Variation in tolerance of soil acidity among 40 rhizobial strains was assessed in greenhouse trials in which the strains were applied as separate seed inoculants (5 ✕104 cells per seed) to two cultivars of mung bean (Vigna radiata L.) and performance was measured by nodulation, growth, and N‐yield of the host plant. The plants grew in a low N, low Ca acid subsoil, Goldridge fine sandy loam (Typic Hapludult, fine loamy, mixed, mesic), left at its natural pH of 5.0 (saturation paste) or limed with CaCO3 to pH 6.3. Each pH ✕ strain treatment was triplicated in separate pots. Strains demonstrated a large and perhaps continuous variation in acid tolerance. A few were very sensitive: they failed to nodulate at pH 5.0. About half were moderately sensitive: nodulation and growth were significantly impaired at pH 5.0. The remainder were tolerant: like NH4NO3 they supported similar plant growth at both soil pH values. Some strains combined high tolerance with high effectiveness. A strain's acid tolerance could not be predicted from the abundance or effectiveness with which it nodulated at favorable pH, or from its growth rate or acid production in conventional yeast mannitol medium. A few strains were sensitive on one host cultivar and tolerant on the other, implying that acid tolerances of symbiotic legumes cannot be compared validly in trials with only one inoculant.

Greenhouse and field trials were done to test the likelihood that impairment of N fixation contributes to late summer decline in productivity of Medicago sativa L. In greenhouse solution cultures, acetylene reduction and N fixation by nodules were impaired by daily heating of the culture solutions to 32 C, and eliminated by repeated exposures to 36 C or a single exposure to 40 C. However, in a field trial conducted on a Yolo silt loam (Typic Xerorthent, fine silty mixed non‐acid thermic), alfalfa showed no signs of N deficiency and responded little to application of 500 kg NH4NO3/ha although soil temperatures above 32 C were recorded for 5 to 6 hours each day during a 6‐day hot spell following a late summer cut, with peak soil temperatures above 40 C at 2 cm depth. Lack of response in the field trial is explained by observations of temperature profiles and nodule distribution. Only the topmost 5 cm of soil heated above 30 C, and this contained less than 10% of the nodules. Most of the nodules were at depth 10 to 30 cm, and remained at nearly optimal temperatures of 22 C to 27 C. Heat inhibition of nitrogen fixation should not significantly limit growth of alfalfa in the field unless nodule‐formation is constrained to shallow depths.

The salt effect on germination is often thought of as a decrease in the percentage of seeds successfully germinated in a given time. It might be more illuminating, however, to consider the depression in percentage of germination as an extreme of the phenomenon of inhibition of seedling growth (4).Imbibition by a seed initiates a sequence of interdependent physiological events which becomes, in a favorable environment, the mechanism of germination, growth, and development. Valuable RESULTSColeoptiles of control plants elongated logarithmically from approximately 24 to 80 hr (Fig. 1)