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

ELSEVIER
Field Crops Research 51 (1997) 65-82
Field
Crops
Research
Yield formation in rice in response to drainage and nitrogen
application
S. Ramasamy ‘, H.F.M. ten Berge b- * , S. Purushothaman a
a Tamil Nadu Agricultural Unievxity. Coinzhatore 641003. India
b DLO Research Institute for Agrobiolog! and Soil Fertility CAB-DLOJ, P.O. Box 14, 6700 AA Wageningen, The Netherlands
Abstract
Four field experiments were carried out in Tamil Nadu, India. to determine effects of internal drainage (percolation rate)
on growth and yield formation in rice, in interaction with nitrogen (N) management. Grain yield response to drainage was
positive at all N levels in all experiments. Yields (averaged over N treatments) increased by 14% (Expt. 1). 10% (Expt. 21,
25% (Expt. 3) and 22% (Expt. 4) in response to drainage. Higher yield responses to N application were found under well
drained than poorly drained soil conditions. In late-season crops under poor drainage, grain yields decreased in response to
high doses of N. The positive yield response to drainage was always associated with increased number of filled grains per
panicle (+ 13 to + 17%). increased translocation of stored reserves, increased root biomass at harvest (+20 to +36%0),
increased N concentration in roots and increased root activity (as measured by a-naphthylamine oxidation). Of the yield
increment resulting from drainage, 25 to 35% could be accounted for by increased translocation of stem reserves in Expt. 1,
5 to 25% in Expt. 2 and 30 to 40% in Expt. 3. Yield increase in response to drainage was not always associated with more
green leaf area, leaf longevity. leaf N content, crop N uptake, nor with a larger dry matter production at given levels of the
leaf N pool and global radiation, as quantified with the help of a calibration factor f,,. A literature review is presented
leading to two hypotheses to explain the effect of drainage on yield: (I ) a possible potassium deficiency is expressed more in
reduced (low-redox) soil due to a lowering of K availability per se; or to increased K demand associated with maintaining
the root’s ‘oxidizing power’; (2) improved root condition resulting from an increase in soil redox potential induces a
prolonged synthesis and transport of cytokinins in roots. resulting in an extended photosynthetic activity, enhanced sink
strength of grains and more translocation and deposition of carbohydrates in the grains. These hypotheses are not mutually
exclusive. They were not explicitly tested in this study. 0 Elsevier Sciene B.V. All rights reserved.
Kqwords: 0~~ safii.a: Rice: Dramage; Root condition: Redox: Nitrogen: Yield formation; Translocation: Cytokinins; Senescence
1. Introduction uptake. crop N utilization and rice yield are not well
understood. Periods of fallow (i.e. non-flooded) con-
Water and nitrogen (N) management are crucial to
dition between successive rice crops seem to have a
high yields. The numerous studies on interaction
consistently positive effect on yield (Lian, 1977;
between the two factors have often revealed contra-
Willett and Muirhead. 1984). Drainage during the
dictory results. Notably, the effects of drainage on N
cropping season. however, increased grain yields in
many (e.g., Miyasaka, 1970; Nho et al., 1975;
* Corresponding author.
Onikura. 1976; Shirakura et al., 1977; Khind and
037%4290/97/$17,00 Copyright 0 1997 Elsevier Science B.V. All rights reserved
PII SO378-4290(96)01039-8

66 S. Ramasamy et al. / Field Crops Research 51 (1997) 65-82
Kazibwe, 1983; Doi et al., 1988) but not all cases
(e.g. Watanabe et al.. 1974; Snitwongse and Jirathana,
1981; Singandhupe and Rajput, 1987;
Kogano et al., 1991).
One explanation for this apparent inconsistency of
crop response may be the method applied to impose
drainage conditions experimentally. Manipulation of
external drainage is via the depth of the flood water
level or the duration of flooding (Upadhya et al.,
1973; Veltkamp and Manniesing, 1974; Onikura,
1976; Snitwongse and Jirathana, 1981; Pate1 et al..
1984; Kogano et al., 1991). Internal drainage is
governed by removal of subsurface water via mole
drains, tube drains, or open drainage channels, and
affects the rate of percolation via a lowering of the
subsurface water table (Watanabe et al., 1974; Shirakura
et al., 1977; Habibullah et al., 1977; Lee et
al., 1979; Khind and Kazibwe. 1983). In addition,
the crop development stage at which changes in
water management are imposed may determine crop
response.
Depending on soil characteristics, crop development
stage and the type of drainage measures taken,
the physico-chemical conditions of the root zone are
affected differently by water management practices.
Various studies indicate a beneficial effect of relatively
high percolation rates of 10 to 25 mm/day.
especially for heavy clay soils rich in organic matter
(e.g., De Datta, 1981 and Hasehawa et al., 1985).
Whereas leaching as a result of increased flooding
duration is beneficial in salt-affected soils
(Singandhupe and Rajput, 1987), leaching may increase
N losses in well-drained soils (Kudeyarov et
al., 1989) and thereby reduce yield. Yet, some welldrained
soils provided greater N uptake under high
than under low percolation (Khind and Kazibwe.
1983). Yamamuro (1983) reported that flooding increased
N uptake in soils with good internal drainage.
but decreased uptake in poorly drained soil. Nonflooded
conditions during the entire grain-filling period
increased both N uptake and yield in the study
by Miyasaka (1970), whereas negative effects of
early drainage on rice growth observed by Kogano et
al. (1991) were attributed to enhanced N losses
through nitrification-denitrification. High N availability
itself may have positive or negative effects on
grain yield, depending on cultivar and surface
drainage (Rai and Murty, 1979).
When the soil surface remains non-flooded for
several days, the effects of drought stress might
overrule the potential benefits of drainage (Snitwongse
and Jirathana, 1981; Singandhupe and Rajput,
1987). Alternatively, draining all surface water
to halt tillering, as practiced in China (Doi et al.,
19881, may be beneficial for yield formation. Other
more detailed analyses (Nho et al., 1975; Trolldenier,
1977; Pate1 et al., 1984; Cheng, 1983; Iida et
al., 1990) linked drainage and soil redox condition
with root activity and with the uptake of various
micro-nutrients (Lee et al., 1979; Pate1 et al., 1979).
To develop a rational basis for improving water
and N management, better understanding is required
of the mechanisms by which drainage interacts with
crop growth and N utilization. We conducted a series
of field experiments and studied yield formation
processes as affected by internal drainage: N uptake;
growth; the utilization of radiation and leaf nitrogen
to produce dry matter; sink formation; grain filling;
and translocation of stored non-structural carbohydrates.
Root characteristics were also studied in
detail and are related to crop performance.
2. Materials and methods
Four field experiments were conducted at Tamil
Nadu Agricultural University, Coimbatore (11” N,
77” E), India, in 1990 to 1992. The soils were of
loamy clay texture (40% clay, 22% silt, 22% fine
sand, 16% coarse sand) with low available N and
high P and K contents. The field was under paddy
cultivation already for many years prior to the experiment.
Experiments 1, 2 and 3 focused on the interaction
between internal drainage (groundwater table; percolation
rate) and nitrogen application level. The
drainage treatments were ‘non-drained’ (0,) -
where no provisions were made to change the existing
percolation rate of 3.0 mm day-‘; and ‘drained’
(0,) - where open trenches (width and depth 0.60
m) were dug to lower the groundwater level and
increase the mean field percolation rate to 12 to 14
mm day-‘. Water was removed from the trenches by
pumping.
The N levels were 100, 150 and 200 kg N ha-‘.
N was applied as prilled urea in three splits with

