Humboldt-Universitšt zu Berlin, Department of Crop Science in the Tropics and Subtropics, Germany
Due to the principal dependence of plant growth on water supply and to the increasing worldwide demand of water, there is an urgent need to save water in irrigated farming and to grow a larger portion of staple crops under rainfed conditions. Climatic water shortage occurs most frequently in arid and semiarid tropics and subtropics. However, productivity is also lowered widely in subhumid climates by `latent water shortage'.
Water can be saved in irrigated crop production by improving supply channels, irrigation systems and water management. Interdependencies among these possibilities and limited financial resources will strongly restrict progress and, hence, integrated approaches are required.
It is of fundamental importance for crop production -- whether irrigated or rainfed -- to avoid latent water shortage during the most susceptible phases of yield formation. Agronomic strategies to save water can be developed according to the concept of the resource-use efficiency, or rather the water-use efficiency with its 3 components: uptake-, conversion-, and transformation-efficiency.
Improving uptake-efficiency is useful at locations where rainfall before or during the vegetation period saturates soil moisture down to deeper layers which can be exploited by deep-rooting of crops and, thus, bridge lack of water during the later season. Such an increased water-uptake can be achieved by agronomic measures or by breeding.
With regard to the evapotranspiration-efficiency (ETE), the second component of WUE, differences among species are well known. Unfortunately, determination of varietal differences are very laborious and time-consuming and, hence, rarely available. Varietal ETE can be indirectly determined by D C13-discrimination analysis. However, expensive sophisticated devices are preventing a wide-spread use of this indirect method. Evidence is provided that short-term monitoring of the ratio of photosynthesis and transpiration may be a useful indicator of ETE.
The third WUE component is the harvest index representing the efficiency of the transformation of biomass into yield. Since strong increment of this index has been the basis for all high-yielding varieties, larger room for varietal improvement exists only in species that have not been subjected to intensive breeding.
Keywords: Breeding , husbandry, irrigation, water-saving, water-use