MORPHOLOGICAL ANALYSIS ON THE MECHANISM OF NITRATE-INHIBITION OF ROOT NODULE FORMATION IN SOME LEGUMES ASSOCIATED WITH DIFFERENT RHIZOBIA

硝酸盐对轮叶牡丹和狗尾草根瘤形成初期的抑制作用用显微镜玻片培养法和生长袋培养法对其进行了研究。主要研究结果如下：1.根瘤的形成受硝酸盐浓度的影响。接种后7d，接种硝酸盐的根瘤数在5 mm处最少，0 mm处最多。硝酸盐施用量为5 mm时，侧根数显著减少。这种处理还抑制了根瘤菌在根表面的附着，X-Gluc染色显示根表面有蓝色的区域。树脂包埋切片观察到侧根组织中有根瘤原基的形成。琼脂包埋切片观察表明，硝酸盐浓度为5 mM时，根瘤发育明显延迟。在生长袋中生长的两种狗牙根的根瘤形成和固氮酶活性均受到了硝酸盐的抑制。施氮量为10 MMN的硝酸盐还降低了玉米根的干重、表面积和根长，说明硝酸盐通过抑制根瘤的生长而抑制根瘤的形成。用显微镜载玻片或生长袋培养技术可能更方便地阐明供试豆科植物根系结构与根瘤形成之间的抑制关系。关于硝酸盐对根瘤菌侵染过程和抑制作用的机理的进一步研究正在进行中。

Inhibitory effects of nitrate on initial stages of root nodule formation in S. rostrata and Crotalaria sp. were investigated by using a microscope slide culture method and a growth pouch culture method. The results are summarized as follows ;1. In S. rostrata, formation of root, nodules was affected by the nitrate concentration. Seven days after Inoculating, the number of nodules were the lowest in the presence of nitrate at 5 mM, and the highest at 0 mM. Nitrate applied at 5 mM markedly reduced the number of lateral roots. This treatment also inhibited rhizobial attachment to the root surface, on which the blue colored area was detected by X-Gluc staining. Formation of root nodule primodia in the lateral root tissue was observed in resin-embedded sections. Observations of the agar-embedded sections revealed that the nodule development was definitely delayed in the presence of nitrate at 5 mM.2. Root nodule formation and nitrogenase activity in two species of Crotalaria grown in a growth pouch were inhibited by application of nitrate to culture solution. Dry weight, surface areas, and total length of the roots of C. spectabilis were also reduced by application of nitrate at 10 mMN, suggesting that nitrate inhibited root nodule formation through reduction of the root growth.3. The culture techniques with a microscope slide or a growth pouch might be rather convenient to clarify the inhibitory relationship between the structure of root system and nodule formation in the tested legume species. Further investigation of the mechanisms on the rhizobia-infection process and inhibitory effect of nitrate is now in progress.