S. Rarnrrsamy et al. /Field Crops Resrarch 51 (19971 65-82 67
50% at transplanting (basal). 25% topdressed 15 or
20 days after transplanting (DAT), and 25% topdressed
30 or 40 DAT, depending on the cultivar.
Other nutrients applied were 22 kg P (as Pz05), 40
kg K (as K 20), 10 kg Zn and 5 kg S (as ZnSO,) per
ha. Green manure (Sesbania rosrruta) was applied
uniformly and incorporated before transplanting at
12.5 t ha-’ (fresh weight, corresponding to 75 kg N
ha- ’ >.
Seedlings of the short-stature, short-duration rice
cv. IR50, 25 d old, were transplanted at 15 X 10 cm
spacing on 13 July 1990 in Expt. 1; and 19 June
1991 in Expt. 3. In Expt. 2 (late season), the
medium-duration, medium-stature cv. IR20 was
transplanted on 3 November 1990. with 20 X 10 cm
spacing. The field remained fallow for 15 days after
the harvest of the first crop, was then flooded for 10
days (field preparation) and was planted to the second
crop.
A detailed root study (Expt. 4) was conducted to
collect information on roots under the two drainage
treatments as imposed in Expts. 1 to 3. The treatments
comprised drained and undrained plots in
combination with 120 kg N ha- ’ (60 kg basal at
transplanting; 30 kg at 20 DAT and 30 kg at 40
DAT) or 150 kg N ha- ’ (60 kg basal at transplanting,
30 kg at 20 DAT, 30 kg at 40 DAT and 30 kg N
ha-’ at heading). Nutrients other than N and green
manure were applied as in Expts. l-3. Seedlings of
rice cv. IR20, 24 d old, were transplanted on December
24 at 20 X 10 cm spacing.
A factorial split-plot design with four replications
(blocks) was used in Expts. 1-3. Plots were strips of
20 X 5 m. Each block consisted of one D,. and one
D, plot, randomly chosen, separated by one buffer
plot with plastered walls to minimize border effects
arising from the trenches surrounding D, plots.
Each plot was subdivided into two subplots (20 X 2.5
m) for planting density, each with three sub-subplots
(6 X 2.5 m) for N levels. The results of only one
planting density are presented in this paper, as no
major differences were found between the two density
treatments. A randomized complete block design
with five replications was used in Expt. 4. Adequate
plant protection measures were taken in all experiments
and irrigation was provided daily to maintain
a ponding depth of approximately 5 cm until 7 days
before harvest.
In Experiments 1-3, observations of biomass of
green leaves, senescent leaves, stems, roots and panicles
were made at 10 d intervals, from the tillering
stage onward. Five plants from sample rows were
removed carefully along with the roots and plant
parts were separated (green leaves, ‘dead’ leaves,
stems plus leaf sheaths, roots, panicles) and ovendried
at 80 C for 72 h. Dry weight of the biomass of
all organs was determined and N contents were
measured by the micro-Kjeldahl method (Humphries,
1956). The amount of N translocated (N,) from
leaves and stems was assessed as the difference
between the highest value (A!,,,,,) of N accumulation
recorded in those organs at any time and the amount
of N retained at harvest. Grain yield was determined
from entire plots, excluding border rows and rows
used for periodical sampling. In recording grain
yields, panicles were threshed, grain moisture was
determined and grain yields were calculated by correction
to 14% moisture content. The number of
productive tillers per hill was determined at harvest
from 10 hills. Filled and non-filled grains were
counted after threshing individual panicles. lOOO-
Grain weight was determined for each treatment.
Roots were separated into active (white), moderately
active (brown) and less active (black), as suggested
by Cheng (1983). The proportion of differently colored
roots was determined at various growth stages
and was expressed as percentage of the total number
of roots.
In Expt. 4, the roots were carefully rinsed and
detached from their nodal bases. Their volume was
measured by the displacement method, after removing
adhering surface moisture. The length and weight
of 10 randomly selected roots were measured. Total
length per plant was derived from this sub-sample
and total root weight. The diameters of roots from
the third. fifth and seventh nodes of the culm were
measured, at 3.0 cm from the base. The root CEC
was measured as suggested by Crooke (1964).
The oxidizing activity of the roots was determined
by measuring oxidation of alpha-naphthylamine ( (Y-
NA) as proposed by Ota (1970). One gram of fresh
roots was transferred into a 150 ml flask containing
50 ml of 20 ppm a-NA. The flasks were incubated
for 2 h at room temperature in an end-over-end
shaker. After inoculation, the aliquots were filtered
and 2 ml of aliquot was mixed with 1 ml NaNO,

