Abstract: Growth characteristics of three alfalfa (Medicago sativa L.) cultivars (Eureka, Victoria and Ameristand 201) under shading were studied in summer at climate transitional regions. Results showed that the daily average of photosynthetic active radiation(PAR)and air temperature(Ta)significantly decreased, whereas the daily average of relative air humidity (RH) had no obvious change with light intensity decreasing. The leaf withering and plant death rates of three alfalfa cultivars first decreased then increased, while the biomasses of root, stem, leaf and individual had an opposite trend. The ratios of aboveground to underground biomass increased significantly with light intensity decreasing. The variation ranges of growth characteristics differed among tested cultivars under shading. Correlation analysis showed that the individual biomass of alfalfa had significant positive correlation with root, stem, and leaf biomass, and the individual biomass of Eureka had distinct positive and negative correlation with the daily average of Ta and RH, respectively. These results indicated that mild shading could alleviate the stress of high temperature and high light intensity and increase the individual biomass and plant survival rate of alfalfa in summer at climate transitional regions,but that might reduce the reproductive capacity of alfalfa.

Key words: Alfalfa, Summer, Shading, Leaf withering rate, Plant death rate

摘要： 采用遮阴试验，研究了我国南北气候过渡地区3个不同紫花苜蓿（Medicago sativa L.）品种（游客、维多利亚和巨人201）在夏季不同遮阴条件下的生长特性。结果表明：越夏期随遮阴程度增加，紫花苜蓿光合有效辐射（PAR）和大气温度（Ta）日平均值明显降低，大气相对湿度（RH）日平均值无明显变化；3个紫花苜蓿品种的叶片凋落率和植株死亡率均呈先下降后上升的趋势，根、茎、叶及单株生物量则先上升后下降，地上部生物量和地下部生物量的比值随光强减弱明显上升，品种间各参数变幅存在较大差异。相关分析结果表明，越夏期各苜蓿品种单株生物量与根、茎、叶生物量成极显著正相关；游客单株生物量与Ta和RH日平均值分别成显著正相关和负相关，其他品种与各生态因子无显著相关性。说明我国南北气候过渡地区夏季高温条件下，轻度遮阴可减轻高温强光对苜蓿生长的胁迫，有利于增加营养体生物量的积累，提高其越夏率，但可能以降低其生殖能力为代价。

关键词: 紫花苜蓿, 越夏期, 遮阴, 叶片凋落率, 植株死亡率