ACC-independent ethylene formation in brown Norway spruce needles involves organic peroxides rather than hydroperoxides as possible precursors

Abstract

ACC-independent ethylene formation (AIEF) by intact acidic mist-stressed brown needles of Norway spruce is enhanced by nitrogen, or at high temperatures (90 °C) or in the presence of Fe(II/III), Cu(I/II) and l-ascorbate. Aqueous extracts made from brown needles respond in a similar manner. By contrast, treatment of intact needles or aqueous extracts with pure oxygen, or free radical scavengers such as di- tert-butylnitroxide (DTBN), 2,2,6,6,-tetramethyl-1-piperidine-N-oxyl and 2-methylnitrosopropane, or treatment of extracts with EDTA or passing them through cationic exchange resins, all inhibit AIEF. Furthermore, decay of spin label from DTBN in extracts from brown needles can be followed by electron spin resonance spectroscopy. All of which suggest that AIEF is a free radical-mediated decomposition of peroxides catalysed by transition metal ions. Several precursors of AIEF quickly decompose to form ethylene only in the presence of FeSO 4 but most of them cannot be removed by the addition of glutathione peroxidase activity which suggests that organic hydroperoxides (ROOH) are not major precursors. However, under acidic conditions, both crude and TLC-purified extracts are able to oxidize iodide to iodine while precursors of AIEF can be reduced by stannous chloride, sodium sulphite or sodium dithionite. Consequently, the precursors of AIEF in brown Norway spruce needles are most likely organic peroxides (ROOR′).