Development of drought tolerant groundnut mutants for the rainfed areas of Western Sudan

Groundnut (Arachis hypogaea L.) is an important oilseed crop in the rainfed sector of western Sudan. The low and highly variable productivity are characteristic features of this sector where terminal drought stress is the main production constraint. This study was conducted at El Obeid Research Station Farm, El Obeid, North Kordofan State, Sudan. The main objectives of this study were creating genetic variation for selecting groundnut mutants with terminal drought tolerance and identifying drought tolerant traits that could be effectively used in groundnut breeding programs for drought tolerance. Dry uniform seeds of six groundnut cultivars, viz., Sodari, Barberton, lCGV 89104, lCGV 86744, lCGV 86743 and lCG 221, all belonging to the Spanish-type groundnut cultivars (Arachis hypogaea 1. subsp. fastigiata var. vulgaris) were irradiated in Pretoria, South Africa in 1999 using gamma rays at total doses of 20 and 30 Krad. Pedigree breedin g method and visual morphological plant characters were utilized in generation advancement and selecting the mutants. Nine promising mutants were selected on the basis of their performance under terminal drought stress during 2001/2002 and 2002/2003 cropping seasons. These mutants and their respective parents were grown under sprinkler irrigation system (unstressed) and rainout shelter (terminal drought stress) for two consecutive seasons (2003/2004 and 2004/2005) in a randomized complete block design with 4 replications. Mutants were evaluated for pod yield, days to 75% emergence, days to 75% flowering, dry matter production (DM), harvest index (HI), specific leaf area (SLA), reproductive efficiency (RE), shelling percentage (SH%), 100-seed weight (g), leaf relative water content (LR WC), leaf senescence (LSENS), and canopy temperature (CT). Pod yield under unstressed environment was significantly higher than that under stressed environment. Some mutants under the stresse d environment exceeded their parents in pod yield, DM, HI, SH%, RE, LR WC and 100-seed weight whereas most mutants showed lower SLA, CT, and LSENS than their parents. Better performance under stressed environment indicated better drought tolerance. Mutants did not differ from their parents in days to attain 75% emergence and days to attain 75% flowering, which were respectively attained within 7 and 30 days. Thus, days to 75% emergence and days to 75% flowering as traits demonstrating little or no genetic variation in this study are of limited use as selection criteria to improve crop performance under terminal drought stress conditions. Pod yield showed significant positive genetic correlation with DM, LR WC, RE, HI, SH%, and 100-seed weight. On the other hand, pod yield showed significant negative correlation with SLA, LSENS, and CT. Path coefficient analysis using genotypic correlations revealed positive direct effects of DM, HI, SH%, CT, and LR WC on pod yield, while, SL A , 100¬seed weight, RE, and LSENS showed negative direct effects on pod yield. Hence, direct selection under terminal drought stress for high values of DM, HI, SH% and LR WC could result in improvement in pod yield. Direct selection for. low SLA, CT, and LSENS could also result in improvement In pod yield. Since most of these traits are easily measurable under field conditions, they could be used as reliable traits in screening for drought tolerance in groundnut breeding programs. Results had also shown that the mutant Barberton-B-30-3 produced the highest pod yield, DM, HI, RE, 100-seed weight, and LR WC under terminal drought stress. Barberton-B-30-3 was also among the mutants with the lowest values of SLA, CT, and LSENS. Barberton-B-30-3 was thus, considered the best drought tolerant mutant in this study. Nevertheless, further evaluation of this mutant over years at several locations is necessary before its suggestion for release as a drought tolerant groundnut mutant. M o reover, genetic studies are needed to elucidate the type of gene action involved in drought tolerance under terminal drought stress conditions