Department of Viticulture and Enology, University of California, Davis, California 95616.
School of Forestry and Environmental Studies, Yale University, New Haven, Connecticut 06511.
Plant Physiol. 2019 Apr;179(4):1658-1668. doi: 10.1104/pp.18.01303. Epub 2019 Feb 4.
Water discharge from stem internal storage compartments is thought to minimize the risk of vessel cavitation. Based on this concept, one would expect that water storage compartments involved in the buffering of xylem tensions empty before the onset of vessel cavitation under drought stress, and potentially refill after soil saturation. However, scant in vivo data exist that elucidate this localized spatiotemporal coupling. In this study on intact saplings of American chestnut (), x-ray computed microtomography (microCT) showed that the xylem matrix surrounding vessels releases stored water and becomes air-filled either concurrent to or after vessel cavitation under progressive drought stress. Among annual growth rings, the xylem matrix of the current year remained largely water-filled even under severe drought stress. In comparison, microtomography images collected on excised stems showed that applied pressures of much greater than 0 MPa were required to induce water release from the xylem matrix. Viability staining highlighted that water release from the xylem matrix was associated primarily with emptying of dead fibers. Refilling of the xylem matrix and vessels was detected in intact saplings when the canopy was bagged and stem water potential was close to 0 MPa, and in leafless saplings over the winter period. In conclusion, this study indicates that the bulk of water stored in the xylem matrix is released after the onset of vessel cavitation, and suggests that capillary water contributes to overall stem water storage under drought but is not used primarily for the prevention of drought-induced vessel cavitation in this species.
人们认为，从茎内储存室排出水分可以最大限度地降低空蚀风险。基于这一概念，人们预计在干旱胁迫下木质部张力缓冲所涉及的储水室会在空蚀发生之前排空，并在土壤饱和后重新充满。然而，目前还缺乏阐明这种局部时空耦联的体内数据。在这项对美洲栗完整实生苗的研究中，X 射线计算机微断层扫描（microCT）显示，在渐进性干旱胁迫下，与空蚀同时或之后，围绕导管的木质部基质会释放储存的水分并充满空气。在每年的生长轮中，即使在严重干旱胁迫下，当年的木质部基质仍主要充满水分。相比之下，从离体茎上采集的微断层扫描图像表明，需要施加远大于 0 MPa 的压力才能从木质部基质中释放水分。活力染色突出表明，木质部基质的水分释放主要与死纤维的排空有关。当树冠被包裹并且茎水势接近 0 MPa 时，在完整的实生苗中可以检测到木质部基质和导管的再填充，并且在冬季无叶的实生苗中也可以检测到再填充。总之，这项研究表明，木质部基质中储存的大部分水分是在导管空蚀发生后释放的，并表明在干旱条件下，毛细水有助于维持整体茎水储存，但不是主要用于防止该物种因干旱引起的导管空蚀。
