
%0 Journal Article
%A Petrie, CL 
%A Hall, AE 
%T Water Relations in Cowpea and Pearl Millet Under Soil Water Deficits. III. Extent of Predawn Equilibrium in Leaf Water Potential
%J Functional Plant Biology
%D 1992/12/01
%V 19
%N 6
%P 601-609
%K 
%R https://doi.org/10.1071/PP9920601
%U https://www.publish.csiro.au/paper/PP9920601 
%X 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 (&#936;L) values during the dry treatment were similar to values of root water potential (&#936;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, &#936;L and &#936;R values in millet were similar, but with small pots, predawn &#936;L was lower than &#936;R. When the surfaces of these small pots were covered with evaporation barriers, however, no significant differences developed between &#936;L and &#936;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 &#936;L that developed between cowpea and millet. In order to determine whether the lower predawn &#936;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 &#936;L had developed, and &#936;L was measured. Measurements taken 9 h later indicated that predawn &#936;L had recovered in millet, and the values of &#936;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 &#936;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. 
