ABSTRACT
External pressure physically altering development has been the historical explanation for the response of roots to mechanical impedance. The inhibition of root elongation in response to mechanical impedance was similar to inhibition by exogenous ethylene, and the researchers therefore suggested a causal relationship. Developmental changes induced by growth in beds of beads were correlated with those induced by exogenous ethylene. It is apparent from these studies that ethylene may be the controlling factor in the growth response to mechanical impedance. 1-aminocyclopropane-1-carboxylic acid (ACC) concentrations in impeded roots were well within the levels that are normally converted to ethylene. It appears that the biochemical conversion of ACC to ethylene was inhibited in some fashion by impedance. Impedance inhibited ethylene evolution by decapped roots to the same extent as intact roots. Genetic mutants and inhibitors of plant growth substances have not yet been used in studies of mechanical impedance.
