Influence of solar radiation and temperature on irrigated rice grain yield in Bangladesh

Field Crops Research, 30 (1992) 13-28 Elsevier Science Publishers B.V. Amsterdam

13

Influence of solar radiation and temperature on irrigated rice grain yield in Bangladesh

M.S. I s l a m ~ a n d J . I . L . M o r i s o n 2 Dept. of Meteoro!ogy, University of Reading, P.O. Box 239, 2 Farley Gate. Whiteknights,

Reading RG6 2AU, UK

(Accepted 22 March 1991 )

&BSTRACT

Islam, M.S. and Morison, J.I.L., 1992. Influence of solar radiation and temperature on irri$atrd rice grain yield in Bangladesh. Field Crops Res., 30:13-28.

Results are presented from several shading and time-of-planting experiments to investigate the effects of total incident solar radiation (irradiance) and temperature on irrigated rice grain yield in Bangladesh. The substantial influence of irradiaace in the reproductive (panicle initiation to anthesis) and ripening periods (anthesis to me~urity) on yield and yield components is derr.onstrated. In all experiments where radiation varied substantially there was a linear relation between grain yield and irradiance during the reproductive and ripening stages. In contrast, grain yield was negatively correlated with mean temperature during the 30 days preceding anthesis. The effects of temperature and irradiance could be combined by the use of the photothermal quotient (Q, ratio of mean daily irradiance to mean temperature above a base temperature), which explained most of the variation in grain yield for any one cultivar with different amounts of shaal ~g and different planting dates. The relation between Q and yield varied between varieties because of differences in spikelet numbers, grain weight and percentage sterile spikelets, as well as other unidentified factors. The relation also varied significantly between seasons for one of the three varieties for which data were available. The mean annual time course and variation of Q were calculated for Joydebpur, and the yield of trial plots related to it.

i N T ~ O D U C T I O N

The rice-growing calendar in Bangladesh is divided into three seasons: Aus, transplant and deepwater Aman, and Boro. Table 1 gives some cropping details. Cultivars grown in the Aus season are photoperiod insensitive and are generally grown as a rainfed crop, both broadcast and transplanted, from March to August. Aman is the most important rice crop and covers about 40% of the total rice area in Bangladesh. All indigenous Aman cultivars are sensitive to photoperiod, but modern transplant cultivars are either insensi-

~Present address: Plant Physiology Division, Bangladesh Rice Research Institute, Joydebpur, Gaz'.:pur, Bangladesh. ZTo whom correspondence should be addressed.

0378-4290/92/$05,00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

14

TABLE 1

Rice cropping details in Bangladesh

M.S. ISLAM AND J.I.L. MORISON

Rice seasons Growing Total area Area with Mean grain Yield of season (Mha) modem yield modem

cultivars (kg ha-~ ) varieties (%) (% of mean)

Aus Mar.-Aug. 2.97 16 1527 189 Aman

Transplant June-Jan. 4.56 24 2214 137 Broadcast /Deepwater Mar.-Jan. 1.35 0 1553 -

Boro Nov.-June 1.50 78 3633 111

Source: BBS (1987), averages of 1983/84, 1984/85 and 1985/86.

tive or only slightly sensitive to photoperiod (BRRI, 1984). The deepwater or "broadcast' Aman crop is either sown in March or April, alone or mixed with the Aus crop and transplanted in May. While the Aus crop is harvested in July or August the broadcast Aman crop copes with the Monsoon floods at water depths of 0.5 to 4.0 m from June to September and is harvested in December or January. The ' t ransplant ' Aman rice crop is t ransplanted from July to September in areas where water depths usually do not exceed 0.5 m. The Boro crop is grown during the dry winter as an entirely irrigated crop. Seedbeds are made from October to December, seedlings are transplanted from December to February and the crop is harvested from late April to June. Fur ther details have been given by Haque (1987) . Over all the seasons the cultivation of m o d e m rice cultivars has increased from less than 3% of the total area in 1969/70 to an average of 26% in the years 1983/84 to 1985/86 and now produces over 40% of the total rice harvest in Bangladesh (Table 1 ).

The yield of m o d e m cultivars grown as irrigated crops in Bangladesh varies seasonally from 4 to 7 t h a - ~. The largest grain yields are usually obtained in the Boro season, followed by the Aman season crop with the smallest yields occurring in the Aus season. This pattern reflects the differing environmental conditions, in particular, variat ion in tempera~:ure and total incident solar radiation ( irradiance). Yield variation has been attr ibuted to high temperature in the reproductive phase (defined as the period from panicle initiation to anthesis; Robertson, 1975 ) during the Boro season, low temperature in the same phase during the Aman season and low irradiance or a combinat ion of low irradiance and high temperature over the whole growth period of the Aus crop (Haque et al., 1983; Haque and Islam, 1984; Haque, 1987). Lower yields of irrigated rice during the wet season compared to the dry season have long been at tr ibuted to low radiation and there have been several studies examin- ing the effect of variation in irradiance and temperature on rice grain yield in the tropics. Results from date-of-planting experiments and multi-location trials

