Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/4/2003
Publication Date: 12/1/2003
Citation: Jenkins, M. 2003. Rhizobial and bradyrhizobial symbionts of mesquite from the Sonoran Desert: Salt tolerance, facultative halophily, and nitrate respiration. Soil Biology and Biochemistry. 35:1675-1682.
Interpretive Summary: Mesquite is a desert tree legume native to South Western North America that in association with a particular group of root bacteria called rhizobia can "fix" or trap nitrogen from the atmosphere into a form useful to the plant. This process allows it to supply its own nitrogen fertilization in soils low in nitrogen fertility. Because of mesquite's nitrogen-fixing ability, its ability to grow in salty soils, and because it can become a source of wood, food, and fodder it is becoming an important component of agriculture in arid environments of Africa, South America, and India. This study investigated a population of rhizobia from a mature mesquite woodland in the Sonoran Desert near the Salton Sea of California. In this woodland the soil is salty, and the mesquite trees form two distinct root systems; one in the surface soil and another in wet soil just above a permanent source of groundwater four meters below the surface. Results of tests demonstrated that the rhizobia associated with the roots of this woodland were more salt tolerant than any previously reported; and some rhizobia from the deeper and wetter root environment had developed the ability to use nitrate instead of oxygen to grow and survive. These survival characteristics of mesquite rhizobia are important for establishment and long-term productivity of mesquite in marginal desert soils, and may someday provide novel types of rhizobia for food crops growing in harsh environments.
Technical Abstract: Rhizobial symbionts from surface and phreatic absorbing root environments of a mature mesquite woodland in the Sonoran Desert of Southern California were tested for their ability to tolerate high salinity, and respire nitrate as mechanisms of free-living survival. Isolates were grown in yeast-extract mannitol (YEM) broth at NaCl concentrations ranging from base-level of 0.002 M to 0.6 M, and their specific growth rate (µ), cell dry weight, and lag times were determined. Isolates were grown in YEM broth under anaerobic conditions with or without 10 mM KNO3. Three categories of salt tolerance were observed: 1. µ of isolates decreased with increased salinity but they remained viable at the highest NaCl concentrations; 2. µ of isolates increased significantly at 0.1 and 0.3 M NaCl; and 3. µ of isolates were significantly greater than base-level at all increased levels of NaCl. None of the isolates were stimulated by KCl. None of the Bradyrhizobium isolates appeared to respire nitrate, but three Rhizobium isolates from the deep (4 to 6 m) phreatic rhizobial population denitrified and displayed anaerobic growth. Long-term interactions between rhizobial and bradyrhizobial populations and the surface and phreatic absorbing root environments of the mature Sonoran Desert mesquite woodland appear to have selected for strains of nitrate respiring Rhizobium, general salt tolerance of both rhizobial and bradyrhizobial symbionts, and strains of weak facultative halophilic Bradyrhizobium.