The Glycine‐Glomus‐Rhizobium symbiosis

Four consecutive trifoliate leaves of 56-day-old symbiotic or nonsymbiotic soybean plants were evaluated individually for CO2 exchange rates (CER), leaf area and dry weight, and leaf N, P, and starch concentrations. Plants had been inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae and Rhizobium japonicum, with either of the endophytes alone, or with neither at time of planting. Plants lacking one or both endophytes received N and/or P fertilizers to produce plants of equal total leaf dry weight in all four treatments. Photosynthetic P-use efficiency (CER per unit leaf P) was higher in the leaves of VAM plants than in P-fertilized plants regardless of the N source (N2 fixation or combined N). Photosynthetic N-use efficiency was also higher in VAM than in non-VAM plants, but it was affected by the N source, with higher CER in the nodulated plants. The greatest differences in CER, starch accumulation and leaf area were found between the nonsymbiotic plants and those with both endophytes. Statistical evaluations of leaf parameters for treatment or nutrient concentration (N and P) effects between the tripartite and the nonsymbiotic treatments showed significant changes in concentration of P, but not N, with decreasing leaf age. Both endophytes apparently enhance CO2 fixation at N and/or P concentrations lower than those of the nonsymbiotic plants. The effects of the endophytes on CO2 fixation were additive. The microsymbionts of the legume root, the VAM2 fungus and the diazotrophic bacterium Rhizobium, may be regarded as the primary sources of P and N to legumes growing in soils deficient in plant-available forms of those nutrients (4). The sink demand resulting from the growth and metabolic activity of these root endophytes has been associated with a compensatory increase in activity by the C source of the association, the chloroplast (21). Given these premises, endophyte effects on source activity for C and sink demand for N and P by leaves should be measurable in terms of CER and N and P concentrations, and these parameters should be different from those found in nonsymbiotic plants of equivalent development. In particular, since the effects of sinks and sources on one another (especially in young annual plants) are inversely proportional to the distances separating them (28), there should be a shift in endophyte effects on CER in individual leaves with position along the stem. This change, as influenced by the complex three-way source-sink relationships of the tripartite legume association (6), should be different in magnitude from that found in nonsymbiotic plants if the symbiotic N or P sources differ in C-sink activity from the nonsymbiotic ones. Changes in the activity of a plant in providing a product relative to the utilization or availability of a necessary requirement affect the efficiency of its resource utilization. Efficiency has many facets. Generally, nutrient-use efficiency has been defined in terms of dry matter produced per unit of nutrient applied or absorbed (10, 11, 24). A more specialized aspect of nutrient-use efficiency relates to the activity of a specific plant process as a function of that nutrient in the tissue involved. Our use of the term photosynthetic nutrient-use efficiency sensu Sage and Pearcy (23) is defined as CER per unit of leaf N or P, and is particularly useful in comparing the nutrient-use potentials of symbiotic legumes relative to nonsymbiotic controls. The purpose of this study was to investigate the effects of symbiotic and nonsymbiotic N and P supplies on CER in order to determine the response of CER to the presence or absence of the endophytes and to differences in leaf N and P concentrations resulting from the nature of the supply. MATERIALS AND METHODS Biological Materials. Soybean (Glycine max [L.] Merr. cv Hobbit) plants were inoculated with the diazotrophic bacterium Rhizobium japonicum strain USDA 136 (abbreviated 'R') and/or with the VAM fungus Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe ('G'). Plants without R. japonicum received N fertilizer ('N'), and those without G. mosseae received P fertilizer ('P'). A control set of nonsymbiotic plants received only the combined fertilizer ('PN'). Fertilizer regimes were adjusted to produce plants of the same weight when harvested after 56 to 59 d of growth. Details of soil, inocula, and growing conditions