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.