68 S. Ramasamy et al./Field Crops Research 51 (1997165-82
(1.18 mmol I- ’ > and 1 ml sulphanilic acid and the
resulting color was measured by spectrophotometer.
Culm wall thickness was determined by vernier
caliper from the difference in diameter of culm and
medullary cavity, at internodes n - 2, n - 3, n - 4,
where n is the panicle bearing internode.
3. Results
3.1. Biomass production
Undrained field conditions induced slightly more
growth during the tillering stage (Fig. 1). The differ-
- DnNlOO - hNI50 - E,, N200 - - -:I. Dw NIOO
---A-.',,wN,50
.-.C’-.,,wN200
s 20000
f
g 15000
3
m 10000
E
z 5000
z
5 0
0 10 20 30 40 50 60 70 80 90
110
s 2oooo
f
g 15000
Expt 2
8
5 a 10000
.-
p 5000
f CI
0
0 10 20 30 40 50 60 70 80 90 100 110
;;i
f
20000
2 15000
In
m 10000
E
a
z
3
5000
0
Expt3
0 10 20 30 40 50 60 70 80 90 100 110
Days after transplanting
Fig. 1. Effects of drainage and N application on total crop biomass of rice at Coimbatore, Tamil Nadu, India. Expt. 1: rice cultivar IR50,
1990; Expt. 2: rice cultivar IRZO, 1990-1991: Expt. 3: rice cultivar IR50, 1991, Standard error of biomass at harvest: 128 (Expt. I), 125
(Expt. 2). 80 (Expt. 3) kg/ha. For treatment details see text.

S. Ramasamy et al. /Field Crops Research 51 f I9971 65-82 69
ence narrowed as growth advanced and reversed
during grain filling. In Expt. 1, however, the net
effect of drainage on final biomass was small. The
increased response (1991 2nd season versus 1990
2nd season) indicates a longer term effect of drainage.
This, however, cannot be further substantiated based
on our current data.
Higher N levels resulted in greater biomass
throughout the growth period in all three experiments,
but effects were more pronounced under
drained conditions. The growth curves in D,, and D,
were generally very similar up to mid-heading and
diverged only thereafter.
3.2. Grain yield, yield attributes and hanlest index
Drainage increased grain yield substantially in all
four experiments, with a maximum difference of 1.7
t/ha in Expt. 3. The effect was greater in the
first-season crop (IR50) than in the second-season
crop (IR20). A significant positive response to N
application was generally seen up to 1.50 kg N ha-‘,
particularly in Expt. 3. There was a clear negative
effect of N application in Expt. 2 at 200 kg N and a
negligible advantage for 150 over 100 kg in that
experiment. There was a positive interaction between
the effects of N application and drainage on grain
yield (Table 1).
The more pronounced tillering observed in D,
had no effect on the number of productive tillers.
Nor did enhanced tillering due to N application
(Expt. 2) affect the number of productive tillers. The
number of spikelets per panicle was slightly higher
in undrained than in drained plots, and was generally
higher in IR20 than in IR50. Nitrogen application
increased the number of spikelets per panicle.
Drained field conditions raised the number of
filled spikelets per panicle by 13 to 17%. Higher N
levels improved this parameter under drained conditions
(0,). The reverse effect of N application was
found in undrained plots. The largest fraction of
unfilled spikelets (42%) was found in the treatment
D, with N200, with cv. IR20. Individual grain weight
was increased by both N application and drainage in
the first season (Experiments I-3). but more so by
drainage in Expt. 2.
Grain yields recorded in Expt. 4 were similar to
those in Experiments l-3 for drained and undrained
conditions. The additional N applied at the time of
heading increased the number of filled grains from
89 to 95 per panicle in the drained plots, with yield
increasing from 7390 to 7740 kg/ha, whereas the
effect on both variables was negative in the undrained
plots (Table 1 j. Harvest index increased in response
to drainage. N application decreased this index in all
cases. The effect was more severe in undrained plots.
Short-duration IR50 had a larger harvest index than
IR20.
3.3. Nitrogen uptake
N uptake (Table 2) generally continued up to
maturity. Uptake rates peaked between maximum
tillering and panicle initiation (30-40 DAT in IR50
and 40-50 DAT in IR20), reaching maximum values
of 4.1, 6.6 and 5.6 kg N ha-’ dd’ in Expts. 1-3,
respectively. Peak values were reached a little earlier
(between early tillering and maximum tillering) in
undrained plots. The mean (over N levels and time)
uptake rate between flowering and maturity was
greater (1.5, 1.2 and 1.2 kg N ha-’ d- ‘) in drained
than in undrained (1.3, 1.0 and 0.9 kg N ha-’ dd ‘)
plots, for Expts. 1, 2 and 3, respectively.
N uptake between flowering and maturity was
relatively small under undrained field conditions
when 200 kg N was applied: 1.2, 0.6 and 0.7 kg N
haa’ d-’ in Expts. 1-3, versus 1.3; 1.4 and 1.0 kg
N ha-’ dd’ when only 100 kg fertilizer N was
applied. The corresponding values for drained fields
were 1.5, 0.9 and 1.0 kg N haa’ d-l for Expts. l-3
at200kgN;and 1.4kgNhaa’dd’ at 100kgNin
all three experiments.
The highest total N uptake, 243 kg N/ha, was
recorded in Expt. 2 and was associated with a small
harvest index. The effect of drainage on total N
uptake was not clearly exposed in Expt. 2.
3.4. Lrqf N content; N translocation
The total amount of N in the green leaf biomass
increased till boot leaf stage (50, 61 and 53 DAT in
Expts. 1, 2 and 3, respectively) and decreased thereafter
(Fig. 2). Leaf N concentration peaked earlier at
30 to 33 DAT. During initial growth stages, the total
amount of leaf N was greater in undrained plots but,
after flowering. higher levels were retained in plants

