NITRATE-INDUCED ETHYLENE BIOSYNTHESIS AND THE CONTROL OF NODULATION IN ALFALFA

We previously reported that inhibition of ethylene biosynthesis with a minoethoxyvinylglycine (AVG) eliminated the inhibitory effect of NO3- on nodulation of alfalfa (Medicago sativa L. cv. Aragon) plants grown aeroponically, In this work, the effect of Ag+, as an inhibitor of eth ylene action, has been studied in plants growing aeroponically or in d arkened tubes with vermiculite, and low-nitrate or high-nitrate soluti on. Vermiculite-grown plants developed up to 3 times as many nodules a s did those growing aeroponically. Nodule formation was mirrored by dr y-matter accumulation, High (10 mol m(-3)) NO3- applied from planting inhibited nodulation to an equal extent (c. 50%) in the two growth con ditions, In contrast, Agi treatment increased nodule formation at all NO3- concentrations assayed under the two growth conditions, with the stimulation being higher in plants grown aeroponically, Finally, no ef fect of Ag+ (10 mmol m(-3)) on plant growth was observed in either of the growth conditions, The effectiveness of NO3- as a nodulation inhib itor and enhancer of ethylene biosynthesis in roots of alfalfa was als o studied. Within 24 h after inoculation, 10 mol m(-3) NO3- exerted mo st of its inhibitory effect on nodulation, At the same time, both 1-am inocyclopropane-1-carboxylic acid (ACC) oxidase activity and ethylene evolution rates markedly increased in inoculated and uninoculated alfa lfa roots treated with NO3-. Support for a role of endogenous ethylene in the control of nodule formation in legumes is discussed.