水分胁迫对不同年龄沙地樟子松幼苗存活与光合特性影响

樟子松以其抗寒、抗旱和速生性,自20世纪50年代在科尔沁沙地南缘人工引种用于固沙造林试验成功以来,已成为我国北方荒漠化地区防风固沙造林的首选树种。然而,进入20世纪90年代以来,早期引种的沙地樟子松人工林出现了衰退现象;虽然从理论上分析,原因可能有病虫害、地理位置、水分条件、营林技术等,但其中水分条件应该是沙地樟子松人工林提早衰退的最重要的原因之一。以1~5年生樟子松幼苗为材料,采用盆栽控水处理法对苗木进行水分胁迫试验,观测水分胁迫条件下樟子松苗木成活与光合特征及其水分利用效率的变化。结果表明,樟子松苗木成活的临界土壤含水率随苗龄的变化没有显著差异,1~5年生苗木成活的临界土壤含水率均在1.5%~1.8%之间。4种不同水分胁迫处理(对照、20%、30%和40%田间持水量)对光合特性的影响为:轻度胁迫(40%田间持水量)时对光合特性的各个指标影响不大;随胁迫程度加重,光合速率、气孔导度、胞间CO2浓度和蒸腾速率逐渐降低;导致樟子松苗木光合速率降低的主要原因应是气孔因素,即在水分胁迫下,气孔的开张度减小,导致胞间CO2浓度和蒸腾速率下降,进而影响光合速率;另外,水作为光合作用的原料之一,当其供应不足时,也直接导致光合速率的降低。2年生、4年生的樟子松幼苗在相同的土壤干旱胁迫条件下,各生理指标比较接近,即生理指标与苗龄之间并没有表现出明显区别。樟子松苗木的水分利用效率在较重度胁迫(20%田间持水量,3.5%)条件下没有降低,而在轻度胁迫条件下,水分利用效率有升高趋势;表明樟子松在较低的土壤含水量条件下,具有忍耐一定干旱胁迫的能力。综合研究表明:樟子松只有在极度水分胁迫时(土壤含水率接近成活的临界土壤含水率值:对于1~5年生苗木约为1.7%)才会出现死亡,这对研究水分与樟子松人工林衰退关系具有参考价值。

Impact of water stress on survival and photosynthesis of Mongolian pine seedlings on sandy land

Mongolian pine(Pinus sylvestris L.var.mongolica Litv.) is one of the key tree species in plantation establishment.Due to well adapted nature for drought condition,it has been broadly introduced on sandy land in "Three North" regions of China(north,northwest and northeast).Since environmental conditions of sandy land ecosystems are not very favorable for all kind of vegetation development due to poor water holding capacity and less soil moisture level in the soil;such a problem as a rapid decline has occurred in the earliest Mongolian pine plantations on Keerqin sandy land located at Zhangwu county,Liaoning province,northeastern China.In order to examine the effects of water stress on survival and rate of photosynthesis for Mongolian pine seedlings,we established an experiment on sandy land.We measured the drought tolerance characteristics and the responses of water stress levels on the photosynthetic efficiency of Mongolian pine seedlings at different ages.Results indicated that the threshold value of soil water content for Mongolian pine seedling survival ranged from 1.5 to 1.8%(8.6 to 10.3% of the field capacity).However,these values were not significantly different among seedling ages.In the controlled environment study,there were three watering regimes which were watered at various levels of soil water content,i.e.,40,30 and 20% field capacity,respectively.The instantaneous photosynthesis rate(IPR) in response to water stress showed that IPR decreased with increasing level of water stress.The stress of 40% field capacity did not influence IPR of the seedlings. The physiological variables of stomatal conductance,CO_2 concentration inside the cell and transpiration rate showed a same tendency to IPR under the water stress.These results may be explained by the changes in stomatal conductance induced by soil water depletion.In addition,as the material of photosynthesis,when water is not sufficient,IPR also decreased.Therefore,there were no significant differences on IPR and physiological variables between 2-year old seedlings and 4-year old seedlings under different water stress.However,the water-use efficiency(WUE) of seedlings was not substantially influenced under the stress of 40% field capacity,while increased under the water stresses of 30% and 20% field capacity,respectively.This result evidently indicated that Mongolian pine is a drought tolerance,hardy species in nature;as well as exhibited saving water-use strategy in a sandy land ecosystem.