INFLUENCE OF SOLAR RADIATION AND TEMPERATURE ON RICE GRAIN YIELD 15

have demonstrated high correlations between grain yield and solar radiation during the ripening period (Moomaw et al., 1967; Sahu et al., 1983; Seshu and Cady, 1984), during the reproductive period (Yoshida and Parao, 1976) or during the whole period from panicle initiation to maturity (IRRI, 1967; De Datta and Zarate, 1970; Evans and De Datta, 1979). In particular, Evans and De Datta (1979) fc.und that the highest correlations between grain yield and solar radiation were in the 20-day periods before and after flowering in both older and more modem varieties, and that the positive response to i_rra- diance was greater at higher levels of N fertiliser application. In a shading experiment, Yoshida and Parao (1976) found that the reproductive phase was more responsive to irradiance than the ripening phase and that the overall effect of irradiance during the vegetative phase was extreme!y small. Clearly, solar radiation in the ripening period is important for crop photosyn- thesis and hence grain filling, and is also important in the reproductive period when the number of spikelets is being determined (Yoshida and Parao, 1976; Yoshida, 1983 ). The length of these two periods is determined by the varietal characteristics and the temperature so that within a moderate range, cooler temperatures in the reproductive period increase the number of spikelets (Yoshida and Parao, 1976; Yoshida, 1983; Oldeman et al., 1987).

Multiple regression studies have incorporated the influence of temperature into the relationships between radiation and rice yield, but only using linear or quadratic terms (e.g. Yoshida and Parao, 1976; Seshu and Cady, 1984). However, as temperature affects the developmental rate (the reciprocal of the duration of any phase) it seems attractive to incorporate temperature in a more physiologically meaningful way. In particular, Fischer ( 1985 ) has sug- gested describing the influence of solar radiation and temperature on wheat grain growth by the use of the photothermal quotient (Q), defined by Nix (1976) as the ratio of the mean daily total incident solar radiation for an interval to the mean temperature less the base temperature in units of MJ m-2 day-~ ° C - ' . The photothermal quotient can therefore be considered as an index of growth per unit of developmental time. The influence of irradiance and temperature on rice grain yield has not been examined in this framework before. In addition, rice is potentially easier to understand than wheat as there is only one floret in each spikelet and so yield is simply the product of the number of panicles m-2, the number of filled spikelets per panicle and the grain weight (Murata and Matsushima, 1975; Yoshida, 1983). In this paper we analyse grain yield and yield components in a number of shading experiments and field trials which permit the testing of the Q model at the Bangladesh Rice Research Institute.

M A T E R I A L S A N D M E T H O D S

All the experiments reported here were at the Bangladesh Rice Research Institute at Joydebpur, Gazipur (lat. 24°0.0'N, long. 90°26.1 'E) during 1982,

16 M.S. ISLAM AND J.I.L. MORISON

1984 and 1988. All experimental crops were well-managed with irrigation, high fertility, and adequate weed and disease control. The long-duration cultivar BR3 ( 'Biplab') was used in all experiments because this is the only cultivar suitable as Boro, Aus and transplant Aman crops. A number of other cultivars and lines were used in 1984 and 1988, with the short-duration cultivars BR 1 and BR9 being used in both these experiments.

Air temperature was meaeured in a standard meteorological screen and the solar irradiance measured with an integrating pyranometer (model R/EIP, Rimco) regularly checked against a pyroheliometer.

The dates of panicle initiation, anthesis and maturity, and the final grain yield (Y) and yield components were recorded in all experiments. In some, the above-ground dry-matter production was also measured. The spikelet number m -2 (S) consists of filled and unfilled spikelets, whereas grain yield m -2 consists of wholly filled grain determined after harvest. Partially filled grains were further separated from sterile spikelets. The methods used in each experiment are described below.

Effect of shading at different growth phases Forty-five-day old seedlings of BR3 were transplanted on 15 January 1982

at a spacing of 25X 15 era. Nitrogen fertiliser was applied in three instal- ments: 30 kg N ha - ~ at the time of land preparation, 30 kg N ha- ~ at 15 days after transplanting (DAT) and 20 kg N ha - ~ at panicle initiation. Both P and K were applied at the time of land preparation at rates of 60 and 40 kg ha - 1, respectively. Fields were prepared as puddled wetland and fully irrigated (flooded to a depth of 3-5 cm ) during the entire crop season. Shading treatments were given either during the (i) vegetative, (ii) reproductive or (iii) ripening phases. A randomised complete block design (RCB) was used with three replications. Plot size was 1.0 × 1.05 m with suitable guard rows and to avoid disturbance in the experimental plots an additional 10 × 10-m plot was transplanted with similar seedlings for panicle initiation determination. Shaded plots covered with cloth during the daytime received about 13% of the total solar irradiance.

