Abstract: The change of the root system hydraulic conductivity and photosynthetic parameters in alfalfa(Medicago sativa L var.Longdong and Algonquin) seedlings were measured under water stress(mimicked by PEG-6000 solution,ψ_s=-0.2 MPa,stress time:48 h) and subsequent rehydration(time:48 h).These measurements were used to investigate photosynthetic response mechanism,change regulation on ability of root water uptake and relationship of above-and underground under variable water conditions in alfalfa.The results showed that the ability of root water uptake declined rapidly in the initial phase and slowly in the later phase,indicating that the root system hydraulic conductivity(Lp_r) was affected obviously by water stress and rehydration.The Lp_r declined with prolonged water stress.Simultaneously,the stomatal conductance(Gs),transpiration rate(Tr),net phorosynthetic rate(Pn) and leaf water potential declined gradually with the reduction of Lp_r and intercellular CO₂ concentration(Ci) decreased at first and accumulated at the end.After rehydration,Lp_r recovered slowly.Pn,Gs,Tr and leaf water potential increased and Ci decreased gradually with the recovery of Lp_r.All the parameters except Ci did not recover to the level of control.The extent of Lp_r recovery and the photosynthetic parameters suggested that the tolerance of water stress in alfalfa was weak,while that of Longdong was stronger than Algonquin.

Key words: Water stress, Rehydration, Root system hydraulic conductivity, Photosynthetic parameters, Alfalfa

摘要： 变水条件下(利用PEG-6000模拟水分胁迫48 h,ψs=-0.2 MPa,之后复水48 h),测定紫花苜蓿(Medicago sativa L,品种阿尔冈金和陇东苜蓿)幼苗根系水力学导度与光合参数变化规律,旨在研究变水条件下紫花苜蓿光合作用的响应机制、植株水分吸收能力的变化规律及地上与地下部可能的相关关系。结果表明:干旱胁迫使得紫花苜蓿根系水力学导度(Lp_r)受到显著影响。随着水分胁迫时间的延长,根系吸水能力呈现快速——缓慢下降趋势,即根系水力学导度逐渐下降,气孔导度(Gs)、蒸腾速率(Tr)、净光合速率(Pn)和叶片水势亦随之而显著降低,胞间二氧化碳浓度(Ci)先随之降低但最终上升而累积。复水后,根系水导呈现缓慢——慢速的恢复趋势,Pn、Gs、Tr、叶片水势随着根系水导速率的增加而逐渐恢复;Ci则随复水时间的延长而逐渐下降。但各参数除Ci外,均没有恢复到胁迫前水平。紫花苜蓿根系水导与光合参数在复水后的恢复程度说明,紫花苜蓿对干旱逆境的抵御与适应能力相对较弱,但陇东苜蓿对水分胁迫的忍耐能力强于阿尔冈金。

关键词: 干旱胁迫及复水, 根系水导, 光合参数, 紫花苜蓿