EFFECTS OF MYCORRHIZAL FUNGI ON GROWTH, NODULATION, AND NITROGEN FIXATION OF ALFALFA (MEDICAGO SATIVA L.) SELECTED FOR HIGH AND LOW NITROGENASE ACTIVITY.

Abstract

Twelve F(,1) families of alfalfa (Medicago sativa L.) plants having different potential for nitrogenase activity, and the two parental populations were tested for response to mycorrhizal inoculation in a low-phosphate soil mixture in the greenhouse. The purpose of this study was to: (a) determine the effects of vesicular-arbuscular mycorrhizae on growth, nutrition and nodulation of these 14 populations, (b) determine if differences existed between the populations with regard to several morphological characteristics, and (c) determine if certain characteristics can be transmitted across generations. The 14 populations were evaluated under four treatments: control no Myorrhizae, no Rhizobium; Rhizobium alone; Mycorrhizae alone; and the combination of Mycorrhizae and Rhizobium. The growth parameters measured differed significantly among the treatments and among the 14 populations studied, and no significant interaction between the populations and the treatments were found. The dual Mycorrhizae and Rhizobium treatment significantly increased plant height at 30 days and 60 days after planting, leaf area per plant, and plant top-dry-weight at two harvest dates. Mycorrhizal inoculation, however, decreased specific-leaf-weight significantly. Nitrogen fixation parameters such as nodule mass score, fibrous root score, and nitrogenase activity were increased significantly by the dual inoculation of mycorrhizae and rhizobium. The 14 populations differed significantly in nodule mass score, fibrous root score, and nitrogenase activity. Mycorrhizal inoculation increased nitrogen fixation more than plant growth. Correlation coefficients indicated that increased Nitrogenase activity is positively correlated with increased nodule mass, increased fibrous root mass, greater top-dry-weight, and leaf area. A step wise multiple regression showed that 49% of the variation in nitrogenase activity can be explained by the variation due to nodule mass, fibrous root mass, top-dry-weight, and leaf area. Several morphological characters showed a heritable response. Plants selected for high nitrogenase activity and high top dry weight transmitted these characteristics to their progenies.