70
S. Ramasamy et al. /Field Crops Research 51 (1997) 65-82
Table 1
Effects of drainage and nitrogen on yield components and grain yield of rice. Coimbatore. India, 1990-1992
Treatment Root wt at Productive tillers Spikelets per panicle (No.) Grain weight Harvest Index Grain yield
harvest (kg/ha) (No. per hill)
total filled unfilled
(mg) (ratio) (kg/ha)
Expt. I
D;NlOO 1070
D;Nl50 1150
D;N200 1190
D;N100 1290
Q-N150 1410
Q-N200 1400
10.9 75.5 55.3 20.2 17.8 0.5 1 6990
11.4 76.4 52.8 23.6 18.3 0.48 7140
10.2 76.6 50.0 26.6 18.5 0.44 6680
10.4 73.0 59.8 13.2 18.5 0.53 7500
10.5 77.5 62.6 14.9 18.6 0.53 7860
10.9 78.8 62.7 16.1 18.5 0.53 8140
Drainage (D)
SEd 37
CD 84
0.31 1.37 0.97 0.7 I 0.24 0.02 34
NS NS 2.2 I .h NS 0.04 107
Nitrogen (N)
SEd - 43 0.28 I .36 1.25 0.68 0.14 0.01 103
CD 88 NS 2.8 NS I .4 0.28 0.03 240
D at N
SEd
Cd
N at D
SEd
CD
62 0.45 2.08 I .75 I .Oh 0.29 0.04 165
NS NS NS NS 2.2 NS NS 313
60 0.40 I .92 1.78 0.96 0.19 0.05 144
NS NS NS 3.7 2.0 NS NS 313
Expt. 2
D;N100
D;N150
D,-N200
D;NlOO
D;N 150
D, -N200
1180 9.0 118 75.1
1210 9.2 119 73.7
1200 9.2 120 69.3
1570 8.3 113 83.0
1630 8.4 114 83.2
1700 8.2 115 81.1
43.0
45.2
50.3
30.3
30.7
33.4
19.0 0.38 6230
19.1 0.37 6180
19.0 0.34 5830
20.5 0.39 6740
20.4 0.38 6780
20.5 0.35 6520
Drainage
SEd
CD
55 0.2 1.5 I.2
125 0.5 3.5 2.6
0.8
2.0
0.30 0.01 53
0.68 NS 168
Nitrogen
SEd
CD
43 0.2 1.1 0.9
NS NS NS I .9
I .o
2.1
0.20 0.01 65
NS 0.02 162
D at N
SEd
Cd
74 0.3 1.9 1.x
NS NS NS NS
I .6
3.6
0.47 0.02 169
NS NS 370
N at D
SEd
CD
61 0.4 2.1 2.1
NS NS NS 4.4
2.7
4.6
0.5 1 0.02 152
NS NS NS
Expt. 3
D”.NlOO
D;N150
D;N200
919 10.8 81.2 60.1
1010 11.7 82.4 58.6
1040 10.7 82.7 54.2
21.1
24.8
‘8.5
17.8 0.50 6830
18.3 0.48 7400
18.5 0.44 6370

S. Ramasamy et al. /Field Crops Research 51 (19971 65-82 71
Table
Treatment
D;N100
D,-Nl50
D, -N200
1 (continued)
Root wt at Productive tillers Spikelets per panicle (No.)
harvest (kg/ha) (No. pa hill)
total filled unfilled
1240 10.8 75.2 65.1
1300 11.1 78.6 66.3
1310 11.5 82.6 67.5
10.2
12.3
15.3
Grain weight Harvest Index Grain yield
hg) (ratio) (kg/ha)
18.5 0.53 8380
18.6 0.52 8460
18.5 0.52 8870
Drainage
SEd
CD
Nitrogen
SEd
CD
D at IX
SEd
Cd
N at D
SEd
CD
31 0.3 0.93 0.71 0.62 0.24 0.01 51
71 NS NS 1.60 1.40 N’s 0.03 162
29 0.3 1.11 0.58 0.73 0.14 0.0 1 79
NS 0.54 2.3 NS 1.5 0.28 0.02 172
4s 0.32 1.59 0.69 1.04 0.29 0.02 160
NS NS NS NS 1.2 NS 0.04 350
39 0.37 1.58 0.82 1.03 0.19 0.03 152
NS NS NS 1.69 2.1 NS NS 331
Expt. 4 *
4
D, + N
DW
D, + N
2. I
2.1
4.3
3.2
9.1
9.1
8.3
8.3
120 71.1
121 70.2
116 89.3
116 95.0
43.2
50.6
26.7
21.2
18.9 n.d. 6500
18.7 n.d. 5850
20.3 n.d. 7390
20.3 n.d. 7740
SEd
CD
0.05
0.11
0.3
0.6
1.3 3.1
2.8 6.7
2.3
4.9
0.35 n.d. 209
0.75 n.d. 455
NS: not significant at P = 0.05; CD: critical difference at P = 0.05; n.d.: not determined: 0,: undrained; D,: drained; NlOO, N150, N200:
100, 150 and 200 kg fertilizer N ha-’ : + N (Expt. 4): 30 kg N ha ’ at heading.
’ Root oxidizing power in mg h- ’ per g of fresh root.
of the drained plots. Larger applications of N resulted
in greater leaf N concentrations and total
amounts of N in leaves. The maximum amount of N
in green leaves recorded in all three experiments was
68 kg/ha (Expt. 2, at 61 DAT with 200 kg N). The
amount of N in green leaves just before harvest was
significantly larger in plants from drained than in
those from undrained plots (Table 3; Fig. 2).
Drainage increased the translocation of N accumulated
in stems, from 24 to 34 kg N haa’ (i.e.
57-64% of accumulated stem N) to 46 to 56 kg
ha- ’ (SO-82%). More N was translocated from stems
than from leaves, under drained conditions (Table 3).
3.5. Root growth. color und actidty; culms
The amount of N in dead and senescent leaves Total root dry weight increased up to heading and
and also the corresponding N concentrations, in- was greater under drained conditions. It decreased 14
creased as crop age advanced. A largest quantity of to 23% in drained field conditions after reaching its
25 kg N in dead and senescent leaves was recorded peak value, compared to 26 to 29% in undrained
for IR20 at harvest (Expt. 2). The N concentration in plots, leading to final values (at harvest) that were 20
these tissues increased from 0.3% of dry matter soon to 36% greater in drained than in undrained treatafter
heading to 0.7% at maturity. Values were ments. Root biomass increased with N application
slightly lower in drained than undrained plots. level. The largest amounts were observed in Expt. 2

