Water Relations in Cowpea and Pearl Millet Under Soil Water Deficits. II. Water Use and Root Distribution

Abstract

Pearl millet [Pennisetum americanum (L.) Leeke] develops lower predawn leaf water potentials than cowpea [Vigna unguiculata (L.) Walp.] when plants are subjected to progressive soil dehydration. Water use and root distribution were studied as possible explanations for this difference using plants grown in pots and in tubes in a glasshouse. In experiments grown in pots, weights of each container were measured daily after initiation of the dry treatment to determine water use, and roots and soil were sampled at the end of the experiment to determine root length density and soil water content. In experiments conducted in 1 m long tubes, plants were grown in either Turface Plus or a Turface Plus/sand mixture which had a high water-holding capacity and could be easily separated from roots. Roots and rooting medium were sampled during the dry treatment. Water content and root length density in the same sample were measured, and root distributions at various depths were determined. Pearl millet (millet) did not use more bulk water than cowpea by the time the difference in predawn leaf water potential developed. Millet root length density was at least twice as great as cowpea, and the decline in predawn leaf water potential was greater in millet, even when its root system was more extensive than that of cowpea.