Geraldton waxflower (Chamelaucium uncinatum Schauer) is a native Australian plant traded internationally for its cut flowers. Marketability of flowering stems is, however, often limited by ethylene-induced floral bud and flower abscission during export handling and transport. The general objective of this study was to characterise the anatomy and physiology of ethylene-induced floral organ abscission in C. uncinatum. A sustained 1-methylcyclopropene (1-MCP) release treatment with potential to prevent abscission during long distance export was also developed and evaluated.
The morphology and anatomy of floral organ abscission was studied using light and electron microscopy. Abscission occurs at a distinct zone located at the junction of the pedicel and floral tube. In longitudinal and transverse tissue sections, the abscission zone is visible as a five to six row-wide band of small, laterally elongated-to-rounded, closely packed highly protoplasmic parenchyma cells. Abscission involves extensive degradation of the middle lamella from the walls of a two to four cell-wide separation layer at the distal end of the abscission zone. Following separation, the cells of the separation layer are generally intact, spherical, loosely packed and contain degenerating protoplasm. Vascular strand tissues that pass through the abscission zone fracture unevenly. Bracteoles that enclose the floral tube abscission zone of young floral buds also abscise from pedicels at two separate abscission zones on opposite sides of pedicels.
Intracellular inclusions were observed in the pedicel and floral tube tissues of C. uncinatum. These inclusions were hard and damaged glass knives during tissue sectioning. They were shown, using polarised light and scanning electron microscopy, to be birefringent 8.9 to 29.5 μm druse (i.e. aggregate) crystals. Energy-dispersive x-ray spectroscopy showed that these crystals were predominantly composed of calcium. Histochemical and acid solubility tests and Raman microprobe spectroscopy indicated that the crystals were calcium oxalate. The calcium oxalate crystals were located in xylem vessel lumens and also in parenchyma cells adjacent to vascular tissues.
A screening study evaluated the relative sensitivity of 51 cultivated C. uncinatum and related Chamelaucium cut flower genotypes to ethylene-induced flower abscission. Sensitivity varied among genotypes both between and within different species and crosses and also between harvests. C. uncinatum x C. micranthum cv. 'Sweet Georgia' and C. uncinatum cw. 'Early Nir', 'Paddy's Late', 'Purple Pride', 'CWA Pink' and 'Early Hard' shed 10 % of their flowers in response to a 12 h treatment with < 0.01 μL ethylene/L at 20 °C. In contrast, C. megalopetalum cvv. 'Winter White' and 'Iceberg' flowers were insensitive to ethylene even at 100 μL/L for 12 h at 20 °C. Sensitivity of C. uncinatum cv. 'Purple Pride' flowers to treatment with μ/L ethylene/L decreased towards the end of vase life in association with slight dehydration.
Maintenance of C. uncinatum flowers at low temperature was evaluated for its potential to minimise ethylene-mediated flower abscission. At 20 °C, flower abscission from C. uncinatum cv. 'Purple Pride' occurred upon 12 h exposure to 1 μL ethylene/L. This ethylene treatment did not cause flower abscission at either 10 or 2 °C. Moreover, flowers held at 2 °C were insensitive to 48 h exposure to 1, 10 and 100 μL ethylene/L. However, increasing the duration of treatment with 1 μL ethylene/L at 10 and 2 °C to 48 and 144 h, respectively, induced flower abscission.
A sustained release device for the gaseous anti-ethylene agent, 1-methylcyclopropene (1-MCP), was developed to protect floral organs during long distance transport where low temperature cannot always be maintained. Plastic and rubber membrane-limited devices containing either 1-MCP solution or dissolved EthylBloc® powder (a.i. 0.14 % 1-MCP) were tested for their capacity to maintain sustained release. Polyvinylchloride (PVC) tubes containing a reservoir of 1-MCP solution or dissolved SmartFresh™ powder (a.i. 3.3 % 1-MCP) sustained the release of 1-MCP at > 0.03 μL/L into fibreboard flower cartons for up to 120 to 132 h (i.e. 5 to 5.5 days).
Efficacy of the PVC tube-based sustained 1-MCP release devices to protect C. uncinatum flower bunches during export by air from Australia to the UK was examined. These devices protected flowers against abscission induced by subsequent exposures to ethylene (10 μL/L, 12 h, 20 °C) for 3 to 5 days after arrival. In contrast, pre-shipment treatments with either a single application of 790 nL 1-MCP/L for 14 h at 2 °C or a 0.2 mM Ag+ (as silver thiosulphate; STS) pulse for 14 h at 2 °C protected flowers against exogenous ethylene for only 1 to 2 days of post-export life. However, the pre-shipment 1-MCP fumigation was up to ca. 3-fold more effective than the sustained 1-MCP release in reducing floral organ abscission during export. Thus, a combination of pre-shipment 1-MCP fumigation with sustained 1-MCP release during shipment should maximise efficacy against postharvest floral organ abscission in C. uncinatum.