[Optimum soil water supply range of Malus pumila]

By means of the determination of net photosynthetic rate and transpiration rate of the leaves of seven-year field and two-year spotted Malus pumila (Goldspur) under different soil water contents and illumination and calculation of water use efficiency (WUE), the optimum soil water supply range of Malus pumila was determined. The results showed that because photosynthetic advantageous radiation (PAR) and soil water content (SWC) decided the value of photosynthetic rate and transpiration rate of Goldspur, thus they influenced the value of WUE. The response of WUE to illumination had relations with soil water. When SWC was 10% (50% of field capacity-FC) or so, the maximum of WUE was the highest value among all the soil water treatment and reached about 230 mumolCO2.g-1H2O. When the water supply was adequate (SWC > 15%), the maximum was only 160 mumolCO2.g-1H2O or so, the difference between each SWC level was not obvious and simulated curve fundamentally overlapped, although WUE decreased slowly after reached the maximum. The variation of WUE with SWC had relations with illumination. WUE was the highest while PAR was in the range of 500-1,000 mumol.m-2.s-1). Because the state of soil water decided the values of stomatic resistance (RS) and leaf water potential, RS and psi 1 also had influence on the changes of WUE. When RS and psi 1 were respectively 2.0 s.cm-1 and -3.0 MPa, WUE was the highest, but Pn was relatively low, which was not good to the normal growth and fruitage of trees. From the overall consideration of WUE, Pn and Tr, the scopes of RS and psi 1 were 2-6 s.cm-1 and -2.1 approximately -1.65 MPa, respectively, which not only guaranteed the normal photosynthesis of Goldspur, but also benefited the increase of WUE. The scope of the corresponding SWC was 11%-15%, which was equal to 55%-75% of FC and this scope could be used as theoretical index of field water supply for Goldspur.