土壤水分状况对小麦叶片超声波信号发射及木质部脆弱性的影响

土壤干旱引发植物木质部导管发生“空穴化”，对植物生长发育及抗旱生理具有重要影响。本研究通过检测小麦叶片超声波（AE）信号发射的变化，在田间充分灌水及控水条件下对冬小麦（Triticum aestivum L.）品种石新733主要生育期叶片木质部导管空穴化及气栓塞发生特性进行了研究。结果表明，叶片AE信号一般6:00~8:00开始发生，夜间21:00~3:00期间停止。土壤干旱时植株叶片木质部气栓塞发生程度显著高于良好供水的植株。拔节至灌浆盛期控水处理24 h AE 总量高于足水处理，其中开花期至籽粒形成初期处理间差别最大，控水处理24 h AE平均为足水处理的4.6倍。但籽粒灌浆中后期控水处理土壤水分胁迫进一步加重时，24 h AE 反而下降至接近甚至低于足水处理，出现“AE衰减现象”。叶片AE累积发生值和叶水势(Ψl)的变化关系符合Logistic曲线模型（脆弱曲线）。拟合结果表明，一般小麦Ψl降至约－1.00 MPa 时导管空穴化发生速率最高。土壤干旱条件下植株叶片木质部脆弱性显著增高，木质部安全性明显降低。土壤干旱时叶片空穴化和气栓塞增高的主要原因是导管对空穴化的抵御能力下降，而叶水势的影响较小。

Effects of Soil Water Conditions on Wheat Xylem Vulnerability and Leaf Ultrasound Acoustic Emission

Water stress-induced xylem embolism has important physiological consequences for plant growth and survival in most species. Effects of soil water conditions on leaf xylem cavitation and embolism occurrence were investigated throughout the major growth stages in field-grown winter wheat (Triticum aestivum L. cv. Shixin 733) plants. The xylem cavitation and embolism were measured by monitoring ultrasonic acoustic emission (AE) from leaf xylem. It was found that daily AE generally started around 6:00–8:00 a.m. and ceased between 21:00 p.m. and 3:00 a.m. of the following day. Embolism occurred to a much greater extent in leaves of water stress (WS) plants than in well-watered (WW) control. The total number of AE occurred in a period of 24 hours (24 h AEs) from WS plants was higher than that from WW control between jointing and mid grain filling stage. The difference between treatments peaked between anthesis and initial grain filling, with 24 h AEs being 4.6 times greater in WS than in WW plants. The number of 24 h AEs from WS plants, however, dropped greatly to values similar to or even lower than that of WW plants under more severe soil water stress after mid grain filling, which phenomenon is termed “AE depletion”. Vulnerability curves, which were fitted to Logistic function by plotting cumulative AEs versus corresponding leaf water potential (Ψl), showed that leaf xylem vessels were generally most vulnerable at Ψl of about －1.00 MPa. Soil water stress led to much more vulnerable leaf xylem, which greatly reduced leaf embolism resistance and consequently impaired xylem safety. The higher degree of cavitation and embolism occurrence in WS plants resulted basically from a more vulnerable water conducting system, and much less from the greater xylem water tensions, as compared with WW control.