Genetic Analysis of Vegetative-Stage Drought Tolerance in Cowpea

The continued improvement in the yield of cowpea for the semi-arid regions where it is a dominant crop will record rapid progress if the genetic basis for yield under different moisture regimes is better understood. This study investigated the genetic combining abilities and heterosis for grain yield, yield components, earliness to flower and biomass production among diallel hybrids derived from nine cowpea genotypes. Hybrids and their parents were evaluated under adequate soil moisture conditions and conditions of soil moisture stress during the first 30 days of growth. Results showed that soil moisture stress significantly reduced grain yield, biomass production, and the number of pods per plant. Grain size however, increased under moisture stress. Only for grain yield was a significant interaction of moisture regimes and genotypes observed. Genetic control for grain size, number of seeds per pod and days to flowering was dominated by additive genetic effects. For these traits, mass selection is judged adequate to make selection gains. For grain yield, both additive and non-additive genetic effects were dominant, but the greater influence of non-additive effects was observed under both adequate soil moisture and moisture stress conditions. The genetic control of yield under the two soil moisture conditions indicated that grain yield improvement will be more challenging for drought prone regions compared with regions with adequate soil moisture. Nonetheless, careful selection of genotypes that have complementary but different alleles for yield under different soil moisture conditions might permit yield improvement through recurrent selection systems. This study has identified parental lines, developed populations, and provided a breeding strategy for developing cowpea cultivars with higher grain yield potential for the Guinea and Sudan savannah regions of West Africa.


Introduction
Cowpea (Vigna unguiculata) [L.] Walp) is a source of livelihood to millions of relatively poor people in less developed countries of the tropics.It is one of the ancient grain legume crops cultivated in semi-arid West Africa where rainfall is characteristically low (mean annual range of 300-600 mm), variable in time and space and undependable (Fussell et al., 1991).The grain is valued for its high protein content of about 23-25% and 50-67% starch.The fresh young leaves, immature pods and peas are used as vegetables, whilst several snacks and main dishes are prepared from the grain.The haulms are used as fodder for livestock particularly during the dry season (Blade et al., 1997).Although cowpea contributes significantly to the income of farmers in the northern savannah zones of West Africa where varieties with grain yield potentials in excess of 3.0 t/ha have been developed for cultivation, farm level yields of cowpea are within the range of 0.3 to 0.65 t/ha (SARI, 1996 and1997).Besides the problem of insect pests (Singh and Jackai, 1985), sensitivity to soil moisture stress during the various growth stages of the crop significantly reduces grain yield.The timing and intensity of drought in relation to the crop phenology, sensitivity of flower, pod and seed development to high night temperatures are important constraints to sustainable cowpea production.The Northern sector of Ghana produces the bulk of cowpea for Ghana.After the cowpea is planted, drought incidence of between 20 and 30 days are not uncommon.Under this production system however, the variety of cowpea that is planted need to be particularly tolerant to drought during the vegetative phase.Identification and separation of varieties that tolerate unfavourable conditions such as drought would be of great value to farmers in the semi-arid zones including Northern Ghana.

Material and Methods
Nine genotypes of cowpea were used for the study.Four of these genotypes, Omondaw, Apagbaala, Milo and Padi Tuya were obtained from CSIR -Savannah Agricultural Institute (CSIR-SARI) and used as female lines.These lines have been developed by SARI for general cultivation to increase productivity in the three Northern Regions of Ghana.The other five genotypes which were among the best drought-tolerant cowpea lines came from the International Institute of Tropical Agriculture (IITA) Kano, Nigeria.These lines included IT 93K-503-1, IT 89KD-374-57, IT 97K-499-35, IT 99K-241-2 and IT 97K-568-19.Seeds of the parental lines were planted in 32cm-diameter plastic pots filled with black, loamy top soil in the screen house at CSIR-SARI.At flowering, the female parents were each crossed with the same male parents to generate F1 populations.Soil physical and chemical analyses Soil samples were taken from the two main plots (i.e stressed and non-stressed plots) and the physical and chemical analyses was carried in the laboratory for each of the block.
After the characterization of the two plots, both the non stress and water stress experiments were irrigated to its water holding capacity and planted on the same day.For the water-stress experiment no further irrigation was done until after 30 days after planting.The non-stress experiment however received water based on the crop water demand established by daily field observations.After the 30 days had elapsed, both fields received water till pod maturity.
The experimental design was split-plot with watering regimes as main plots and test genotypes as sub-plots in three replications and completely randomized.The data collected were analyzed using the GenStat Statistical Program (Discovery Edition 3.0).First, the data on each trait was separated into moisture stress or non-stress set and analysed, followed by analysis of variance using the North Carolina II Design.The ANOVA of this cross-classified experiment, involving n 1 males crossed to n 2 females and genotypes replicated r times.

Results and Discussion
The ANOVA following the North Carolina II design provided estimates of the general combining ability (GCA) for the five advanced breeding lines used as males and for the four cultivars used as females, and the specific combining ability (SCA) for their crosses.In general, the GCA estimates for males were not significant (P > 0.05) for the majority of traits studied.Only for biomass and days to flowering under non-stress condition (Table 2) and hundred seed weight (Table 2) were the GCA estimates for males significant.On the other hand, female GCA estimates were significant for all six traits studied under both stress and non-stress conditions.For grain yield, Apagbaala and Padi Tuya contributed positively but their GCA estimates were significant only under adequate soil moisture conditions.The Milo cultivar was the best female under moisture stress conditions, contributing to increasing grain yield.Omondaw was the worst female as its GCA indicated that it reduced grain yield under both moisture regimes.3).Omondaw × IT 89KD-374-57 produced a significant negative SCA estimate for grain yield under moisture stress.The additive and dominance variance components were estimated for each trait under stress or adequate soil moisture conditions.Days to flowering, weight of hundred seeds and number of seeds per pod were conditioned mainly by additive genes.For biomass production, number of pods per plant and grain yield, dominance variance was higher than the additive variance component.
Pearson's correlation coefficient was determined between the six traits recorded in the experiment.Late flowering genotypes were associated with high biomass yields with r = 0.547.Under adequate soil moisture conditions, genotypes that were late to flower produced lower grain yields.In the present study, Padi Tuya combined well for yield under adequate soil moisture conditions, but not for stress conditions.On the other hand, Milo combined well for yield under stress conditions.A recurrent selection program that makes use of favourable alleles from different genetic backgrounds might help develop cultivars with increased yield under both adequate and moisture stress conditions.

Table 1 :
Crossing scheme to generate F 1 seeds

Table 2 :
General combining abilities for grain yield, biomass and days to flowering of nine cowpea lines evaluated under contrasting moisture regimes during the vegetative phase

Table 3 :
Specific combining ability estimates for grain yield in a diallel mating of nine cowpea genotypes under moisture stress during the vegetative phase