Water Relations Bibliography

Stomatal gas exchange and zeatin riboside levels (as determined by ELISA) of flax (Linum usitatissimum L. ) were investigated with regard to an observed growth response of these plants to vesicular-arbuscular mycorrhizal infection, which was shown not to be related to increased nitrogeen, phosphorus or potassimum contents of plants. Additionally, the stomatal gas exchange responses of non-mycorrhizal plants to zeatin and abscisic acid were studied by xylem application experiments. In comparison to non-mycorrhizal plants highly infected plants revealed increased transpiration and CO₂ assimilation rates, while stomatal density was not affected and the shoot water potential was unchanged or even lowered. These findings indicated that enhanced stomatal opening was not primarily caused by an improved water supply of the shoots. Additionally, respiration rates of leaves of mycorrhizal plants were lower when compared with non-mycorrhizal plants at the end of the experiments. During the beginning of the mycorrhizal infection zeatin riboside levels in roots were temporarely decreased when compared to non-mycorrhizal plants, whereas levels where increased in shoots. However, when the symbiosis had established colonized roots revealed significantly higher zeatin riboside than those of non-mycorrhizal plants. Significant growth responses of shoots and roots due to mycorrhizal infection were preceded by higher zeatin riboside levels in the respective organs. Zeatin applied alone into the vascular system of non-mycorrhizal flax did not affect stomatal gas exchange, whereas abscisic acid applied alone decreased transpiration and CO₂assimilation rates. Additional application of zeatin, however, partially reversed abscisic acid-mediated effects and improved transpiration and CO₂ assimilation rates showing analogy to the mycorrhizal infection. These results lead to the conclusion, that the enhanced internal cytokinin levels are involved in the improved photosynthesis and growth of mycorrhizal flax.