72 S. Ramasamy et al./ Field Crops Research 51 (1997) 65-82
Table 2
Effects of drainage and nitrogen application on N uptake (in kg ha- ’ ) by rice, Coimbatore, India
Treatment a Expt. 1 (IR50, 1990) Expt. 2 (lR20, 1990-91) Expt. 3 (IR50, 1991)
b FF’
final MT b FF’ final MT b FFc final
::DAT) (50 DAT) harvest (41 DAT) (71 DAT) harvest (33 DAT) (53 DAT) harvest
(88 DAT) (IIODAT) (91 DAT)
D;NIOO 28.3 84.1 135.1 54.5 171.3 221.9 50.8 100.2 137.7
D;N 150 34.3 102.0 150.1 61.3 186.1 235.2 61.8 115.1 156.0
D,-N200 37.7 107.7 154.9 71.1 188.6 239.4 73.3 127.3 153.3
D,.-NlOO 27.3 89.1 143.8 55.0 169.4 230.4 50.3 110.7 162.8
D;N150 30.0 102.5 157.8 61.6 184.5 236.9 55.7 126.9 110.2
D;N200 33.6 111.0 169.5 70.4 194.1 242.8 61.8 138.8 180.3
Drainage
SEd 0.3 3.4 2.2 0.6 5.5 2.9 1.9 0.6 3.8
CD * 1.0 NS 7.0 NS NS NS 5.9 1.9 12.1
Nitrogen
SEd 0.8 2.3 1.6 1 .O 2.1 3.2 1.1 1.6 2.3
CD 1 .I 4.9 3.6 2.3 5.9 7.0 2.3 3.6 5.1
D at N
SEd 0.9 4.3 2.9 1.3 6.4 4.1 2.2 2.0 4.6
CD NS NS 6.0 NS NS NS 4.8 NS 10.1
N at D
SEd 1.1 3.2 2.3 1.5 3.8 4.5 1.5 2.3 3.3
CD NS NS 4.8 NS NS NS NS NS 7.2
’ 0,: undrained: D,: drained; NlOO, N150 and N200: 100, I50 and 700 kg fertilizer N ha- ’ .’ MT: maximum tillering stage.’ FF: first
flowering stage.” NS: not significant at P = 0.05: CD: critical difference at P = 0.05.
(Table 1). The root-shoot mass ratio decreased from
0.20 at active tillering to 0.07 at harvest. It was only
slightly affected by drainage, with values 0.01 to
0.02 higher in drained plots at all stages. Nitrogen
application had a small negative effect on the
root:shoot ratio with differences of up to 0.03 between
NlOO and N200 treatments. Expt. 4 revealed
that root volume in drained plots was 65% larger
than in undrained plots, while root length was 50%
larger (Fig. 3). root diameter was 6-47% larger
(depending on nodal position) and root CEC 69%.
(All values determined just before maturity.)
The root oxidizing power in drained and undrained
plots was similar during early growth, but fell sharply
after panicle initiation in undrained plots. This was
not altered by N application at heading (Fig. 4).
The effects of drainage on root color were nearly
identical in Experiments l-3. Fig. 5 illustrates for
Experiment 3, as an example. the larger fraction of
white and brown roots found under drained condi-
tions at all stages of crop growth. In undrained plots,
the proportion of white and brown roots decreased
sharply after heading, and the fraction of black roots
increased to 68% at maturity. At that stage, roughly
two thirds of the root mass were still brown in
drained plots. versus one third in undrained plots.
Larger proportions of brown and white roots were
found in Expt. 2 than in Expts. 1 and 3. This may
indicate a cultivar difference although observed
trends were similar in all experiments.
Observations made in Expt. 4 on the thickness of
the culm wall and medullary cavity size indicated
differences with drainage treatment. Culm walls were
thicker in drained plots and cavity diameter was
smaller.
4. Growth analysis
In analyzing the above results, grain yield is
viewed as the net result of a series of crop physio-

