THE GRADIENT BETWEEN PRE-DAWN RHIZOPLANE AND BULK SOIL MATRIC POTENTIALS, AND ITS RELATION TO THE PRE-DAWN ROOT AND LEAF WATER POTENTIALS OF 4 SPECIES

Uptake of soil water by plants may result in significant gradients bet ween bulk soil and soil in the vicinity of roots, Few experimental stu dies of water potential gradients in close proximity to roots, and no studies on the relationship of water potential gradients to the root a nd leaf water potentials, have been conducted, The occurrence and impo rtance of pre-dawn gradients in the soil and their relation to the pre -dawn root and leaf water potentials were investigated with seedlings of four species, Pre-germinated seeds were grown without watering for 7 and 11 d in a silt loam soil with initial soil matric potentials of -0.02, -0.1 and -0.22 MPa, Significant gradients, independent of the s pecies, were observed only at pre-dawn soil matric potentials lower th an -0.25 MPa; the initial soil matric potentials mere greater than or equal to -0.1 MPa, At an initial bulk soil matric potential of -0.22 M Pa, a steep gradient between bulk and rhizoplane soil was observed aft er 7 d for maize (Zea mays L. cv, Issa) and sunflower (Helianthus annu us L. cv, Nanus), in contrast to barley (Hordeum vulgare L. cv, Athos) and wheat (Triticum aestivum L. cv, Kolibri), Pre-dawn root water pot entials were usually about the same as the bulk soil matric potential and were higher than the rhizoplane soil matric potential, Pre-dawn ro ot and leaf water potentials tended to be much higher than rhizoplane soil matric potentials when the latter were lower than -0.5 MPa, It is concluded that plants tend to become equilibrated overnight with the wetter bulk soil or with wetter zones in the bulk soil, Plants can thu s circumvent negative effects of localized steep pre-dawn soil matric potential gradients, This may be of considerable importance for water uptake and growth in drying soil.