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.