Inheritance of Ineffective Nodulation and Non‐nodulation Traits in Alfalfa1

Abstract

Although the roots of alfalfa (Medicago sativa L.) usually form pink nodules capable of fixing atmospheric N2 when inoculated with Rhizobium meliloti Dang., one clone incapable of forming nodules (non‐nod) and five clones which produced ineffective nodules were recently discovered. These unusual nodulation traits are useful for research on root infection, nodule formation, and N2 fixation. The objectives of this study were to determine the inheritance of the non‐nodulating trait and of the ineffective nodulation in each of the five clones, to compare the genetic controls for ineffective nodulation in the five clones, and to determine the genetic relationship between ineffective nodules and non‐nodulation.The non‐nod clone and the five ineffective clones were self pollinated, crossed in a diallel mating design, and crossed to two normal effective nodulating clones. F2 and backcross seed was produced on non‐nod ✕ normal and ineffective ✕ normal crosses. F2 seed was also produced on non‐nod ✕ ineffective crosses and ineffective ✕ ineffective crosses. Progenies were evaluated under nil‐nitrate greenhouse sand culture. Six weeks after planting, non‐nod plants were small, chlorotic, and had no nodules. Plants with ineffective nodules were small, chlorotic, and had white nodules. Plants with effective nodules were vigorous, dark green, and had pink nodules. Four of the five clones had a different basis for inheritance of ineffectiveness. Three were conditioned by a single tetrasomically inherited recessive gene (in1, in2, or in3), the nulliplex genotype in each instance producing ineffective nodules. The fourth was conditioned by two recessive genes (in4 and in5), the nulliplex condition at both loci being required for production of ineffective nodules. The non‐nod trait was conditioned by two tetrasomically inherited recessive genes (nn1, and nn2). The nulliplex condition at both loci resulted in non‐nodulation. The non‐nod trait and the four different types of ineffective nodules appeared to segregate independently of one another.