S. Ramasamy et ~1. /Field Crops Research 51 (1997165-82 73
logical processes. During pre-flowering crop development,
the key processes are light interception, N
uptake and assimilation, dry matter production and
the formation of sink capacity (panicles, spikelets).
After flowering, the dominant processes are carbohydrate
production, mobilization of stored nonstructural
carbohydrate reserves (i.e., starch in stems
and leaf sheaths) and the partitioning of these substances
to the growing spikelets. Sustained functioning
of roots is indispensable for continued absorption
of nutrients, notably N, thus delaying leaf senescence
and ensuring continued biomass accumulation
(Sinclair and De Wit, 1975). Roots might, however,
also affect crop processes via other mechanisms. The
70 -
- IhNlOO - hN150 -hNZOO ‘--..--.DwNlOO
0 10 20 30 40 50 60 70 80 90 100 110
75 60
+ m 50
C
z 40
t 30
; 20
5 10
0
0 IO 20 30 40 50 60 70 80 90 100 110
70
s 60
+ m 50
c
z 40
5
[
30
20
5 10
0
0 10 20 30 40 50 60 70 80 90 100 110
Days
aftertransplanting
Fig. 2. Effects of drainage and N application on N content in green leaves of rice at Coimbatore, Tamil Nadu. India. Expt. 1: rice cultivar
IR50, 1990: Expt. 2: rice cultivar IR20, 1990- 1991: Expt. 3: rice cultivar lR.50. 1991. Standard error of N uptake at peak values: 1.6 (Expt.
1, at SO days after transplanting (DAT)). 2.3 (Expt. 2, 60 DAT), 1.2 (Expt. 3. 43 DAT) kg N/ha. For treatment details see text.

14 S. Ramasamy et al. / Field Crops Research 51 (1997165-82
observed effects of drainage on each of the above
intermediary processes are summarized in Table 4
and discussed below.
4. I. Yield components
Yield differences between drained and undrained
plots were directly related to the number of filled
grains per panicle. The number of panicles per m’
and the number of spikelets per panicle were not
much affected, or were even slightly smaller under
drained conditions. The formation of panicles and
spikelets is determined by the rate of crop growth at
the tillering and early ear development stages, respectively
(e.g., Yoshida and Parao. 1976; Seito et
al., 1990; Islam and Morison, 1992; Hasegawa et al.,
1994). The absence of differences in sink capacity
due to drainage is therefore consistent with the absence
of drainage effects on pre-flowering growth.
4.2. Pre-flowering growth, N uptake and leaf area
Dry matter production is determined by light interception
(leaf area) and light use efficiency, both of
which are closely related to leaf nitrogen content
(e.g. Sinclair and Horie, 1989). During the preflowering
stage, drainage did not significantly affect
growth in our experiments, nor did it alter the total
amount of N in leaves. Leaf area was reduced slightly
in response to drainage. These observations were
consistent in Expts. l-3.
4.3. Post-flowering growth, N uptake and green leaf
urea
During the grain filling stage, biomass accumulation
increased significantly due to drainage in all
three experiments. In Expt. 1, however, the extra
biomass production under drained conditions could
account for only 20% of the grain yield increase.
Miyasaka (19701 found that N uptake increased
after the panicle initiation stage with improved
drainage. N uptake in drained fields was indeed
greater in our Expts. 1 and 3, but not in 2 (Tables 2
and 4). Because larger grain yields were, nevertheless,
also attained in Expt. 2, it is concluded that
yield response to drainage is not necessarily based
on increased N uptake.
Table 3
Effects of drainage and N application on translocation of stem carbohydrate reserves and nitrogen (N,) in race cultivar IR50 (Expt. 3) at
Coimbatore, Tamil Nadu. India, during 1991
Treatments N,,, N at maturity (kg/ha) a N, (kg/ha) Wkwx Stem carbohydrates
(kg/ha) a
green leaves dead leaves ’
translocated (kg/ha)
total h
Leaves
0”.N-loo
D;N150
Q-N200
42.7 I .l 14.2 (0.33) 15.9 26.8 0.63
51.1 2.1 15.2 (0.30) 17.3 33.0 0.66
57.8 2.5 18.8 (0.32) 21.3 36.5 0.63
Q-N100
D;N150
D, -N200
45.2 8.1 12.1 (0.27) 20.2 25.0 0.55
5 1.2 9.3 13.1 (0.26) 22.4 28.8 0.56
51.2 10.0 13.8 (0.24) 23.8 33.4 0.50
Stems
D;NlOO
D;N150
D;N200
D;NIOO
D;N150
D, -N200
40. I 16.7 (0.40) 23.9 0.60 2400
53.2 18.9 (0.36) 34.3 0.64 2647
56.3 - 24.0 (0.43) 32.3 0.57 2483
57.6 - 11.3 (0.20) 46.3 0.80 2857
64.3 - II.3 (0.18) 53.0 0.82 3100
68.6 12.2 CO.18) 56.4 0.82 3187
* N,,, IS the maximum amount of N observed in plant parts (leaves. stems) at any ttme during the growth period. Calculation of N, is
explained in the text.
’ Figures in parentheses are amount of N in dead tissue, relative to N mar of the corresponding organ.

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.

