USDA-ARS, Crops Pathology and Genetics Research Unit, Davis, CA 95616, USA.
Tree Physiol. 2010 Jun;30(6):761-72. doi: 10.1093/treephys/tpq026. Epub 2010 May 6.
In the Central Valley of California, mature walnut trees afflicted with apoplexy disorder exhibit rapid and complete crown defoliation within a few weeks of symptom initiation. Symptoms are typically found throughout the entire crown and are initially expressed as wilting and chlorosis followed by scorching of leaves. Since the cause of apoplexy disorder is unknown, we set out to elucidate the water relations physiology underlying this condition. Stem water potential (Psi(s)) of healthy, asymptomatic trees remained high throughout the 2007 growing season, while that of apoplexy-afflicted trees decreased significantly with the onset of symptoms for both healthy-appearing and symptomatic portions of these trees. Psi(s)s were significantly reduced by at least 0.7 MPa in the lower, middle and upper portions of the symptomatic canopies compared with those from asymptomatic trees. Heat pulse velocities measured in the main trunk at three radial depths consistently decreased prior to the onset of symptoms and eventually reached zero with complete crown defoliation. Comparison of sap flow with predicted water use based on walnut evapotranspiration suggests that stomata of symptomatic trees were closing at higher evaporative demand prior to symptom formation. Specific hydraulic conductivity (K(s)) of symptomatic stems was significantly lower than that of asymptomatic stems, and no detectable K(s) could be measured on several of the symptomatic stem samples. However, shallow root K(s) did not significantly differ between symptomatic and asymptomatic trees, suggesting that hydraulic failure was isolated to the crown of these grafted trees. Light and scanning electron microscopy of stem and trunk sapwood revealed extensive tylose development in vessels throughout the crown of symptomatic trees. Analysis of the formation of tyloses on multiple dates revealed rapid development of these vessel occlusions in conjunction with visual symptom formation and dramatic decreases in sap flow. In 2008, tylose development was associated with elevated ethylene production in the active sapwood of symptomatic trees. The cause of elevated ethylene associated with tylose production and symptom formation is yet to be determined.
在加利福尼亚中央山谷，患有中风失调症的成熟核桃树在症状开始后的几周内迅速完全落叶。症状通常出现在整个树冠上，最初表现为萎蔫和黄化，随后叶片烧焦。由于中风失调症的病因尚不清楚，我们着手阐明这种情况的水关系生理学。在 2007 年整个生长季节，健康、无症状树木的茎水势 (Psi(s)) 一直保持高位，而患有中风失调症树木的 Psi(s) 在这些树木的健康和有症状部分出现症状时显著下降。与无症状树木相比，症状树冠的下部、中部和上部 Psi(s) 至少降低了 0.7 MPa。在三个径向深度的主干上测量的热脉冲速度在症状出现前一直下降，最终随着树冠完全落叶而降至零。与基于核桃蒸散的预测耗水量相比， sap 流量的比较表明，在症状形成之前，有症状树木的气孔在更高的蒸发需求下关闭。有症状茎的比水力导率 (K(s)) 明显低于无症状茎，并且在几个有症状的茎样本上无法测量到可检测的 K(s)。然而，有症状和无症状树木的浅层根 K(s) 没有显著差异，这表明水力衰竭仅限于这些嫁接树木的树冠。茎和树干边材的光和扫描电子显微镜显示，症状树冠中的所有导管中都有广泛的侵填体发育。对侵填体形成日期的分析表明，这些导管闭塞在与视觉症状形成和 sap 流量急剧下降同时迅速发展。2008 年，与有症状树木活跃边材中乙烯产量升高相关的侵填体发育。与侵填体形成和症状形成相关的乙烯升高的原因尚未确定。
