Solute leakage, lipid peroxidation, and protein degradation during the senescence of Iris tepals

A colour change and inrolling of the tepal edges are the first symptoms of senescence of Iris flowers (Iris x hollandica Tub., cv. Blue Magic). Tepals showed an increase in leakage of both ions and anthocyanins, prior to the visible senescence symptoms. Increased leakage occurred irrespective of the time at which the tepals were severed and placed in water, indicating that the senescence process is inherent in the tepal cells. Net loss of proteins in the tepal edges started after flower opening, and after two more days, when the first symptoms of senescence were observed, the protein level was only 20% of that at harvest. Cycloheximide delayed senescence and resulted in a lower rate of protein loss. Phenylmethylsulfony fluoride (PMSF), a protease inhibitor, had a similar effect on protein levels but did not affect the time to visible senescence, and also several other protease inhibitors did not affect the time to senescence. During senescence the rate of respiration of the tepals remained unchanged and their rate of ethylene production decreaased. The rate of ethane production, an indicator of lipid peroxidation, was very low and remained unaltered. Antioxidants (lascorbic acid, benzoic acid, butylated hydroxytoluene, diphenylamine, propyl gallate, propyl-p-hydroxybenzoate and sodium benzoate) had no effect on the time to tepal senescence. It is concluded that tepal wilting is due to transfer of solutes from the symplast to the apoplast. Although net protein degradation occurs early during the senescence process, its inhibition is not correlated with a delay in the time to senescence. Furthermore, the results do not support the hypothesis that the increase in solute leakage is due to (free radical-mediated) peroxidation of membrane lipids. The present results are in contrast with the ethylene-regulated petal senescence of carnation, which is accompanied by lipid peroxidation.