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.

Light interception as a source of variation for nitrogen fixation in groundnut genotypes

“…838. 83 Tonn and Weaver, 1981). Despite these shortcomings, observations of acetylene reduction activity (ARA) on groundnuts (grown in conditions of limited soil N) can correlate well with final N accumulation (Arunachalam et al, 1984;Dutta et al, 1988), suggesting that this method can provide a quantitative index of N2 fixing ability for use in breeding programs (Coale et al, 1985;Duhigg et al, 1978;Graham and Temple, 1984). Usually the described genotype variations in ARA are based on isolated plants, or on plants in widely spaced rows, i.e., conditions where the fraction of incoming light intercepted by the crop is less than one.…”

Isolation and properties of soybean [ Glycine max (L.) Merr.] mutants that nodulate in the presence of high nitrate concentrations

“…The most striking variation has resulted in decreased nodulation or absence of nodulation (8). Nevertheless, variants within existing germ plasms with increased nodulation have been demonstrated in some species (9)(10)(11)(12)(13)(14)(15). These promising variants with enhanced nodulation have previously been selected in the absence of nitrate (9,15).…”

Effects of nodulation on resistance to alfalfa sickness among ten alfalfa cultivars

“…Similar results had been reported previously 14. Seetin and Barnes a~ , Duhigg et al 4 and Tan ~3 showed that acetylene reduction rate and other characters associated with nitrogen fixation in alfalfa were genetically controlled. Tan and Tan is further reported that a large portion of the genetic variation for acetylene reduction rate was attributed to the interaction between alfalfa cultivars and Rhizobium strains.…”