Net photosynthetic rate (P_n) of seven-year-old field and two-year-old potted the Goldspur apple tree (Malus pumila CV.Goldspur) was determined through the use of controlling different soil moisture (SWC) and irradiation intensity (Photosynthetically Active Radiation,for short PAR).The experimental site was located in Tuqiaogou watersheds,Yukou town,Fangshan county,Shanxi Province,China,a part of gully-hilly area of loessial plateau in the middle reaches of Yellow River,37°36′58″N,110°02′55″E.The average annual precipitation here amounts 416 mm,and the precipitation from June to August is more than 70%.The vegetation type is forest bush steppe.The soil moisture was 15.01 percent (equal to 75 percent of field capacity,FC) on August 17,1996,when P_n was measured in the field.To avoid the measured error caused by too high soil temperature due to the straightly irradiating of the sunlight when the pots are put in the air,the pot was buried in the field soil so that the temperature of the pot soil could be the same as the field soil,and the soil moisture of the pots was artificially controlled.In the study on response of the net photosynthetic rate to light,all experimental pots were divided into six groups (three pots per group),and represented with letter from a to f.In the first five days out of ten days before the observation on June 22,1996,each pot was watered adequately every.Since the beginning of the sixth day,the watering of the six groups of the pots was stopped in turn from a to f.As a result,SWC was controlled by six levels such as a (6.04%),b(9.11%),c(11.34%),d(15.46%),e(18.78%) and f(22.56%).In the study on response of net photosynthetic rate to water,the soil moisture of twenty-one potted samples was controlled in the successive variable system range of 5%～23% in the same way before the observation day (August 17,1996).SWC was determined through the use of soil sample drying and weighing,and field capacity (FC) of soil,measured with ring knife,was 20.01%.PAR was controlled at six levels,that is PAR<50,50<PAR<100,100<PAR<500,500<PAR<1000,1000<PAR<1400,PAR>1400 μmol·s^-1·m^-2 by using differently thick plastic membrane layers and pledget layers.P_n under different SWC and PAR was measured with a portable photosynthetic system (L1-6200,LI-COR Co.USA).At the same time PAR was determined using the quantorecorder sensor on the leaf chamber of L1-6200 photosynthetic system.Predawn leaf water potential and soil moisture characteristic curve was determined respectively employing a pressure chamber (ZLZ-4,Lanzhou University,China) and the method of pressure membrane.;The results showed that P_n related closely to PAR and SWC.When enough water was provided to tree and no water stress existed,or SWC(>15%) was more than 75 percent of field capacity,the curve of P_n response to PAR was a right-angled hyperbola (including pots and field site),the maximum photosynthetic rate (P_max) as 24.40 μmolCO₂·s^-1·m^-2 was recorded for pots sample.The photoinhibition stresses didn’t occur in the range of determinable PAR.The curve of P_n response to PAR was a quadratic parabola when SWC is below the level,which means the arising of photoinhibition stresses.Water stress was more serious,and the photoinhibition stresses arose earlier.When SWC was in the range of 11%~15%(accounted for 55%~75% of FC),P max retarding with SWC increasing was 12~18 μmolCO₂·s^-1·m^-2.Under the serious soil water stress (SWC was 6.04% equal to 30% of FC),and PAR reached 300 μmolCO₂·s^-1·m^-2,P max was only 1.0832 μmolCO₂·s^-1·m^-2.;Through the determination of P_n of seven-years-old field Goldspur apple tree,it showed the decrease pattern at noon (noon break pattern),which was the same as the daily change of P_n of most plants and the phenomenon of noon break was obvious when water stress didn’t occur.The daily change law of P_n of Goldspur apple tree differed from different SWC.The time for noon break was longer if the water stress was more serious,which was also when SWC was smaller than 11% (below 55% of FC...