Euphytica 110: 109–126, 1999.© 1999Kluwer Academic Publishers. Printed in the Netherlands.

109

Phenological responses ofOryza sativa, O. glaberrimaand inter-specific ricecultivars on a toposquence in West Africa

Michael Dingkuhn & Folkard AschWest Africa Rice Development Association (WARDA/ADRAO), B.P. 2551, Bouak´e 01, Cote d’Ivoire

Received 10 July 1998; accepted 2 June 1999

Key words:Basic vegetative phase (BVP), crop duration, ecosystems, flowering; photoperiod-sensitivity, ecotypes

Summary

Phenological properties of rice cultivars, particularly crop duration, determine their yield potential, local agronomicsuitability and ability to escape from drought. Crop duration of a given cultivar depends mainly on photoperiod (PP)and temperature, but is also affected by the crop establishment practice and environmental stresses. A sample of84 ecologically and genetically diverse rice cultivars was sown on five dates between May and September 1997 onthe flooded-lowland (transplant), hydromorphic and upland levels of a toposequence at 7◦52′ N in Cote d’Ivoire,in order to characterize the cultivars’ phenological responses. In the upland, life-saving sprinkler irrigation wasapplied when drought symptoms were visible. A non-replicated design augmented with four replicated checks(four replications per ecosystem) was used. Phenology was characterized by date of emergence, first heading, 50%flowering and maturity. The period from emergence to flowering was subdivided into three phases following asimple model used at IRRI to characterize germplasm for photoperiodism. For each ecosystem and cultivar, thebasic vegetative period (BVP) was estimated by subtracting 30 d from the duration to flowering at the sowing dateassociated with the shortest duration, and expressed in degree-days (dd), assuming a base temperature of 10◦C.The PP-sensitive phase (PSP) was estimated by subtracting BVP+30 d from the time to flowering. PP-sensitivity(PS) was calculated from the apparent change in PSP between 12.0 and 12.5 h mean astronomic daylength duringthe PSP, by regression across dates. Cultivars differed strongly in BVP (300 to 1200 dd) and PS (0 to 1000 dd).The BVP was generally longer in the lowland than in the hydromorph, and mostly longer in the upland than inthe hydromorph, possibly due to transplanting shock (lowland) and drought (upland). Many cultivars, particularlyupland-adapted japonicas, had a greater PS in the lowland than in the upland. Principal-component and clusteranalyses based on BVP and PS in each of the three ecosystems established three large and three small groups ofcultivars having common phenological responses. The linkage groups were associated with ecotypes (lowland vsupland, traditional vs improved) and genetic groups (O. sativajaponica and indica,O. glaberrima, inter-specificprogenies). The groups were seen to represent past selection strategies by farmers and recent breeding strategies,with respect to achieving yield stability in the various ecosystems. For example, indigenous selection strategiesfor O. sativaupland rices seem to have favored a long BVP, whereas fromO. glaberrima, which generally has asuperior initial vigor, cultivars with a short BVP have been selected. The authors conclude that the modern uplandrice breeding strategy for the region on the basisO. sativa, which aims at drought escape using a short BVP,is paralleled by existing indigenousO. glaberrimamaterials. Efforts to utilize these materials for breeding areongoing.

Introduction

Crop duration is a major parameter for the localagonomic fit of a rice cultivar. In irrigated systems,crop duration determines the calendar options for rice

double cropping and intensified crop rotation systems(Dingkuhn, 1995). Duration also determines the abil-ity of the crop to escape from climatic stresses, forexample thermal stresses occurring during the cold orhot season in arid, irrigated rice systems (Dingkuhn

110

and Sow, 1998; Dingkuhn et al., 1998c). In rainfedrice systems, crop duration is pivotal to temporal es-cape from drought or floods (Bagchi et al., 1995), orto ensure that maturity does not occur before floodrecession (Greenland, 1997). Equally important butfrequently overseen is the place of planting dates andcrop duration in seasonal patterns for labor use andfood availability, for example in labor-limited andhighly diversified cropping systems in African uplands(Ruthenberg, 1976).

Crop duration is interactively determined by thegenotype and the environment. Rice cultivars are gen-erally short-day plants (Chang and Vergara, 1985;Dingkuhn and Miezan, 1995). They vary strongly intheir sensitivity to photoperiod (PP) and to a lesser ex-tent in their sensitivity to temperature (Ritchie, 1993;Dingkuhn and Miezan, 1995). Under phenologicallyoptimal temperatures (approximately between 20 and30◦C) and PP (< 12 h), crop duration depends mainlyon the cultivar-specific duration of the basic vegetativephase (BVP), which varies between about 20 and 70 dfor cultivated rice (Chang and Vergara, 1985). Underfully inductive conditions, the BVP is followed withina few days by panicle initiation (PI). If conditions arenot inductive, however, BVP is followed by an exten-ded photoperiod-sensitive phase (PSP) during whichfloral induction takes place, and which ends with theinitiation of the inflorescence (Collinson et al., 1992;Sie et al., 1998ab).

The subsequent reproductive and ripening phasesare thought to be of comparatively uniform duration(about 30 d for each phase; Yoshida, 1981). Where wa-ter resources are not limiting, the temperature at of theapical growing point governs the duration of the BVPof any given cultivar (Dingkuhn et al., 1995; Sie et al.,1998a), and PP and temperature govern interactivelythe duration of PSP (Summerfield et al., 1992; Ellis etal., 1993; Yan and Wallace, 1996; Sie et al., 1998ab).These simple, two-phase models for floral initiationdo not capture the full complexity of photoperiodism.Collinson et al. (1992) showed that they over-estimatethe duration of the BVP and under-estimate that of thePSP because (i) even under highly inductive condi-tions, PSP can be substantial; and (ii) because the PSPextends somewhat beyond the time of floral initiationand into what is commonly called reproductive phase(Yoshida, 1981), thereby apparently affecting the rateat which the inflorescence develops after its initiation.For the present study, which aims at the classificationof phenological types as an orientation to breeders, weconsidered Chang and Vergara’s concept sufficiently

robust, but rendered it sensitive to temperature by cal-culating phase duration in thermal time units insteadof time units.