Effect of different levels of shading during two growth phases Three 20-day old seedlings of BR3 were transplanted into 25-cm diame-

terX 20-era deep pots containing soil fertilised with N : P : K at a rate equivalent to 60:100:80 kg ha-~ and were thinned to single plants after 10 days establishment. Additional nitrogen was applied 20 DAT and at panicle initiation in lots of 50 and 40 kg N ha - ~, respectively. After the last application, 70 similar pots were divided into groups of ten assigned to t~:ree shading treatments (receiving 78, 48 and 23% of incident irradiance) at two different periods and one control plot receiving full sunlight throughout. An RCB design was used with each pot as a replicate within each group.

TAB

LE 2

Gra

in y

ield

and

yiel

d co

mpo

nent

s of

BR

3 w

ith sh

adin

g du

ring

thre

e gr

owth

pha

ses i

n B

oro

seas

on

Trea

tmen

t ~

Gra

in y

ield

P

anic

les

Spik

elet

s Fi

lled

Gra

in w

eigh

t H

arve

st

(gm

-2 )

(No.

hil

l-l )

gr

ain

(rag

) in

dex

No.

pan

icle

-I

No.

m -2

(%

)

Abo

ve-g

roun

d st

raw

wei

ght

(gm

-2)

Con

trol

566a

2 1 l

.la

88.3

a 26

268

a 84

.2a

27.7

a 0.

59a

397a

Sh

adin

g dur

ing

474a

lO

.4a

82.1

a 23

061

a 84

.9a

27.3

a 0.

55ab

36

8a

tiile

ring

Shad

ing d

~rin

g 11

2b

9.0a

53

.4b

13 0

38b

59.7

b 25

.1b

0.38

b 17

5c

repr

oduc

tive

phas

e Sh

adin

g dur

ing

94b

10.6

a 82

.0a

;23

273a

37

.4c

22.6

c 0.

3 lb

24

2b

ripen

ing

phas

e

tSha

ding

was

abo

ut 1

3% o

f the

con

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reat

men

t. M

ean

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r rad

iatio

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cide

nt o

n co

ntro

l= 2

2.2

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day

-t a

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over

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row

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d,

and

22.9

, 21.

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rage

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d rip

enin

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ases

, res

pect

ivel

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ring

the

sam

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ter(

s) w

ithin

a c

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t sig

nific

antly

diff

eren

t at P

= 0

.01,

usi

ng D

unca

n's

mul

tiple

rang

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t.

tn,a

-,

d

18 M.S. I S L A M A N D J . L L . M O R I S O N

Effect o f naturally varying temperature and solar radiation Thirty- to 45-day old seedlings of BR3 were planted at 15-day intervals

during December through to March (Boro season) 1982 under field condi- t ions giving eight planting times. Cultural condit ions were as described in Exper iment 1 and an RCB design was used with plot sizes of 6 × 7 m.

Effect o f climatic factors on different cultivars and lines Thirty-day old seedlings ~f BRI , Suweon 287, BR3, IR:~9660-311, BR9,

BR9-HR7 and BR8 were t ransplanted in pairs on the 15th and 30th day of each m o n t h (29th in February) in 1984 to hills spaced at 2 5 × 15 cm with each cult ivar in six rows of 5 m length. Fertiliser was applied at 60 kg N, 26 kg P and 33 kg K ha-~ and fields we;e prepared as puddled wetland as in Experiment 1. Measurements were taken from 20 randomly selected hills from the middle four rows.

A similar exper iment was carded out in 1988 using the modern cultivars BR 1, BR3, BRg, BR 14 and the t radi t ional short-durat ion cultivar Dharial.

RESULTS

Responses to shading Shading to 13% of the irradiance of the control plots during the vegetative

phase did not have any significant overall effect on yield or on any of the yield components of cult ivar BR3 grown during the Boro season in 1982 (Experi-

1 2 0 , , , , , , . . . . , , , , 'I " ' , , ' , " ' - ' , " '

.--I

o 1 0 0 ~ n.,. i - . .

0 u

8 0 I t . 0

6O _ J

> -

"7 4 0

2 0

% \

\

0 0 2 0 8 0 1 O 0

P E R C E N T S H A D I N G

Fig. 1. Grain yield as a percentage of the control caused by three levels of shading during the reproductive ( [] ) or ripening ( A ) phases of growth of irrigated rice, cultivar BR3 (Boro season, 1983).


