Combining ability and heterosis for tolerance to low soil phosphorous and nitrogen in common beans (phaseolus vulgaris L).

Abstract

Common bean in Kenyans are predominantly grown by subsistence, resource-poor farmers who use little or no soil amendment chemicals. Production is limited by infertile soils with inherently low P and N, diseases, pests, use of unimproved cultivars and poor general agronomic practices. Eight common bean (Phaseolus vulgaris L.) cultivars, their 28 F, progenies derived from a half-diallel cross were evaluated under P and N stress (-) and non-stress (+) conditions at two locations (Kabete and Thika) in 2000 long rain season. Data were recorded on basal root length, root dry weight; number of nodules per plant, mid-leaf area, leaf area per plant, leaf dry weight, ,shoot dry weight, day to 50% flowering and maturity, pods per plant, seeds per plant, 100-seed mass and grain yield. The experimental design was a randomized complete block design (RCBD) with three replications at each location. Analysis of variance and combining ability analysis were performed for genotypes according to Griffing's (1956a) Method 2 Modell, while the GCA: SCA ratio "vascalculated as suggested by Baker (1978). Analysis of variance (ANOVA) showed highly significant differences (P = 0.01) among the genotypes for all the traits studied under both P stress and non-stress conditions at Kabete and Thika. General combining ability analysis of variance indicated that at Kabete and Thika, both the GCA and SCA mean squares were highly significant (P = 0.01) for all the characters studied under both P stress and non-stress conditions. Root traits, especially the basal root length, were well developed in crosses involving the efficient parents. These generally had good yields thus indicating improved tolerance to P stress at Kabete and Thika. CIM 9314-36 and CAL 143 were the best performers in grain yield under both P stress conditions and locations. In general a marked response of yield to P application of 21.1 % at Kabete and 15.3 % at Thika was observed. The general combining ability was more important than the specific combining ability for all the traits studied for both P stress conditions and locations. This in effect implies that the fixable additive genetic effects were more important than the interactions in controlling inheritance of these traits. The combining ability analysis showed highly significant differences for all the traits studied at both N stress and non-stress conditions and locations. Root traits especially the basal root length for both parents and F1 progerues influenced tolerance to N stress as indicated by higher yields for both locations. CIM 9314-36 and CAL 143 performed well again in grain yield under non-stress conditions at Thika. The overall response to N application for yield was 28.7 % at Thika and 17 % at Kabete. The general combining ability was more important than the specific combining ability for all the traits studied for both N stress conditions and locations in controlling the inheritance of tolerance to low soil N.