Effect of alkaline and saline substrates on ABA contents, distribution and transport in plant roots

Abstract

The distribution of the phytohormone abscisic acid (ABA) between plant and soil and within plants growing on an alkaline substrate has been studied in order to separate the true effect of high soil pH from any effects that might be a result of the high salinity normally observed in alkaline soils. Leaves of a range of plants grown in an alkaline and saline solid substrate (municipal solid waste incinerator bottom slag) exhibited higher ABA levels than leaves of control plants. In contrast, roots of most plants grown on alkaline and saline substrates, particularly those without an exodermis (various species of Fabaceae), had slightly lower than or comparable ABA contents to control roots. However, in corn roots (Zea maysL. cv. Garant FAO 240) which possess a well-developed exodermis, alkaline and saline conditions in the rhizosphere did not reduce the endogenous ABA concentration, because the leaching of ABA from corn roots into the rhizosphere was lower than that from Vicia faba (variety Dreifache Weisse) roots. ABA efflux from corn and Vicia roots into the soil solution was observed only during the first days of the experiments and thereafter became substantially decreased. Because the leaching of ABA from Vicia faba roots into the rhizosphere was higher than that from corn roots, the leaves of Vicia plants grown in alkaline soil at low salinity no longer exhibited an elevated ABA concentration. However, whilst the roots of corn plants grown on desalted slag retained ABA levels that were higher than those of the control, the ABA content of leaves was not significant higher than the controls. For this reason, root ABA retention must be enough to induce tolerance to alkalinity in corn plants and there is no need to implicate changes in ABA concentrations in the aerial parts of the plant as having a role in this tolerance. In alkaline soil substrates, considerable portions of the ABA synthesised in the roots leached out into the soil solution of the rhizosphere according to the anion trap concept. An exodermis substantially reduces this leakage. The transient nature of ABA efflux into the rhizosphere was a result of the fact that the salt stress itself was only a transient phenomenon due to a washout of salt by irrigation. The results match predictions of mathematical models describing the effect of alkaline pH on the distribution of abscisic acid within plants and between roots and the rhizosphere. Species that can retain root ABA in the face of its tendency to leach into the more alkaline compartment are able to tolerate these normally harmful sites.