中文摘要：

【目的】研究春、夏两季连翘叶片光合作用多级土壤水分梯度的响应过程,阐明连翘光合作用的土壤水分阈值效应和生产力分级范围,并进行季节间比较,为春、夏季土壤水分胁迫下连翘的风险诊断和田间植物的土壤水分管理提供科学依据。【方法】以2年生连翘苗木为材料开展温室盆栽试验,通过测定人工给水后植物自然耗水的方法分别获取春季14组和夏季10组土壤水分梯度,利用Li-6400光合作用系统测定2个季节连翘叶片的光合作用光响应过程,并以净光合速率（P_n）和水分利用效率（WUE）分别作为评价土壤水分对连翘的＂产＂、＂效＂指标,研究2个季节连翘的土壤水分阈值效应及其生产力分级。【结果】1）当春、夏季土壤相对含水量（RSWC）分别降低至37.5%和46.2%时,随着RSWC继续降低,P_n和WUE显著降低,连翘光合作用的气孔限制转为非气孔限制,P_n和Gs之间由线性正比关系转为非线性正比关系;2）春、夏季P_n,WUE对RSWC的响应过程均可用多项式较好地拟合,由此可确定2个季节连翘的P_n水分补偿点、P_n均值水分点、WUE均值水分点、P_n水分饱和点和WUE水分高效点,借助坐标轴图示,可将土壤水分对连翘的有效性分为无产无效水（春季RSWC≤26.1%,夏季RSWC≤26.4%）、低产低效水（春季RSWC为26.1%～37.5%,夏季RSWC为26.4%～46.2%）、中产中效水（春季RSWC为37.5%～39.5%或80%～100%,夏季RSWC为46.2%～47.8%或95.7%～100%）、高产中效水（夏季RSWC为47.8%～49.2%,83.7%～95.7%）和高产高效水（春季RSWC为39.5%～80.0%,夏季RSWC为49.2%～83.7%）;3）夏季土壤水分阈值（水分补偿点除外）均高于春季,有效水分范围（低产低效水、中产中效水、高产中效水、高产高效水）内夏季各级＂产-效＂水的上限、下限均高于春季,表明连翘对土壤水分含量的要求因生长发育时期不同而异。【结论】植物水分管理中要

英文摘要：

【Objective】Under the influence of global climate change,the Northern China is experiencing more and more severe volatility in soil water contents,which may significantly affect photosynthesis. The objective was to elucidate the threshold effects of photosynthesis in leaves of Forsythia suspense to soil moisture,clarify the threshold range of photosynthetic productivity,and define the regional water adaptability,by investing the photosynthetic responses of F.suspense to different soil water conditions in spring and summer. 【Method 】A potted experiment was carried out in a greenhouse with two-year-old F. suspense seedlings. During the experiment,14 levels of soil water regimes in spring and 10 levels in summer were applied with the artificial water supply and the natural water consumption by plants. At each soil water level in both spring and summer,light response of photosynthesis of F. suspensa was measured by using a Li- 6400 portable photosynthesis system. By defining the net photosynthetic rate（ P_n） and water use efficiency（WUE） of photosynthesis as the index of plant productivity and soil water efficiency,respectively,key thresholds and the grading of soil water in summer and spring were investigated. 【Result】The results showed that when the relative soil water content（ RSWC） decreased to 37. 5% in spring and to 46. 2% in summer,both P_nand WUE showed an obvious decrease with the continuous decreasing of RSWC. At the soil water point,the stomatal limitation changed into non-stomatal limitation in both seasons and the relationship between P_nand stomatal conductance（G_s） was also changed from linear relationship into non-linear relationship as RSWC decreased. All these indicated that when soil water decreased to the turning water point of stomatal mechanism of P_n（ RSWC_（SL→nSL））,P_nbegun to decrease seriously as the non-stomatal limitation dominated the decrease of photosynthesis. Thus,we defined RSWC_（SL→nSL）as the lower limit of soil water maintaining high produc