African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 5, Num. 4, 1997, pp. 341-350

ASSESSING THE POTENTIALS OF COWPEA GENOTYPES BASED ON SOME YIELD DETERMINANTS OF A SIMPLE PHYSIOLOGICAL MODEL

K. O. MARFO, W. PAYNE^1 and F. WALIYAR^2

Savanna Agricultural Research Institute, P. O. Box 52, Tamale, Ghana
^1 ICRISAT Sahelian Centre, B. P. 12404, Niamey, Niger
^2 ICRISAT , B. P. 320, Bamako, Mali

(Received 4 March, 1997; accepted 2 September, 1997)

Code Number:CS97040
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ABSTRACT

A simple crop physiological model was employed to determine the yield basis of thirty-three cowpea genotypes of early, medium and late maturities evaluated across the major agro-ecologies in Northern Ghana between 1992 and 1994. Among the lines, significant genotypic variations were observed for pod yields (Y), crop and pod growth rates (CGR and PGR), partitioning coefficient (p), and the harvest index (HI) in all the maturity groups. Pod yields averaged 2032 kg ha^-1 for the early lines, 2170 kg ha^-1 for the medium group and 1983 kg ha^-1 for the late lines. Most of the lines had p values exceeding 1.0 indicating high efficiency in resource partitioning. Whilst drought adversely affected Y, CGR and PGR, partition coefficient and harvest index values rather increased, especially within the early genotypes. This indicates that under dry conditions, early cowpea varieties mobilise most of their photosynthetic assimilates to reproductive sinks through increased partitioning. Very close association was observed between high pod yield and CGR within the medium and late genotypes. This signifies the possibility of identifying lines which can serve "dual roles" as haulm producers for feeding livestock and grain for human consumption. The simple physiological model in this study has not only assisted in providing a better understanding of varietal performance but could enhance selection efficiency in the breeding programme.

Key Words: Crop growth rate, harvest index, partition coefficient, reproductive growth, Vigna unguiculata

Resume

Un model physiologique simple de la culture a ete utilise en vue de determiner le rendement de base de trente-trois genotypes de niebe a maturite hative, intermediaire et tardive a ete evalue a travers les principales zones agro-ecologiques au nord du Ghana entre 1992 et 1994. Parmi les lignees, les variations genotypiques significatives ont ete observees concernant les rendements des gousses (y), les taux de croissance de la culture et des gousses (CGR et PGR), le coefficient de repartition (p), et l'indice de recolte (IR) dans tous les groupes de maturite. Les rendements moyens des gousses etaient de 2032 kg/ha pour les lignees hatives, jeunes, de 2170 kg/ha pour le groupe a maturite intermediaire et de 1983 kg/ha pour les lignees tardives. La plupart des lignees presentaient des valeurs p excedent 1,0 indiquant ainsi une grande efficacite dans la repartition de resource. Alors que la secheresse affectait defavorablement y, CGR et PGR au contraire le coefficient de repartition et les valeurs de l'incidence de recolte augmentaient en particulier dans les lignees hatives. Cela indique que dans les conditions de secheresse, les varietes hatives de niebe mobilisent la plupart de leurs produits photosynthetiques dans la reproduction grece a une repartition accrue. On a observe une tres proche relation entre le rendement eleve en gousses et la CGR dans les genotypes a maturite intermediaire et tardive. Cela signifique que la possibilite d'identifier les lignees susceptibles de jouer un "double role" a la fois comme producteur de fanes pour le betail et de graines pour la consommation humaine. Le modele physioloqique simple dans cette etude a non seulement aide a fournir une meillure comprehension de la performance varietale mais aussi a accroitre l'efficacite de la selection dans le programme d'elevage.

Mots Cles: Taux de croissance de la culture, indice de recoltes, coefficient de repartition, croissance reproductryice, Vigna unguiculata

INTRODUCTION

Cowpea (Vigna unguiculata (L.) Walp.) is becoming increasingly important in sub-Saharan Africa. This may be partly due to the ability of the crop to produce reasonable grain and haulm yields in areas with limited rainfall as low as 200 mm per annum. Emphasis, of late, is being placed on the development of varieties which mature early enough to avoid terminal drought. Despite the "hardy" nature of the crop, little or no efforts have been made in the dry areas of northern Ghana to understand the physiological basis of the yields of the crop to enhance selection efficiency. The reason for this may be due to the tedious and time consuming nature of most physiological sampling and analysis. A simple physiological yield model developed by Duncan et al. (1978) can be adapted to overcome these constraints. The model states that:

Y       = RD x p x CGR
where Y = Pod or fruit yields
RD      = Reproductive duration
p       = The ratio of photosynthetic assimilates partitioned to            
          reproductive sinks compared with the sources, referred to as      
          partitioning coefficient.
CGR     = Crop growth rate. 

