Effects of light intensity and CO2 concentration on the kinetics of 1st month growth and nitrogen fixation of alfalfa

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Medicago sativa L. cv. Algonquin seedlings were grown for 28 days in growth rooms at several intensities of light and concentrations of CO2. Optimal or deficient NO3− concentrations were provided, the latter with or without inoculation and nodulation by Rhizobium meliloti str. 102F70 (Burton). All growth coefficients (k1′) were hyperbolically dependent on the intensity of light. Light saturation of plant k1′ was achieved, but the k1′ for nitrogenase development the highest in value, was not light saturated at high CO2 by the highest light intensity (555 μE∙m−2∙s−1). That intensity also did not saturate the photosynthesis of plants grown at that intensity nor the amount (yield) and absolute rate of plant growth. The latter were very much reduced at intensities below the compensation point (100 μE∙m−2∙s−1) of net photosynthesis. The data for k1′ at low light intensity indicated that photosynthate was utilized with equal efficiency for N2 and NO3− reduction. Fourfold enrichment of CO2 concentration did not influence the k1′ of plant growth in optimum NO3− and high light intensity but increased the yield by 78%. In the absence of high NO3− concentration, however, it nearly doubled the nitrogenase growth k1′, to a doubling time of 1.4 days, increased the nodule yield fourfold, the plant (symbiotic) yield threefold, and N content twofold. Sevenfold enrichment of CO2 was inhibitory to yields of N-deficient plants and nodules. The previous conclusion that added (combined) N chiefly limited seedling growth was supported by the lack of effect on plant k1′ of the stimulation of photosynthesis by high light intensity and CO2 concentration. A limitation on the value of the k1′ for shoot elongation in deficient combined N raised CO2 and high light was relieved symbiotically.