig. 1), suggesting that the hydraulic conductance of roots in CAPRI and FPRI trees was enhanced, and that the roots had a greater water uptake capacity than in WRI when the average soil water content in the root zone was the same.
Such an approach was also encouraged by more recent investigations on grapevines, including some field experiments on Shiraz, Cabernet sauvignon, and Riesling (Loveys et al., 1997, 1998; Dry and Loveys, 1998, 1999, 2000a, b; Dry et al., 2000; Stoll et al., 2000). A consistent feature of all these trials was that there was no significant reduction in yield due to partial root-zone drying treatment, even though the amount of irrigation was halved. As a result, yield per unit of water applied doubled in response to partial root-zone drying. The results of partial root-zone drying on fruit composition with respect to wine-making attributes indicate that quality is at least maintained if not improved. Some experiments revealed no apparent effect on fruit quality as indicated by concentrations of anthocyanins and phenolics in the fruit. In these cases, the control vines were well balanced with relatively open canopies and partial root-zone drying did not substantially alter the canopy microclimate. The concentration of the derivatives of delphinidin, cyanidin, and petunidin in fruit from partial root-zone drying vines increased relatively more than the derivatives of malvidin and peonidin. Furthermore, they found that partial root-zone drying enhanced the formation of the coumarate forms of anthocyanins. This may be a response to bunch exposure, because shading of Shiraz bunches in a hot climate was founded to enhance the proportion of coumarate forms.
Gu et al. (2000) also reported the effect of PRI on vine water relations, vegetative growth, mineral nutrition, yield, and fruit quality in field-grown mature sauvignon blanc grapevines grown in the San Joaquin Valley in California, compared with conventional drip irrigation (CDI). This was the first time that PRI had been tested in the United States. Their results showed that partial stomatal closure due to PRI resulted in a decrease in stomatal conductance, transpiration rate, and vine vegetative growth (laterals and pruning weight) and, in turn, an improvement in WUE. Yield and fruit composition were not significantly affected by PRI treatment or by the amount of water applied. Petiole mineral nutrient contents were not affected by PRI treatments or the amount of water applied. Preliminary experiments demonstrated that PRI offers a way to produce a vine with a better balance between vegetative and reproductive development, reducing water use, and controlling vine vigour and canopy density, while maintaining crop yields compared with standard vineyard irrigation practice.
The results described above show that PRI can improve WUE, and may or may not reduce yield, improve fruit quality, and control vegetative growth in the relatively short term. It is still necessary to know whether CAPRI in different canopies, field crops, or fruit tree orchards can show such effects in the longer term.
Projects have been started with field crops (maize, wheat, and cotton), vegetables (hot pepper, tomato), and fruit trees (apple, peach, pear, and persimmon) in a systematic way for the long-term evaluation of PRI. The aim is to assess how root hydraulic conductivity changes for different parts of the root zone, how the ability of roots to take up water and nutrition changes for different parts of the root system, how the major nutrients are distributed in the plants, and how the stomatal adjustment and changes in root uptake ability regulate and improve WUE under CAPRI. It is necessary to reveal the response differences from crop to crop to CAPRI, to understand the variations of root and trunk sap flow, transpiration rate, evapotranspiration rate, water cycle and balance in the field under CAPRI. Crop coefficients under CAPRI should be evaluated to supplement and expand the results recommended by FAO (Allen et al., 1998) so that they can be applied in irrigation water management. It is also necessary to analyse the relationship between salt accumulation in different parts of the root zone and use of water for irrigation because salt accumulation in soils with a higher salinity content needs to be controlled to avoid any possible negative effect of CAPRI.
There are several groups in Australia working on PRI. Agriculture in Australia uses a lot of irrigation and increasing WUE is a challenging goal. Brian Loveys and his group continue to work with grapevines using PRI, while Ron Hutton from New South Wales Agricultural Institute, and Ian Goodwin and Harold Adem in Natural Resources and Environment Victoria, are concentrating on stone fruits and pears at Tatura (Land and Water Australia, 2002). A central theme of all the work is to assess how the PRI technique can alter the crop's water requirement and it is becoming evident that this is often a lot less than the amount currently supplied through irrigation.
