INHIBITION OF ETHYLENE ACTION PREVENTS ROOT PENETRATION THROUGH COMPRESSED MEDIA IN TOMATO (LYCOPERSICON-ESCULENTUM) SEEDLINGS

Tomato (Lycopersicon esculentum L. cv. Rutgers) seedlings germinated on 2% agar containing the inhibitor of ethylene action, silver thiosulfate (STS), failed to insert their radicles into the agar. The growing seedlings developed a corkscrew morphology because of their inability to develop a gravitropic axis. Treatment with STS increased root length, and decreased hypocotyl elongation. These effects were also observed in seedlings germinated in atmospheres containing 2 000 mul l-1 2,5-norbornadiene (NBD), another inhibitor of ethylene action. Neither of the inhibitors suppressed root penetration by seedlings grown on soft agar (0.5%). The altered pattern of growth appears to be due to restricted water uptake, since water application restored normal growth but not root penetration. Under low impedance, the rate of ethylene production was higher on STS- or NBD-treated roots. Increasing the concentration of agar in the growing medium increased ethylene production in untreated roots, and reduced that of those treated with inhibitors. The inability of roots to penetrate impeding agar surfaces was also observed in seedlings of an ethylene-resistant mutant (er) of Arabidopsis thaliana. These data indicate that ethylene plays a role in the response of roots to mechanical impedance, and that inhibition of their action impaired root insertion through compressed media. Possible mechanisms involved in these processes are discussed.