Water Relations in Cowpea and Pearl Millet Under Soil Water Deficits. I. Contrasting Leaf Water Relations

Cowpea [Vigna unguiculata (L.) Walp.] can survive soil water deficits more effectively than pearl millet [Pennisetum americanum (L.) Leeke]. Cowpea and millet were grown in a glasshouse in different rooting media and different sizes of container, under wet and dry treatments, and as sole crops and intercrops to evaluate any differences in leaf water potential. Millet developed significantly lower predawn leaf water potentials (ΨL) than cowpea under the dry treatment of all of the rooting media and container sizes used, but both millet and cowpea maintained high predawn ΨL in the well-watered treatment. With the dry treatment, the same difference in predawn ΨL between cowpea and millet developed in plants grown either as sole crops or as intercrops in the same pot. These results suggest that plants grown as intercrops were somehow isolated from each other, even though their root systems may have overlapped, and that competition for water was probably not occurring. Differences in predawn ΨL between cowpea and millet were detected with either a pressure chamber or psychrometers, but values of ΨL varied with measurement method. Compared with psychrometer values, pressure chamber values became significantly lower in millet late in the dry treatment but were higher in cowpea. Agreement between the methods for measuring ΨL improved in cowpea when predawn xylem osmotic potential was added to the pressure chamber value. At the end of the experiments, leaf surface conductance to water vapour and leaf area were lower in millet than cowpea. Consequently, it is possible that the significantly lower predawn ΨL in millet was not due to greater water use by millet compared with cowpea.