Members of the genera Protea, Leucospermum R. Br., and Leucadendron R. Br. are predominantly grown in South Africa for export to Europe. Manipulation of flowering time in these cut flowers is important in taking advantage of high-spring to midsummer (September–January) prices as well as to smooth supply and reduce periods of overproduction. Leucospermum (Malan and Jacobs, 1990) and Leucadendron (Hettasch and Jacobs, 2006) flowering times can be controlled by daylength manipulation. However, poor shoot qualities result under artificial long days applied in winter in the case of Leucospermum (Malan and Jacobs, 1994). Manual removal of the Leucospermum primary inflorescence that permits a secondary, and later maturing inflorescence to develop, is therefore practiced to smooth out overproduction during September and October (Jacobs and Minnaar, 1980).
A number of Protea species such as P. magnifica Link, P. grandiceps Trattinnick, P. eximia (Salisb. Ex Knight) Fourcade, and some ecotypes of P. cynaroides (L.) L. do flower in the desired window from spring to midsummer (September–January). However, many of the superior hybrid Protea cultivars flower from March to June. Protea eximia exhibits an ability to initiate flowers throughout the year and although hybrids of P. eximia such as ‘Sylvia’ (P. eximia × P. susannae Phill.) and ‘Cardinal’ (P. eximia × P. susannae) have inherited this characteristic, most inflorescences predominantly develop on the spring flush with harvest dates then spread over January to April (Gerber, 2000). ‘Sylvia’ shoot growth is synchronized by pruning plants in winter (July or August) to effect a flowering peak 14 to 16 months later in the spring and early summer (October–December) (Gerber et al., 2001a).
Although effective in ‘Sylvia’ and also commercially applicable to ‘Cardinal’, this approach has been less successful on hybrid cultivars that initiate flowers almost exclusively on the spring flush such as ‘Pink Ice’ (P. compacta R. Br. × P. sussannae Phill.) and ‘Carnival’ (Nieuwoudt, 2006).
Gerber et al. (2001b) showed that flower initiation in ‘Carnival’ coincided with the early phases of spring flush elongation. In a number of perennial woody plants, spring budbreak is correlated with an increased concentration of cytokinins in the xylem sap just before budbreak (Tromp and Ovaa, 1990; Van Staden and Davey, 1979). Furthermore, exogenous application of cytokinins can promote budbreak during late dormancy (Jones, 1973; O'Hare and Turnbill, 2004). Cytokinins have also been implicated in flower initiation in many herbaceous species (Bernier et al., 1998; Corbesier et al., 2003) and a number of perennial horticultural crops (Davenport, 2000).
The objectives of this study were to determine the concentration of zeatin riboside in the xylem sap of ‘Carnival’ before and after flower initiation in spring and to induce “out-of-season” flowers by treating shoots or terminal buds in autumn with BA.
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