Sensu Gerd Research Articles

Defoliation effects on mycorrhizal colonization, nitrogen fixation and photosynthesis in the Glycine‐Glomus‐Rhizobium symbiosis

Soybean [Glycine max (L.) Merr. cv. Wells] plants grown in a greenhouse were inoculated with Rhizobium japonicum strain 61A118 and the vesicular‐arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum (Thaxt. sensu Gerd.) Gerd. & Trappe. Plants were defoliated (26, 48 and 66%) throughout the growth period and evaluated for VAM colonization, N2, fixation and photosynthesis at harvest (six weeks). Photosynthate stress as a result of defoliation affected nodulation and nodule activity most severely. Colonization of the roots by the VAM fungus was little affected in comparison, and the intensity of colonization increased with increasing stress. The CO2‐exchange rate decreased less with defoliation than did leaf mass, and photosynthetic efficiency increased with the severity of defoliation. The increase in photosynthetic efficiency 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.

Ecological interaction of little bluestem and vesicular–arbuscular mycorrhizal fungi

The percentage of vesicular–arbuscular mycorrhizal (VAM) colonization of little bluestem (Schizachyrium scoparium) (Michx.) Nash.) and VAM spore populations in the rhizosphere of little bluestem from four prairie areas in Illinois were studied. At Goose Lake Prairie, a significant positive correlation (r = 0.77, p < 0.01) was found between soil moisture and density of VAM spores, but soil moisture and percent VAM colonization were not significantly correlated at any of its sites. Percentage of VAM colonization tended to be higher at study areas where little bluestem was a dominant species. However, spore numbers and abundance of little bluestem tended to be inversely related. Colonization decreased from May to July at all areas and this trend frequently continued into September. Month of sample collection had no significant effect on spore numbers except at Reavis Hill Prairie. The endophyte isolated at all areas was Glomus fasciculatum (Thaxter sensu Gerd.) Gerd. & Trappe. Other species, such as Gigaspora heterogama (Nicol. & Gerd.) Gerd. & Trappe and Acaulospora laevis Gerd. & Trappe, were more restricted in their distribution.

THE EFFECT OF PHOSPHORUS ON THE FORMATION OF HYPHAE IN SOIL BY THEVESICULAR‐ARBUSCULAR MYCORRHIZAL FUNGUS, GLOMUS FASCICULATUM

SummaryHyphae formed in soil by the vesicular‐arbuscular (VA) mycorrhizal fungus, Glomusfasciculatum (Thaxter sensu Gerd.) Gerd. and Trappe were extracted using the membrane filter techniqe and their length was estimated by the grid intersection method. The effect of phosphorus on the formation of this external mycelium was assessed after sampling procedures had been investigated. Phosphorus supply was varied from amounts severely deficient to those adequate for the growth of subterranean clover. After 6 weeks, the alleviation of severe phosphorus deficiency increased both the length of infected root and the length of external hyphae per centimetre of infected root. Further additions of phosphorus decreased both of these measurements. However, the level of added phosphorus at which the most external hyphae was formed per centimetre of infected root was higher than the level of phosphorus which gave the greatest length of infected root. The increase in phosphorus supply which gave the greatest increase in the length of external hyphae per centimetre of infected root also decreased the formation of vesicles within infected roots. At phosphate levels adequate for growth of mycorrhizal plants, there was little development of either external hyphae in soil or vesicles within the mycorrhizal roots.

Mycorrhizae, Water Relations, Growth, and Nutrient Uptake of Geranium Grown under Moderately High Phosphorus Regimes

Abstract To determine the role of endomycorrhizae on water relations and plant growth of geraniums under nonrate-limiting soil P conditions, hybrid seedlings (Pelargonium X hortorum Bailey cv. Cherry Glow) were planted in moderately high P (40 ppm) media, grown under high (−0.4 MPa) or low (−1 MPa) soil water potential (ψs), and either inoculated with VA mycorrhizal fungi [Glomus mosseae (Nicol. & Gerd.) and G. fasciculatus (Thaxt. sensu Gerd.) Gerd. & Trappe] (VAM) or left as noninoculated controls. Geraniums grown under high moisture had greater shoot growth, more advanced floral development, and higher P uptake than the low moisture plants. Mycorrhizal plants under high moisture conditions had higher P levels than noninoculated plants, while mycorrhizal plants under low moisture regimes had greater shoot growth, more advanced floral formation, and greater N uptake than noninoculated plants. Xylem water potential in leaves (ψL) was lower under −1 MPa than −0.4 MPa ψS moisture regimes and lowest in mycorrhizal geraniums grown under −1 MPa ψS. One hundred minutes after plants had recovered from water stress, the greatest change in ψL was recorded for mycorrhizal geraniums acclimatized to low moisture regimes. Geraniums under low moisture regimes are more mycorrhizal-dependent. Total estimated root length and root water conductivity were lower with mycorrhizal geraniums under high water and P regime. Under water stress, the larger mycorrhizal geraniums have greater total water demands than controls. Consequently, mycorrhizal geraniums stressed more rapidly yet more efficiently recovered from water deficits. Data suggests that VAM plants acclimate more efficiently to water stress because of more frequent or extreme drought.

