Water Deficit Indicators Of Winter Wheat And Their Diagnosing Thresholds Based On Response To Grain Yield

Clarifying relationship between water deficit and crop physiology, growth and yield is the scientific basis for implementing deficit irrigation scheduling. By varying irrigation times（zero to four irrigations, treatments 1 to 7） to differentiate levels of soil water deficit among treatments, we investigated the response of soil moisture, leaf photosynthetic rate, plant height, leaf area, canopy temperature and NDVI（Normalized Difference Vegetation Index） to yield of winter wheat and their relations under field conditions in climate of Hengshui in North China. The main results were as follows:（1）Soil water threshold varied with increase of soil depth and generally decreased with the progress of wheat growths. The soil water thresholds in 0-100 cm at seeding-wintering, wintering-greening, greening-jointing, jointing-heading, heading-flowring, flowring-grain filling, grain filling-maturing was respectively 78.9%～81.4%、78.3%～79.5%、74.8%～77.5%、70.6%～74.7%、62.2%～71.1%、65.1%～67.5% and 64.4%～65.2%。（2）Photosynthetic rate for the severe water stressed treatments decreased significantly. The average photosynthetic rate of T7 were 15% lower than that of control. Relation of photosynthetic rate with yield depended on growth stage: it showed binomial relationship at grain filling（P<0.05）, meaning when photosynthetic rate is 14.3 μmol·m^-2·s^-1, the yield would be the highest. While at the heading and flowering stage, the yield increased with increasing photosynthetic rate（P<0.05）, so in order to maintain a yield level of 7t/hm², the photosynthetic rate should remain higher than 18.2 μmol·m^-2·s^-1 and 22.7 μmol·m^-2·s^-1 respectively.（3）Canopy temperature for the severe water stressed treatments was significantly higher than that for the control. The highest canopy temperature was obtained by T7 being 17.7% higher than T1. The correlations of canopy temperature with yield at jointing, flowering, grain filling and maturing were negative（P<0.05）. In order to maintain a yield level of 7t/hm², the canopy temperature should be lower than 15.6℃～16.1℃、14.7℃～18.1℃、19.1℃～21.4℃ and 19.6℃～23.2℃ in those period. In contrast, the correlations of temperature differences between air and canopy with yield in jointing, flowering, grain filling and maturing were positive（P<0.05）. In order to maintain a yield level of 7t/hm², the temperature differences between air and canopy should be more than 9.4℃～11.3℃、8.2℃～10.2℃、8.1℃～8.2℃ and 8.8℃～9.4℃in those growth stages.（4）Compared with the control, NDVI for winter wheat under severe water stress decreased significantly（P<0.05）, the average NDVI of T7 was 30.4% less than the control. The NDVI correlated significantly and positively with yield（P<0.05） at jointing to maturing, with the highest correlation（P<0.01） at jointing, flowering and maturing. To maintain a yield level of 7t/hm², the NDVI should be more than 0.83, 0.92, 0.85 and 0.80 in those stages.