Effects of root environment on the kinetics of 1st month growth and nodulation of alfalfa

Measurements were made of plant, root, and nodule dry matter, nodule number, acetylene reduction by nitrogenase, plant N content, and shoot height of Medicago sativa L. cv. Algonquin and expressed chiefly as rate constants of growth (k1′). The effects of the nature and quantity of solid substratum, of the form and concentration of combined N and of symbiotic and non-symbiotic growth, were compared in optimum growth room conditions. Plants grew at the same maximum k1′ in vermiculite with or without gravel and in a soil mix when supplied with 15 mM NO3− in the nutrient solution. Plant growth was retarded with decreasing pot size but maximum nodule growth k1′ occurred in 7-cm pots. Nodulation and nitrogenase activity showed maximum k1′ with least added N but moderate additions produced larger yields of roots and nodules. Plant growth in dry matter and N content, expressed as k1′, yield, or absolute rate (k1′∙yield), was under no circumstance increased by symbiosis in this phase of exponential growth. Nodulation was completely inhibited by 15 mM NO3− and higher concentrations of N were generally inhibitory. A transient, postgerminative treatment with 15 mM NO3− provided a sustained boost to growth. Combined N supplied as NO2− or NH4+ at 15 mM in the nutrient solution without NO3− suported k1′ values comparable to those obtained with less than 1.5 mM NO3−.