TABI,E 3

Grain yield and yield components o f BR3 with three levels o f shading during either the reproductive or ripeni~,o- phases in Boro season, 1983

Shading t Grain yield Panicles Spikelets Filled Grain Harvest Aboveff~round (%) (g pot-1 ) (No. po t - l) grain weight index straw weight

No. panicle -I No. pot - I (%) (rag) ( gpo t -~)

Control 0 59.9a 2 42.5c 102.1a 4338a 76.3a 27.5ab 0.41a 85.5a

(100) 3

Reproductive phase 22 48.8b 42.5bc 87.1bc 3703d 75.8a 27.6a 0.38a 78.2ab

(81.4) 52 40.4c 40.2c 79.1cd 3181e 78.3a 27.lab 0.34bc 76.7b

(67.5) 77 31.8d 43.5bc 68.9d 2998f 78.2a 26.8b 0.30c 76.4b

(53.1)

Ripening phase 22 47.2b 49.3a 83.8bc 4133ab 70.1b 27.0ab 0.38a 74.2b

(78.8) 52 40.9c 46.2ab 86.9bc 4015be 63.3c 25.7c 0.35b 72.7b

(68.3) 77 16,1e 42.5bc 90.4b 3843cd 44.7d 24.3d 0.22d 57.2c

(26.9)

t Mean solar radiation incident on control---- 19.2 MJ m -2 day-t averaged over the reproductive period, and 22.7 MJ m -2 day - t over the ripening period. 2Data bearing the same letter ( s ) within a column are not significantly different at P=0.01 , using Dvncan's multiple range test. SIndicates grain yield as a percentage o f the control.

ment 1, Table 2) . In contrast, shading during the reproductive and ripening phases caused significant reductions in yield by reducing panicle number, final spikelet number, filled grain percentage, grain weight and in consequence reduced the harvest index. In the reproductive period the relative sensitivities (relative change in component as a ratio of relative change in irradiance) o f panicles m-2 , spikelets per panicle, grain weight and spikelets m - 2 were 0.22, 0.44, 0.11 and 0.58, respectively. In particular, reduction of irradiance in the ripening period caused a 35% reduction in yield because of a decreased pro- portion of filled grains and reduced individual grain weights. Examination of unfilled spikelets showed an increase in partially filled grain but not an increase in sterility. Reduced light in the reproductive phase affected above- ground dry-matter production drastically whereas in the ripening phase grain yield alone was most affected.

Figure 1 shows the grain yield of BR3 plants grown in pots during the Boro season in 1983 with three levels of shading during the reproductive or ripening phases (Experiment 2). Grain yield was reduced progressively as irradiance was reduced. The effect of 22% and 52% shading during either the repro-


800

E o~ 600

4 0 0

0 200

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e. A A

% - - " " ' ~ ' • • ~ ' 8 " " ' s o TEMPERATURE, °C

, , . . . . . . . . . . . . . . . . . ~

Q

A I I A Q

j C

,rl

6 ~

, 2 ' ' " i , ' " " , ~ ' ' ' , ~ ' " ' 2 ~ " " ' 2 2 o S ' ' o . ' 9 ' ' ' , : , ' ' ' ~ . ' 3 ' IRRADIANCE, MJ m'2day -1 Q, MJ rn-aday "l *C "I

Fig. 2. Grain yield (Y) as a function of (a) mean temperature, (b) mean irradiance and (c) photothermal quotient (Q) during the 30 days prior to anthesis. Symbols: /x, cultivar BR3 in Boro season 1982; iq, cultivar BRI in Boro and Aus seasons 1984. Lines drawn are fitted regressions with equations: (a) BR3 Y= 3003-- 90.6 × Mean temperature**

BR1 Y - - 2391 - - 73.3 × Mean temperature* ( b ) B R 1 Y = - 546 + 52.0 X Mean irradianee** (c) BR3 Y= -2200+2365×(2***

BR 1 Y= - 395 + 762.1 × Q~**. where *, ** and *** indicate regression is significant at P< 0.05, < 0.01 and < 0.001, respectively.

ductive or r ipening phase was the same but 77% shading had a much larger effect when applied dur ing the r ipening phase. The yield component data (Table 3 ) indicate the very different nature of the effects of dense shading between the two periods: dur ing the reproductive phase, spikelet number is tbe componen t most reduced by shading but dur ing r ipening the proport ion of filled grain and the individual grain weight are more affected. Indeed, the relative sensitivity of spikelet n u m b e r per pot to irradiance was not constant with different degrees o f shading, being 0.67, 0.51 and 0.40 for the reproductive phase with shading levels of 22, 52 and 77%, and 0.21, 0.14 and 0.15 at corresponding shading for the r ipening phase. This suggests some ability to adjust assimilate supply to the growing panicle at lower irradiances in the earlier period. The significance of this is hard to assess for field crops as pot exper iments lack the natural pat tern of competi t ion.

The results demonstra te the substantial effects of shading on the yield and yield components and also suggest that the relation between grain yield and irradiance (Fig. 1 ) during the r ipening and reproductive phases of the rice crop is linear.