In rainfed rice systems, drought or excess watermay alter the duration of the various phenologicalphases. Drought generally delays development if it oc-curs between germination and flowering (Wopereis etal., 1996), but hastens development if it occurs dur-ing the ripening period (Dingkuhn and Le Gal, 1996).Little is known on the effects on rice phenology ofexcess water and the redox potential of the soil. Ricecultivars generally tolerate both flooded and aeratedsoil conditions, but differ substantially in drought res-istance (IRRI, 1982) and tolerance to continuous waterlogging (A. Audebert, WARDA, personal communic-ation, 1997). Recent comparisons between rice cul-tivars of different ecological adaptation, which wereplanted across a hydrological toposequence, indic-ated strong genotype-specific effects of the ecosystemon growth and crop duration (M. Becker, WARDA,personal communication, 1997).

The present study seeks to characterize the phen-ological responses to PP and hydrological conditionsfor genetically and ecologically diverse rice cultivarsin West Africa. The experimental site in Ivory Coast,located in the transition zone between the Guinea sa-vannah and the humid forest, is representative of thoserainfed rice environments where the genotypic andenvironment-induced variability of crop duration areparticularly important: further north in the Guinea andSudan savannah zones, the short but intense wet sea-son allows for little flexibility in sowing dates; andfurther south in equatorial forest, environment effectson crop duration are small because temperatures andPP are stable and rainfall is abundant.

The cultivars used in the study were selected torepresent all genetic and ecological rice types relev-ant to rainfed systems in West Africa. In particular, arange ofO. glaberrimalandraces indigenous to the re-gion, as well as interspecific crosses betweenO. sativaandO. glaberrima,were included because upland riceimprovement in Africa is increasingly drawing fromthis germplasm (Jones et al., 1997; Dingkuhn et al.,1998ab). These materials were compared with tra-ditional and improvedO. sativa cultivars from thetropical-japonica (mostly upland-adapted) and indica(mostly lowland-adapted) sub-species.

111

Table

1.C

hara

cter

istic

sof

the

gene

ticm

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used

inth

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for

June

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irrig

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wla

nd;

UP

L,

upla

nd;

coun

try

code

s:B

F,B

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naF

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BR

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razi

l;C

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Cam

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n;C

H,C

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Rep

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SE

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ZA

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aire

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ries

prin

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No.

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1A

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rad

131××

PH

PH

43T

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4004

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1-2-

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2B

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189

sat

ind

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129××

INC

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tin

dIm

pr12

8××

NI

3C

G14

gla

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rad

104

××

SE

SE

45T

OX

4204

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T-14

7-2-

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ind

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135××

NI

4C

G17

gla

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rad

106

××

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SE

46T

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4251

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tin

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5××

NI

5C

G20

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98××

SE

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728-

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NI,

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NI

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85-1

sat

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128××

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NI

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135×

SL

SL

49W

AB

126-

15-H

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tja

pIm

pr11

3×

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8C

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111×

SL

SL

50W

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126-

18-H

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137

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11sy

n.O

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110

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100

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FAR

O46

syn.

ITA

150

sat

jap

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91×

NI

NI

53W

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189-

B-B

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sat

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100

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O48

syn.

ITA

301

sat

jap

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112

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H-H

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pr10

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CI

13FA

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8sa

tin

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INN

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jap

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108

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FK

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ind

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128××

BF

BF

56W

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32-8

0sa

tja

pIm

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2×

CI

15F

KR

28sy

n.IT

A12

3sa

tin

dIm

pr12

1××

NI

BF

57W

AB

340-

B-B

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H1

sat

jap

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112

×C

I16

FK

R48

sat

ind

Impr

121××

BF

BF

58W

AB

340-

B-B

-9-L

3-L

1-L

Bsa

tja

pIm

pr10

8×

CI

17G

ambi

aka

sat

ind

Tra

d13

0×

CI

CI,

MA

59W

AB

377-

B-1

6-L

3-L

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18ID

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10sy

n.IR

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262

sat

jap

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100

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pro

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pr10

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19ID

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6sy

n.IR

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216

sat

jap

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116××

CI

CI

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450-

24-2

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106

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sat

jap

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113

×C

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4-3

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18-H

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o–

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103

×C

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gla

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rad

138

××

CI

CI

63W

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450-

I-B

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29-H

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91×

CI

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eto

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jap

Tra

d12

7××

CI

CI

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450-

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33-H

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o–

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98×

CI

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54sa

tin

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PH

PH

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450-

I-B

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60-H

Bpr

o–

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114

×C

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ITA

257

sat

jap

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91×

NI

NI

66W

AB

450-

I-B

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68-3

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114

×C

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ITA

321

sat

jap

Impr

117

×N

IN

I67

WA

B45

0-I-

B-P

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1-1

pro

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pr11

3×

CI

26IT

A33

1sa

tja

pIm

pr11

4×

NI

NI

68W

AB

450-

I-B

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8-H

Bpr

o–

Impr

100

×C

I27

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min

sin

sat

jap

Tra

d12

6×

CI

CI

69W

AB

56-1

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sat

jap

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100

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IC

I28

M22

sat

ind

Tra

d12

6×

70W

AB

56-1

25sy

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pr98

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n.L

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132

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ko8

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75W

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tin

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NI

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NI

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tin

dIm

pr13

2××

NI

82W

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tin

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0××

NI

CI

41T

OX

3880

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1-1-

2sa

tin

dIm

pr13

5××

NI

83W

ITA

8sa

tin

dIm

pr12

5××

NI

CI,

NR

42T

OX

3882

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3-1-

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tin

dIm

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84W

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9sa

tin

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5×

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112

Materials and methods

Genetic materials

Eighty-four rice cultivars were selected for phenolo-gical analysis, with the objective to cover the geneticdiversity of rice currently cultivated in various eco-systems in the humid forest and moist savannah en-vironments in West Africa (Table 1). Nine geneticallystable, inter-specific (O. glaberrima× O. sativa) pro-genies and a number of promisingO. sativa indicabreeding lines were included because they representmajor advances in rice breeding for the region. Seedwas obtained from the WARDA germplasm coordin-ation unit, or in the case of breeding lines, fromthe breeder. The tropical japonica, traditional, uplandcultivar Azucena from the Philippines was includedbecause it has previously been subject to phenolo-gical studies (Chang and Vergara, 1985; Summerfieldet al., 1992). Four cultivars identified as checks onthe basis of their known agronomic and physiologicalproperties and importance in local production systems(Table 1). The check cultivar WAB 450-24-3-2-P18-HB, an interspecific hybrid, is henceforth named V4for brevity.

