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

S. Ramsamy et al. /Field Crops Research 51 (1997) 65-82 15
Nodal root length
8;
l- ‘-.
6.
E 5- &\
f 4.
Aa -- D”
E 3- - Dn+N
Flowering
/
;2- FL . -~I--- .~~ =:+,
0 20 40 60 SO 100 120
Days after transplanting
Fig. 3. Effects of drainage and N application at heading on the
length of nodal roots of rice cultivar IR20, Coimbatore, Tamil
Nadu, India, 1991- 1992, Expt. 4. D,: undrained, D,: drained, N:
nitrogen application at heading. For further treatment details see
text.
Alpha-naphthylamine oxidation power by roots
0 50 100 150
Days after transplanting
Fig. 4. Effects of drainage and N application at heading on the
oxidizing power of roots in rice cultivar IR20, Coimbatore, Tamil
Nadu, India, 1991-1992. Expt. 4. 0,: undrained, D,: drained, N:
nitrogen application at heading. For further treatment details see
text
In two experiments (Expts. 1 and 3), drainage was
associated with greater green leaf area and a larger
amount of bulk leaf nitrogen NL (g N per m’ ground
area). To analyze whether photosynthetic activity per
unit bulk leaf N increased during grain filling in
response to drainage, we determined the factor fc,,,
defined by the relation
G=f,, .p.NL[l -e-EK/(P*v~ ‘1
(Ten Berge et al., 19961, where G (g m-* d-‘) is
the crop growth rate, R is the daily incident global
radiation (MJ m-’ d-l), p is the initial leaf nitrogen
use coefficient (fixed at 10 g g-’ d-l) and E the
initial global radiation use coefficient (fixed at 2.5 g
MJ-‘1. The factor f,, is an empirical calibration
coefficient, accounting for all variations in growth
arising from factors other than R and N,. This
coefficient was determined by fitting the growth
curves G(t) calculated by Eq. (1) to the observed
growth curves, using measured time series R(t) and
N,(t). A significant increase of the post-flowering
f,, value in response to drainage was observed in
Expts. 2 and 3, but not in Expt. 1. (During the
pre-flowering phase, effects of drainage on f,, were
absent in all experiments.)
Earlier leaf senescence was recorded in the
undrained plots, but must be eliminated as the primary
cause of poorer grain filling, because smaller
grain yields in undrained soil were not always (Expt.
1) associated with an equivalent decrease in total dry
matter production (see also Fig. 1).
Leaf senescence is often viewed as the result of N
extraction from leaves and translocation to growing
(1)
panicles (e.g., Sinclair, 1990). The present study
does not fully confirm this concept, because Expt. 2
produced more grain in drained conditions, without
additional N uptake, yet with less leaf senescence
than in undrained plots. Moreover, N content in
‘early’ senescent leaves (just after flowering) was
lower (0.3% N) than in leaves that senesced later
(0.7% N near maturity); and N concentration in
senescent leaves was greater in undrained plots.
These observations indicate that senescence is not
just the result of N extraction from leaves. This view
is supported by Herzog and Geisler (1982) who
proposed that leaf senescence in wheat is induced by
competition (ear versus leaf) for a limited pool of
growth regulators, rather than carbohydrates or N.
Un-drained
Drained
AT PI T-F MR
Fig. 5. Effect of drainage on root color in rice cultivar IR50 at
Coimbatore, Tamil Nadu. India, 1991. Expt. 3. at crop stages AT
(active tillering), PI (panicle imtiation). FF (first flowering) and
MR (maturity). Means over all N levels. Observations in Expts. 1,
2 and 4 were similar. For treatment details see text.