Responses to natural variations in temperature and irradiance Results f rom serial plantings of BR3 during the Boro season in 1982 (Ex-

i3edment 3 ) and BRI during both the Boro and Aus seasons in 1984 (Exper- Llaent 4) are shown in Fig. 2. The ranges in temperature ( 16-26 °C) and ir-


TABLE 4

Intercept, slope and standard en _.., for the linear relations office grain yield and photothermal quotient

Season and year cultivar a ( g m -2)

s.e. b s.e. r F N (gday °C MJ -I )

Boro 1982 BR3 --2200 (213) 2365 (184)

Boro 1984 BRI --417 (149) 782 (137) BR3 -- 1102 (286) 1570 (290) BR9 --997 (225) 1378 (217) 6CVs I -- 1022 (120) 1446 (117)

Boro and Aus 1984 BRI --395 (45) 762 (95)

Boro 1988 BRI --438 (75) 777 (73) BR3 -- 507 (95) 954 ( 105 ) BR9 --656 (99) 1123 (102) BRI4 --104 (105) 481 (112) Dharial --315 (87) 523 (83)

0.98 165.2"** 7

0.92 32.4** 8 0.91 29.3** 8 0.93 40.5*** 8 0.88 152.6"** 48

0.93 64.5*** 12

0.98 t13.3"** 7 0.97 83.0*** 7 0.98 121.1"** 7 0.89 18.4** 7 0.94 39.9** 7

Yield trials in i 984 and 19872 Boro, Ausandtranspl . --243 (66) 715 (72) 0.74 97.9*** 84 Aman B o r o a n d A u s o n l y --360 (73) 836 (7d) 0.81 114.3"** 60

'The six cultiv~rs/lines were Senweon 287, BR3, IR 19660-311, BR9, BR9-HR7 and BR8. 2For details of yield trials see Discussion; 14-20 cultivars pooled. ** and *** indicate regression is significant at P < 0.01 and < 0.001, respectively.

radiance ( 14---20 MJ m - 2 d a y - t ) were much larger in Experiment 4. Grain yield was negatively correlated with the mean temperature during the 30 days preceding anthesis in both experime nts (Fig. 2a ). The yield components were assessed in Experiment 3 and show large reductions in spikelet number (from 35 to 25 × 103 m -2 ), large increases in sterility (from 13.5 to 30.0%) and also small decreases in grain mass (from 30.1 to 28.1 mg gra in - t ) with decreasing temperature. Because of the small range of irradiance occurring in Boro 1982 (20-22 MJ m -2 day-1 ) there was no real correlation between radiation and grain yield in this season, though a positive relationship is clear from the results of the 1984 experiment (Fig. 2b ). However, in both experiments grain yield was positively and closely correlated with the photothermal quotient, Q (calculated with a base temperature of 10 ° C; Fig. 2c). The correlations were closer with Q than with either temperature or irradiance alone, though different for the different cultivars (see Table 4 for details).

Similarly, data for six cultivars or lines grown in the Boro season of 1984 (BR3, Suweon 287, IR19660-311, BR9, BR9-HR7 and BR8) are shown in Fig. 3 and again, the clearest relation is between grain yield and Q. The data

22 M,S. ISLAM A N D J.I.L. M O R I S O N

800 ~ ; . . . . . . . . . . . . . . . . . . . . . . . ¢, . . . . . . . . . . . . .

,, ~ o 8 = 8 = = 8 9" E "= I % =s00 o~ ~

a ~ " ~ 0 . e

024 ~ ' 2=6 ' ' ' 218 " ' ~ 3 0 1 2 ~ " I " ~ ,~ ' ' I ' 6 " ' ' I ' 8 " ' " 2 ( ) " ' ' 2 2 0 . 7 ~ " "0.~9 " ' " I . ' I " ' ' 1 . ~ 3 "

TEMPERATURE, *C IRRADIANCE, MJ m-Zday -: Q, MJ m-2day - I °C -t

Fig. 3. Grain yield of cultivars Suweon 287 ( O ) , BR3 (F'I), ]R 19660-3 ] 1 ( A ), BR9 (<>), BR9-]-]R7 ( • ) and BR8 ( & ) grown in Boro season, ]984, as a function of (a) mean temperature, (b) mean irradiance and (c) photothermal quotient ((2) during the 30 days prior to anthesis. Lines drawn are fitted regressions with equations: (a) Y= 2453 -- 7 l. 8 X Mean temperature* (b) Y = - - 1 0 8 2 + 8 4 . 7 X M e a n irradiance** (e ) Y = - - 1022+ 1146×(2"** where *, ** and *** indicate regression is significant at P < 0.05, < 0.01 and < 0.001, respectively.

800

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Z ,.¢. el,,.

