SUMMARY T h e feasibility of t h e acetylene r e d u c t i o n t e c h n i q u e for e v a l u a t i o n ofc o m p a r a t i v e effectiveness in Rhizobium was tested in R. leguminosarum, R. tri[olii, a n d R. meliloti w i t h strains which were closely related b u t differed widely in effectiveness. Several variables in sampling a n d h a n d l i n g of nodules were found to introduce significant error into this sensitive assay. Freezing of nodules destroyed all reducing activity. R e m o v a l of nodules from t h e roots, storage of d e t a c h e d nodutes for several hours before assay, a n d t h e d r y -w e t condition of nodules during assay c o n t r i b u t e d to lowered ethylene production. T h e t i m e p a t t e r n of appearance, increase, a n d decline of acetylene reducing a c t i v i t y paralleled t h a t of leghemoglobin c o n t e n t in effective pea nodules assayed a t different stages of development. In a comparisoll of strains from all 3 species, there was generally good a g r e e m e n t b e t w e e n the rate of acetylene reduction (assayed a t a stage of peak a c t i v i t y for effective nodules only) a n d p l a n t d r y weight, p l a n t nitrogen, or visual ratings of effectiveness. Several exceptional m u t a n t s which were r a t e d as p a r t l y effective on t h e basis of nodule t y p e or leghemoglobin c o n t e n t showed little acetylene r e d u c t i o n or N2 fixation. S u i t a b i l i t y of t h e acetylene assay for s t r a i n comparison a n d t h e question of hi!its of comparison w i t h o t h e r criteria of N2 fixation are considered.

A rapid, simple nutrient solution technique was developed for evaluating Al tolerance of wheat selections (Triticum aestivum Vill., Host). The effective Al concentrations in the screening solutions were maintained within narrow limits by controlling the phosphorus concentration and the pH. Differences in tolerance, notably for root elongation, were evident shortly after germination and were of the same relative order as when plants were grown in acid soils of varying exchangeable Al levels. Approximately 50 varieties were screened for their tolerance to Al, and they fell into three groups: (1) tolerant at 6.4 ppm, (2) tolerant at 2.4 ppm but sensitive at 6.4 ppm, and (3) sensitive at 2.4 ppm. The advantages of the nutrient solution method over the soil method for screening for Al tolerance are discussed. A biological rest was proposed for assessing Al in soils based on the differential response of lines of widely varying tolerance.

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When Rhizobium trifolii was inoculated into 13 seed pellet coating materials, a wide variation in survival of the organisms was found. Five of six adhesives examined supported rhizobia growth. A tenfold improvement in rhizobia survival was obtained on pelleted subterranean clover seeds when the rhizobia were suspended in peat rather than broth. Only one of seven pelleting treatments using a peat suspension of rhizobia in 40% gum arabic had adequate numbers after 8 days.