Location, experimental design and crop management

A phenological experiment based on five sowing dates(15 May, 15 Jun, 15 Jul, 15 Aug, 15 Sep) wasconducted at WARDA’s experimental station at Mbéin Cote d’Ivoire (7E52′ N, 5E6′ W, 300 m asl) ona toposequence ranging from the well-drained up-land to the permanently flooded lowland (5 to 10 cmdepth). The site was characterized by a bimodal rain-fall pattern (ca. 1200 mm annual rainfall) typical ofthe forest-savannah transition zone. The soil was aPlinthic Ferralsol in the upland, a Gleyic Arenosol inthe hydromorph and an Eutric Gleysol in the lowland(FAO soil classification system).

Eighty-four rice cultivars (Table 1) were sown atthree levels of the toposequence (well-drained upland,intermittently water-logged but non-flooded hydro-morphic and flooded lowland ecosystems). At eachlevel and for each sowing date, four quadratic blockswere identified and a subset of twenty cultivars andfour replicated check cultivars were sown in 1 m× 1 mplots in a completely randomized arrangement. Thedesign was a non-replicated, completely randomizeddesign augmented with replicated checks.

In the upland and hydromorphic plots, dry seedwas dibbled, whereas for the lowland plots, pre-

germinated seed was sown in a nearby seedbed nurseryand transplanted into the main plots at 24 days aftersowing. Plant population was 16 hills m−2 on 1.25 m2,with two plants per hill, and at a spacing of 0.25 m×0.25 m. Plots were fertilized basally with 20 kg ha−1

of N (as urea), P (as triple superphosphate) and K (asKCl), and topdressed with 20 kg ha−1 of N as ureaat 45 d after sowing. Weeding was by hand to avoidpossible herbicide effects on crop phenology. In theupland plots, sporadic life-saving sprinkler irrigationwas used whenever severe drought symptoms were ob-served (leaf rolling in the early morning), particularlybetween July and September. Lodging was observedonly for someO. glaberrimacultivars, from headingstage onwards.

No herbicides or insecticides were used, andabove-ground pest pr0oblems were restricted to mildattacks of African rice gall midge andTrichispa ssp.on some cultivars in the lowland, and mild symp-toms of brown spot (Helminthosporus spp.) in theupland. Many of the upland-adaptedO. sativa ja-ponica cultivars and inter-specific progenies had largepopulations of parasitic root nematodes (Hirschman-niella oryzae) in the lowland, but showed no wiltingor leaf discoloration.

Phenological observations and interpretation

Seedling emergence, first heading, 50% floweringand maturity were visually assessed on a daily basison all plots throughout the experiment. Observationswere made in the center of plots for main culms and1st-order tillers only. The period from emergence toflowering was subdivided into three phases follow-ing the simple model used by Chang and Vergara(1985) to characterize large numbers of germplasmfor photoperiodic reactions. For each ecosystem andcultivar, the basic vegetative period (BVP) was estim-ated by subtracting 30 d from the duration to floweringat the sowing date associated with the shortest dur-ation, and expressed in degree-days (dd), assuminga base temperature of 10◦C (Dingkuhn and Miezan,1995). The PP-sensitive phase (PSP) was calculatedby subtracting BVP+30 d (in dd) from the time toflowering. The model is presented graphically in Fig-ure 1. PP-sensitivity (PS) was estimated from theapparent change in PSP between 12.0 and 12.5 h meanastronomic daylength during the PSP, by 2nd orderregression across dates.

113

Figure 1. Schematic diagram representing the hypothetical fractionation of the rice crop’s life cycle into a basic vegetative phase (BVP), aphotoperiod-sensitive phase (PSP), a reproductive phase assumed to be constant at 30 d, and a grain ripening phase (RIP). The respectivedevelopmental stages are emergence (E), panicle initiation (PI), flowering (F) and maturity (M). D1 through D5 indicate sowing dates and PPindicates photoperiod.

Statistical analysis

Regression analyses and the calculation of confidenceintervals were conducted with SigmaPlot V4.0 (Win-dows). Principal component and cluster analyses weredone with Statistica V.5.0 (Windows).

Results and discussion

Variability of environmental conditions and cropduration

Rainfall and temperature distribution showed abimodal pattern typical of the site (Figure 2, bot-tom). The drought period in July and August, whichin 1997 was more severe than normal for the site, was

bridged with life-saving irrigation whenever severedrought symptoms occurred. The photoperiod (as-tronomic daylength, PP) varied between 11.64 and12.57 h, with early plantings (May, June) experiencingthe longest and late plantings (August, September)experiencing the shortest days. Mean daily air tem-peratures fluctuated between 23 and 29 EC (monthlyaverages between May and December between 24.3and 26.3◦C), and therefore, were within the optimalrange for rice development (Yoshida, 1981; Dingkuhnet al., 1995; Sie et al., 1998a).

Crop duration from emergence to flowering var-ied between 60 and 130 d, depending on the sowingdate and cultivar. (Durations are expressed in ab-solute terms in this section to permit comparisonswith weather conditions; further phenological ana-

114

Figure 2. Rice crop duration in different environments. Top: duration from emergence to flowering for three contrasting cultivars, at threedifferent sowing dates and in two ecosystems. Botton: Astronomic daylength, mean air temerature and rainfall for the same period.

115

lyses are based on thermal time.) Patterns for threecultivars with contrasting phenology are shown Fig-ure 2 (top). TheO. glaberrimalandrace IG10 had aprogressively shorter duration as seed was sown laterin the season. Effects of the ecosystem (lowland vsupland) on duration were small. Crops sown in earlyMay, June and July flowered almost at the same time(mid to late September), indicating a pronounced PP-sensitivity. IG10 is cultivated in the rainfed upland buttolerates water-logged environments equally well. Bycontrast, theO. sativa japonica cultivar WAB 638-1, a tall-traditional type adapted to rainfed-lowlandand hydromorphic conditions, had an almost constantand comparatively long duration of 110 to 120 d inthe upland. Under lowland conditions, crop durationdecreased slightly as seed was sown later in the sea-son, indicating a mild PP-sensitivity under floodedconditions.

