Petal senescing processes monitored by dynamic states of water in orchid plants exposed to exogenous ethylene

Ethylene induces premature senescence of orchid flowers when plants are particularly transported with fruits and vegetables stored nearby. Exogeneous ethylene regulated a complex physiological changes of petals in intact flowering clones of Dendrobium phalaenopsis during seven days; decrease in distance between petals, water content, NMR relaxation times (T₁s and T ₂S) and degree in vital staining, while increase in ion leakage and translucence. Petal wilting and decrease in water content in florets were closely correlated with loss in viability of petal parenchyma tissues adjacent to the vascular bundles in the ethylene-exposed orchid flowers. Highly mobile water measured by T₁ values of the long fraction in the petals was considered to free water derived from intact vacuoles. The loss of membrane integrity and free water of the orchid petals can be modulated by ethylene. After ethylene exposure treatment, the free water gradually decreased at 5 d. Additionally, T₂S of the short fraction in the petals markedly decreased at 5 d after the ethylene exposure and they reached to 10 ms at 7 d. From these results, the decrease in the T₁ and T₂ of the orchid petals due to ethylene exposure indicated decreased vacuolar water, followed by turgor loss and increased bound water, thus decrease in diffusion of substrates and metabolism.