The mechanism of genetic control for low soil nitrogen (N) tolerance in common beans (Phaseolus vulgaris L.)

The study was conducted to determine the mode of inheritance and the genetic mechanisms associated with bean nitrogen use efficiency under low soil nitrogen. Eight widely grown and well adapted low soil N tolerant lines (CAL143, CIM9314-36, and AFR708) and non-tolerant varieties (E5, E8, GLP-2, CAL96, and SCAM80-CM/15) were crossed in a half diallel mating design. The parents and their F₁ generations were evaluated under low soil N and moderate soil N conditions. Significant genetic differences were observed for basal root length, root dry weight, nodule numbers, leaf area and grain yield for both N conditions. Inheritance of the traits associated with better performance under low soil N was found to be under both additive fixable and non-fixable genetic effects. Genotypes that had well-developed basal root systems performed better than those with less developed basal roots. F₁ hybrids from crosses between low N tolerant and non-tolerant parents performed better under nitrogen stressed conditions compared with those between susceptible parents. Hybrids involving CAL143 as one of the parents showed high levels of tolerance to low soil nitrogen conditions. Among parents, CAL143 showed the best performance for basal root length, root dry weight, nodule numbers, leaf area, and grain yield. This parent had highly significant GCA effects for the traits under both N conditions, except basal root length which was significant at Kabete and non-significant leaf area at Thika. The results show that CAL143 would be a good parent to use in a low soil N bean breeding program.