This non-destructive growth analysis model has been used by Greenberg et al. (1992) and Ndunguru et al. (1992) to study the effects of drought on groundnut in the Sahel region. Ntare and Williams (1993) also employed this model to interpret the performance of cowpea cultivars under cool season production in the Sahel. The differences in seed yield between 120 chickpea genotypes were determined by Williams and Saxena (1991) who attributed differences in yields mainly to CGR with RD and p having no direct effects on yields. This was contrary to observations by Duncan et al.(1978) who attributed yield differences in groundnut mainly to partitioning. However, the observations of Williams and Saxena (1991) on chickpeas are in agreement with the conclusions on the role of CGR by Ntare et al. (1993) in their studies on the effect of phosphorus fertilizer and sowing dates in cowpea intercropped with millet. Witzenberger et al. (1988) were able to interpret the effects of photoperiod in some groundnut cultivars based on Duncan et al. (1978) model.

The objective of our study is to employ this model to determine the physiological factors controlling the yields of some cowpea breeding lines in Northern Ghana.

MATERIALS AND METHODS

Thirty-three advanced cowpea breeding lines consisting of 11 early, 12 medium and 10 late maturity types were tested at several sites in Northern Ghana between 1992 and 1994. Some of the lines were originally developed by the International Institute of Tropical Agriculture (IITA) based in Ibadan, Nigeria. The test locations were Damongo and Nyankpala in the Guinea savanna, Wa in the transition between Guinea and Sudan savanna and Manga in the Sudan savanna. Based on FAO soils classification, the soils at Damongo may be described as being Chronic- Haplic Lixisol while those of Wa were Ferric Lixisol. Those of Manga and Nyankpala were Haplic Lixisol and Ferric- Haplic Lixisol, respectively.

After land preparation by disc ploughing, 45 kg P2O5 ha^-1 as single superphosphate was broadcast and harrowed in before planting. The design used was randomised complete blocks with four replications. In 1994, however, only three replications were used for the early lines due to shortage of seeds. Plots consisted of four 4-m rows spaced 60 cm apart with 20 cm distance between plants within rows. The number of plants per hill were two. Weeds were controlled by hand weeding as and when necessary. Insect pests were also controlled by the application of KarateR insecticide at the rate of 600 ml ha^-1 on three occasions beginning with floral bud initiation.

The number of days from sowing to flowering (DFF) and pod maturity (DH) were noted. At pod maturity, i.e., between 60 and 85 days after sowing, the two inner rows of each plot were harvested. Pod and crop residues were dried for 10 continuous days in the sunlight to a moisture level of about 5%, measured with a moisture meter. Shed leaves on the ground at the time of harvest were added to the crop residue. Based on Duncan et al. (1978) model, which approximates crop growth to be linear, the following parameters were computed:

DFF = No. of days to 50% flowering
DH  = No. of days to maturity
RD  = Reproductive duration = DH - DFF
Y   = Pod weight in kg ha^-1 
H   = Crop residue weight in kg ha^-1
T   = Total biomass = Y+H in kg ha^-1
CGR = Crop growth rate = T/DH in kg ha^-1 day-1
PGR = Pod growth rate =Y/RD in kg ha^-1 day-1
p   = PGR/CGR

The harvest index was also calculated as: Y/T. Analysis of variance was performed on pod yield and the other yield components across sites and locations using GENSTAT 5 Release 3.1R.

RESULTS AND DISCUSSION

The rainfall distributions for 1993 and 1994 were almost normal. However, in 1992, rainfall was inadequate and erratic in distribution. In August 1992 when reproductive development of the plants should have reached its highest level, only 37, 44, 76 and 47% of the expected rains were received at Damongo (Fig.1), Nyankpala (Fig. 2), Manga (Fig.3) and Wa (Fig.4), respectively.

Early maturing lines. Significant genotypic variations were noted for pod yields, crop and pod growth rates, partitioning and harvest index for the early lines. Pod yields were high (Table 1), averaging 2032 kg ha^-1 and were highest at Damongo, with a mean value of 3281 kg ha^-1. All genotypes had partition coefficients exceeding 1.0 indicating that pod growth rates were higher than crop growth rates. Ndunguru et al. (1992) and Greenberg et al. (1992) attributed the adaptation of groundnut lines to dry conditions in the Sahel to efficiency in partitioning. High partitioning also appears to be a desirable attribute for cowpea adaptation to northern Ghana. Two of the advanced breeding lines, Val x BB1 and Val x BB2 now under tests on farmers' fields, were among the top performing lines in combining both high pod yields and high partitioning.