The improved upland rice cultivar WAB 56-104had a short crop duration throughout (65 to 85 d),which decreased gradually as the crop was sown laterin the season. Again, this effect was more pronouncedin the lowland than in the upland, indicating mild PP-sensitivity in the upland and moderate PP-sensitivityin the lowland.

The three examples represent different agro-ecological strategies. The near-synchronization offlowering for IG10 towards the end of the wet sea-son can be interpreted as a natural survival strategy(escape from drought). IG10 is a landrace with manywild (‘weedy’) characteristics (Johnson et al., 1998).WAB 56-104 was bred for drought resistance and shortduration, in order to provide maximal flexibility forcropping calendars. Lastly, WAB 638-1 is suited tolowlands with variable hydrology such as traditionalsystems, where dry seed is sown before the onset ofshallow to medium-deep, uncontrolled floods. Withappropriate timing of sowing, cultivars of this typemature late enough to permit harvesting after floodrecession. Gambiaka, the most commonly grown cul-tivar grown in floodprone, traditional lowlands in WestAfrica, showed the same phenological behavior (datanot presented).

Phenological responses of the check cultivars indifferent ecosystems

The duration from emergence to flowering for thefour replicated check cultivars varied between 54 d(WAB 56-104, hydromorph) and 112 d (Bouaké 189,irrigated lowland) (Table 2). The greatest difference

between the minimal and maximal duration for a givencultivar and ecosystem was 33 d (interspecific pro-geny V4, lowland), and the smallest difference was10 d (WAB 56-104, upland). For all check cultivars,the shortest duration to flowering was significantly(P < 0.05, by T-test) longer in the lowland and up-land ecosystems than in the hydromorphic ecosystem.The extended crop duration in the lowland was prob-ably caused by the transplanting shock (Dingkuhn etal., 1990). In addition, occasional diurnal recordingsof floodwater temperature showed slightly lower val-ues compared to the mean air temperature (data notpresented). This effect is known to increase crop dura-tion as well (Dingkuhn et al., 1995). Yan et al. (1995)and Yan and Wallace (1996) presented models de-scribing photoperiod and temperature interactions inpea andSorghum. Sie et al. (1998abc) observed andsimulated an interactive enhancement of the durationof PSP by low water temperatures and long days inirrigated rice. The warm and humid climate of the siteand the probable existence of an optimum temperaturefor crop development within the range of the observedmean air temperatures (Dingkuhn and Miezan, 1995;Yan et al., 1995), however, suggest that thermal effectswere negligible in this study, and that transplantingshock was the main cause of extended crop durationunder irrigation.

The extended crop duration in the upland wasprobably due to water limitation. In a separate studyusing the same upland soil, WAB 56-104 showeda progressive delay in flowering when grown underconstant levels of 64% and 41% field capacity, ascompared to field capacity (Asch, unpublished, 1999).

The maximal crop duration, which was generallyobserved for the May plantings that also experiencedthe longest days, was not significantly longer in the up-land than in the hydromorphic ecosystem, but longerin the lowland than the hydromorph (+9 d, CG 14[P < 0.01]; +10 d, Bouaké 189 [P < 0.05]; +11 d,WAB 56-104 [P < 0.01]; +29 d, V4 [P < 0.01]).

Based on the simple model described by Changand Vergara (1985), we divided the period from emer-gence to flowering into three phases (Figure 1; detailsin materials and methods section). This concept re-quires less empirical data than the more detailed mod-els developed by Collinson et al. (1992). The approachof Chang and Vergara was modified, however, bymeasuring phase durations in degree-days (dd) insteadof days with 10 EC as base temperature, in order totake into account temperature effects on developmentrate (Ebata, 1990; Lecoeur et al., 1995).

116

Table 2. Phenological parameters observed for four check cultivars in three ecosystems (meansand SE of four replications). BVP, basic vegetative phase expressed in degree-days (dd); PS, pho-toperiod-sensitivity calculated as the difference in duration (dd) of the photoperiod-sensitive phase(PSP) between 12.0 h and 12.5 h mean astronomic daylength during PSP. V4 is the inter-specifichybrid WAB 450-24-3-2-P18-HB (Table 1)

Parameter CG 14 Bouake 189 WAB 5-6-104 V4

Upland

Min. duration to flowering (d) 69 ± 1 93 ± 1 61 ± 1 63 ± 1

Max. duration to flowering (d) 90 ± 2 109 ± 2 71 ± 2 84 ± 4

BVP (dd) 617 ±18 951 ±10 487 ±15 521 ±13

PS (dd) 241 ±17 257 ±32 86 ±14 269 ±38

Hydromorph

Min. duration to flowering (d) 61 ± 2 85 ± 1 54 ± 1 56 ± 2

Max. duration to flowering (d) 87 ± 2 102 ± 2 78 ± 2 79 ± 4

BVP (dd) 502 ±26 895 ±22 386 ±14 414 ±30

PS (dd) 214 ± 8 108 ±19 166 ±18 216 ± 8

Lowland

Min. duration to flowering (d) 66 ± 1 89 ± 1 65 ± 2 75 ± 1

Max. duration to flowering (d) 96 ± 1 112 ± 3 89 ± 2 108 ± 5

BVP (dd) 586 ±18 962 ±14 568 ±32 729 ±22

PS (dd) 331 ± 3 333 ±58 237 ±10 443 ±74

The BVP, which varied among the checks between386 (WAB 56-105, hydromorph) and 962 dd (Bouaké189, lowland), was significantly (P < 0.05) longerin the upland and lowland than in the hydromorphicecosystem for all check cultivars except Bouaké 189.The variability of the BVP across ecosystem appearedto be indicative of the hydrological adaptation of thecultivars. Bouaké 189, a cultivar bred for irrigatedand rainfed lowlands, had the most constant BVP.Considering the fact that transplanting in the lowlandgenerally delays vegetative development by five to tendays (Dingkuhn et al., 1990), corresponding to 85to 170 dd in this study, Bouaké 189 would have hadthe shortest BVP in the lowland if it had been direct-seeded as in the other ecosystems. By contrast, theupland-adapted cultivars WAB 56-104 and V4 showeda marked extension of the BVP in the lowland. Indeed,these cultivars showed poor growth and tillering in thelowland as well (data not presented). CG 14, an uplandrice with extremely wide hydrological adaptation, wasintermediate.

