1989. The Glycirte-Glorrtus-Bradyrhizobium symbiosis. IX. Nutritional, morphological and physiological responses of nodulated soybean to geographic isolates of the mycorrhizal fungus Glomus mosseae. -Physiol. Plant. 76: 226-232.The objective of the work was to determine differences in plant response to geographic isolates of a vesicular-arbuscular mycorrhizal (VAM) fungus, and to demonstrate the need for such determinations in the selection of desirable host-endophyte combinations for practical applications. Soybean {Glycine max (L.) Merr.) plants were inoculated with Bradyrhizoblum japonlcum and isolates of the VAM-fungal morphospecies Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe, collected from an arid (AR), semiarid (SA) or mesic (ME) area. Inoculum potentials of the VAMfungal isolates were determined and the inocula equalized, achieving the same level of root colonization (41%, /'>0.05) at harvest (50 days). Plants of the three VAM treatments (AR, SA and ME) were evaluated against nonVAM controls. Significant differences in plant response to colonization were found in dry mass, leaf K, N and P concentrations, and in root/shoot, nodule/root, root length/leaf area and root length/ root mass ratios. The differences were most pronounced and consistent between the AR and all other treatments. Photosynthesis and nodule activity were higher (P<0.05) in all VAM treatments, but only the AR plants had higher (P<0.05) photosynthetic water-use efficiency than the controls. Nodule activity, evaluated by HT evolution and CjH, reduction, differed significantly between treatments. The results are discussed in terms of nutritional and non-nutritional effects of VAM colonization on the development and physiology of the tripartite soybean association in the light of intraspecific variability within the fungal endophyte.

1988. Effects of drought on host and endophyte development in mycorrhizal soybeans in relation to water use and phosphate uptake. -Physiol. Plant. 72: 565-571. Soybean [Glycine max (L.) Merr.] plants were grown in pot cultures and inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus GIomus mosseae (Nicol. & Gerd.) Gerd. and Trappe or provided with P fertilizer (non-VAM plants). After an initial growth period (21 days), plants were exposed to cycles of severe, moderate or no drought stress over a subsequent 28-day period by rewatering at soil water potentials of -1.0, -0.3 or -0.05 MPa. Dry weights of VAM plants were greater at severe stress and smaller at no stress than those of non-VAM plants. Phosphorus fertilization was applied to produce VAM and non-VAM plants of the same size at moderate stress. Root and leaf P concentrations were higher in non-VAM plants at all stress levels. All plants were stressed to permanent wilting prior to harvest. VAM plants had lower soil moisture content at harvest than non-VAM plants. Colonization of roots by G. mosseae did not vary with stress, but the biomass and length of the extraradical mycelium was greater in severely stressed than in non-stressed plants. Growth enhancement of VAM plants relative to P-fertilized non-VAM plants under severe stress was attributed to increased uptake of water as well as to more efficient P uptake. The ability of VAM plants to deplete soil water to a greater extent than non-VAM plants suggests lower permanent wilting potentials for the former.

The texture of green beans after blanching, freezing and cooking was measured with a shear press. Each processing step reduced shear resistance about the same amount. Visible damage to the vegetable tissues was caused by freezing, but not by blanching or cooking. Very rapid freezing by immersion in liquid nitrogen prevented damage and reduced the texture degradation. When the freezing rate was slightly slower than that required for perfect preservation, immature cells of the inner parenchyma were the first to be damaged. Further reduction of the freezing rate caused breakage of other cell walls in this tissue, and separation of walls in the outer part of the pod. When the freezing rate was changed during freezing, the part of the bean that froze rapidly was not damaged, whereas that which froze slowly was damaged. Decreasing the freezing rate during freezing did not damage the portion of the bean already frozen. Sensory appraisal panels were able to distinguish texture differences in beans frozen at various rates when there were visible differences in the amounts of cell wall damage.

SUMMARYSymbiotic associations of soybeans and the vesicular-arbuscular mycorrhizal fungus Glomus fasciculatus were grown to maturity in a sand-perlite rooting medium watered with a nutrient solution containing growth-limiting amounts of soluble phosphorus. Development of fungal mycelia external and internal to the host plant's root system was measured by determining chitin in isolated fungal mycelium, in the rooting medium, and in tbe mycorrhizae. The biomasses of the extra-and intraradical mycelia were calculated from the values of chitin obtained spectrophotometrically. The amount of total fungal biomass relative to that of the host plant varied throughout the lifespan of the association and reached a maximum of 2*3 % 10 weeks after planting. The amount of intraradical mycelium increased throughout the host plant's life span. Extraradical fungal structures attained a maximum weight at the onset of logarithmic growth of the soybean pods and decreased thereafter. Cessation of the rapid growth phase of G. fasciculatus lagged behind that of the host plant's vegetative structures, but appeared to be related to pod development. The fungus to root dry wt ratio was 12-3% at senescence. The ratio of extra-to intraradical mycelium decreased throughout the association's lifespan. Since the extraradical hyphae are the organs responsible for enhanced nutrient uptake this ratio is proposed as an index of the endophyte's usefulness to the host. High values for this usefulness index coincided with significant growth enhancement of the host plant. Source-sink relationships in the host appear to be a determining factor in the growth of the fungal endophyte.

1984. Defoliation effects on mycorrhizal colonization, nitrogen fixation and photosynthesis in the Glycine-Glomus-Rhizobium symbiosis. -Physiol. Plant, 62: 576-580. Soybean [Glycine max (L.) Merr. cv. Wells] plants grown in a greenhouse were inoculated with Rhizobium japonicum strain 61A118 and the vesicular-arbuscular mycorrbizal (VAM) fungus Glomus fasciculatum (Thaxt. sensu Gerd.) Gerd. & Trappe.Plants were defoliated (26,48 and 66%) throughout the growth period and evaluated for VAM eolonization, Nj fixation and photosynthesis at harvest (six weeks), Photosynthate stress as a result of defoliation affected nodulation and nodule activity most severely. Colonization of tbe roots by the VAM fungus was little affected in comparison, and tbe intensity of colonization increased with increasing stress. The COj-exchange rate decreased less witb defoliation than did leaf mass, and photosynthetic efficiency increased with the severity of defoliation. Tbe increase in pbotosynthetic effieiency was significantly correlated with increases in leaf P (r = 0.91) and N (r = 0.97) concentrations. The results suggest that the VAM fungus should not be regarded as a simple P source and C sink in the tripartite legume association, Threeway source/sink relationships (VAM-P, Rhizobium-N, and host leaf-C) are discussed.Additional key words -CO^-exchange rate, hydrogen evolution, nitrogen content, phosphorus content, root nodule, soybean.

Infection of soybean [Glycine max (L.) Merr.] roots by the vesicular‐arbuscular mycorrhizal fungus Glomus fasciculatus (Thaxt. sensu Gerd.) Gerd. and Trappe was assessed throughout the ontogeny of the symbiotic association. Degree of infection was evaluated by a histological method as percent infection and colorimetrically as mg chitin/g mycorrhiza. Correlation of data by the two methods was highly significant (r = 0.99) below 60% infection and not significant (r = 0.62) at higher levels. Assessment of infection by the histological method did not yield significantly different data above 60% infection. Data by the colorimetric method were statistically distinct at all levels of infection. The effects of biological and chemical contaminants on the colorimetric assay were determined. The fungal component of mycorrhizae was compared to purified chitin standards. Use of chitin for standard curves in the absence of degradation products from uninfected root materials significantly (p < 0.05) overstated the degree of infection. Colorimetric determination of fungal infection in mycorrhizae is recommended when high levels of infection are expected.

No abstract

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.