Abstract: In order to study the spatial differences in assimilation capacity and their impact factors and provide the scientific basis for variety improvement and field management in the Loess Plateau,the diurnal dynamics of photosynthetic characteristics in different alfalfa leaf layers were monitored using portable LI-6400 photosynthesis system in 2006.The results show that the curves of diurnal variation in net photosynthetic rate(Pn) and transpiration rate(Tr) of different leaf layers had three peaks,and an obvious midday depression appeared at 12:00 o’clock.There was a significant difference in Pn as the upper leaf layer>the middle leaf layer>the lower leaf layer;a distinct difference in intercellular CO₂ concentration(Ci) as the lower leaf layer>the middle leaf layer>the upper leaf layer;an opposite trend for stomal limit value(Ls) to Ci.;no distinct difference in Tr among different leaf layers.Based on the tendency of changes in Pn,Ci,and Ls,it was speculated that Pn depression was controlled by stomatal factors before the highest PAR,and then by non-stomatal factors.Alfalfa had different photosynthetic characteristics for leaf layers: the upper was a high photosynthetic layer;the lower was a limit both for photosynthesis and light energy capture;the middle layer between the upper and the lower layers.

Key words: Alfalfa, Leaf layer, Diurnal dynamics, Net photosynthetic rate, Transpiration rate, Intercellular CO₂ concentration, Stomatal limit value

摘要： 为了解紫花苜蓿(Medicago sativa L.)冠层同化能力的差异及影响因子,为其品种改良和田间管理提供依据,用LI-6400型便携式光合测定仪于2006年观测了不同叶位叶片光合作用日变化特征。结果表明:现蕾期苜蓿不同叶位叶片的净光合速率(Pn)、蒸腾速率(Tr)日变化均呈现"三峰"曲线,有明显的光合"午休"现象;不同叶位之间Pn存在极显著差异(P<0.01),高低表现为上位叶>中位叶>下位叶;且胞间CO₂浓度(Ci)存在显著差异(P<0.05),表现为下叶位>中位叶>上位叶,气孔限制值(Ls)与Ci表现出相反趋势,即上位叶>中位叶>下位叶,Tr差异不显著;根据Pn、Ci、Ls的变化方向,苜蓿上、中、下叶位光合速率的下降在光合有效辐射(PAR)达到全天最大值之前受气孔因素限制,之后受非气孔因素限制;苜蓿叶片的光合速率具有分层的特点:上层强光区为高光合速率层,下层弱光区为低光合速率维持层,中层为中光区,光合速率介于上层与下层之间。

关键词: 苜蓿, 叶位, 日变化,净光合速率, 蒸腾速率, 胞间CO₂浓度, 气孔限制值