Abstract To raise yield and water use efficiency(WUE) of crops synchronously is a major target of modern water saving agriculture. In order to understand the high efficient utilization of limited water on yield, WUE and its mechanism, by the methods of plot and pot experiments used three kind varieties of spring wheat and corn as test materials under rainproof awnings and plant growth chambers. The correlation of Yield-WUE-Water, character of material accumulation and WUE in difference growth stages of crops, the correlation between water and photosynthesis or transpiration, WUE daily changes and its physio ecological bases, and the adjustment effect on WUE of environmental factors(such as air humidity) were analysed. Meanwhile, WUE of crops were systematically studied in the difference levels from population, individual to physiology. The main results are as follows: 1. Correlation of Yields-WUE-Water. It is showed that the limited water supply can produce high yield and WUE, the jointing stage of spring wheat in difference varieties is a critical period of water requirement, grain filling period also is important to the dryland variety in semi arid of Loess Plateau; To maintain limited water supply at every stages of corn, the WUE is higher than that of the full water treatment. The pot test found to keep 40% capillary water of soil (CW) at jointing stage is a threshold value of increasing WUE and yield together, and 60%CW is the lowest limited value to get more high WUE and yield. Under limited water supply, the relationship between grain yield(Y) and evapor transpiration(ET) exists a straight line relation, WUE-ET curve or the negative line. 2. Yield formation and growth as effected by limited water supply. One of the important mechanism is compensatory effects on growth and yield, which including the long term and short term effects. The key action of limited water supply at jointing stage is to increase the length and grain number of sprinkle, to improve total leaf area and flag development, to enhance absorption and use of crops to soil water. The research indicated that the net accumulation rate (NAR) and WUE of drymatter in jointing grain filling stages are related closely to grain yield and its WUE.
3. WUE daily change characteristics of limited water supply and its physio ecological base. It is proven that limited water supply can raise photosynthesis(Pn) and adjust transpiration rate(Tr) efficiently. The WUE daily change have non symmetry features between morning and afternoon, the clear difference on WUE between water supply and drought is in the afternoon. The main reaction of Pn to water stress is stomata limit in the morning, and non stomata limit in the afternoon. The major reason of higher Tr in the afternoon than that in the morning is low relative humidity(RH) and high temperature of air. The correlative analyses found between WUE and stomata resistance(Rs) are curve line type markedly, WUE has an adaptive range to leaf water potential (ψw), and tightly related to RH and leaf temperature(Tl). ψw and Tl can be used as plant indexes to guide irrigation of crops. To control RH and Tl can improve growth environment of crops. These viewpoints have important meanings on exploring new measures of water saving agriculture. 4. Action and mechanism of environmental factors under limited water supply condition. Based on the test of plant growth chambers, the results indicated to increase RH can improve the water condition of plants and physiological metabolism on some degree, decrease Tr, and remove non stomata limit of Pn by water stress of soil, especially relieve and compensate growth limited of crops by water stress of soil, and raise use efficiency of soil water markedly. This result also has a great reference meaning on instanllative agriculture.