Effects of So ilMo isture on Photosyn thesis and Transpira tion Parametersof Am ygda lus davidiana Franch in Loess Hilly Pla teau

Diurnal variations of leaf－air interface physiological parameters in leaves of three－year－old Am ygdalusdavidiana were studied under different soilmoisture conditions with C IRAS－2 portable photosynthesis system in theloess hilly p lateau. The results indicated that the diurnal variation p rocess of photosynthesis, transp iration and wateruse efficiency under different soilmoisture conditions took on different regulations. When soil relative water contentwas 57. 3% , 70. 1% , 80. 2% , the time that maximum disp lay was at 13: 00, 9: 00 and 15: 00, respectively, and the maximum values of photosynthetic rate were 10. 51, 12. 52, 9. 25 μmol·m-2·s-1 , respectively, the dailyaverage valueswere 6. 83 ±4. 92, 6. 40 ±4. 43 and 6. 32 ±3. 90 μmol·m-2·s-1 , respectively. In other soilmoisture conditions, all the photosynthetic rate followed a two－peak pattern; The style of diurnal variation curves oftransp iration rate was similar with that of photosynthetic rate, when soil relative water contents were 57. 3% ,70. 1% , 80.2% , the time thatmaximum disp laywas always at 13: 00, and the daily average values of transp irationrate were 1.91 ±1. 03, 3. 27 ±2. 09, 2. 90 ±1. 70μmol·mol- 1 , respectively; The maximum ofwater use efficiencybefore 13: 00 appeared at 7: 00 or 9: 00, but itwas very distinct after 13: 00. Analysis showed the dominating reasoncausing the photosynthetic rate fallwas stoma limitation when relative water contentwas over 57. 3%. In other soilmoisture conditions, stoma limitation was the dominating reason before 13: 00, while non2stoma limitation becamedominant after 13: 00. According to the connections between the photosynthetic rate, transp iration rate , water useefficiency and soil moisture, in loess hilly p lateau, the fitting soil relative water content maintaining Am ygdalusdavidiana natural growth was between 39. 7% and 57. 3%. At the same time, it should be p lanted in shady slope orsemi2shady slope in site design.