Ramasamy et al. - 1997 - Yield formation in rice in response to drainage an

S. Rarnasanq et al. / Field Crops Resrurch 51 i 19971 65-82 71
ual starch reserves are also found in cultivars of
longer duration (Dat and Peterson, 1983).
We did not measure translocation of non-structural
stem reserves directly, but estimated it as the
decrease in stem weight from around flowering.
when peak stem weight is reached, up to maturity
(Table 3). This amount was 40 to 50% (depending
on treatment) of peak stem weight in Expt. 1; 17 to
23% in Expt. 2; and 45 to 55% in Expt. 3. These
amounts correspond to 25 to 40% (Expt. l), 20%
(Expt. 2) and 35 to 40% (Expt. 3) of final grain
weight. Absolute amounts were larger under welldrained
conditions, in all treatments of Expts. l-3.
Of the yield increments resulting from drainage,
25-35% could be accounted for by increased
translocation from stem reserves in Expt. 1. This
figure was 5 to 25% in Expt. 2 and 30 to 40% in
Expt. 3. A literature survey did not provide us with
mechanisms explaining effects of drainage on
translocation.
4.5. Root growth and root condition
Post-flowering root mass was greater, root N contents
were higher and fewer black roots were found
under drained conditions in Expts. l-3 (Table 4).
Where root oxidizing power and root length were
measured (Expt. 4), these were greater, too, under
drained conditions.
4.6. Growth analysis summarized
Drainage increased post-flowering growth and
yield in all experiments, but none of the ‘source-related’
variables - green leaf area, N uptake, green
leaf N and f,, - could be identified as the common
factor explaining, for all experiments, the extra
growth and yield in drained plots. In Expt. 1. the
increment of total crop biomass was much smaller
than that of grain yield, rendering invalid the directly
source-related explanations. In Expt. 2, crop N uptake
and the amount of N in green leaves was
unaffected by drainage and green leaf area was
affected only slightly and at the very end of the
grain-filling stage. It may well be that positive responses
of the source-related variables to drainage
have no direct causal relation with the improved
filling of spikelets.
The association found, however, between the
translocation of stem reserves and grain yield in
response to drainage was consistent in Expts. l-3.
The same is concluded for post-flowering root growth
and root condition as expressed in root color (Expts.
l-3), N content (Expts. l-3) and a-NA oxidative
activity (Expt. 4). Further research is required to
reveal the mechanisms behind these associations.
5. Discussion
If it is not the uptake and subsequent exploitation
of N by which rice crops benefit from altered soil
conditions in response to drainage, explanatory
mechanisms can be sought in two other directions:
(i) drainage e n h antes the availability of specific
nutrients other than N, or lowers the crop’s demand
for these, or increases the ability of roots to
acquire these (the latter option is supported by
Kumazawa (1984) and Pate1 et al. (1984)); and
(ii) d rama g e r ed uces the presence of toxic compounds
in the root zone and in the plant and
thereby affects other grain formation processes.
A brief literature review will permit discussion of
these alternatives. Because our experiments reveal
consistent effects of drainage on indicators of root
condition. the discussion is limited to aspects of
those mechanisms wherein a change of root condition
is explicitly recognized as a cause or as a
symptom.
5.1. Mechanisms qf t)?ve fil
Root condition is closely related to the redox
status of the soil (Ota, 1970; Yanatori, 1981; Cheng,
1983; Kumazawa, 1984; Tseng and Yang, 1990;
Kludze et al., 1993; Jiang et al., 1994a). It seems
likely, therefore, that the beneficial effect of drainage
is due to improved soil redox conditions. The mechanisms
by which this might occur are not quantified
in literature. One aspect is the increased flow of
dissolved oxygen through the root zone via increased
water percolation rate, increasing the redox potential
E,. This lowers the corresponding concentrations of
toxic reduction products such as Fe’+, H, S and
organic compounds. Another aspect is the removal of