Similar trends were observed for the apparent PP-sensitivity (PS), as represented in Table 2 by thedifference in PSP between 12.5 and 12.0 h pho-toperiod (refer to materials and methods for interpola-

tion method). PS was generally greater in the lowlandthan in the hydromorphic treatment (P < 0.01). In theupland, however, only lowland-adapted Bouaké 189exhibited an increased PS, possibly due to physiolo-gical drought. It therefore seems that BVP and PS ofany given cultivar are not the same between ecosys-tems if the cultivar shows differential adaptation tothese ecosystems.

Figure 3 shows for the inter-specific cross V4 andits parents the relationship between the duration ofPSP and the mean PP during PSP in the upland andthe lowland. Particularly the upland-adapted, japonicaWAB 56-104 showed large differences in PSP betweenecosystems, but the differences were also significant(P < 0.05) for CG 14, which is well adapted acrossthe environments. The ecosystem effect on PSP wassmaller for V4, but on the basis of a greater apparentPP-sensitivity. The increase in PSP at about 11.8 hPP in the upland could not be explained. Many ofthe japonica rices tested by Chang and Vergara (1985)showed an increase in duration to flowering whenexposed to very short days, despite their confirmedgeneral behavior as short-day plants.

117

Figure 3. Changes in the duration of the photoperiod-sensitive phase (PSP; degree-days, dd) as a function of mean photoperiod (astronomicdaylength) during PSP, for theO. glaberrimalandrace CG 14, theO. sativacultivar WAB 56-104 and an inter-specific progeny (four replicationspooled), for the upland and flooded-lowland ecosystems. Solid lines indicate 2nd-order regressions, dotted lines the 0.05-confidence interval.

Cultivar differences in BVP andphotoperiod-sensitivity

Figure 4 shows the thermal requirements for the BVP(assumed to be independent of PP) and the PSP (rep-resented as a function of PP), for 12 genetically andecologically contrasting rice cultivars.O. glaberrimacultivars had generally a short BVP (200 to 600 dd).One cultivar (IG 10) was extremely PP-sensitive (PSP> 1000 dd at PPPSP= 12.5 h), whereas the otherO.glaberrima cultivars (CG 14, CG 17, CG 20, TOG6545) were moderately PP-sensitive (PSP between300 and 600 dd, equivalent to about 18 to 36 d).

The three traditional, upland, tropical japonica,O.sativa cultivars (Figure 4: Moroberekan) were onlymildly PP-sensitive and had a long BVP. Amongthe improved upland japonicas, however, many wereultra-short duration types (maturity at #100 d; Table 1)and had a short BVP (example in Figure 4: WAB56-104, ca. 400 dd), or were medium-duration types(110 to 125 d) with an intermediate BVP (IDSA 6; ca.750 dd). WAB 638-1, a tropical-japonica type adaptedto rainfed lowland and hydromorphic environments,was nearly PP-insensitive and had a long BVP (ca.1000 dd).

The O. sativa indica cultivars, most of whichwere adapted to rainfed or irrigated lowland condi-tions, were mostly more PP-sensitive than the tropical-japonica cultivars, although no highly PP-sensitivematerials were found among them. Traditional in-dica types such as Suakoko 8 and Gambiaka had avery long BVP (example in Figure 4: Suakoko 8, ca.1200 dd) and were moderately PP-sensitive. Most ofthe improved indica cultivars, such as the medium-duration types Bouake 189 and the WITA lines bredat WARDA and IITA, had an intermediate BVP (800to 1000 dd).

Interspecific progeny from anO. glaberrima(CG14)×O. sativa(WAB 56-104) cross were mostly sim-ilar to their parents, both of which had a short BVPand moderate PP-sensitivity. The progeny were char-acterized by a marked dependency of the BVP, andin some cases of the PP-sensitivity, on the ecosystem(one example given in Figure 4). These lines had alonger BVP and greater PP-sensitivity under floodedlowland conditions than under the upland conditionsthey had been selected for. The same phenomenon wasalso observed for some, but not all upland-adaptedtropical-japonica cultivars. Conversely, some of the

118

Figure 4. Duration of the basic vegetative phase (BVP) and the photoperiod-sensitive phase (PSP), the latter as a function of the meanphotoperiod (PP) during PSP, for 12 contrasting rice cultivars for the upland, hydromorphic and flooded lowland ecosystems, expressed asdegree-days (dd). Gla,O. glaberrima, Jap,O. sativajaponica; Ind,O. sativaindica; Pro, inter-specific progeny.

119

lowland-adapted indica cultivars (Figure 4: Suakoko8 and the rainfed-lowland adapted japonica cultivarWAB 638-1 had a markedly longer BVP in the uplandthan in the lowland ecosystem.

Caution should be exercised when extrapolatingthe results to environments with different ranges oftemperature and PP. Sie et al. (1998abc) reported thatlow temperatures can prolong strongly both the BVPand PSP. Moreover, the mostly exponential responseof the PSP to PP observed between 11.8 h and 12.6 hmight become linear or level off when PP extends to13 h or more (Chang and Vergara, 1985; Summerfieldet al., 1992). The present results are valid only for thewarm and humid tropics.

Validation of the results with independent reports

Independent, published information to validate thepresent findings is scarce. Four upland-adapted ja-ponica cultivars were previously characterized undercontrolled daylength by Chang and Vergara (1985).For these, a good agreement was found between thetwo studies for the BVP (Table 3). The mean BVPobserved in this study correlated well with that re-ported by Chang and Vergara (R2 = 0.99). No suchcorrelation was found for PP-sensitivity, possibly be-cause the data were quite incomparable. Chang andVergara used a PP range between 10 and 16 h at 2-h steps, whereas the present study observed a muchnarrower range (11.8 to 12.6 h) at greater resolution.The response of PSP to PP was generally stronger inthis than in Chang and Vergara’s study.

