Earlier work has shown that rice plants growing in reduced soil
are able
to solubilize P and thereby increase their
P uptake by inducing an acidification in the rhizosphere; the acidification
is
caused by H+ produced in Fe2+
oxidation by root-released O2, and by the direct release of
H+
from the roots to balance cation–anion intake. Here,
we report rates of release of O2 and H+ from P-stressed
and
P-sufficient rice plants into sand cultures continuously
perfused with deoxygenated nutrient solution. The P stress was sufficient
to
reduce plant dry mass by roughly half,
but root dry mass increased roughly twofold and root surface area 2·5-fold.
The proportion of fine roots increased from 11 to 21% of root length under
P
deficiency; root porosity, averaged over the whole root system, increased
from 0·25 to 0·40. Apparent rates of O2 release
were
0·8–3·3 μmol per plant d−1, or
22–87 μmol g−1
(root dry mass) d−1. Assuming that the bulk of the O2
was released from medium and fine roots, the fluxes of O2 were
0·02–0·13 nmol dm−2 (root surface)
s−1, which is in the range found for soil-grown plants.
The
release per plant was twofold greater in the low P treatment, although
rates of
release per unit root mass were slightly lower. The
increased release under P deficiency is consistent with the increased length
of fine roots and increased porosity.
Rates of H+ release were 0·7–1·2 mmol per
plant−1 d−1, or 1·4–6·1
mmol
g−1 (root dry mass) d−1. The H+
release per unit plant dry mass was 60% greater in the low P treatment,
but the
release per unit root mass was 2·5-fold lower. The increased H+
release under P deficiency was associated with increased NH4+
intake and decreased NO3−
intake, and a tenfold increase in plant NO3-N. This suggests
that P
deficiency reduced NO3− assimilation, causing
reduced NO3− influx and/or increased efflux.