Selection for Acetylene Reduction Rates in ‘Mesilla’ Alfalfa1

Abstract

The rate of N fixation by leguminous plants depends on both the effectiveness of the Rhizobium strain and the genotype of the host plant. Most of the previous work has involved attempts to improve the bacterial strain. The objective of this study was to determine the variability in ‘Mesilla’ alfalfa (Medicago sativa L.) for plant characteristics considered to be indices of N fixation and to determine the response of these indices to selection based on acetylene reduction rate. Alfalfa seedlings were grown for a 10‐week period in a N‐free sand culture. Before planting, seeds were inoculated with a commercial peat base alfalfa inoculum. Data were obtained on nodulation score, nodule color score, root score, dry weight of the top growth, % N in the top growth, total N in the top growth, and acetylene reduction rate. Considerable variation existed for all traits measured among 278 plants of the cultivar Mesilla. Plants selected for high rates of acetylene reduction were higher in nodulation score, nodule color score, root score, dry weight of top growth and total N in the top growth than the original cultivar. Fifteen plants with high acetylene reduction rates and 15 with low acetylene reduction rates were selected. These plants were intercrossed within groups by hand pollination without emasculation. Progeny of the high selections showed an 82% increase in acetylene reduction, a 57% increase in dry weight of the top growth, and a 60% increase in total nitrogen in the top growth as compared to the original cultivar. These progeny also had a significant increase in nodulation. Nodule score and root score were highly correlated with each other and with dry weight of the top growth. Dry weight of the top growth was more closely correlated with total nitrogen in the top growth than percent nitrogen in the top growth. Dry weight of the top growth and total nitrogen in the top growth were highly correlated with acetylene reduction rates. A selection procedure for nilnitrate conditions was suggested involving 1) a 10‐week growth period, 2) initial selection on the basis of dry weight of the top growth, and 3) acetylene reduction assays of selected plants.