For Azucena grown in flooded pots, Summerfieldet al. (1992) observed a delay in flowering by 11.5 d(161 dd) the PP was increased from 11.5 to 12.5 h.The respective, extrapolated values in this study were108 dd for the upland, 140 dd for the hydromorphictreatment and 478 dd for the lowland, or about 6, 8 and28 d, respectively. Chang and Vergara (1985) observeda delay by about 5 d (ecosystem not reported).

Phenology-based linkage groups

On the basis of the phenological results, six paramet-ers were calculated to establish a Euclidian linkagemap using a cluster analysis (Figure 5). The paramet-ers were the duration of the BVP, expressed as dd,for each of the three ecosystems, and the PS (inter-polated difference in PSP between PP = 12.5 h andPP = 12.0 h) for the same ecosystems.

Seven linkage groups A through F were identifiedon the basis of Euclidian distances among cultivars.

The large groups D, F and G, which together included92% of the cultivars tested, were divided into two sub-groups each, denominated by upper or lower case. Thecomposition of the groups in terms of genetic groupsand ecotypes is presented in Table 4.

Group A consisted of only one cultivar, theO.glaberrima landrace IG10 (Figure 5). Group B con-sisted of two upland-adapted,O. glaberrima× O.sativa progenies. Group C included twoO. glaber-rima landraces. Group D+d was made up mainly ofupland rices (20 of 21), most of which were improvedO. sativajaponica (12) andO. glaberrima× O. sativainterspecific lines (6). Group E consisted of two tall-traditional, japonica, strictly upland-adapted types.Group F+f had 21 entries and consisted mainly oflowland-adapted rice (20 of 21), 19 of which belongedto theO. sativaindica group. Group G+g (34 entries)was less homogenous. Its sub-group G (15 entries)had mainly improved, lowland-adapted indicas (11),whereas sub-group G consisted mainly of improvedupland types (14 of 19), 13 of which were japonicas.

Principal component analyses of linkage groups

The linkage groups were further characterized by prin-cipal component analysis (PCA) using the same para-meters as for the cluster analysis (Table 5). An initialanalysis for all entries identified two axes which to-gether explained 81% of the variance. Axis 1 wasmainly composed of the BVP parameters and axis 2described the PP-sensitivity of the entries. The axis 1vs axis 2 scatter plot (Figure 6) shows that this analysisdifferentiated well between the main linkage groups,but not between the sub-groups. The large groups D/d,G/g and F/f differed strongly in BVP (axis 1; BVPlong for F/f, short for D/d, intermediate for G/g), butvaried widely in PP-sensitivity (axis 2).

The single cultivar in group A was not included inthis scatter plot because it was extremely far removedfrom the rest of the entries (left-top, out of range ofFigure 6). Groups B and C were similar to A (PP-sensitive and short BVP), but were less extreme. Allentries in A, B and C wereO. glaberrimacultivars ortheir descendants. Group E, consisting of two tradi-tional japonicas, was also comparatively PP-sensitive,but had a long BVP.

The large groups D/d, F/f and G/g were alsoanalyzed individually by PCA (Figure 7), using thethree PP-sensitivity parameters across which they hadscattered in the pooled PCA analysis. For each ofthese groups, two near-identical axes were identified

120

Table 3. Duration of the basic vegetative phase (BVP), expressed as degree-days(dd), for four rice cultivars as observed in this study and reported by Chang andVergara (1985, units adapted; hydrological conditions not reported)

Cultivar Duration of BVP (dd)

Lowland Hydromorph Upland Mean, Chang &

this study Vergara

Azucena 882 791 829 834 782

OS6 789 777 852 806 799

Moroberekan 902 919 1059 960 1003

LAC 23 935 949 1048 977 1071

Mean 877 904 947 894 914

Figure 5. Cluster analysis on the basis of Euclidian distances for 83 rice cultivars, using the duration of the basic vegetative phase (BVP)in the upland, hydromorphic and lowland ecosystems the photoperiod-sensitivity in the same ecosystems as parameters. Encircled symbolsdenominate linkage groups. Numbers indicate cultivars according to Table 1. GLA,O. glaberrima; JAP,O. sativajaponica; IND,O. sativaindica; PRO, inter-specific progeny; Impr., improved; Trad., traditional.

(Table 5) that distinguished between PP-sensitivity inthe upland and hydromorphic ecosystems (axis 1) andthat in the lowland (axis 2). This analysis separatedreasonably well the sub-groups D and d, F and f, andG and g on the basis of ecosystem-specific responses.Group D, as compared to d, was particularly PP-sensitive in the lowland; group f, compared to F, wasPP-sensitive in the upland; and group G, compared tog, was more PP-sensitive in the upland as well.

Agro-ecological properties of the linkage groups andimplications for breeding

In the following description, we will take into ac-count the groups’ genetic composition and predomin-ant ecotypes (Table 4), their phenological parameters(Table 5) and their duration from emergence to matur-ity for the most common local sowing date (early June)

and ecosystem where they are grown (group means inTable 4, individual data in Table 1).

Groups A through C, comprisingO. glaberrimacultivars and their descendants, were PP-sensitive andhad a very short BVP. Some lesser PP-sensitiveO.glaberrima landraces were also found in groups Dand d. Although the sample ofO. glaberrima ricesin this study is small, there appears to be a trendtowards greater PP-sensitivity the further south theywere collected. The most PP-sensitive cultivar, IG10,originated from Cote d’Ivoire at a latitude of about6 to 9 ◦N, the least sensitive cultivars, CG 20, CG14 and CG 17, originated from Senegal (Casamanceregion) at about 12◦N, and TOG 6545, which wasintermediate, originated from Togo (8 to 10◦N). If theecological function these cultivars’ PP-sensitivity is toensure flowering before the end of the rainy season inOctober (Figure 2: IG10), a greater sensitivity wouldindeed be needed at lower latitudes.

