Auxin and Ethylene Control of Growth in Seedlings ofZea maysL. andAvena sativaL

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Abstract
The effects of applied ethylene on the growth of coleoptiles and mesocotyls of etiolated monocot seedlings (oat and maize) have been compared with those on the epicotyl of a dicot seedling (the etiolated pea). Significant inhibition of elongation by ethylene (10 μll−1 for 24 h) was found in intact seedlings of all three species, but lateral expansion growth was observed only in the pea internode and oat mesocotyl tissue. The sensitivity of the growth of seedling parts to ethylene is in the decreasing order pea internode, oat coleoptile and oat mesocotyl, with maize exhibiting the least growth response. Although excised segments of mesocotyl and coleoptile or pea internode all exhibit enhanced elongation growth in IAA solutions (10−6−2 × 10−5 mol l−1), no consistent effects were found in ethylene. Ethylene production in segments was significantly enhanced by application of auxin (IAA, 10−5 mol l−6 or less) in all tissues except those of the eat mesocotyl. Segments of maize show a slow rate of metabolism of applied [2-14C]IAA (30 per cent converted to other metabolites within 9 h) and a high capacity for polar auxin transport. Ethylene (10 μl l−1 for 24 h) has little effect on either of these processes. The oat has a smaller capacity for polar transport than maize and the rate ef metabolism of auxin is as fast as in the pea (90 per cent metabolized in 6 h). Although ethylene pretreatment does not change the rate of auxin metabolism in oat, there is a marked reduction in auxin transport. It is proposed that the insensitivity of maize seedlings to ethylene is related to the supply and persistence of auxin which could protect the seedling against the effects of applied or endogenously produced ethylene. Although the mesocotyl of oat is sensitive to applied ethylene it may be in part protected against ethylene in vivo by the absence of an auxin-enhanced ethylene production system. The results are discussed in relation to a model for the auxin and ethylene control of cell growth in the pea.

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