Causes of Injury to Flooded Tobacco Plants.

Abstract

Tobacco is one of the most susceptible of crop plants to injury from flooding or saturation of the soil with water. It really exhibits two types of reaction to flooding, immediate but temporary wilting accompanying temporary flooding, and severe permanent injury caused by longer periods of flooding. If the soil is suddenly saturated by a downpour of rain and the sun later shines 'bright and hot, sudden wilting of the leaves, often termed flopping by tobacco growers, sometimes occurs. This sudden wilting occurs where drainage is slow and the soil remains saturated for at least a few hours ?fter a rain. High air temperatures and bright sun accentuate this sudden wilting and its occurrence may depend also on the condition of the plants, apparently being much more severe if the tobacco has been growing rapidly and therefore is somewhat soft and succulent. If the excess soil moisture drains away within a few hours the plants usually recover from this type of wilting with little or no permanent injury. Where because of topography or an impermeable subsoil the soil in the root zone remains saturated for several days serious permanent injury or death usually results. The leaves of such plants turn yellow and die, beginning with the lower ones and progressing up the stem. Such injury is most common and severe on clay soils because they drain more slowly than sandy soils, but injury sometimes occurs even on sandy soil if poorly drained. Considerable discussion has occurred among tobacco workers concerning the causes of the sudden wilting which often occurs immediately after a rain, but apparently little or no research has been done. Of course, the immediate effect of saturating the soil with water is to displace the air which normally occupies the noncapillary pore space of the soil. This causes a deficiency of oxygen and an accumulation of carbon dioxide, either of which might injure or kill the roots. Damage to the root systems has been assumed to result in reduction of water intake and injury to the tops from desiccation. This explanation seems inadequate, however, because wilting often occurs within an hour or two after flooding, too soon for serious injury to the roots. Furthermore, as Kramer (8) pointed out earlier, some of the symptoms of flooding injury are different from those expected in plants dying from desiccation. Wilting is often most severe in the middle leaves and in cloudy weather leaves of flooded plants often turn yellow and die with little or no wilting. Epinasty also is common, though the change in angle of curvature is so small in tobacco that it is generally overlooked. These observations suggested the need for a careful study both of the visible symptoms of flooding injury and of the physiological causes of these symptoms.