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.
), and in a pear orchard in Victoria, Australia by using a flood irrigation system (Kang et al.
). 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.