121

Table

4.C

hara

cter

istic

sof

linka

gegr

oups

(Ath

roug

hG

,re

fer

toF

igur

e5)

iden

tified

onth

eba

sis

ofph

enol

ogic

alpa

ram

eter

s.P

rinci

pal

com

pone

nts

asin

Fig

ure

s6

and

7;sc

at,

scat

tere

d;P

P,

phot

oper

iod;

SD

,sho

rtdu

ratio

n;U

SD

,ultr

a-sh

ortd

urat

ion;

MD

,med

ium

dura

tion;

LD

,lon

gdu

ratio

n;al

loth

erab

brev

iatio

nsas

inTa

ble

2

Lin

kage

Tota

lgl

apr

osa

tsa

tU

plan

dL

owla

ndM

atur

ity(d

)P

rinci

palc

ompo

nent

sP

redo

min

ante

coty

pes

grou

pcv

s.ja

pin

dtr

ad.

impr

.tr

ad.

impr

.M

ean

SE

alll

ines

w/in

grou

ps

Axi

s1A

xis2

Axi

s1A

xis2

A1

11

138

−−−

++

+H

ighl

yP

P-s

ens.

,tra

d.up

land

(O.g

labe

rrim

a)

B2

22

108

5,9−−

++

SD

,mod

.PP

-sen

s.,i

mpr

.upl

and

(inte

rspe

cific

s)

C2

22

109,

53,

4−−

++

SD

,mod

.PP

-sen

s.,t

rad.

upla

nd(O

.gla

berr

ima)

D6

11

41

599

,80,

4−−

+sc

at+

+U

SD

,im

pr.u

plan

d,P

P-s

ens.

inlo

wla

nd(ja

poni

ca)

d15

15

81

113

110

0,7

1,8−−

scat

scat

−U

SD

,im

pr.u

plan

d(ja

poni

ca&

inte

rspe

cific

s)

E2

22

134,

52,

5+

++

+L

D,m

od.P

P-s

ens.

,tra

d.up

land

(japo

nica

)

F10

19

37

130,

41,

4+

+−

−−+

MD

-LD

,PP

-inse

ns.,

impr

.&tr

ad.l

owla

nd(in

dica

)

f11

110

11

912

9,5

2,2

++

++

+sc

atM

D-L

D,i

mpr

.low

land

,P

P-s

ens.

inup

land

(indi

ca)

G15

411

31

1112

7,9

1,4

+sc

at+

+sc

atM

D,i

mpr

.low

land

,PP

-sen

s.in

upla

nd(in

dica

)

g19

113

514

511

4,2

1,5

scat−

−sc

atS

D-M

D,P

P-in

sens

.,im

pr.u

plan

d(ja

poni

ca)

835

933

3611

355

3211

7,9

122

Table 5. Contributions of parameters (P1 through P6) to the principal components of the phenological variability observed among 83 ricecultivars (Table 1; No. 40 excluded dur to missing data) sown in Cote d’Ivoire on five dates and three ecosystems (upland, hydromorphicand lowland) during the 1997 wet season. The first analysis used six parameters and all cultivars (refer to Figure 6). Groups Dd, Ff andGg (Figure 5) were further characterized using a subset of three parameters (refer to Figure 7abc). BVP, basic vegetative phase expressedin degree-days (dd); PS, photoperiod-sensitivity, estimated as the difference in the photoperiod-sensitive phase between 12.0 and 12.5 hastronomic daylength, expressed in dd

Parameter Parameter contribution (factor loadings)

All lines (Figure 6) Group Dd (7a) Group Ff (7b) Group Gg (7c)

Axis1 Axis2 Axis1 Axis2 Axis1 Axis2 Axis1 Axis2

P1 BVP (upland) 0.98∗ –0.04 – – – – – –

P2 BVP (hydromorph) 0.98∗ –0.08 – – – – – –

P3 BVP (lowland) 0.97∗ –0.05 – – – – – –

P4 PS (upland) 0.25 0.78∗ 0.91∗ 0.08 0.82∗ –0.27 0.89∗ 0.16

P5 PS (hydromorph) –0.18 0.83∗ 0.90∗ –0.13 0.88∗ 0.14 0.70∗ –0.46

P6 PS (lowland) –0.18 0.76∗ 0.03 –1.0∗ 0.04 –0.98∗ –0.01 –0.94∗

Variance 2.88 1.88 1.64 1.02 1.44 1.05 1.28 1.12

Contribution to variance 50% 31% 55% 34% 48% 35% 43% 37%

Figure 6. Scatter plot for principal component analysis, based onall 83 rice cultivars pooled, for phenological parameters and linkagegroups as in Figure 5. Axis components are listed in Table 5.

The weed-like phenological characteristics of theO. glaberrimalandraces are ecologically in line with– although physiologically independent of – theirleafy and weed-competitive plant type, particularlyfor IG10 (Johnson et al., 1998). Although agronom-ically quite inadequate, the large phenological plas-ticity and adaptation to multiple ecosystems identify

IG10 and similar landraces as being interesting ge-netic resources for crop improvement for low-inputand stress-prone environments (WARDA, 1996; Joneset al., 1997; Dingkuhn et al., 1998a). The present ana-lysis suggests, however, that the agronomically moreappropriate types with low to moderate PP-sensitivityare more likely to be found at higher latitudes in WestAfrica.

Group D/d had clear-cut genetic and ecologicalcharacteristics (Table 5). It was almost exclusivelymade up of ultra-short to short duration, improvedupland rices, which belonged either to the tropical-japonica group or were descendants of a CG 14 (O.glaberrima)×WAB 56-104 (O. sativajaponica) cross(Jones et al., 1997). The only indica-type cultivar inthis group was the highly drought resistant Salumpikitfrom the Philippines. Group-D/d cultivars had an ex-tremely short BVP, and were only mildly PP-sensitive.Sub-group D was characterized by high PP-sensitivityunder lowland conditions, possibly indicating poorphysiological tolerance to water logging.

Group D/d represents the explicit or implicit phen-ological objectives of many upland rice breeders inWest Africa, who seek to combine short durationand photo-period-insensitivity with drought resist-ance, thus enabling plants to both temporally escapeand withstand drought. This breeding strategy standsin marked contrast to the phenological characterist-ics of many traditional upland rices, such as LAC23 and Danane, which have a long BVP and are

123

PP-sensitive (group E), or like Moroberekan (f) andIguape Cateto (G), which also have a long BVP butare less PP-sensitive. Under low-input conditions andlimited seasonal rainfall, the traditional combinationof a long BVP (resulting in much biomass produc-tion and tall plants to outgrow weeds), PP-sensitivity(to advance flowering if crop establishment was late)and early sowing (to escape terminal drought) aims atmaximizing yield stability. Conversely, it is likely thattraditional, short duration, upland-adapted japonicashave not evolved before the intervention of modernbreeding because they lack in yield stability.

