Abstract: The effects of different phosphorus levels on photosynthesis and root nodule nitrogen-fixing characteristic of ‘Gannong No.3' alfalfa were investigated using nutrient culture. Results showed that stomatal conductance and photosynthetic rate were markedly increased with increased phosphorus levels, whereas transpiration rate was decreased. Simultaneously, chlorophyll content, leaf area, leaf number, root nodule number, root nodule weight and nitrogenase activity of alfalfa were improved. Photosynthesis was best at the phosphorus level of 1500 μmol·L^-1. Transpiration rate, stomatal conductance, photosynthetic rate and CO₂ concentration reached 4.27 mmol·m^-2·s^-1, 347.54 mmol·m^-2·s^-1,10.37 μmol·mol^-1 and 307.28 μmol·mol^-1 at this phosphate level. Chlorophyll content, leaf area and leaf number were first increased then decreased. Leaf area reached the largest (1.21 cm²) at 1500 μmol·L^-1 of phosphorous. Leaf number was 26.50 at the 250 μmol·L^-1, and increased with more phosphorus. Root nodule number and root nodule weight increased with the increase of phosphorus level. Nitrogenase activity was the best (57.1 μg·g^-1·h^-1) at 1500 μmol·L^-1. Phosphorus level of 1500 μmol·L^-1 is optimum for alfalfa. Excessive high and low phosphorus levels all prejudice normal growth and development of Gannong No.3 alfalfa.

Key words: Phosphorus levels, Alfalfa, Photosynthesis, Nitrogenase activity

摘要： 采用砂培方法,在营养液条件下研究不同磷水平对甘农3号紫花苜蓿(Medicago sativa ‘Gannong No.3')光合作用和根瘤固氮特性的影响。结果表明:在其他营养充分供应的基础上,适当提高磷水平可以显著增大甘农3号紫花苜蓿叶片的气孔导度和光合速率,减小蒸腾速率,并提高叶片中叶绿素含量,增大叶面积,增加叶片数,同时使根瘤数目增多,根瘤重增加,固氮酶活性显著增强。光合作用在磷水平为1500 μmol·L^-1时最强,此时叶片蒸腾速率为4.27 mmol·m^-2·s^-1,气孔导度为347.54 mmol·m^-2·s^-1,光合速率为10.37 μmol·mol^-1,胞间CO₂浓度为307.28 μmol·mol^-1;叶绿素含量、叶面积和叶片数都随着磷水平的增大而呈现出先增大后减小的趋势,叶面积在磷水平为1500 μmol·L^-1时达到最大值1.21 cm²;叶片数在磷水平为250 μmol·L^-1时达到最大值26.50 个,继续增大磷浓度至1500 μmol·L^-1时叶片数仍显著高于对照;根瘤数和根瘤重随磷水平的增大持续增大;固氮酶活性在磷水平为1500 μmol·L^-1时最强达57.1 μg·g^-1·h^-1。综合考虑,磷水平在1500 μmol·L^-1时为最佳值,过高或过低的磷水平都不利于甘农3号紫花苜蓿的正常生长发育。

关键词: 磷水平, 紫花苜蓿, 光合作用, 固氮酶活性