200

t

~ ° i, []

• % 0 %

b

o

O = = a O o ~ ° A - -

OD 0 • ~ A&

% ~ " "2 '6~ " " z ~ " ' ' 3 o 1 2 ' " 1 ~ , ' " 1 ~ ' " I ~ ' " z ' o ' " 2 2 o . 6 . . . . . . . . . . . . . o.e ,.o ~.'2' TEMPERATURE, *C IRRADIANCE, MJ rn-=doy - I Q, MJ rn'Zday "1 *C "1

Fig. 4. Grain yield o f cultivars BRI (!-1) BR3 ( / x ) , BR9 ( O ) , BRI4 (<>) and Dharial ( & ) grown in the Boro season, 1988, as a function of (a) mean temperature, (b) mean irradiance and ~c) photothermal quotient (Q) during the 30 days prior to anthesis. Lines drawn in (c ) for each cultivar are fitted regressions with equations in Table 4.

for the individual cultivars and lines were not significantly different and were pooled to fit the linear relations shown.

The experiment carried out in the Boro season o f 1988 with one traditional (Dharia!) and four m o d e m cultivars (BR1, BR3, BR9 and B R I 4 ) provided more information on the yield components . Again, grain yield is better related to Q than to irradiance or temperature alone (Fig. 4 ) . However, there were substantial differences between cultivars: grain yield of Dharial and BR 14, for example, showed considerably smaller responses to Q than that of BR9. Indeed, the photothermal quotient can be used to separate the different characteristic,: o f the five cultivars. The highest yielding cultivars BR3 and

INFLUENCE OF SOLAR RADIATION AND TEMPERATURE ON RICE GRAIN YIELD 2 3

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Fig. 5. Relation o f yield components (a ) spikelet number, (b ) grain weight and ( c ) percent sterility to photothermal quotient (Q) during the 30 days prior to anthesis. Cultivars BR 1, BR3, BR9, B R I 4 and Dharial grown in the Boro season, 1988. Symbols as in Fig. 4.

BR9 had large numbers of spikelets (Fig. 5a), high or medium weight grains (Fig. 5b) and low sterility (Fig. 5c). The two cultivars BR 1 and BRI 4 had intermediate yield for different reasons: BRI4 because o f low numbers of spikelets m - 2 and BR 1 primarily because of higher sterility than the other modern cultivars and low grain weight. Grain weight differs substantially between cultivars but is relatively conservative within a cultivar over the range of Q observed and across different seasons. Spikelet number is very responsive to Q and is much higher in the four modern cultivars than in Dharial.

DISCUSSION

The results from these shading experiments are very similar to those of Yoshida and Parao ( i 976) and confirm the view that solar radiation variations in the range normally encountered during the vegetative period have very little effect on yield and yield components. In contrast, when any level of shading was imposed during the reproductive or the ripening periods, yield was significantly affected.


The major determinants of rice yield are the number of filled spikelets m-2 and the grain weight (e.g. Yoshida, 1983 ). As both the number of panicles m -2 and the grain weight are relatively insensitive to irradiance (Tables 2 and 3 ), yield variation is most closely related to factors influencing the number of spikelets per panicle during the reproductive stage and those affecting the percentage of filled spikelets during both the reproductive and ripening stages. The use of the photothermal quotient (Q) which expresses the influence of mean temperature on panicle development rate and solar radiation on assimilate supply to the growing panicle is therefore a logical framework for analysing yield variation (Fischer, 1985). While Fischer applied the method to the analysis of wheat grain number, we have used it to expla~, the variation in grain yield. Indeed, the photothermal quotient for the period prior to anthesis explained more of the variation in rice grain yield than either mean temperature or irradiance alone in a number of experiments planted at different t imes of the year (Figs. 2-5 ). While the data presented here indicate that the calculation of Q may be a useful predictor of grain yield for these irrigated, pest- and disease-free crops, there is some substantial variation in the value of the calculated coefficients for the relation. Regressions for all the data in the paper are shown in Table 4. Part of the variation appears to be varietal (e.g. Fig. 4c) attr ibutable to differences in gra~.n weight and percent sterility (Fig. 5 ) and some is between years. Fig. 6 brings together the different data for cultivars BR3, BR1 and BR9 (3, 2 and 2 years" data, respectively). The relations for each of the three cultivars were closely similar in 1988 and 1984, but different for BR3 in 1982. There is no obvious explanation for this, but it may be caused by differences in the percentage of filled grain due to varia- t ions in solar radiat ion in the reproductive phase after anthesis (Yoshida and Parao, 1976). This variation is not included explicitly in the calculation of Q,

800

~' 600 E

~ 400 , -J

Z 200

r y

( . 9

0

0 . 6

[] a

, , , r , , , , ~ ~ ~ ~ , ,

0 . 8 1 . 0 1 . 2

. . . . . . . . . . . . . . t )

o

o

[ ]

r , . . , . . , , , , , , . . . . . . , . , . , . . , , . , .