The poor rains in 1992 adversely affected pod yield, T, CGR and PGR. Nevertheless, it was in the same year that the highest p and HI values were obtained indicating how important these two traits may be under abiotic stresses. Reproductive duration (RD) did not have any direct effect on pod yields (Table 1) which is consistent with the observations made by Williams and Saxena (1991) in chickpea. Pod yields, however, were strongly correlated with partitioning and HI. This implies that, under unstable rainfall conditions, selection pressure should be in favour of high levels of p and HI. Crop growth rate did not appear to have any direct association with pod yields, making any attempts to select early varieties which combine high haulm and grain yields difficult.

Medium maturing lines. Significant differences were observed among the medium lines for pod yield and other physiological parameters such as CGR, RD and p (Table 2). Pod yields averaged 2170 kg ha^-1, about 100 kg higher than the early genotypes. As in the early group, partitioning coefficient for each of the lines was higher than unity which indicates that pod growth rates exceeded crop growth rates. As expected, reproductive duration was about four days longer than the early lines and was shorter in the dry year compared with normal years. The average p was lower in the medium lines compared with the early lines. This indicates the relative efficiency of the early lines in mobilising photosynthetic assimilates to reproductive sinks. The driest year, 1992, produced the highest p values, and the shortest reproductive duration. This also indicates that under dry conditions, pod growth rates appear to be faster than crop growth rates.

In the medium lines, strong associations were observed between pod yields on one hand and CGR, p and HI on the other. This shows that it is possible to identify genotypes which can combine high crop residue production for livestock with high pod yields for human consumption.

Late maturing lines. Genotypic differences were observed for pod yields, reproductive duration, total biomass, crop and pod growth rates as well as partitioning and harvest index. Pod yields of late maturing lines were unexpectedly lower than for the early varieties, with an average of 1983 kg ha^-1 (Table 3). Partitioning coefficient was lowest in this group compared with the earlier varieties. Similar to the earlier genotypes, the highest partitioning and harvest index were obtained in 1992. Pod yields were highly correlated with CGR, p and HI with r values of about 0.9, 0.7 and 0.9, respectively (Table 4). It is, therefore, also feasible to select for "dual purpose" genotypes which combine high crop residue production with high pod yields in the late materials.

CONCLUSION

Although pod yields and other physiological traits were adversely affected by erratic rainfall, as in 1992, efficiency in partitioning of photosynthates, especially in the early lines was enhanced. Thus p and HI appear to be more reliable parameters to emphasise in areas with unpredictable rainfall distributions. Although RD did not appear to have any direct influence on pod yields in all the maturity groups, in years with drought, such as in 1992, the duration was shorter indicating hastening of photosynthate partitioning to fruits. The studies also showed that medium and late cowpea genotypes can be selected with high haulm production to support the livestock industry as well as supply adequate grain for human consumption.

The use of this simple physiological model has made it possible to gain an insight into the basis of cowpea varietal performance for a large number of varieties. This would not have been possible using conventional physiological analyses, which would have also involved substantial investment in time and other resources.

ACKNOWLEDGEMENT

The authors wish to thank Mariama I. Yacouba for her assistance during the preparation of the paper.

REFERENCES

Duncan, W.G., McCloud, D.E., McGraw, R.L. and Boote, K.J. 1978. Physiological aspects of peanut yield improvement. Crop Science 18:1015-1020.

Greenberg, D.C., Williams, J.H. and Ndunguru, B.J. 1992. Differences in yield determining processes of groundnut (Arachis hypogea L.) genotypes in varied drought environments. Annals of Applied Biology 120:557-566.

Ndunguru, B.J., Williams, J.H. Stern, R.D. and Ntare, B.R. 1992. Physiological models and agronomic data applied to experimental analysis and interpretation. En. Summaries in En, Pt. Pages 3-8. In: Proceedings of the Fifth Regional Groundnut Workshop for Southern Africa, Lilongwe, Malawi, 9-12 March, 1992, Lilongwe, Malawi. Nageswara Rao, R.C. and Subrahmanyam, P. (Eds.). Patancheru 502 324, Andhra Pradesh, India: International Crops Research Institute for the Semi-Arid Tropics.

Ntare, B.R and Williams, J.H. 1993. Selection of cowpea cultivars for cool season production in the Sahel. Field Crops Research, 32:37-39.

Ntare, B.R., Williams, J.H. and Batiano, A. 1993. Physiological determinants of cowpea seed yields as affected by phosphorus fertilizer and sowing dates in intercrop with millet. Field Crops Research 35:151-158.

Williams, J.H. and Saxena, N.P. 1991. The use of non-destructive measurement and physio-logical models of yield determination to investigate factors determining differences in seed yield between genotypes of "desi" chick-peas. Annals of Applied Biology, 119:105-112.

Witzenberger, A., Williams, J.H. and Lenz, F. 1988. The influence of daylength on yield determining processes in six groundnut cultivars (Arachis hypogea L.). Field Crops Research 18:89-100.

Copyright 1997, African Crop Science Society

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