Influence of carbon supply on developmental kinetics of symbiotic and nonsymbiotic alfalfa cv. Algonquin through first flowering

Biphasic first-order growth kinetics of NO3-supported or symbiotic seedlings of Medicago sativa L. cv. Algonquin were followed over a range of light intensities and at two concentrations of CO2. The initial, [Formula: see text]-supported growth coefficients ([Formula: see text] or relative growth rate) decreased with decreasing light intensity, but those for symbiotic growth showed relief from high light inhibition by passing through a maximum [Formula: see text] at an intermediate light intensity. In low light intensity (60 μE∙m−2∙s−1) the low initial growth coefficient persisted to 40 days in Hoagland's solution, or for 58 days symbiotically at which time the corresponding biomass was reached. At high light intensity (550 μE∙m−2∙s−1) the initial values of [Formula: see text] were insensitive to the enrichment of CO2 (1325 μL∙L−1), but after 27 days values of [Formula: see text] were enhanced by the raised CO2 concentration. The initial growth phase, which is N limited at a high C supply, was followed by a phase of growth that was C limited at a high N supply. The symbiotic N supply, unlike the combined N supply, was dependent only on the C supply because when the CO2 concentration was raised the acceleration of symbiotic seedling growth equalled the maximum on [Formula: see text] nutrition. The results support a hypothesis that the change in kinetic phase is controlled by developmental morphogenesis independent of N source and C supply and that a plant pool of organic N metabolites plays a role in the regulation of the N metabolism that is involved in the growth effects.