GENETIC ANALYSIS OF EARLINESS AND DROUGHT TOLERANCE IN GROUNDNUT (Arachis hypogaea L.) IN NIGER, ABSTRACT
End-of-season drought, characterized by low and erratic rainfall, is the most important factor limiting groundnut (Arachis hypogaea L.) production in Niger. Information about farmers' varietal preferences and production constraints are limited. Genotypes have not yet been screened for earliness and tolerance to end-of-season drought. Data on combining ability and heritability estimates are not available. Development of varieties that have better ability to use limited available water or that mature early and avoid drought is needed.
A participatory rural appraisal (PRA) through Focus Group Discussion session was conducted in 2011 to assess farmers' perceptions on groundnut production constraints and to gather information on preferences for elite groundnut varieties. One hundred and fifty genotypes were screened in 2010 under well watered conditions in an alpha lattice design in an unreplicated trial to select five early maturing parental lines for genetic studies. One hundred intermediate maturing genotypes were selected and, evaluated in 2011 offseason under both well watered and end-of-season drought conditions in an alpha lattice design with two replications to select five drought tolerant parental lines for genetic studies. The selected parental lines were crossed with the three farmers preferred varieties in a North Carolina II mating design and 7 F4 populations for earliness and 7 F4 populations for drought tolerance were developed by selfing. Seven F3 populations were evaluated for earliness and agronomic traits in RCBD replicated three times under well watered conditions. Seven F3 and F4 populations were evaluated for drought related traits in RCBD replicated three times. Data recorded included: % emergence, 50% plants flowering, pod yield, seed weight, seed number, shelling %, pod length, maturity index, and biomass. Among the physiological traits measured were SPAD chlorophyll meter reading and harvest index. The data were analyzed using appropriate statistical procedures and experimental designs. The PRA survey revealed that drought was a major production constraint followed by low soil fertility. In the study regions, the varieties 55-437, RRB and JL24, were the most grown. There was variation among the genotypes screened for the various characteristics. The PCA analysis revealed that 79.62% of the total variation among the genotypes was explained by maturity, pod yield, biomass, harvest index and 50% plants flowering. Out of the 150 genotypes, two extra early maturing genotypes (75 days) Chico and ICIAR19BT, three early maturing genotypes (80 days) ICG3584, 796, and ICGV02022 and five drought tolerant genotypes ICGVSM99511, Tainan-9, ICG11249, ICG6703 and ICGIS01820 were identified. In the early maturity experiment, the analysis of variance for the eight traits revealed significant differences (P ? 0.01 and P ? 0.05) among parents for pod yield, seed weight, pod length and maturity index. Crosses showed highly significant differences for pod yield (P ? 0.001), seed number (P ? 0.001), shelling % (P ? 0.01) and pod length (P ? 0.05). Female and male General combining ability (GCA) mean squares were highly significant (P ? 0.01) only for pod yield. GCA mean squares (119.26) for females were greater than males GCA mean square (32.11) for pod weight indicating that the major contribution to additive variance for this trait was due to the female parents. Specific combining ability (SCA) mean squares differed significantly (P ? 0.01 and P ? 0.05) for seed number and shelling percentage indicating the importance of both additive and non-additive variance for these characters. Chico, ICIAR19BT, 55-437 and RRB were the best general combiners. The estimates of phenotypic coefficient of variation (PCV) were greater than genotypic coefficients of variation (GCV) for all the traits studied. None of the traits recorded high PCV and GCV. PCV ranged from 4.36 to 19.34% and GCV from 2.02 to 11.99%. Narrow sense heritability estimates ranged from 35.4 to 95.5%; for days to emergence (95.5%), pod yield (85%), shelling percentage (84.3%), seed number (80%) and 50% plants flowering (72%). Moderate to low heritability estimates were obtained for maturity index (66.1%), pod length (54.4%) and seed weight (35.4%). Highly significant positive correlations were found between pod yield and seed weight (r = 0.97, P ? 0.001) and pod length (r = 0.72, P ? 0.01); between pod length and seed yield (r = 0.66, P ? 0.01); and between maturity index and days to emergence (r = 0.72, P ? 0.001). Correlation between shelling percentage and seed weight was positively significant at P ? 0.05 (r = 0.61). In the drought tolerant experiment, combined analysis of variance showed large and significant differences (P ? 0.01) between all 14 entries for all traits except for pod yield. Highly significant differences (P ? 0.01 and P ? 0.05) in chlorophyll content among the crosses at 60 and 80 DAS were found. The overall means of chlorophyll content under end-of-season drought stress conditions were 41.55 and 39.65, at 60 and 80 DAS respectively compared to 35.27 and 38.04 under irrigated conditions. The analysis of variance showed highly significant differences (P ? 0.01) between the crosses in water regimes for pod yield. The drought tolerance index for pod yield ranged from 0.44 to 1.37 for seven crosses. GCA mean squares for males and females were significant (P ? 0.05) for pod yield and biomass. GCA mean squares for females were greater than males GCA mean square for pod yield and total biomass indicating that the major contribution to additive variance for these traits was due the female parents. SCA mean squares differed significantly (P ? 0.05) for pod yield and harvest index indicating the importance of both additive and non-additive variance for these traits. Tainan-9, ICGV-SM99511, 55-437 and RRB were the best general combiners. The estimates of phenotypic coefficient of variation (PCV) were greater than genotypic coefficients of variation (GCV) for all the physiological traits. PCV ranged from 7.27 to 40.20% and GCV from 2.71 to 15.91%. Narrow sense heritability estimates for physiological traits were higher than for agronomic traits, and varied under both well-watered and end-of-season drought conditions. The heritability estimates for pod yield (0.26) and biomass (0.18) were low, but they were high for harvest index (0.78) and SPAD chlorophyll meter reading (SCMR), estimate of 0.71 and 0.68 respectively, at 60 and 80 days after sowing. Fifty percent (50%) to flowering showed moderate heritability estimate of 0.56. End-of-season drought decreased narrow sense heritability estimates for all the physiological traits. Heritability estimates decreased from 71.6% to 36.8% for SCMR 60 DAS; from 68.2% to 10.6% for SCMR 80 DAS and from 78.9% to 37.8% for harvest index. Highly significant positive association between pod yield and harvest index was found in both water regimes. The correlation coefficient was higher (r = 0.77, P ? 0.001) under end-of-season drought stress conditions than under well watered conditions (r = 0.74, P ? 0.01). Strong negative significant (P ? 0.05) correlation was found between biomass and harvest index under well watered and end-of-season drought conditions r = -0.66, -0.58, respectively. Weak positive and non-significant correlation was found between SCMR and pod yield, under well watered conditions (r = 0.38). The results show that development of early maturing and drought tolerant varieties may ensure better yields of groundnut in the Sahel.