Agriculture Reference
In-Depth Information
Research on the effects of phenolic acids on rice has measured effects on
seed germination and rates of root elongation and seedling growth. This has
shown that concentrations greater than a few mM of the more noxious acids are
required to substantially impair root elongation (Olofsdotter
et al
., 2002). Under
tropical conditions, without large additions of organic matter, total concentrations
of alkali-soluble phenolic acids in the soil during rice cropping are generally
less than this (Tsutsuki and Ponnamperuma, 1987), though concentrations may
be greater locally, such as in the rhizosphere. Factors that exacerbate the toxic
effects of phenolic acids include low plant N status (Vaughan and Ord, 1990); low
soil pH, the undissociated acid having greater membrane permeability (Tanaka
and Navaesero, 1967); and the nature of the acid, cinnamic acid derivatives being
more inhibitory than benzoic acid derivatives, and lipid-soluble acids being more
inhibitory than lipid-insoluble (Glass, 1973). Little research has been done on the
effects of phenolic acids on ion uptake by rice roots, but ion uptake is presumably
impaired at much smaller concentrations than root elongation.
Recent experiments at the International Rice Research Institute (IRRI) on the
maximum yields of high-yielding rice cultivars developed over the last 40 years
have shown that the older cultivars no longer perform as well as they used
to (Peng
et al
., 2000). The first modern high-yielding cultivar released by IRRI,
IR8, which often produced 9-10Mg of grain ha
−
1
at the IRRI farm in the
1960s under optimal management, now yields only 7-8Mg ha
−
1
under sim-
ilar conditions, whereas its most recent successors yield 9-10Mg ha
−
1
.This
does not appear to be due to deterioration in seed stocks over the years through
repeated multiplication cycles (Peng, unpublished), or to new or increased pests
or diseases. It appears that new abiotic stresses have arisen over the 40 years
of continuous rice cropping on the IRRI farm, with two or three crops per year
in flooded soil, and the more recent cultivars may be better adapted to these
stresses. Changes in soil conditions have been observed, most notably changes
in the nature of the soil organic matter associated with prolonged soil flood-
ing and more strongly reducing conditions in the soil, particularly increased
concentrations of phenolic compounds (Olk and Senesi, 2000). The decrease in
grain yield of the older cultivars is associated with poor grain filling and harvest
index, and impaired acquisition of soil nitrogen. A possible explanation is that
the changes in soil conditions have led to impaired root function in the older cul-
tivars through toxins associated with prolonged soil flooding, such as phenolic
or other organic acids.
7.2.4 SALINITY
Salinity occurs in coastal areas affected by seawater, in areas receiving salty
water by lateral flow from salt-bearing rocks upstream, and also in otherwise
non-saline environments as a result of soil waterlogging, mainly due to high
ground water. In waterlogged land, whatever dissolved salt is brought in with the
