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 fine localization of mineral matter in spores of Bacillus megaterium and Bacillus cereus was studied by the technique of microincineration adapted for use with the electron microscope. The specimens, which included intact and thin-sectioned spores as well as shed spore coats, were burned either in the conventional way at high temperature or by a new technique using electrically excited oxygen at nearly room temperature. The ash residues were examined by bright field, dark field, and diffraction in the electron microscope and also with the phase contrast microscope. In some cases, the specimen was previewed in both microscopes before incineration. The results do not support a previous report that the mineral elements of the spore are confined to a peripheral layer, but rather indicate that the spore core as well as the coat are mineral-rich. The cortex may be deficient in minerals, but the possibility of artifact prevents a clear decision on this point. Incinerated B. megaterium spores show a highly ordered fine structure displaying 100 A periodicity in the ash of the middle layer of the coat. The nature of this structure is discussed, as is the technique which demonstrated it. The fine definition of the ash patterns, particularly those obtained with the low-temperature, excited-oxygen technique, suggests that microincineration may be generally useful in the study of fine structure.
Isolated rat liver mitochondria were incubated in vitro under conditions supporting the massive accumulation of calcium and phosphate. Samples were embedded, thin sectioned, and examined in the electron microscope. The intramitochondrial distribution of insoluble or structure-bound mineral substances was studied by electron microscopy coupled with recently developed techniques of high resolution microincineration. As shown previously, the ion-loaded mitochondria acquire large, internal granules which have inherent electron opacity indicative of high mineral content. Study of ash patterns in preselected areas of sections directly confirmed the high mineral content of the granules, and the appearance of the residues was consistent with the copresence in the granules of some organic material. Other mitochondrial structures were almost devoid of mineral. Thin sections of unincubated control mitochondria also were incinerated. They were found to contain appreciable amounts of intrinsic mineral, seemingly associated with membranes. The normal, dense matrix granules commonly seen in unaltered mitochondria could be seen in intact sections of these control preparations, but after burning no definite correspondence of any ash to the granules could be demonstrated. The normal granules perhaps do not contain mineral. Heating experiments on ash patterns of all the preparations demonstrated the thermal stability and crystallizability of the ash. The crystallized ash of the in vitro-produced dense granules was tentatively shown by electron diffraction to be 3-tricalcium phosphate (whitlockite). This, together with evidence from the literature, suggests that the original, noncrystalline mineral may be a colloidal, subcrystalline precursor of calcium-deficient hydroxyapatite. Experiments were performed on synthetic calcium phosphates for comparison. Other possible applications of the microincineration techniques are briefly discussed.
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