基于液流和叶片水势测定的沙柳水分利用特征分析

Analyses on water use characteristics of Salix psammophila based on sap flow and leaf water potential

【目的】干旱半干旱区植物的水分传输过程依赖于一套有效的调控机制,研究典型沙生植物对土壤干旱的响应机制有助于预测未来气候条件下荒漠生态系统的结构和功能变化。【方法】采用包裹式茎流仪于2017年5-10月对毛乌素沙地沙柳液流进行长期连续观测,期间选择天气晴朗的19 d测得黎明前叶水势与正午叶水势,同步连续监测林冠上方太阳辐射、气温、空气相对湿度与土壤含水量。【结果】(1)短期内土壤水势与枝条液流具有较好的相关性(5-6月、7-9月),整个生长季内液流密度与土壤叶片水势差(ΨL-ΨS)呈正相关。(2)叶片水势(ΨL)、大气水汽压亏缺(VPD)均对叶片蒸腾速率(EL)有调控作用,小于阈值呈正相关,大于阈值呈负相关。VPD对EL调控阈值为1.9 kPa,ΨL对EL调控阈值为-3.7 MPa。VPD对叶片导度(g L)有显著调控作用,调控阈值为0.9 kPa,小于阈值呈正相关,大于阈值呈负相关。(3)沙柳木质部栓塞脆弱性曲线呈"r"形,引起枝条50%导水率损失的压力值(P50)为0.73 MPa。(4)整体上看,正午木质部传导度(Ks)与正午叶片传导度(g L)是正相关的。K_s与黎明前叶水势(ΨS)呈现正相关关系,而g L受VPD影响较大导致其与Ψ_S相关性弱。g_L与K_s对于土壤干旱(Ψ_S降低)的相对敏感性(?)为1.035。【结论】以上结果表明,随着水分胁迫加重,木质部在水势较高时便通过降低导水率来减少水分丧失,木质部栓塞到一定程度也不关闭气孔,而是能保持一定的气孔开度。这些适应策略在一定程度上反映了沙柳最大化蒸腾和同化速率的一种机制,对正确认识干旱地区沙柳的水分利用特征有明显的理论意义,为深入研究沙柳的水力限制补偿机理奠定基础。

[Objective] In arid and semi-arid areas, water transfer process of plants is regulated by a set of effective mechanisms. Understanding the mechanisms of responses of xerophytic plants to soil drought is important for predicting the structure and functioning of dryland ecosystems under changing climate.[Method] The stem sap flow of Salix psammophila was monitored continuously using five heat balance sensor, and the leaf water potential of the plant was measured at predawn and midday in nineteen sunny days during 1 May to 10 October, 2017. The photosynthetic active radiation above canopy, air temperature, air relative humidity, and soil moisture content were monitored simultaneously. [Result] The results were that the sap flux density and soil water potential were highly correlated in short term(May to June, July to September). The sap flux density and the water potential drop(ΨL-ΨS) were positively correlated during the growing season. Leaf transpiration rate increased with leaf water potential and vapor pressure deficit(VPD),respectively, saturating at-3.7 MPa and 1.9 kPa, leaf conductance for water vapour increased positively with VPD, saturating at 0.9 kPa, then decreasing with these variables when greater than their respective threshold. The vulnerability curve was "r" shape for Salix psammophila. The water potential, at which 50%of hydraulic conductivity was lost as a result of xylem embolism(P50), was 0.72 MPa. The whole-branch hydraulic conductance per unit basal sapwood cross-sectional area(Ks) increased with soil water potential(ΨS) and leaf conductance(g L). The correlation between g L and ΨS was low because of the influence of VPD on g L. The relative sensitivity of stomata and plant hydraulic conductance to declining soil water potentials(?) was 1.035. [Conclusion] The results show that as water stress develops, Salix psammophila controls the water loss by reducing the Ks and certain level of xylem embolism does not induce the closure of the stomata. This mechanisms may be advantageous in terms of maximizing transpiration and assimilation rates.It has obvious theoretical significance to understand the water use characteristics of plants in arid areas, and lays a foundation for further study on the mechanism of the compensation for hydraulic limitation of Salix psammophila.