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The influence of temperature and leaf maturity on ethephon-promoted abscission was examined by simulta- neously applying either ethylene (10.5 µl·liter -1 ) or ethephon (0 to 4 ml·liter -1 ) to potted Camellia plants at four constant temperatures (10 to 30C). The abscission rate (time to 50% abscission) and extent of abscission of leaves, and vegetative and floral buds was measured. Increased temperature promoted the rate and extent of ethephon-promoted abscission and increased ethylene-promoted abscission rate of all organs of Camelliu. Lower temperatures reduced the abscission rate after ethephon application more than that following ethylene application. Sensitivity to ethephon was greater for leaves on newly extending shoots, although once shoot elongation and leaf expansion had ceased, leaves became less sensitive. Ethephon sensitivity increased progressively with maturation over the following 2 years. Optimal thinning of floral buds . at low temperatures required high ethephon concentrations, while at high temperatures, low ethephon concentrations were optimal. The influence on abscission of the time of year when ethephon was applied, is suggested to be due to tissue maturity, which affects tissue ethylene sensitivity, and temperature, which affects ethylene release from ethephon and tissue response to ethylene. Chemical name used: (2-chloroethyl) phosphoric acid (ethephon). Camellia plants are currently exported from New Zealand to the northern hemisphere in late summer and fall. At this time, floral buds must be removed manually, since the presence of floral buds is undesirable for subsequent vegetative budbreak and plant growth. An alternative is chemical thinning, where the aim is to promote target organ abscission with minimal abscission of nontarget organs. Ethephon is an ethylene-releasing compound (ERC), which is widely available and relatively simple to apply in a commercial system. Previous work has demonstrated that ethephon can selec- tively remove, or thin, floral buds from Camellia (Woolf et al., 1992). The efficacy of ethephon application in removing target organs is influenced by a wide range of factors including tempera- ture, concentration, organ type, and organ maturity. Temperature is the most important environmental factor. Ap- plying ethephon at low temperatures results in significantly lower organ abscission (Jones and Keen, 1985; Klein et al., 1978). Temperature influences ethylene release from ethephon (Klein et al., 1978). The rate of ethylene evolution from ethephon-treated Prunus leaves, for example, strongly depends on temperature with an activation energy (Ea) value of 125.6 kJ·mol -1 (Olien and Bukovac; 1978). Because high temperature increases the rate of ethylene release from ethephon, the duration of exposure to ethyl-

Leaf Maturity and Temperature Affect Removal of Floral Buds from Camellia Ethephon