76 S. Ramasamy et al./ Field Crops Research 51 (1997165-82
4.4. Translocation
Though a large fraction of the carbohydrates accumulated
in grains is derived from photosynthesis
during grain filling, translocation from previously
stored reserves seems essential (Weng et al., 1982)
and is highly correlated with grain filling and spikelet
sterility (Lin et al., 1994). Apart from providing
carbohydrates for grain filling, it has been suggested
that translocation plays an important role in favoring
the transport of NADH-dependent glutamate syn-
thase (GOGAT) protein, which in turn favors grain
filling (Hayakawa et al., 1993). Others (Vorob-ev
and Skazhennik, 1987) reported that mobilization,
translocation and grain filling are linked with crop N,
P and K uptake, but mentioned no mechanisms.
Reported contributions of remobilized carbohydrates
to grain biomass vary from 20% (Cock and Yoshida,
1972) to 54- 62% (Vorob-ev and Skazhennik, 1987).
Starch from vegetative tissues near to the panicle is
generally mobilized, but remote vegetative parts may
be left with large residues at maturity. Larger resid-
Table 4
The effects of drainage on the key yield formation processes, observed in rice cultivars IR50 (Expts. I. 3) and IR20 (Expts. 2, 4) at
Coimbatore.Tamil Nadu. India, during 1990- 1992
Grain yield
panicles (m - ’ )
total spikelets cm-‘)
filled spikelets (m ’ )
individual grain wt
Expt. 1 Expt. 2 Expt. 3 Expt. 4
+
0
-
+
0
-
+
+
+
0
-
+
0
+
-
-
+
+
Pre tlowering biomass accumulation
green leaf area
green leaf N
green leaf f,,
0
-
0
-
0
-
0
0
0
-
0
n.d.
n.d.
n.d.
n.d.
Post flowering biomass accumulation
green leaf area
green leaf N
green leaf f,,
dead leaf biomass
+J
+
+
0 h
-
+
o’h’
+
-
+
+
+
+
-
n.d.
n.d.
n.d.
n.d.
n.d.
Translocation of non structural reserves
+
+
+
nd
Root growth
pre flowering
pre flowering
post flowering
post flowering
root mass
root length
root mass
root length
0
n.d.
+
n.d.
0
n.d.
+
n.d.
0
n.d.
+
n.d.
0
0
+
+
Root function. activity
root color after panicle initiation
pre flowering crop N uptake
post flowering crop N uptake
post flowering root N concentration
pre flowering oxidizing power
post flowering oxidizing power
N removal from senescent leaves
+
+
+
n.d.
n.d.
+L
+
0
0
+
n.d.
n.d.
+’
+
-
+
+
n.d.
n.d.
+
n.d.
n.d.
n.d.
n.d.
0
+
n.d.
( + ) significant increasing effect of drainage. (- ) significant decreasing effect of drainage. 0 no significant effect of drainage (P = 0.05).
For explanation of f,, see text.
A Drainage effect on crop biomass (200 kg/ha) was much less than on grain yield (1000 kg/ha).
’ Drainage effect after flowering was first slightly negative, then slightly positive.
L Drainage effect only noticed at end of grain filling (last 2 weeks).

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

78 S. Ramasam~ et al. /Field Crops Research 51 (19971 65-82
these substances through leaching (Govindasamy and
Chandrasekaran, 1979).
Healthy rice roots oxidize their immediate environment,
developing a brown coating of ferric oxides
and hydroxides (Shioiri, 1944). Japanese literature
refers to this ability as ‘oxidizing power’. It involves
the production and release of both peroxidase and
hydrogen peroxide by root tissues (e.g., Kumazawa,
1984). The oxidized rhizosphere protects the root
against free H,S, which inhibits root respiration
(Mitsui et al., 19511, and Fe’+, which also affects
root metabolism (Kumazawa, 1984). Black coloration
is due to FeS precipitation, implying the
presence of high concentrations of Fez+ and S’-
near the root surface. Black roots have low functional
activity and little oxidizing power (Yanatori,
1981; Cheng, 1983). Black coloration of roots has
also been attributed to other causes, including potassium
deficiency (Mitsui and Kumazawa, 1964;
Trolldenier, 1977; Kumazawa, 1984); and, in the
special case of old degraded paddy soils from which
Fe and Mn have been leached out (Kumazawa,
1984), insufficient availability of Fe’+ to buffer the
redox potential above the point where SOi- is
reduced. The latter cause can be eliminated in the
present study, but K deficiency remains a possible
cause of black coloration in our undrained treatments.
Doberman et al. (1996a) indeed reported that
the K supply rate from soils at Coimbatore near the
site of our experiments was only modest. Grain
yields in their experiment (Doberman et al., 1996bl
increased from 4840 to 5590 kg ha-’ upon application
of 41 kg K ha- ‘. This argument is, however,
weakened by the facts that our yield levels were
1200 to 2000 kg ha- ’ greater than those of Doberman
et al., even in undrained plots, and that all plots
in our experiment received K at the rate applied in
Doberman’s ‘plus K’ treatments.
If the K deficiency hypothesis is nevertheless
correct, two mechanisms can be forwarded to explain
why black coloration due to K deficiency should be
more pronounced in undrained plots: (a) K’ availability
is likely to be less in more reduced soil
because higher Fe” concentrations tend to eliminate
K+ from the adsorption complex, and subsequently
from the root zone by leaching (high Fe’+ also
impairs K‘ transport through the rhizosphere by the
lowering of rhizospheric HCO, concentration, re-
sulting from the acidification associated with Fe*’
oxidation near the root); (b) K is a key factor in the
processes underlying ‘oxidizing power’ and Kf
would thus be specifically required in reduced conditions
to maintain healthy roots. The second idea is
suggested in the summary made by Kumazawa (1984)
about early Japanese research on rhizosphere oxidation.
Phosphorus deficiency would be another explanation
of the above type (i), and documentation of
the Coimbatore site by Doberman et al. (1996~) can
support this hypothesis, although the same counterarguments
as given in the case of K would apply
(i.e., differing yield levels and blanket P doses applied).
Moreover, an explanation is lacking why P
deficiency would be expressed more under poor
drainage. Yet, we cannot reject the (K or PI deficiency
hypothesis for our experiments based on
available information.
5.2. Mechanisms of type (ii)
The presence of toxic compounds would involve
an adjustment in the plant’s functioning, arising either
from entrance of the compounds in the plant, or
from deterioration of the root system in the low-redox
environment, curtailing root functions other than
the uptake of nutrients. An hypothesis can be put
forward that the crop response to drainage is caused
by an effect of soil condition on the production of
growth substances in roots. Evidence in favor of this
mechanism as the primary cause of yield response to
drainage is summarized below.
Grain filling in cereals is mediated in part by
cytokinins (e.g.. Ray and Choudhuri, 1981; Gabali et
al., 1986; Yoshida, 1987; Soejima et al., 1992; Smiciklas
and Below, 1992). Cytokinins have a positive
effect on leaf longevity (Ray and Choudhuri, 1981;
Herzog, 1982; Ambler et al., 1983; Soejima et al.,
1992): on early cell division in developing grains
and grain size (Herzog, 1982); on the number of
vascular bundles in peduncle cross sections (Kaur
and Singh, 1987); and on stomata1 aperture (Davies
and Zhang, 1991). Cytokinins are produced in roots
and, in rice, their presence in root tissue is strongly
affected by root condition (Soejima et al., 1992;
Bano et al., 1993) and root development (Jiang et al.,
1994a; Jiang et al.. 1994b). These findings, combined
with our own observations that drainage posi-

