Causes of Injury to Plants Resulting from Flooding of the Soil.

This study was made in an effort to learn why saturating the soil with water causes almost immediate injury or death of many species of plants. While it has been shown that the deficient aeration accompanying flooding' injures or kills the root systems, this fact does not explain why the shoots are injured so quickly. Injury is usually attributed to desiccation, caused by decreased water absorption through the injured roots; but this explanation seems inadequate. Plants can live for some days after the root systems are killed if the soil is kept saturated (11), and cut shoots can 'be preserved in good condition for many days in containers of water if the water is changed occasionally. Most species of plants can be grown fairly satisfactorily in shallow tanks of nut.rient solution without forced aeration; but if the same species are grown in soil which is later flooded, the shoots are quickly injured. The injury of shoots cannot be caused entirely by injury to the roots as absorbing systems, because reduced absorption of water or of minerals cannot explain all of the symptoms observed in the shoots of flooded plants. Although wilting of leaves is often observed after flooding, this is not the only nor even the most characteristic symptom of injury. Among the conspicuous symptoms of flooding injury is yellowing and death of the leaves, beginning with the lower ones and progressing up the stem. This chlorosis superficially somewhat resembles nitrogen deficiency, but often develops within four to six days after flooding, much too soon to be caused by nitrogen deficiency. The middle leaves of tomato show epinastic curvature within twenty-four to forty-eight hours after the soil is flooded; and lumps of callus tissue develop along the stem, particularly at the water level. In many species adventitious roots develop at the soil surface or just below the water surface where the water level is above the soil surface. These occurrences cannot be explained fully as a simple result of desiccation caused by a damaged root system. Desiccation certainly would hinder the development of adventitious roots rather than stimulate their development. The epinastic curvature of leaf tips and the change in angle of petioles are also more characteristic of the reaction of turgid cells than of flaccid cells and wilted tissue. The rapid yellowing and death of the lower leaves might be caused by desiccation, or by toxic substances escaping from the dying roots or being produced in the soil which are carried upward in the transpiration stream. It, therefore, appears probable that 722