GENETIC VARIABILITY FOR DROUGHT TOLERANCE IN COWPEA RECOMBINANT INBRED LINES [Vigna unguiculata (L.) WALP.] IN GHANA
Abstract
Drought is more prevalent in the semi-arid areas where cowpea has the greatest potential. It is unpredictable in occurrence so timing to avoid drought is not easy. Drought affects both vegetative and reproductive growth of cowpea resulting in significant yield losses. This study was therefore undertaken to: (1) assess farmers’ perceptions on the effects of drought on cowpea production in northern Ghana and identify farmer preferred traits in cowpea varieties, (2) identify drought tolerant cowpea recombinant inbred lines using wooden box screening technique, (3) determine grain yield and biomass loss of cowpea inbred lines under low soil moisture conditions, (4) assess genotype by environment interaction and stability of cowpea recombinant inbred lines in Guinea, Sudan and transition ecologies of Ghana, and (5) determine the effect of drought on cowpea inbred lines’ grain nutrient content, minerals and phytochemical variability among 22 inbred lines seed samples. Results from the Participatory rural appraisal (PRA) study indicated that, the farmers demonstrated a high understanding of climate variability. Insect pest infestation, drought, and low soil fertility were constraints faced by farmers in those production zones. Seventy percent of the farmers preferred varieties with large grain size, smooth or rough textured seeds with white coats depending on the location, and high expansion ratio. From the focus group discussions, 83.7% of farmers preferred varieties that were early and drought tolerant. Results obtained from Screen-house screening for seedling tolerance indicates that the chlorophyll content gradually decreased over the period of water stress. Chlorophyll contents taken at 7, 14, 21 and 28 days after water-stress application significantly varied among the inbred lines, but no significant differences were observed for the parental checks. Relative water content for the recombinant inbred lines during water-stress ranged between 20-70% for drought susceptible and drought tolerant inbred lines respectively across the population for screening. The parental lines used as checks both had relative water contents of 60% for the first screen-house experiment. Relative water content for the second experiment ranged between 22-74% for susceptible and tolerant recombinant inbred lines respectively. Leaf wilting after 7 days of water-stress correlated positively with chlorophyll content at 7, 14, 21 and 28 days of water stress, however, leaf wilting on 14, 21 and 28 days negatively correlated with chlorophyll content at 7, 14, 21 and 28 days of stress imposition. Relative water contents for 14 days of water stress correlated negatively with chlorophyll content at 7, and 14 days but correlated positively with chlorophyll content at 7, 14, 21 and 28 days of stress. Potential seedling tolerant inbred lines were 84, 406, 325, 223, 75, 186, 131, 20, 28, 230, 398 and 353. The genetic variability found for this morphological trait among these inbred lines in the screen-house study suggest the opportunity for selecting superior genotypes under water limited conditions in the field. The results obtained from the field drought study indicated that, water stress resulted in percentage yield reduction for most of the inbred lines and the parental checks. The highest percentage yield reduction was observed for inbred line 20 (57.83%). Inbred line 84 rather had yield increase under drought (-10.57) Inbred lines 255, 353, and 186, had mean grain yields of 1.83, 1.74, and 1.67 tonnes per hectare (t/ha). The lowest mean grain yields were obtained from inbred line 28 with grain yield of 0.89 t/ha. The parental checks IT93K-503-1 and 97K-279-3, had mean grain yields of 1.60 and 1.07 t/ha respectively. The highest mean grain yield under water stress conditions was obtained from inbred lines 186 and 255 with mean yields of 1.48t/ha whereas the lowest mean yield was obtained for inbred line 38 with grain yield of 0.68t/ha. Additive main effects and multiplicative interaction (AMMI) analysis indicate significant differences for all the traits across all the six environments. In this current study, the highest yielding inbred lines across all the six environments were 75, 186, 353, 57, 255 and 131. Under the three stress environments, inbred lines 131, 75, 353 and 255, were the best performing inbred lines. Inbred lines 396, 28, 189, 116, 38, 325, 131 and 84 performed well under well-watered conditions. The most stable inbred line across the six environments was 75, whereas the highest yielding inbred lines were 255 and 131. In general, the recombinant lines that proved to be drought tolerant both under screen-house and field experiments were 84, 186, 255, 131, 353 and 75. Biochemical analysis for seed related traits revealed that inbred lines responded differently to drought. Significant differences of water regimes on various phytochemical traits were only observed in phosphorus, lead, valine and dl-beta phenyl-alanin. Inbred line with family number 57 had the highest crude protein content of 46.90% under well-watered conditions. Inbred line 84 under water stress conditions had high levels of Mg and K. Inbred line 20 under well-watered conditions had the highest antioxidant content. For phenolic acid content, family 255 scored the highest. Quercetin and rutin were most abundant in inbred line 186 for both water-stress and well-watered conditions. Inbred lines with family numbers 84, 406 and 189 for water-stress treatment had highest amino acids such as glutamine, lysine and histidine. Farmer involvement in the development of new varieties should be encouraged to enable easy adoption of improved varieties for cultivation.