1 . 4 0 . 6 0 . 8 1 . 0 1 . 2 . 1 . 4 0 . 6 0 . 8 1 . 0 1 . 2 1 . 4

Q, MJ rn-2dQy -1 °C -1

C

o []

J Fig. 6. Relation between grain yield and photothermal quotient (Q) in different years for cultivars (a) BR3 in 1982, 1984 and 1988 (b) BR9 in 1984 and 1988 and (c) BRI in 1984 and 1988. Lines shown are linear regressions with equations in Table 4. Symbols: ~ 1982, [] 1984, and A 1988.

INFLUENCE OF SOLAR RADIATION AND TEMPERATURE ON RICE GRAIN "~ ~ L D 25

though there is generally a good correlation between solar radiation in one period and in any subsequent period (Evans and De Datta, 1979; Seshu and Cady, 1984). Another possible cause of the differences in the relation between Q and yield may have been variations in nitrogen nutri t ion which affects the response of rice to radiation (Evans and De Datta, 1979), though nitrogen supply was similar in these experiments. There are also possible problems in the calculation of Q. BR 1 is classified as a short-duration cultivar which may indicate that it has a low base temperature for development, so that calculations using a base temperature of 10°C will overestimate Q. Sim- ilarly, i fphotoper iod influences the Ouration of the reproductive period, then the calculated Q will be in error, but modern cultivars used in the Boro and Aus seasons are photoperiod insensitive.

The significance of the Q value during the pre-anthesis period is that it may be subject to manipulat ion through both the genetic alteration of the crop threshold temperature for development and the t iming of crop development by altering sowing date. In the first case, the above calculations assumed a base temperature of 10°C but variation between cultivars can be as much as _+ 3°C (Nix, 1976), and van Heemst (1986) has reported base temperatures for rice over the whole range from 0 to 10 ° C. The effect of sowing date on Q can be examined by using s tandard climatological information to calculate Q for any given base temperature and any sowing date at a location. An example is shown in Fig. 7 using monthly mean temperature and irradiance data for Joydebpur over the years 1979-85. Monthly mean irradiance figures were calculated from the recorded hours of bright sunshine using Angstrom's rela- t ionship (Angstrom, 1925 ). The variation between years in Q shown in Fig. 7 is relatively small, amount ing to only 10-14% (95% confidence limits) re- flecting the correlation of irradiance with mean temperature. However, the monthly values of Q vary over a two-fold range. Clearly, high values of Q and therefore of grain yield can be obtained only at the cooler times of the year as in the Boro season and with cultivars with high base temperatures (slow development rates to maximise dry-weight allocation to panicles).

Average grain-yield data were available for 14 to 20 cultivars and lines grown under irrigation with normal cultural practices, and with opt imum pest man- agement in the Boro, Aus and transplant Aman seasons in 1984 and 1987 at BRRI (BRRI, 1984, 1987 ). The genotypes were divided into short-, medium- and long-duration types. The photothermal coefficient was then calculated from the daily irradiance and mean temperature data for the 30 days prior to the estimated anthesis date. The patterns of the calculated Q for 1984 and 1987 fit well with the observed pattern of grain yield from these trials (Fig. 8a) and grain yield was closely ~elated to (2 (Fig. 8b). There were no significant differences between the data for 1984 and 1987 or among short, medium or long durat ion cultivars, so a single regression was fitted to the data, giving the relation Y = - - 243 + 715 × Q (for details see Table 4) . It is clear, however,

26

2.0

M.S. ISLAM AND J , I .L MORISON

T

!

0 " 0 ? E I.o

d

0.5

L 0 . 0 ~ ' I , i i I I I I I i I

0 2 4 6 8 1C 12

MONTH

Fig. 7. Seasonal t ime course o f photothermal quotient (Q) for Joydebpur, Bangladesh, calculated from monthly mean climatic data for 1975-1987, using three base temperatures 0 ( A ), 5 ( [ ] ) and 10 ° C ( O ) . Vertical bars show standard errors o f the mean on calculated points for a base temperature of 10 ° C. Monthly mean irradiance figures were calculated from the recorded hours of bright sunshine using Angstrom's relationship: S/S=~t- a + b ( n / N ) with values for the a and b coefficients o f 0.202 + 0.015 and 0.458 + 0.022 respectively (99% confidence l imits) , evaluated from daily irradiance and sunshine hour records for 1984 and 1987 at BRRI.

