B. Degenhardt et al., Effect of alkaline and saline substrates on ABA contents, distribution andtransport in plant roots, PLANT SOIL, 225(1-2), 2000, pp. 83-94
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 mi
ght be a result of the high salinity normally observed in alkaline soils. L
eaves 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 (va
rious species of Fabaceae), had slightly lower than or comparable ABA conte
nts to control roots. However, in corn roots (Zea mays L. cv. Garant FAO 24
0) which possess a well-developed exodermis, alkaline and saline conditions
in the rhizosphere did not reduce the endogenous ABA concentration, becaus
e the leaching of ABA from corn roots into the rhizosphere was lower than t
hat from Vicia faba (variety Dreifache Weisse) roots. ABA efflux from corn
and Vicia roots into the soil solution was observed only during the first d
ays of the experiments and thereafter became substantially decreased. Becau
se the leaching of ABA from Vicia faba roots into the rhizosphere was highe
r than that from corn roots, the leaves of Vicia plants grown in alkaline s
oil at low salinity no longer exhibited an elevated ABA concentration. Howe
ver, whilst the roots of corn plants grown on desalted slag retained ABA le
vels 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 ret
ention 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 p
arts of the plant as having a role in this tolerance. In alkaline soil subs
trates, considerable portions of the ABA synthesised in the roots leached o
ut into the soil solution of the rhizosphere according to the anion trap co
ncept. An exodermis substantially reduces this leakage. The transient natur
e 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 plan
ts 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.