Water Relations in Cowpea and Pearl Millet Under Soil Water Deficits. III. Extent of Predawn Equilibrium in Leaf Water Potential

Abstract

Pearl millet [Penniseturn americanum (L.) Leeke] consistently develops lower predawn leaf water potentials than cowpea [Vigna unguiculata (L.) Walp.] when plants are subjected to progressive soil drying, even when they are grown as intercrops in the same pot. Lack of equilibrium in water potential during the predawn, within the plants and between plants and soil, was studied as a possible explanation for this difference. Experiments were conducted in a glasshouse in pots containing an artificial rooting medium which had a high water-holding capacity and was easy to separate from roots. Leaf and root predawn xylem water potentials were measured with a pressure chamber. In cowpea, leaf water potential (Ψ_L) values during the dry treatment were similar to values of root water potential (Ψ_R), indicating an approximate equilibrium within the plants. In millet, measurements were made on plants grown in both large and small pots. With large pots, Ψ_L and Ψ_R values in millet were similar, but with small pots, predawn Ψ_L was lower than Ψ_R. When the surfaces of these small pots were covered with evaporation barriers, however, no significant differences developed between Ψ_L and Ψ_R at predawn during soil drying, indicating an approximate equilibrium within millet. During the early stages of the dry treatment, leaf surface conductances measured at predawn indicated that significant water flux occurred at night from the leaves of both cowpea and millet. Leaf surface conductances declined to negligible levels as the dry treatment progressed, however, and night-time fluxes of water from leaves cannot explain the significant difference in predawn Ψ_L that developed between cowpea and millet. In order to determine whether the lower predawn Ψ_L in millet was due to a resistance to water uptake at the root surface, an osmoticum (- 0.5 MPa mannitol solution) was applied to pots of cowpea and millet intercrops after substantial differences in predawn Ψ_L had developed, and Ψ_L was measured. Measurements taken 9 h later indicated that predawn Ψ_L had recovered in millet, and the values of Ψ_L in millet and cowpea after mannitol treatment were similar to the osmotic potential of the mannitol solution. These results suggest that the lower predawn Ψ_L in millet than in cowpea under drought is due to the development of a substantial soil resistance to water movement to the root surfaces of millet.