The evaluation of CAPRI has progressed beyond the experimental stage with a significant area of CAPRI installed in fields, orchards, and greenhouses in China, Australia, Indonesia, Spain, Turkey, Yugoslavia, New Zealand, the United States, the Netherlands, South Africa, etc. To date, most installations have involved a second drip line either above or below ground. Kang and his colleagues also designed a new alternate valve to control CAPRI for field crops.
Water is the most limiting factor in plant production. Conventional irrigation cannot be sustained in many areas in the world because of the rapid depletion of water resources. In Northwest China (especially in the Loess Plateau) much land use has now shifted from field crop cultivation to fruit trees in order to save water and to conserve soil. It is believed that conventional irrigation is a luxury use of water and can be reduced without much effect on economic yield. Methods that may cut down irrigation are of considerable interest and should be explored. Earlier work by the present authors about root-to-shoot communication of soil drying has yielded some ideas for designing an irrigation method to manipulate the plant response system so that a continuous soil-drying signal may restrict plant water use in the long term. It is believed that plants must have developed such a self-protection mechanism through evolution to survive in environments where rainfall is unpredictable. This means that plants should be able to regulate their water use according to the availability of water in the soil. The new water-saving technique, CAPRI, has been presented systematically, put into practice and has been expanded into crop production. Such a system explores exactly the plant feed-forward response to soil water availability and may regulate plant water loss, growth, and development in a way not seen in conventional irrigation systems. Certainly a lot more interesting questions are waiting to be answered.
We are grateful for the grant support from RGC of Hong Kong University Grants Council (HKBU 2041/01M) and financial support from the Chinese National Natural Science Fund (Nos 50339030, 50279043). We also wish to thank the anonymous reviewers for their constructive and critical comments which greatly improved the revised version.
Abbreviations: CAPRI, controlled alternate partial root-zone irrigation; WUE, water use efficiency; ABA, abscisic acid; PRI, partial root-zone irrigation; AFI, alternate furrow irrigation; FFI, fixed furrow irrigation; CFI, conventional furrow irrigation; FPRI, fixed partial root-zone irrigation; WRI, whole root-zone irrigation; CDI, conventional drip irrigation; RDI, regulated deficit irrigation.
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Fig. 1. The ratio of trunk water flow to the reference evapotranspiration as a function of soil water content in a pear orchard. APRI, alternative partial root-zone irrigation; FPRI, fixed partial root-zone irrigation; WRI, the whole root-zone irrigation (Kang et al., 2003).
Source: Journal of Experimental Botany 2004 55(407):2437-2446
Table 1. Leaf photosynthesis rate (Pn, µmol CO2 m–2 s–1), transpiration rate (Tr, µmol H2O m–2 s–1) and water use efficiency (WUE, µmol CO2 µmol–1 H2O) of maize grown in arid area in the year of 1998
AFI, FFI and CFI are alternate, fixed and conventional furrow irrigation, respectively. Values are means of three plots for each treatment.
APRI, alternative partial root-zone irrigation; FPRI, fixed partial root-zone irrigation; WRI, the whole root-zone irrigation. Data are means of four measurements under the same treatment. Letters following numbers of consumptive water use indicate statistical significance of difference when the west and east sides within the same treatment were compared at P0.05 level (same letters indicate a non-significant difference). DAB means days after beginning of the experiment (Kang et al., 2002a)
a* Statistical significance at P0.05 level when compared with the corresponding values of WRI at the same experimental period. Statistical significance at P0.05 level of trunk sap flow in APRI compared with the corresponding values of FPRI in the same experimental period.
Source: Journal of Experimental Botany 2004 55(407):2437-2446