Light Effects in Mycorrhizal Soybeans

Soybean (Glycine max. L. Merr.) plants were grown in an experiment with a 3 x 3 factorial design using different levels of light (170, 350, and 700 muE.m(-2).s(-1)) and P as factors. Plants were grown in a greenhouse in pot cultures using a soil low in plant-available P under three P regimes: no additional P, P added as KH(2)PO(4), or P uptake enhanced by colonization of the host plant with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum (Thaxt. sensu Gerd.) Gerd. and Trappe. Development of the VAM fungal endophyte and of plants under all three P regimes was depressed by limiting light. However, the growth response of VAM plants to increasing light relative to non-VAM plants in the absence of additional P increased while the response relative to non-VAM plants with additional P decreased slightly. The highly significant interaction between the factors (P < 0.001) of the experiment was due to differences in the magnitude and direction of simple effects of the factors. The implications of these differences in terms of source-sink relationships of the symbionts and the value of different non-VAM controls in interpreting VAM effects are discussed.

Parasitic and mutualistic associations between a mycorrhizal fungus and soybean: The effect of phosphorus on host plant‐endophyte interactions

Soybean [Glycine max (L.) Merr. cv. Kent] plants were colonized by the vesicular‐arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum (Thaxt. sensu Gerd.) Gerd. and Trappe in pot cultures using an inert medium and a nutrient solution. Phosphorus was provided initially as 0, 25,50, 100 or 200 mg hydroxyapatite [HAP, Ca10(PO4)6(OH)2] per pot. Under the low (0 mg HAP) and high (100 and 200 mg HAP) P regimes, VAM plants showed 20, 25 and 38% growth retardation, respectively, relative to non‐colonized controls. At 50 mg HAP, VAM plant growth was significantly enhanced (14%). Dry weight and P content of both VAM and control plants increased with increased P availability throughout the HAP gradient. Intraradical VAM fungal biomass increased linearly with increasing P availability. Extraradical VAM fungal biomass was smaller than the intraradical component of the fungus at the lowest and highest levels of P addition in the growth medium. The ratio of extra‐ to intraradical mycelium, a suggested index of VAM fungal effectiveness, was greatest for the 50 mg HAP treatment, coinciding with growth enhancement of the host plant. This enhanced growth of the host at an intermediate P level was apparently a result of increased P uptake by the endophyte.

Influence of Selected Vesicular-arbuscular Mycorrhizal Fungi on Seedling Growth and Phosphorus Uptake of Sophora secundiflora1

Abstract Seedlings of Sophora secundiflora (Ortega) Lag. were grown in 10.2-cm diameter pots containing sterilized media individually inoculated with 1 of 4 vesicular-arbuscular (VA) mycorrhizal fungi and fertilized with Omocote (18N-6P-12K) at rates of 0, 1.2, or 4.2 kg/m3. After 263 days, seedlings inoculated with Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe and with Gigaspora margarita Becker and Hall exhibited increased fresh and dry weights of shoots and roots and root quality at the 1.2 kg/m3 fertilizer level. Increased growth responses of seedlings inoculated with either G. mosseae or G. margarita at 1.2 kg/m3 Osmocote were comparable to noninoculated controls at 4.2 kg/m3. G. etunicatus Becker &amp; Gerd. and G. fasciculatus (Thaxt. sensu Gerd.) Gerd. &amp; Trappe did not enhance seedling growth at any fertilizer rate tested, nor were they colonized on seedling roots. Microscopic examination of roots confirmed VA mycorrhizal colonization at all 3 fertilizer levels with G. mosseae, and at the 2 lowest levels with G. margarita. Increased P uptake occurred only with G. mosseae-inoculated seedlings at 0 and 1.2 kg/m3 fertilization rates.

Measurement of Mycorrhizal Infection in Soybeans

AbstractInfection 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 &lt; 0.05) overstated the degree of infection. Colorimetric determination of fungal infection in mycorrhizae is recommended when high levels of infection are expected.

Mycorrhizal Influence on the Nutrition of Strawberries1

Abstract Strawberries (Fragaria × ananassa Duch. cv. Hood) were grown at three levels of soil P and inoculated with either Glomus fasciculatus (Thaxter sensu Gerd.) Gerd. &amp; Trappe or Gigaspora calospora (Nicol. &amp; Gerd.) Gerd. &amp; Trappe mycorrhizal fungus. P and N concentration in the aerial part of inoculated plants were higher than in controls; highest concentrations were found in plants inoculated with Glomus fasciculatus. Inoculation with this fungus also resulted in the greatest length of mycorrhizal roots and the highest percentage of total root length infected.