S. Ramasamy et al. /Field Crops Research 8 I (1997) 65-82 79
tively affected post-flowering root growth, root condition,
grain filling, leaf longevity and mobilization
of reserves (Table 41, support the hypothesis. More
specifically, the primary effect of drainage would be
to improve root condition, inducing a prolonged
synthesis and transport of cytokinins, resulting in
extended photosynthetic activity and increased deposition
of carbohydrates in grains. Numerous studies
have demonstrated that grain filling and grain yield
in rice improved in response to externally applied
cytokinins (Yoshida, 1987; Kaur and Singh, 1987;
Samantasinhar and Sal-m, 1990), as well as in maize
(Smiciklas and Below, 1992), barley (Aufthammer
and Solansky, 1976) and wheat (e.g., Herzog, 1982).
One way to test this hypothesis experimentally would
be to evaluate whether the yield response to external
application is stronger in poorly drained than in
well-drained soils.
5.3. Mechanisms of qpes (i) and (ii) combined
The possibility cannot be ruled out that both
mechanisms (i) and (ii) should be linked to explain
the effect of drainage on rice yield. Thus, strongly
reduced soil conditions might cause increased K
demand for maintaining ‘oxidizing power’; and if
this K demand is not met, root condition deteriorates
due to the entrance of reduced products into the root.
As a result, the production and transport of cytokinins
in roots decreases, which leads to poor grain
filling due to decreased sink strength and enhanced
leaf senescence.
5.4. Excess nitrogen
The last point to address is the effect of excess
nitrogen application. In undrained plots, high N application
levels (200 kg/ha) consistently decreased
grain yield in Expts. l-3 (Table 11. N application at
heading (Expt. 4) also lowered grain yield in
undrained plots (Table 1). Why did this occur only in
undrained plots? Two possible explanations are derived
from the literature. The first is based on further
lowering of soil redox potential, triggered by N
application (Trolldenier, 1977). If conditions are already
poor, the crop may be more susceptible to
further reduction. The second possible explanation is
based on increased N uptake leading to lower Si:N
ratios in the plant, subsequent degeneration of panicle
branches and spikelet sterility (Choi et al., 1986).
For this to be a valid explanation in our case, Si
uptake must have been lower in undrained plots.
Lower Si uptake under poor drainage has indeed
been reported (Kumazawa, 1984), but cannot be
confirmed here as we did not measure Si uptake. The
thin culm walls observed in our undrained plots,
however, are often seen as a symptom of Si deficiency.
Other possibilities include negative effects of
high soluble N concentrations in plants per se (Murty
and Murty, 1982); and possible interactions between
N nutrition and endogenous cytokinin levels
(Smiciklas and Below, 1992, for maize).
Both experiments showing a negative response of
yield to high N were second season experiments. We
presume that the soil was in a more reduced state
during the second season due to (a) the greater
supply of organic matter as the first season’s stubble
was incorporated into the soil and (b) prolonged
flooding as a result of the longer vegetative stage of
IR20 and antecedent (first-season) flooding.
6. Conclusions
Rice grain yields increased in response to improved
internal field drainage, in all four experiments
and at all N application levels. The number of
panicles and the number of spikelets per panicle
were not - or negatively - affected by improved
drainage and the increases in grain yield could thus
be attributed fully to better grain filling in all cases.
Yield responses to drainage were always associated
with larger numbers of filled grains, increased
translocation of stored reserves and improved root
condition.
Yield response to N application in rice was larger
under well-drained than poorly drained soil conditions.
It seems that in ‘second season’ crops (i.e.,
after prolonged flooding) under poor drainage, large
N doses should be avoided because they may lead to
yield loss. N application at heading, too. appears to
be effective only in well-drained soils and should
otherwise probably be avoided.
Yield increases were often associated with increases
in crop biomass, green leaf area and leaf area
duration. leaf N content, crop N uptake; and also

80 S. Ramasamy et al./ Field Crops Research 51 (1997) 65-82
with increased efficiency (f,,) by which leaf N
converted radiation into dry matter. Yet, it is concluded
that yield response to drainage cannot, or not
exclusively, be attributed to increased N uptake and
the enhanced ‘source capacity’ normally associated
with it. This observation, combined with consistently
positive responses of indicators of root condition to
drainage, indicates that the beneficial effect of
drainage is via root functions other than N uptake.
One possible mechanism is that drainage increases
K availability or reduces the crop’s K requirement.
The other is that drainage improves root
condition, which leads in turn to a larger flow of
cytokinins to the shoot, where they retard leaf senescence
and stimulate translocation of reserves from
stems to grains. The two mechanisms are possibly
linked, K deficiency triggering the deterioration of
root condition. Although poor drainage, cytokinin
production in roots and grain yield have not been
connected explicitly in rice literature, the causal
relations between drainage and root condition, between
root condition and cytokinins production, and
between cytokinin levels, grain filling and leaf senescence
are each well documented and provide the
basis for the cytokinin hypothesis. The various hypotheses
were not explicitly tested in this study.
References
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concentrations in xylem sap of senescent and nonsenescent
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Aufthammer, W. and Solansky, S.. 1976. Effect of kinetin on
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Bodenkd., 1976(41: 5033515.
Bano, A.. DoertIling, K., Bettind, D. and Hahn, H., 1993. Abscisic
acid and cytokinins as possible root-to-shoot signals in
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20: 109-115.
Cheng, Y.S., 1983. Effects of drainage on the characteristics of
paddy soils in China. In: Organic Matter of Rice. IRRI, P.O.
Box 933, Manila, pp. 417-427.
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