Allometry of acetylene reduction and nodule growth of Robinia pseudoacacia families subjected to varied root zone nitrate concentrations

The effects of nitrate (NO3(-)) on acetylene reduction and growth were examined in nodulated seedlings from three open-pollinated families of black locust (Robinia pseudoacacia L.) grown in sand culture. In the first study, nine-week-old seedlings were supplied with 0.0, 1.0, 5.0 or 15.0 mM NO3(-) for two weeks during which acetylene reduction and biomass were measured five times. In the second study, eight-week-old seedlings were supplied with 0.0, 1.0 or 5.0 mM NO3(-) for 51 days during which acetylene reduction and biomass were measured six times. Results were analyzed with and without adjustments for seedling size. In the first study, 15.0 mM NO3(-) significantly decreased total acetylene reduction but lower concentrations did not. In seedlings given 15.0 mM NO3(-), both nitrogenase activity and nodule biomass were reduced. Inhibition of nitrogenase activity by NO3(-) was reversible. In the second study, both and 1.0 an 5.0 mM NO3(-) treatments increased plant growth compared to the control (0.0 mM). At the end of the 51-day treatment period, total acetylene reduction and nodule biomass were greatest in the 1.0 mM NO3(-) treatment and least in the 5.0 mM NO3(-) treatment. However, when adjusted for seedling size, total acetylene reduction and nodule biomass were similar in the 0.0 and 1.0 mM NO3(-) treatments. The greater total acetylene reduction and nodule biomass of seedlings grown with 1.0 mM NO3(-) resulted from increased seedling size due to fertilization. After adjustment for plant size, total acetylene reduction, nodule biomass and nitrogenase activity were significantly lower in the 5.0 mM NO3(-) treatment compared with the control of 1.0 mM NO3(-) treatment. Adjustment for seedling size, by means of allometric principles, appears necessary to interpret treatment effects on total acetylene reduction and its components, nodule biomass and nitrogenase activity correctly.