The response of the roots of maize seedlings to physical impedance was studied in glass chambers that enabled oots to be grown either unimpeded or in loosely packed small glass spheres (ballotini) and flushed with humidified iir or other gas mixtures. Impedance decreased root length when measured after approximately 40 h by 30-60% and increased root diameter 10 cm behind the tip by a similar percentage. Fresh weight was reduced by up to 28-40% while ethylene evolution from roots was increased 2- to 2·5-fold. Supplying ethylene (5-13 μl 1-1) in the air flow to unimpeded roots simulated closely the effects of impedance.
The volatile inhibitor of ethylene action, 2,5-norbornadiene (approx. 900μl 1-1), overcame the effects of ethylene (5·4 μl 1-1) completely, but the growth of impeded roots was unchanged by norbornadiene. Similarly, aminoethoxyvinylglyeine ((0·5-0·75 mmol M-3) inhibited endogenous ethylene formation, but did not modify the growth responses of roots to impedance, even though ethylene production was suppressed to rates well below those of unimpeded roots. The concentration of abscisic acid was not affected by impedance.
It is concluded that despite a widespread view to the contrary, ethylene is not the cause of the morphological -esponse of roots to physical impedance. Abscisic acid is also unlikely to be involved.