Anatomy of ethylene-induced floral-organ abscission in Chamelaucium uncinatum (Myrtaceae)
Andrew J. Macnish A C , Donald E. Irving A , Daryl C. Joyce A , Vasanthe Vithanage B and Alan H. Wearing AA The Centre for Native Floriculture, School of Agronomy and Horticulture, The University of Queensland, Gatton, Qld 4343, Australia.
B CSIRO Plant Industry, Queensland Bioscience Precinct, 306 Carmody Rd, St Lucia, Qld 4067, Australia.
C Corresponding author. Current address: Department of Environmental Horticulture, PO Box 110670, University of Florida, Gainesville, FL 32611, USA. Email: amacnish@ifas.ulf.edu
Australian Journal of Botany 53(2) 119-131 https://doi.org/10.1071/BT04075
Submitted: 28 May 2004 Accepted: 1 November 2004 Published: 31 March 2005
Abstract
Postharvest abscission of Geraldton waxflower (Chamelaucium uncinatum Schauer) flower buds and flowers is ethylene-mediated. Exposure of floral organs to exogenous ethylene (1 µL L–1) for 6 h at 20°C induced separation at a morphologically and anatomically distinct abscission zone between the pedicel and floral tube. Flower buds with opening petals and flowers with a nectiferous hypanthium were generally more responsive to exogenous ethylene than were flower buds enclosed in shiny bracteoles and aged (senescing) flowers. The anatomy of abscission-zone cells did not change at sequential stages of floral development from immature buds to aged flowers. The zone comprised a layer of small, laterally elongated-to-rounded, closely packed and highly protoplasmic parenchyma cells. Abscission occurred at a two- to four-cell-wide separation layer within the abscission zone. The process involved degradation of the middle lamella between separation layer cells. Following abscission, cells on both the proximal and distal faces of the separation layer became spherical, loosely packed and contained degenerating protoplasm. Central vascular tissues within the surrounding band of separation layer cells became torn and fractured. For flower buds, bracteoles that enclose the immature floral tube also separated at an abscission zone. However, this secondary abscission zone appeared less sensitive to ethylene than the primary (central) floral-tube abscission zone as bracteoles generally only completely abscised when exposed to 10 µL L–1 ethylene for the longer period of 24 h at 20°C. The smooth surfaces of abscised separation-layer cells suggest that hydrolase enzymes degrade the middle lamella between adjacent cell walls.
Acknowledgments
Technical advice and assistance from Victor Robertson, Rick Webb, Wendy Armstrong, Rob Gould, Cameron McConchie and David Beardsell is gratefully acknowledged. We thank Allan Lisle for advice on statistics and Ken Young and Nick Bennett for providing plant material.
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