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.
Phosphorus is an essential element in both plant and animal nutrition. However, in some alkaline soils alfalfa (Medicago sativa L.) does not always respond to P fertilization despite deficient or low P concentrations in the tissues. Objectives of this study were to determine the feasibility of selection for increased P concentration in alfalfa grown in alkaline soils and effects on nontarget forage yield and quality characteristics. Ten diverse unselected alfalfa populations, and first and second cycle selected populations were evaluated for P and Ca concentration, Ca:P ratio, and forage yield. selected populations were also evaluated for forage yield, nontarget minerals, and forage quality characteristics. Unselected populations differed in P concentration, but variability among individual plants was considerably greater than among populations. Selection on the basis of P concentration of individual plants was effective with 7 to 12% increases over the check in Cycle 1 and an additional gain of 6% with a second cycle of selection. Realized heritabilities with individual plant phenotypic selection were 0.17 and 0.36 in the first and second cycles of selection, respectively. Selection for increased P concentration did not affect Ca concentration, but reduced the Ca:P ratio and increased Zn concentration. No detrimental effects of selection for increased P concentration were found for nontarget minerals, components of forage quality, or yield. Selected populations did not differ from the check population in response to P fertilization. Selection among individual alfalfa plants from any genetic source based on P concentration in forage from the first harvest would be the most efficient breeding procedure to increase P concentration.
Five selected alfalfa (Medicago sativa L.) clones were evaluated for self‐compatibility, cross‐compatibility, and pollen growth characteristics under four temperature treatments. Crosses were made with and without emasculation. An increase in temperature resulted in a decrease in self‐ or cross‐compatibility estimates. Field results were similar to those obtained at 32C in the growth chamber.Clonal differences existed, but a highly significant clone ✕ temperature interaction for all characteristics measured showed that these clones would not be expected to perform the same in all environments. Cross‐ and self‐compatibility or male and female cross‐compatibility estimates were generally not correlated. Some plants performed differently as male and female parents. Female cross‐compatibility was correlated between 21 and 27C and between 27C and the field study temperatures. Higher correlations were found for male cross‐compatibility values from the different temperature regimes. The cross‐compatibility estimates from the crosses produced with and without alcohol emasculation were not correlated. Pollen germination was affected by the temperatures at which the plants were grown, whereas pollen tube elongation was affected more by incubation temperatures. Male cross‐compatibility was correlated with pollen germination.
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