Effect of nitrogen nutrition on the stability of orchid-syimbiont association

在螺旋藻与其菌根共生体的共生培养中，比较了氮源的适宜性。白蛋白是远优于无机氮的氮源。在24种氨基酸中，精氨酸仅是该氮水平下的一种适宜氨基酸，其他氨基酸虽然程度不同，但均较不适宜或有害。在面筋中，氮的阈值水平远远低于几种粉末状谷物，如燕麦或小麦。研究了芽尖褐变、原球茎死亡等病害的发生与原球茎和真菌菌丝中氮含量的关系。原球茎对面筋水平的响应因真菌而异：一种真菌处理的原球茎鲜重随着面筋水平的增加而显著降低，而另一种真菌处理的原球茎鲜重在同一氮水平下受影响不大。鲜重下降的原球茎中，氮浓度在原球茎中升高，在真菌体中降低。在健康原球茎的情况下，原球茎的侵染区平衡地占据不到原球茎截面的下半部分，因此共生关系得以维持，茎和根的伸长迅速进行。在遭受疾病的情况下，受感染的区域迅速增加，最终几乎占据了整个原球茎，导致原球茎死亡。这些观察结果表明，这种疾病是由共生向寄生转变的结果。

In the symbiotic culture of Spiranthes sinensis with its mycorrhizal symbionts, the suitability of nitrogen sources was compared. Alubumin was far superior nitrogen source to inorganic nitrogen. Among 24 amino acids tested, however, arginine was only suitable one at this nitrogen level, and all the others were less suitable or harnful although they varies in their degree. The threshold levels of nitrogen over which detrimental effects appear were far lower at gluten than at powdered sereal grains, e.g. oats or wheats.The relation between the occurrence of the disorder such as browning of shoot tips and death of protocorms ans nitrogen content in protocorms or in fungal mycelia was investigated. The response of protocorms to gluten levels varied according to fungi : the fresh weight of protocorms with a fungus decreased remarkably with an increase of gluten level, whereas that with another fungus was not much affected at this nitrogen level. Of those decreased in the fresh weight of protocorms, suffered severer disorder, and the nitrogen concentration of the them was increased in protocorms and decreased in fungal body.In the case of healthy protocorms, the infected area in a protocorm balanced occupying less than a lower half of a protocorm section, thus symbiosis was sustained and elongation of shoot and root rapidly proceeded. In the case of those suffered disorder, infected area increased rapidly and ultimately occupied almost whole protocorm resulting in the death of the protocorm. These observation suggested that this disorder was the result of change from symbiosis to parasitism.