城市遮阴环境变化对大叶黄杨光合过程的影响

城市遮阴与自然生态系统遮阴环境不同,它由城市中的建筑、基础设施等人工构筑物所产生,并对植物光合过程产生影响。采用Li-6400便携式光合作用测定系统对北京市不同建筑遮阴环境下的生态因子特征及大叶黄杨光合参数特性进行了现场实验对比观测与分析。研究显示:(1)城市建筑或基础设施布局或格局变化导致建筑遮阴环境光、温等生态因子的差异性变化,主要体现在光合有效辐射(PAR)在时间分布及强度上的显著差异。(2)遮阴改变大叶黄杨的光合参数,日净光合速率曲线为不规则单峰型,由遮阴进入太阳直射时,净光合速率(Pn)明显升高,随遮阴程度加重峰值时间有后延现象,完全遮阴时净光合速率最低。(3)大叶黄杨叶片在有一定时间太阳直射的中度遮阴(2.3h和5.5h)下适应性良好,日均净光合速率、光补偿点(LCP)、光合能力(P_nmax)、叶面积(LA)、比叶重(LMA)没有显著差异,植物主要通过光合色素含量及组成的改变来适应太阳直射时间变短(2.3h)的遮阴环境。(4)遮阴改变植物的光暗适应策略,没有阳光直射的重度遮阴下,大叶黄杨通过其光合、形态、光合色素等多方面的改变来适应遮阴环境,建筑重度遮阴严重影响植物光合过程。探讨了不同程度建筑遮阴对植物光合作用的影响及植物的适应机制,为基于提高其生态服务功能的树种配置和城市规划提供科学依据。

Effects of urban shading on photosynthesis of Euonymus japonicas

Urban shading is caused by urban artificial constructive structures and has definite effects on the photosynthetic processes of plants. We used Li-6400 portable photosynthesis system to study the changes in ecological factors and photosynthetic characteristics of Euonymus japonicas planted in different levels of urban shading in Beijing. The results showed that: (1) The pattern of buildings or infrastructures' layout resulted in the differences of ecological factors under different levels of urban shading, mainly represented by temporal distribution and intensity of photosynthetically available radiation (PAR) and atmospheric temperatures. (2) Different levels of urban shading caused the changes in several main photosynthetic parameters of E. japonicas such as net photosynthesis (Pn),presenting an irregular single-peaked curve. When urban light environment changed from shading into direct sunlight, Pn showed an obvious increase and reached a peak value rapidly. The time point of the maximum value of Pn was delayed with the increasing level of shading. The Pn value of plants in the highest level of urban shading (S4) was the lowest in the four sites. (3) The leaves of E. japonicas showed a good ecological adaptability under a moderate urban environment of shading with a certain length of time (2.3 hours and 5.5 hours) when exposed to direct sunlight, and there was no significant variability among some indicators such as daily average of Pn, light compensation point (LCP), P_nmax, leaf area (LA) and leaf mass per area (LMA). The leaves of E. japonicas well adapted to the changes in urban shading environment with a shortened period ( to 2.3 h) of direct sunlight mainly by changing the contents and composition of photosynthetic pigments, namely, Chl a and Chl b, Chl a+b and Car/Chl. (4) Different levels of urban shading caused the changes in adaptive strategies of E. japonicas. When there was no direct sunlight appearing, a highest level of shading, E. japonicas presented obvious adaptive changes in its physiological photosynthetic processes, morphology, photosynthetic pigments and so on, and this type of the greatest shading caused by urban buildings or other infrastructures can obviously affect the growth of plants.