Influence of Bradyrhizobium japonicum Location and Movement on Nodulation and Nitrogen Fixation in Soybeans.

The influence of seed and soil inoculation on bradyrhizobial migration, nodulation, and N(2) fixation was examined by using two Bradyrhizobium japonicum strains of contrasting effectiveness in N(2) fixation. Seed-inoculated strains formed fewer nodules on soybeans (mostly restricted to the tap and crown roots within 0 to 5 cm from the stem base) than did bradyrhizobia distributed throughout the soil or inoculated at specific depths. Nodulation was greater below the depths at which bradyrhizobial cells were located rather than above, even though watering was done from below to minimize passive bradyrhizobial migration with percolating water. The most profuse nodulation occurred within approximately 5 cm below the point of placement and was generally negligible below 10 cm. These and other results suggest that bradyrhizobial migration from the initial point of placement was very limited. Nevertheless, the more competitive strain, effective strain THA 7, migrated into soil to a greater extent than the ineffective strain THA 1 did. Nitrogen fixation resulting from the dual-strain inoculations differed depending on the method of inoculation. For example, the amount of N(2) fixed when both strains were slurried together onto the seed was about half that obtained from mixing the effective strain into the soil with the ineffective strain on the seed. The results indicate the importance of rhizobial distribution or movement into soil for nodulation, nodule distribution, strain competitiveness, and N(2) fixation in soil-grown legumes.

Gastric inhibitory polypeptide responses to nutrients in Caucasians and American Indians with obesity and noninsulin-dependent diabetes mellitus.

Serum gastric inhibitory polypeptide (GIP), insulin, and glucose responses to either a 75-g oral glucose challenge or a 500-cal liquid test meal were determined in 141 Caucasians and American Indians. The Caucasians were normal weight, averaging 101 +/- 3% (+/-SEM) ideal BW (IBW), or were obese (168 +/- 21% IBW) and had normal glucose tolerance (n = 77), impaired glucose tolerance (IGT; n = 12), or noninsulin-dependent diabetes mellitus (NIDDM; n = 19). The American Indians were all obese (144 +/- 6% IBW) and had either normal glucose tolerance (n = 22) or NIDDM (n = 11). In all study subjects, including obese individuals with and without glucose intolerance, diabetic patients both thin and obese, and lean subjects with impaired glucose tolerance, fasting serum insulin and GIP, and incremental glucose, insulin, and GIP were greater than they were in normal lean subjects, especially during the first hour of the tests. Obese subjects and diabetic patients exceeded lean normal subjects by up to 620% for glucose, up to 640% for insulin, and up to 360% for GIP during the first hour after glucose ingestion or the test meal. Exceptions were two groups with the most severe diabetes in whom incremental insulin values after oral glucose were only 70% (thin Caucasians) and 110% (obese Indians) that of lean normal subjects. The smallest differences in GIP responses occurred between lean normal subjects and obese nondiabetic Caucasians tested with either a meal or oral glucose, whereas American Indians consistently had the greatest insulin and GIP responses to the tests. High fasting GIP and exaggerated GIP increments in response to nutrients could be attributed to neither obesity nor diabetes alone nor to the type of nutrient used to stimulate its release, but, instead, may be genetic or dietary in origin or may be due to other as yet unidentified factors. High basal GIP and exaggerated nutrient-stimulated GIP release were associated with hyperinsulinemia, except in the most severe diabetic patients. These observations suggest that exaggerated GIP release, along with a greater rise in serum glucose in response to nutrients, may play a role in the pathogenesis of the hyperinsulinemia of obesity and early NIDDM.

THE EFFECTS OF SOIL TEMPERATURE ON PLANT GROWTH, NODULATION AND NITROGEN FIXATION IN CASUARINA CUNNINGHAMIANA MIQ.

The effects of soil temperatures between 15 and 30°C on plant growth, nodulation and nitrogen fixation in seedlings of Casuarina cunninghamiana Miq. inoculated with Frankia from two different sources were examined. The optimum soil temperature for the growth of plants dependent on symbiotic nitrogen fixation was 25°C. Decreasing the soil temperature below 25°C markedly decreased plant growth that was reliant on symbiotically fixed nitrogen; effects on the growth of plants supplied with mineral nitrogen were much smaller. At 15°C there was no response in plant growth to inoculation after 148 d, whereas plants supplied with nitrogenous fertilizer were 10 times the weight of uninoculated plants. Nodulation was delayed at 15 and 20°C with nodules formed at 15°C fixing no nitrogen in these studies. The production of fewer nodules at 20°C than at 25°C was partly compensated by the production of larger nodules. Nodule growth at 20 to 30°C was a prime determinant of nitrogen fixed, with the exception of one Frankia at 20°C. The amount of nitrogen-fixed g-1 nodule was the same for the two Frankia sources at 25 and 30°C, differences in effectiveness being due to nodule development. However, differences in the effectiveness of the two Frankia sources at 20°C were related to differences both in nodule development and in nitrogen-fixing ability. The absence of nitrogen fixation at 15°C would be expected to limit the natural distribution of Casuarina species reliant on symbiotically fixed nitrogen to areas where soil temperatures exceed 15°C for a major part of the potential growing season.

The influence of root temperature on (14)C assimilate profiles in wheat roots.

The rate of translocation of (14)C assimilates from leaves to seminal roots in wheat seedlings was considerably reduced by lowering root temperature from 20° to 10° or 5° although the total translocation of (14)C to the roots after 24 h was little affected by temperature. The lowered root temperatures (particularly 5°) resulted in a more uniform distribution of assimilate along the roots than did a temperature of 20°, the ratios of radioactivity/cm in the apical cm, elongating zone, and basal parts of the root after 24 h being 14.0:9.6:1 in 20° roots by contrast with 2.8:1:1 in 5° roots. Temperature effects on assimilate distribution may help explain the observations that for roots grown below 15° ion uptake is sustained in older parts and that roots grown at a low temperature are thicker than roots grown at a higher one.

Translocation and loss of phosphate along roots of wheat seedlings.

Sites of phosphate uptake, translocation and loss along sterile roots of 5-day-old wheat seedlings were obtained using an automatic scanning method. Highest uptake occurred in the apical centimetre of the primary root and in the lateral root zone although appreciable uptake occurred along the whole of the root. 68% of absorbed phosphate was translocated from the apex, 84% from the midroot portion and 87% from the lateral root zone after 24 hr. Profuse production of lateral roots is seen as important in phosphate uptake and translocation.Losses of absorbed phosphate from root to solution were small and the patterns of such losses along the root corresponded with the patterns of uptake by the root. Patterns of net flux of phosphate along the root are likely to be identical with those for influx. Patterns of phosphate loss along the root are in marked contrast to those reported for chloride efflux and loss of organic materials.