Effects of shading treatments on photosynthetic characteristics ofJuniperus sabinaAnt. seedlings

为了探明臭柏幼苗在不同遮荫处理下的光合特性和色素含量，通过盆栽实验，测定了0%、25%、50%、75%和90%四个遮荫处理下臭柏光合特性日变化和色素含量。结果表明：（1）随着遮荫率增加，臭柏净光合速率（Pn）、气孔导度（Gs）及蒸腾速率（Tr）随之减小，而胞间CO2浓度（Ci）随之增大，非气孔制限因素是臭柏光合速率下降的主要原因，且90%遮荫会影响臭柏幼苗的正常生长；（2）叶绿素a（Chl a）、叶绿素b（Chl b）和叶绿素总量（Chl a+b）均随着遮荫率增加而增大，Chl a/b则随着遮荫率的增大而呈下降趋势；（3）几乎所有遮荫处理组Gs均与Tr表现出不同程度的相关性，而Gs与Pn的相关性较差；所有遮荫处理组水分利用效率（WUE）与Pn均呈极显著相关，而与Tr相关性不显著，说明臭柏是通过较高的光合速率来实现高的水分利用效率以适应不同的光照条件。同时WUE和Ci也表现出不同程度的相关性。;Juniperus sabina seedlings germinated under Salix cheilophila shrub in the natural state. Therefore, seedlings were in shadow of weak light intensity in its young generation, with its growth, seedlings got rid of the shadow of S. cheilophila shrub gradually and formed pure shrubbery until it was exposed to strong light completely. During this process, J. sabina seedlings experienced the change of light intensity, temperature and soil moisture. A pot experiment with iron tailings as matrix was conducted to study the effects of different shading treatments (shading of 0%, 25%, 50%, 75% and 90%) on photosynthetic characteristics and chlorophyll contents of J. sabina seedlings for attempting to reveal the adaptability of seedlings to changing light conditions in the natural regeneration process. Based on the degree of lush folige of S. cheilophila shrub and the different locations of seedlings germinated in the natural state, the actual shade rate of seedling individuals were different. In order to simulate the changable shade environment of natural state and investigate the response of J. sabina seedlings to different light conditions, five shading gradient were set up and thirty potted seedlings of J. sabina were divided into five groups and placed in 0% (natural light), 25%, 50%, 75% and 90% shade rate conditions in turn. The photosynthetic indicators of J. sabina seedlings were measured two months later. In this study, the Li-6400 (LICOR,Inc. Lincoln,NE,USA) portable photosynthesis system and high performance liquid chromatography (HPLC) were employed to measure diurnal changes in photosynthetic indices and chlorophyll contents, respectively. Photosynthetic parameters were measured on each hour from 7:00 am to 18:00 pm, four pots of every treatment were selected to measured and three times determination of each pot. In the determination of photosynthetic parameters, leaves were clipped with scissors quickly at the time of 8:00, 12:00 and 17:00, respectively, wrapped with aluminum foil, saved in liquid nitrogen and brought back to lab for pigment extraction. The pigments were analyzed with a Spherisorb column (C18, 5 μm, 250 mm×4.6 mm), and were eluted using acetonitrile:methanol(85:15) and methanol:ethyl acetate(68:32) by a linear gradient. The pigments were detected by a spectrophotometer at 445 nm. The results showed as following: (1) With the increase of shading ratio, the net photosynthetic rate (Pn), stomatal conductivity (Gs) and transpiration rate (Tr) decreased in turn, while stomatal CO2 concentration (Ci) increased significantly. The principal cause of the reduction in photosynthesis was attributed to non-stomatal limitation, and the normal growth of J. sabina seedlings was affected by the shading of 90%. (2) With the increase of shading ratio, chlorophyll a (Chl a), chlorophyll b (Chl b) and the total chlorophyll content (Chl a+b) increased markedly, while the ratio of chlorophyll a to chlorophyll b (Chl a/b) decreased. (3) There were correlations between Gs and Tr in almost all shading treatments, while poor correlationships between Gs and Pn. In all shading treatments, water use efficiency (WUE) had a extremely significant correlation with Pn, while had no correlation with Tr, so J. sabina can improve its WUE in yielding its higher Pn so as to adapt to different light regimes. Meanwhile, there were correlations between WUE and Ci in different degrees.