Endogenous ethylene does not regulate opening of unstressed Iris flowers but strongly inhibits it in water-stressed flowers

The floral buds of Iris flowers (Iris x hollandica) are enclosed by two sheath leaves. Flower opening depends on lifting the flower up to a position whereby the tepals can move laterally. This upward movement is carried out by elongation of the subtending pedicel and ovary. In the pedicels and ovaries of unstressed control flowers, the concentration of ACC (1-aminocyclopropane-1-carboxylic acid) and the rate of ethylene production increased during d 0–1 of flower opening, and then decreased. Exposure to ≥200 nL L−1 ethylene for 24 h at 20 °C inhibited elongation of the pedicel + ovary, and inhibited flower opening. However, pulsing of unstressed flowers with solutions containing inhibitors of ethylene synthesis (AOA, AVG), or an inhibitor of ethylene action (STS), did not affect pedicel + ovary elongation or flower opening. When the flowers were dehydrated for 2 d at 20 °C and 60% RH, they did not open when subsequently placed in water, and showed inhibited elongation in the pedicel + ovary. This dehydration treatment resulted in elevated pedicel + ovary ACC levels and in increased ethylene production. Treatment with STS prevented the increase in ACC levels and ethylene production, overcame the effect of dehydration on elongation of the pedicel + ovary, and resulted in full flower opening. It is concluded that flower opening in unstressed Iris flowers is not regulated by endogenous ethylene. An increase in endogenous ethylene above normal levels during stress, by contrast, strongly inhibited flower opening, due to its inhibitory effect on elongation of the pedicel + ovary.