Investigation on the mechanisms of wilting and host defenses in mass mortality of oaks

病原菌Raffaelea quercivora接种栎树幼苗后，水电导率在接种部位附近明显降低，而远端部分电导率没有变化。当远端补水时，接种苗和离体苗的萎蔫发育均有所延迟，提示局部水分阻塞可能是萎蔫机制。接种真菌后，植株萎蔫前期蒸腾速率下降。木材变色/真菌的轴向扩散与枯萎病的发生没有关系。虽然离接种点很近的区域含水量下降，但无水传导的区域从干燥区向更远的区域延伸，说明该病害没有发生明显的空化现象。在没有水传导的情况下通过该区域的血管中观察到作为液上升阻塞的因素。在反应区屏障、木质部分子内层或闭塞层中，抗真菌活性明显增强。木质素样化合物和果胶分别是纤维区和小管管区主要成分。酚类、醌类和脂类也被观察到。接种后3 d木质部纤维闭塞，1周后形成空间连续屏障。2周后，泰氏菌群、幻化菌群和酚类物质积累显著增加。木质素含量在反应区屏障处增加。木质素的增加大部分归因于凝聚木质素，可能源于富含愈创木酰基的木质素。为研究壳斗科不同树种间防御反应的差异，在不同树种的幼苗上接种了槲皮菌。只有锯齿栎和脆皮栎幼苗枯萎。敏感性的差异与木材变色的切向扩散一致，而与轴向扩散不一致。少

Mechanisms of wiltingWater conductivity was conspicuously reduced near the inoculation site of oak seedlings inoculated with the pathogenic fungus Raffaelea quercivora, while the conductivity did not change in the distal part. Wilt development was delayed in both inoculated seedlings and excised seedlings when water was supplied to the distal part, suggesting local water blockage as the wilt mechanism. Transpiration decreased in the early stage of wilting after the fungal inoculation. No relationship was observed between axial spread of wood discoloration / fungi and wilt development. Although water content decreased in very adjacent area to the inoculation site, the area without water conduction extended further from dry zone, suggesting that no marked cavitation occur in this disease. Tyloses were observed in vessels which passed through the area without water conduction as the factor of sap ascent blockage.Defense responsesAlthough antifungal activity of extracts did not increase in … More reaction zone barrier, deposits lining or occluding xylem elements were remarkable. Lignin-like compound and pectin were major constituents in fiber region and in small vessel and tracheid region, respectively. Phenols, quinones and lipids were also observed. Fiber occlusion in the xylem was observed 3 days after the inoculation, and spatially continuous barrier was formed after one week. Tyloses, suberization and phenols accumulation became remarkable after 2 weeks. Lignin content increased at the reaction zone barrier. Most of increased lignin was attributed to condensed lignin, probably originated from guaiacyl-rich lignin.Characteristics in the responses of susceptible and resistant treesTo investigate the differences in defense responses among Fagaceae species, seedlings of several species were inoculated with R.quercivora. Only Quercus serrata and Q.crispula seedlings wilted. Differences in susceptibility coincided with tangential spread of wood discoloration, but not with the axial spread. Less