Ethylene production by the roots of wheat seedlings (Triticum aestivum L-cv. GK Othalom) grown in 1 mM nitrate or in an N-free nutrient solution: a possible role for ethylene in root elongation

Ethylene production by the 1.5 cm long apical and basal zones of the roots of 5-day-old wheat seedlings growing in an N-free nutrient solution decreas ed significantly compared with controls receiving 1 mM nitrate at pH 7.0. A t pH 4.0 the roots generally evolved less ethylene than at pH 7.0 and there was no significant difference between the two N supply levels. The concent ration of 1-aminocyclopropane-1-carboxylic acid (ACC), the precursor of eth ylene, did not exhibit a close correlation with the ethylene production, su ggesting that the activity of ACC oxidase could also be affected. Roots lac king an exogenous N supply accumulated more chloride and hydrogenphosphate ions instead of nitrate. The elongation of the roots at pH 4.0 seemed to be independent of both the ethylene production and the N supply. At pH 7.0 et hylene-inhibiting concentrations of aminoethoxyvinylglycine (AVG) or aminoo xy-acetic acid (AOA), the inhibitors of ACC synthase, were ineffective in i ncreasing the elongation rate of roots supplied with nitrate. As the effect s of ethylene synthesis inhibitors are not entirely specific, they may caus e side effects very frequently; in this case they may inhibit the mobilizat ion of reserves during germination. At neutral pH, Co2+, an inhibitor of AC C oxidase, increased the length of nitrate-treated roots up to that of N-de prived plants. In this experiment the inhibitors of basal ethylene producti on either failed to promote extension (AVG, AOA) or increased the length of the roots (Co2+) under the same conditions. This suggests that the side ef fects of these inhibitors are of great importance when the role of basal et hylene in root elongation is under discussion. On the basis of these result s, the growth-promoting effect of decreased ethylene production in the elon gation zones of N-deprived roots cannot be excluded.