Effects Of CO₂ Concentrations On Physiological Properties And Water Use Efficiency Of Ryegrass Under Phosphorus Deficiency

Phosphorus（P）is a vital macronutrient for plant growth and low P is one of the major growth-limiting factors for plants in many agricultural and natural ecosystems.As the global greenhouse effect increases,the concentration of carbon dioxide（CO₂）has been obviously enhanced from 280μmol mol^-1to currently 400μmol mol^-1 since the 19th Century industrial revolution,and may reach at 800μmol mol^-1by the end of the 21st Century,which is bound to have a significant impact on global agricultural production.Thus,global climate change and nutrient availability have been identified as two serious challenges to world food security and economic development.However,most previous studies on crop response to environmental changes have focused on single factors,such as CO₂ concentration,water,and nutrients,while studies on the effects of the interaction of elevated CO₂ concentration and low phosphorus stress on photosynthetic processes,water use efficiency,and leaf stomatal traits of annual ryegrass are still poorly understood.In the current study,to evaluate the interactive effect of doubling CO₂ concentration and temperature rise on annual ryegrass（Lolium multiflorum Lam.）growth and physiology,plants were grown in eight controlled growth chambers with a range of P supply（0.004,0.012,0.02,0.06,0.1 and 0.5 m M）under two levels of CO₂(400 and 800μmol mol^-1).We found that:（1）The stomatal density on the abaxial leaf surface of annual ryegrass was significantly increased by doubling CO₂ under sufficient P supply,while substantially decreased under P deficiency.Meanwhile,doubling CO₂ significantly reduced the stomatal length and width at P_0.1,which indirectly led to the reduction of stomatal openness.（2）In general,the spatial pattern of stomata distributed on leaves of annual ryegrass followed a regular pattern at small scales（<150μm）and a random distribution at larger scales（>200μm）.Additionally,doubling CO₂ resulted in more regular stomatal distribution pattern at small scales,while P deficiency resulted in more regular stomatal distribution pattern at ambient CO₂ concentrations.（3）Aboveground and total biomass of annual ryegrass gradually decreased with decreasing P supply,while belowground biomass and below/above biomass ratio increased.In addition,doubling CO₂ significantly increased aboveground biomass only when P supply was adequate,while P deficiency resulted in a significant decrease in belowground and total biomass as well as below/above biomass ratio.（4）P deficiency resulted in a significant decrease in net photosynthetic rate（P_n）and water use efficiency（WUE）of annual ryegrass with multiplication of CO₂ concentration.In addition,P_n and WUE of annual ryegrass were significantly increased by doubling CO₂under sufficient P supply,while obviously decreased under P deficiency.Furthermore,doubling CO₂ resulted in a significant decrease in stomatal conductance（G_s）at any level of P supply.（5）Doubling CO₂ reduced the P content of shoot and root of annual ryegrass and decreased the P content in each tissue with decreasing P supply.In addition,doubling CO₂resulted in a significant increase in shoot C/N ratio of annual ryegrass under P deficiency,but did not substantially affect the shoot C as well as root C and N.