A plant type concept for labor-limited, low-inputupland rice systems recently developed at WARDAseeks to improve the breeding strategy seen in groupD (extremely short BVP) by increasing yield stabil-ity through genetic weed competitiveness (Jones etal., 1997; Dingkuhn et al., 1998b). Contrary to thetraditional strategy realized in group E, weed compet-itiveness would not be achieved with a long BVP butinstead, with enhanced leaf area production during ashort BVP, usingO. glaberrimacultivars as geneticdonors for vigor.O. glaberrimacultivars produce alarge leaf area and biomass in a shorter time thanO.sativa japonica upland rices (Dingkuhn et al., 1998).This approach has so far been using as parentsO.sativa materials from group D/d andO. glaberrimamaterials from the same group (CG 14, CG 20) or themore PP-sensitive group C (CG 17).

Almost all improved, lowland-adapted indicastested belonged to linkage groups F/f and G, whichshared a long BVP and differed slightly in PP-sensitivity. Traditional, lowland-adapted rices werefound in group F/f, which had a longer BVP thangroup G. No highly PP-sensitive lowland rices werefound. The absence of short-duration, lowland indicasin the sample, which was fairly comprehensive for thehumid zone of West Africa, can probably be explainedwith the relative unimportance of rice double croppingin the region so far, and with the large investmentof labor and fertilizer that is needed to achieve highyields with short-duration cultivars.

Group g was the least homogenous of the link-age groups, and included both upland and lowlandcultivars having an intermediate duration to maturitybetween 107 and 125 d and very low PP-sensitivity.Some of the most successful improved cultivars andearly introductions to the region were found in thisgroup, such as OS6 and IDSA 6 (upland), and ITA123 and TOX 728-1 (lowland).

Synthesis of results

The empirical, phenology-based linkage groups dis-criminated to some extent between genetic groups ofcultivated rice, and more clearly between ecotypes.For lowland rices (groups F/f and G), no phenolo-gical difference was found between traditional andmodern (improved) cultivars. For example, group Fincluded improved semidwarf types such as Bouaké189, medium-tall traditional types such as Suakoko 8,and tall, flood-tolerant types such as Gambiaka. Weconclude that many traditional, lowland-adaptedO.sativacultivars grown in the forest and savannah zonesin West Africa are not particularly PP-sensitive withinthe range of daylengths studied here, and derive theirlong duration from a long BVP. For traditional low-land rice systems in the humid zone of West Africa,highly PP-sensitive cultivars such as IG10 might in-deed flower too early to permit harvesting before therecession of natural floods. Our data do not permitextrapolation to the large floodplains of the Sudansavannah and Sahel, however, because PP and tem-perature patterns in those environments deviate fromthe experimental conditions.

Two highly contrasting groups composed ofupland-adapted cultivars were identified. Improved,short-duration, comparatively PP-insensitive typeswere concentrated in group D, whereas traditional,PP-sensitive, medium- to long-duration types werefound in group E. These types represent alternativestrategies to achieve yield stability either through tem-poral escape from drought (D) or extended duration ofvegetative growth (E). Some other traditional uplandjaponicas were comparatively PP-insensitive, but alsohad a long BVP. In striking contrast to the traditionaljaponicas, the traditional, indigenousO. glaberrimacultivars were either similar to the ‘modern’ group D(short BVP, low-PP-sensitivity), or combined an ex-tremely short BVP with high PP-sensitivity (groups Aand B).

These findings are relevant to current breedingefforts in West Africa on the basis of inter-specificcrosses (Jones et al., 1997) because they show thatthe phenological type D preferred by most uplandrice breeders occurs indigenously inO. glaberrimagenus. Independent studies (Dingkuhn et al., 1998ab)indicated thatO. glaberrimaupland rice types over-come the major disadvantages of short duration,namely, poor weed competitiveness and biomass pro-duction, with specific physiological traits enablingearly ground cover. OngoingO. glaberrima× O.

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Figure 7. Scatter plot for principal component analysis, conducted individually for linkage groups D+d, F+f and G+g as shown in Figure 5,on the basis of photoperiod-sensitivity in the upland, hydromorphic and lowland ecosystems. Numbers indicate cultivars according to codes inTable 1. Axis components are listed in Table 5.

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sativa breeding programs using parental materialsfrom within group D (e.g., CG 14× WAB 56-104) may therefore have the potential of developingshort-duration types with improved yield stability.

Conclusion

We conclude that the cultivated rice germplasm in thehumid forests and moist savannahs of West Africafall into different phenological groups that are as-sociated with genetic groups, ecotypes and apparentfarmers’ and breeders’ selection strategies for yieldstability. Among the traditional and improved low-land rices, no short-duration materials were found.For traditional lowland types, this was seen to re-flect the farmers’ preference to harvest the crop afterflood recession. The comparatively long duration ofimproved lowland materials, however, reflected breed-ers’ objectives to achieve high yield potential andstability in systems where mostly one rice crop peryear is grown, and where suboptimal water man-agement requires a relatively long crop duration toachieve high yields (B.N. Singh, WARDA, personalcommunication, 1998).

The phenological types preferred by many uplandrice breeders (short and stable duration to escape fromseasonal drought) are paralleled by some traditionalO. glaberrima landraces, whereas the traditionalO.sativa, tropical-japonica materials from the regionseem to have generally a longer duration and belongto different phenological groups. These findings sup-port current efforts to breed for short-duration uplandrices with superior yield stability on the basis ofO.glaberrima× O. sativacrosses. More information isneeded, however, on the phenological, ecological andgenetic diversity of theO. glaberrimagenus.

Acknowledgements

This research was partially funded by the OverseasDevelopment Agency (ODA) of the United King-dom and the Canada-CGIAR Linkage Fund of theCanadian International Development Agency (CIDA).

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