Effects of soil water and nitrogen availability on photosynthesis and water use efficiency of Robinia pseudoacacia seedlings

The efficient use of water and nitrogen （N） to promote growth and increase yield of fruit trees and crops is well studied.However,little is known about their effects on woody plants growing in arid and semiarid areas with limited water and N availability.To examine the effects of water and N supply on early growth and water use efficiency （WUE） of trees on dry soils,one-year-old seedlings of Robinia pseudoacacia were exposed to three soil water contents （non-limiting,medium drought,and severe drought） as well as to low and high N levels,for four months.Photosynthetic parameters,leaf instantaneous WUE （WUEi） and whole tree WUE （WUEb） were determined.Results showed that,independent of N levels,increasing soil water content enhanced the tree transpiration rate （Tr）,stomatal conductance （Gs）,intercellular CO2 concentration （Ci）,maximum net assimilation rate （Amax）,apparent quantum yield （AQY）,the range of photosynthetically active radiation （PAR） due to both reduced light compensation point and enhanced light saturation point,and dark respiration rate （Rd）,resulting in a higher net photosynthetic rate （Pn） and a significantly increased whole tree biomass.Consequently,WUEi and WUEb were reduced at low N,whereas WUE i was enhanced at high N levels.Irrespective of soil water availability,N supply enhanced Pn in association with an increase of Gs and Ci and a decrease of the stomatal limitation value （Ls）,while Tr remained unchanged.Biomass and WUEi increased under non-limiting water conditions and medium drought,as well as WUEb under all water conditions;but under severe drought,WUEi and biomass were not affected by N application.In conclusion,increasing soil water availability improves photosynthetic capacity and biomass accumulation under low and high N levels,but its effects on WUE vary with soil N levels.N supply increased Pn and WUE,but under severe drought,N supply did not enhance WUEi and biomass.

Photosynthesis and Water Use Efficiency of Platycladus Orientalis and Robinia Pseudoacacia Saplings under Steady Soil Water Stress during Different Stages of Their Annual Growth Period

A simulated drought experiment was conducted in a rain-free shed to test the physiological response of Platycladus orientalis and Robinia pseudoacacia saplings to steady soil water stress during different stages. The five soil water treatments were： 100%, 87.84%, 70%, 52.16% and 40% of field capacity. The results showed that the net photosynthetic rate of R. pseudoacacia decreased as soil water potential decreased in the range between -0.041 MPa and -0.292 MPa. The threshold value at which the net photosynthetic rate changed significantly was -0.12 MPa. The relationship between net photosynthetic rate of P. orientalis and soil water potential could be described as a quadratic parabola in the range between -0.041 MPa and -0.648 MPa. Analysis of variance showed significant differences in the net photosynthetic rate of P. orientalis between soil water potentials of -0.061 MPa ~, -0.648 MPa. Average water use efficiency （WUE） increased as soil water potential decreased, but the influence mechanism of soil water stress on leaf WUE and photosynthetic rate for the two species were different evidently.