极端干旱环境下的胡杨木质部水力特征

Xylem hydraulic traits of Populus euphratica Oliv. in extremely drought environment

胡杨作为我国西北干旱区重要的乔木树种,研究其木质部水力特征对了解此树种适应极端干旱环境的生物学背景具有重要意义。以塔里木河下游的胡杨成株和2年生胡杨幼株为研究材料,对其木质部最大导水能力(ks(max))和自然栓塞程度(PLC)等木质部水力特征及其水力特征有关的木质部导管(或管饱)数量特征进行研究。结果表明,成株胡杨多年生枝条和侧根(2≤d<5 mm)木质部自然栓塞程度均较高,PLC均值高于50%,其中多年生枝条栓塞程度具有一定的日变化规律,6:30—7:30的PLC均值(58%)小于14:00—15:00的(67%);河道边上成株胡杨侧根的均ks(max)和PLC均值都小于距河道200 m处的。随着土壤干旱程度的加剧,幼株胡杨侧根的PLC均值随之增加,而叶片气孔导度随之降低,土壤含水率与侧根PLC均值,叶片气孔导度之间分别存在显著负相关关系(R=-0.9、R=-0.811)。在统一直径范围内(2≤d<5 mm),成株胡杨侧根均导管直径(dmean)和水力直径(d95%、dh)均大于胡杨幼株,而导管密度胡杨幼株高于成株胡杨;胡杨侧根木质部ks(max)与均导管直径、水力直径之间具有显著正相关关系(R>0.9)。

During evolution, plant species developed various mechanisms to enhance their drought adaptation, including physiological adjustment and structural modification. Plants under arid conditions regulate their water status using several tactics such as leaf biochemical and stomata characteristics as well as xylem sap flux. According to the cohesion-tension theory, water moves through drought season increases the xylem under tension during transpiration and therefore, rapid transpiration during the risk of xylem cavitations. This study was carried out on Populus euphratica Oliv. , the mainpoplar species distributed in arid land of northwestern China. Two-year old seedlings of P. euphratica were cultivated in-situ at the ecological observation station and mature P. euphratica trees selected from the natural stands along the river bank on the lower reaches of Tarim River, southern Xinjiang, China. Xylem hydraulic traits including xylem maximum hydraulic conductivity （ks（max））, natural embolism level （PLC,%）, as well as lateral root anatomical structure were determined. Results revealed that the native embolism level in lateral roots and branches （2 ≤ d〈5 mm）of mature P. euphratica trees were relatively higher with mean PLC value of more than 50%. Morning native embolism at branch level （ PLC 58% ） was lower than in the afternoon （ PLC 67% ）, but there were no significant differences （ P〉0. 05 ） between the morning and afternoon native branch xylem embolism. The ks（max） value and native embolism level in lateral roots of mature P. euphraticatrees at 200 m from the river bank were higher than those at the river bed, and there were significant differences （P 〈 0. 05 ）, between the native embolism level but not the k~~~~ value. With the an increased stress from soil drought, the lateral root native embolism level of P. euphratica seedlings increased while stomata conductance decreased, and there was a significant negative correlation between them （ R = -0. 9, R = -0. 811 ）. Anatomical structure of xylem in lateral roots in the same diameter class（2 ≤d〈5 mm） revealed that mean vessel diameter（ dm~, ） and mean hydraulic diameter （d95% , dh） of mature trees were significantly higher （P〈0.01） than that of its seedlings, and seedlings had higher vessel density than mature trees. Maximum hydraulic conductivity were closely correlated with mean vessel diameter and hydraulic diameter in lateral roots （R〉0.9）. Generally, P. eupratica growing at the arid land had relatively higher native embolism level, which is beneficial since it aids in preventing excessive water loss during transpiration and therefore affects positively the water status of the plant. This study of xylem hydraulic traits and its anatomical structure facilitates understanding of the biological background of P. euphratica and acclimatization to drought environment.