800 . . . . . . . . . . . . . . . . . . . . . . . 1.3 800 / . . . . . . . . . . . . . . .

o " - g F -

~-J 200 ~ O ~ [] ~.~ 0.7 O

DATE OF ANTHESIS-SO days Q, MJ m - 2 d a y - I °C-S

Fig. 8. (a ) Season~:l t ime course of grain yield of irrigated rice crops at BRRI in 1984 and 1987 ( E-q, A ), together with calculated photothermal quotient (Q) ( - - , - - - ) for the 30 days preceding anthesis, with a base temperature of 10 ~ C. Yield data are means of 18, 16 and 20 cultivars or lines in 1984 and 15, 14 and 14 cultivars in 1987 in the Boro, Aus and t :ansplant Aman seasons, respectively. (b) Grain yield of irrigated rice crops grown in 1984 and 1987 at BRRI as a function of Fhotothermal quotient (Q) for the 30-day period preceding anlhesis. Symbols: 1987,/x Boro and Aus season, A T. Aman; 1984, [] Boro and Aus season, [] T. Aman. Other details as in Fig. 8a. The regression fitted uses all data from the Boro and Aus seasons: Y= -- 3 6 0 + 836 × Q, details in Table 4.

INFLUENCE OF SOLAR RADIATION ANO TEMPERATURE ON RICE GRAIN YIELD 27

that the data for the transplant Aman season generally have a poor fit to Q ,

perhaps because of the photoperiod sensitivity of the cultivars used in that season and the cold conditions prevailing during the later phases of crop growth. Omitting these data points gives a regression equation Y = - 360+ 836 x Q. The scatter is large, because of the other factors affecting yield, and the standard deviation for the model is equivalent to 0.96 t ha -~. This is slightly smaller than the standard deviation of a linear model relating yield to radiation and mean temperature during the reproductive phase sug- gested by Yoshida and Parao (1976) but about twice that found by Seshu and Cady (1984). Their work derived a multiple regression model of rice yield in a multiple location trial, using mear~ radiation and minimum temperature in the period after flowering. Obviously multiple location trials have considera- ble advantages as they cover a much wider range of environmental conditions than we were able to do at a single site. We suggest that future regression studies should consider using Q as a derived variable, rather than temperature and radiation separately, as Q is more physiologically meaningful. Fur- ther, the simple calculation of Q at an,j location as in Fig. 7 may aid the selec- tion of appropriate cultivars.

ACKNOWLEDGEMENTS

M.S.I. thanks the British Council for awarding a scholarship and the Bang- ladesh Rice Research Institute for permission to undertake MSc studies at the University of Reading. The authors thank Dr. M.Z. Haque, Head of Plant Physiology Division, BRRI, for support to complete this study, and are grate- ful to Dr. H.R.B. Hack, Department of Meteorology, University of Reading, for his critical reading of the manuscript.

REFERENCES

/~ngstrom, A., 1925. On radiation and climate. Geogr. Ann., 7: 122-142. Bangladesh Bureau of Statistics (BBS), 1987. Year Book of Agricultural Statistics, 1985-86.

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Bangladesh Rice Research Institute (BRRI), 1984. 1984 Annual Report. Bangladesh Rice Re- search Institute, Joydebpur, Gazipur, Bangladesh, pp. 9-12.

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De Datta, S.K. and Zarate, P.M., 1970. Envirtmmental conditions affecting the growth characteristics, nitrogen response and grain yield of tropical rice. Biometeorology, 4:71-89.

Evans, L.T. and De Datta, S.K., 1979. The relation between radiation and grain yield of irrigated rice in the tropics, as influ~enced by cultivar, nitrogen fertilizer application and month of planting. Field Crops Res., 2: 1-17.

28 M.S. ISLAM AND J.I.L MORISON

Fischer, R.A., 1985. Number of kernels in wheat crops and the influence of solar radiation and temperature. J. Agric. Sci. (Camb.), 105: 447-461.

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Nix, H.A., 1976. Climate and crop productivity in Australia. In: Climate and Rice. Interna- tional Rice Research Institute, Los Bafios, Philippines, pp. 495-508.

Oldeman, L.R., Seshu, D.V. and Cady, F.B., 1987. Response of rice to weather variables. In: Weather and Rice. International Rice Research Institute, Los Bafios, Philippines, pp. 5-39.

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Sahu, G., Rao, Ch.N. and Murty, K.S., 1983. Influence of temperature and solar radiation on growth and yield of rice. Oryza, 20:51-54.

Seshu, D.V. and Cady, F.B., 1984. Response of rice to solar radiation and temperature estimated from international yield trials. Crop Sci., 24: 649-654.

van Heemst, H.D.J., 1986. Crop phenology and dry matter distribution. In: H. van Keulen and J. Wolf (Editors), Modelling of Agricultural Production: Weather, Soil and Crops. Pudoc, Wageningen, Netherlands, pp. 27-40.

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Yoshida, S. and Parao, F.T., 1976. Climatic influence on yield and yield components of lowland rice in the tropics. In: Climate and Rice. International Rice Research Institute, LOs Bafios, Philippines, pp. 471-494.

Influence of solar radiation and temperature on irrigated rice grain yield in Bangladesh

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