Effects of Ethylene and 2,4-Dichlorophenoxyacetic Acid on Cellular Expansion in Pisum sativum

Ethylene inhibits growth in the subhook region of intact etiolated pea seedlings (Pisum sativum, var. Alaska) by reducing the capacity of the polar auxin transport system supplying auxin to the tissue. Application of 0.1 mm 2,4-dichlorophenoxyacetic acid reverses the growth inhibition caused by ethylene, and stimulates formation of sufficient gas to induce a swelling response in the absence of applied ethylene. Added ethylene causes a further swelling response but no change in growth rate when 0.1 mm 2,4-dichlorophenoxyacetic acid is present. If ethylene produced in response to 0.1 mm 2,4-dichlorophenoxyacetic acid is removed by hypobaric conditions, tissue swelling is prevented but the growth rate is not altered. Reducing the pressure also does not affect the growth rate of control plants. A higher concentration of 2,4-dichlorophenoxyacetic acid (1 mm) acts in a similar manner except that it also depresses growth through direct herbicidal action, whereas 0.1 mm 2,4-dichlorophenoxyacetic acid stimulates growth. Applied and auxin-induced ethylene prolong the phase of cellular expansion in both etiolated and light-grown seedlings. As long as ethylene is present, growth continues, glucose is incorporated into the cell wall, and the wall weight increases in proportion to tissue fresh weight. When ethylene is removed, glucose incorporation into the cell wall decreases and growth ceases.