The Response of Transpiration Rate of Malus pumila cv. Goldspur to Illumination and Soil Moisture

We tested the transpiration rate (Tr) of seven-year-old field and two-year-old potted Malus pumila cv. Goldspur under different conditions of illumination and soil water. The results showed that the interrelationship between Tr of Malus pumila cv. Goldspur and illumination and soil water content (SWC) was quite remarkable. Tr increased with the increase of light intensity and SWC. However, when one of the environmental stresses of illumination and water existed, the improvement of the other condition couldn't make Tr rise greatly. Only when SWC was higher than 11%, which arrived at over 55% of the field content (FC), or the photosynthetic active radiation (PAR)higher than 400 μmol·s^-1m^-2, Tr could rise greatly with the increase of PAR or SWC.But when SWC was higher than 15%, which reached over 75% of FC or PAR higher than 1 000 μmol·s^-1·m^-2, Tr would not change a lot with the change of PAR or SWC. That PAR and SWC influenced the magnitude of stomatic resistance(RS)and leaf water potential (Ψ_l)was the basic reason for the Tr responded to them. Light stress reduced the open degree of stomas, so when severe light stress existed (PAR < 100 μmol·s^-1·m^-2), RS was la rger (RS > 2.0 s·cm^-1), which led to the decrease of Tr(Tr < 5 μg H₂O·s^-1·cm^-2). When severe water stress existed(SWC < 11% and < 55% of FC and soil water potential Ψ_ws < -1.15 MPa), RS was higher than 4.00 s·cm^-1 and Ψ_l lower than -2.10 MPa, which led to Tr lower than 11 μg H₂O·s^-1·cm^-2. When soil water was adequate(SWC > 15% amd over 75% of FC, and Ψ_ws > -0.50 MPa), RS was lower than 2.00 s·cm^-1, Ψ₁ higher than -1.65 MPa and Tr would be higher than 15 μgH₂O·s^-1·m^-2. The range of SWC, 11%~15%, which accounted for 55% to 75% of FC, and correspond RS(2.00~4.00 s·cm^-1)were the turning area, where the variable curve of Tr transited from a variable trend to another variable one. It could be considered as the range to control soil water.