Breeding And Propagation Breeding Narcissus cultivars

Fernandes (1967) defined two sub-genera within Narcissus, Hermione with a base haploid number of chromosomes of 5 (or 10 or 11), and Narcissus with 7 (or 13), and crosses between the sub-genera result in a range of chromosome numbers. Brandham (1986, 1992; see also Kington, 1998, p.12) and Brandham and Kirton (1987) have made extensive studies of the cytogenetics of narcissus: diploid, trip-loid and tetraploid cultivars are common, with high ploidy levels in some species (e.g., N. bulbocodium is hexaploid). Brandham (1992) tabulated data for 731 cultivars. Most narcissus cultivars, interpreted as the optimum level of horticultural fitness, were tetraploids (2n = 28) (Brandham and West, 1993). Other cytogenetic studies of narcissus include those of Kalihaloo (1987), Kalihaloo and Koul (1989) and Gonzalez-Aguilera et al. (1988).

The bulk of narcissus breeding has been carried out by enthusiasts with the show-bench in mind, for example, in Northern Ireland, the USA, New Zealand and Australia. Commercial bulb growers identify new cultivars that may have the right characteristics for commercial exploitation, such as high rates of bulb and flower production. De Hertogh and Kamp (1986) and De Hertogh (1990) listed the desirable characters for commercial cultivars, such as sturdy stems and leaves, reliable bud opening, long-lasting flowers, fragrance, tolerance or resistance to diseases, and critical weights for floral initiation such that a double-nosed bulb will reliably produce two flowers. In some cases, breeding programs may have more specific aims. In the UK, there have been breeding programs aimed at producing yellow trumpet and large-cup flowers similar to 'Golden Harvest' or 'Carlton' but with relative resistance to base rot derived from 'St. Keverne'. One programme, concentrating on the production of early field-grown flowers, has already resulted in several cultivars being commercialised (Pollock, 1989). In another programme investigating the genetic basis of resistance to base rot (Bowes, 1992), new cultivars are presently under evaluation (Bowes et al., 1996). The latter programme exploits the absolute resistance to base rot found in species such as Narcissus jonquilla (Linfield, 1986a, 1990, 1992a,b), rather than the relative (or field) resistance of commercial cultivars such as 'St. Keverne' (Tompsett, 1986). Breeding for resistance to base rot is hampered by the difficulties in screening seedling bulbs because of the development of adult plant resistance (Linfield and Price, 1986). Breeding for resistance to base rot currently utilizes a lengthy screening method that requires large numbers of clonal two-year-old bulbs (Bowes et al., 1992). An in vitro assay using bulb scales is showing promise as a much faster alternative (J.H. Carder, personal communication). Narcissus pollen can be stored long-term in liquid nitrogen (Bowes, 1990).

Mutation breeding of narcissus was reported by Misra (1990). Two narcissus cultivars flowered early without leaves, after exposure to gamma radiation. Rahi et al. (1998) carried out experiments with N. tazetta 'Paper White' in which bulbs were exposed to gamma radiation and then planted in normal or alkaline soil. The performance of irradiated plants in the alkaline soil indicated possibilities for selecting salt-resistant strains.

D.O. Sage (personal communication) is developing a transformation system for narcissus as a possible route to cultivar development. Transgenic callus of narcissus 'Golden Harvest' has been produced, carrying a selectable marker and reporter gene, and attempts are being made to regenerate plants from it. Work will then concentrate on 'clean' transformation technologies for producing trans-genic narcissus ultimately without selectable marker and reporter genes. The technology should then be able to approach pest and disease control by introducing resistance genes to otherwise acceptable cultivars.

Future uses of narcissus plants may require breeding for characteristics such as alkaloid or essential oil content, which have not apparently so far been attempted. Whatever the goals of narcissus breeding, there is a need to conserve genetic material for future use. Because of the huge numbers of commercial cultivars there is a danger that historical but valuable parent cultivars may be lost, while the loss of wild species (and potentially useful subspecific taxa) through over-collecting or habitat destruction has already begun (see Chapter 3, this volume). Historic cultivars and wild types need to be conserved. Koopowitz (1986) has discussed the wider implications of conserving amaryllids, including the need for a large number of each to represent the variation of the gene pool meaningfully, long generation times, specialised cultural requirements and the widespread occurrence of virus diseases.

As well as improving narcissus cultivars, narcissus genes may be useful in the production of other transgenic plants. Booth (1957, 1963) studied carotenoids in narcissus, finding the coronas to be among the richest sources of carotene. Rice contains neither ^-carotene (provitamin A) nor its precursors, and Burkhardt et al. (1997) transformed rice by microprojectile bombardment with a cDNA coding for phytoene synthesis from narcissus.

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