Accumulation of α-Tocopherol in Senescing Organs as Related to Chlorophyll Degradation

Abstract

a-Tocopherol (a-T) has been identified, using gas chromatography-mass spectroscopy and 'H-and 13C-nuclear magnetic resonance, in senescing leaves of Melia azedarach L. The content of a-T increased concomitantly with the breakdown of chlorophyll in senescing Vinca and Melia leaves. An increase in a-T was found also in detached Melia leaves, senescing in either light or darkness and in senescing, ethylene-treated orange leaves and fruit. The possibility that phytol, which is released from chlorophyll by chlorophyllase is utilized for the biosynthesis of a-T is discussed. Senescing leaves of the low chlorophyllase plants, parsley and tobacco, did not contain a-T in measureable amounts.in Petri dishes containing moist filter paper covered with a Saran net, on which the leaves were laid to avoid direct contact with water. Dark green Melia leaflets, picked by late summer (September), were also allowed to senesce by the same method.Orange (Citrus sinensis L. Osbeck cv Shamouti) leaves and mature green fruit were harvested from the orchard and allowed to senesce in glass cylinders under a humid stream of air containing 20dl V-' of ethylene/L at 25C in the dark. ExtractionChl degradation occurs widely in nature as part of the senescence and ripening of green plant organs. The exact fate of the Chl molecule is not known, however (see Refs. 10 and 20 for recent reviews). Removal of the phytol by chlorophyllase has been suggested to be one of the earliest degradative steps, at least in certain plant species (2,14).Notwithstanding the progress made in recent years (13,16) further identification ofbreakdown intermediates seems to be the most important task for the elucidation of the pathway of Chl catabolism.In search for compounds which accumulate during the course of the disappearance of Chl we were intrigued by a peak in our HPLC system which eluted between Chl a and b and which increased markedly during senescence.In the present paper we report the identification of this peak as a-T' and the accumulation of this compound during senescence in several plant species. MATERIALS AND METHODS Plant MaterialLeaves ofMelia azedarach L., China tree, and Vinca major representing different degrees of senescence were collected from plants growing on campus during autumn. Leaves at different degrees of senescence were picked also from tobacco (Nicotiana tabacum L.) growing in pots in the greenhouse.Parsley (Petroselinum sativum L.) and celery (Apium graveolens L.) leaves were purchased fresh in the market. Leaves were allowed to senesce at 25°C either in light or in darkness, ' Abbreviation: a-T, a-tocopherol.Plant materials were homogenized with a Polytron homogenizer in -1 5°C acetone (0.2 g leaf tissue/5 mL; 0.5 g Citrus fruit peel/5 mL) and left in the cold for 10 min. The homogenate was centrifuged to remove the debris and the supernatant filtered through a 0.45 ,um pore size Millipore filter prior to analysis by HPLC. HPLC AnalysisAn LKB HPLC system equipped with a UV detector (Micromeritics 788) and HP3390 integrator w...

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