Environmental Regulation of Embryo Growth, Dormancy Breaking and Germination in Narcissus Alcaracensis (Amaryllidaceae), a Threatened Endemic Iberian Daffodil

Article excerpt

ABSTRACT.-Developing a conservation plan for threatened plant species requires a solid understanding of seed germination ecology. The aims of this study were: (1) to discover the requirements for embryo growth, dormancy breaking, and germination; and (2) to determine the effect of seed age on embryo growth and germination of Narcissus alcaracensis (Amaryllidaceae), an endangered Mediterranean daffodil endemic to southern Iberian Peninsula. The phenologies of germination and embryo growth were studied in natural conditions. Temperature requirements for embryo growth and germination, and the effect of gibberellic acid (GA^sub 3^) on germination were determined by incubating seeds under controlled laboratory conditions. In natural conditions, embryo growth starts in late autumn. Seedlings emerge in late winter shortly after the embryos reach a critical length of 3.3 mm, quite longer than the initial embryo length at dispersal (1.42 mm). In the laboratory, embryo growth occurs during a prolonged incubation period at 5 C. Stratification at 5 C breaks the physiological and morphological dormancy, enabling seeds to germinate at a wide range of temperatures. Cold stratification at alternating day/night temperatures of 9/5 C or constant day/night temperatures of 10 C also promoted germination. The velocity of embryo growth and germination percentages increased with seed age. Seeds of N. alcaracensis show intermediate complex morphophysiological dormancy (MPD). This is the first time that this level of MPD was detected in Narcissus, and the second one reporting an increase of embryo-growth velocity with seed age in morphophysiologically dormant seeds.

INTRODUCTION

Narcissus (Amaryllidaceae) is a genus of perennial geophytes, known as daffodils. In die wild diey occur principally on the Iberian Peninsula and in Morocco but are also found around the Mediterranean Basin and along the Adantic coasts of Europe. Underdeveloped, linear embryos, which must elongate to a critical length before radicle emergence are a common feature of Amaryllidaceae species (Baskin and Baskin, 1998). However, information on the types of dormancy and die conditions necessary for embryo growdi and germination is particularly scarce in this family (Vandelook and Van Assche, 2008). Many Narcissus taxa showing underdeveloped embryos are endemic to specific locations in the Mediterranean Basin, and very few studies have been carried out on their germination ecology.

Descriptions of the genus Narcissus have presented taxonomic difficulties caused by weak reproductive barriers, high hybridization ability, pronounced morphological variability, and tile effects of speciation arising from polyploidy (Zonneveld, 2008). Webb (1980) listed 26 Narcissus species in die Flora Europaea. Blanchard (1990) recognized 65 species, and more recendy Zonneveld (2008) differentiated 36 species, some of them with several subspecies, based on variations in nuclear DNA content.

Narcissus alcaracensis belongs to die section Pseudonarcissi DC. That section is morphologically diverse and unstable, presumably as a result of recent and continuing speciatíon (Fernández-Casas, 1983; Dorda and Fernández-Casas, 1994). The centre of origin of the majority of the section lies in the southern mountain ranges of the Iberian Peninsula (Sierra Nevada, Subbeticas, and Cazorla) where over ten new taxa have been described during the four last decades. However, many of these species are hardly morphologically distinguishable because speciatíon is still occurring (Rios-Ruiz et al, 1999). Although all species readily propagate from bulbs (Bañares et al, 2003), seed germination ecology is poorly known. More information on seed germination is essential for ex situ plant production, both for ornamental aims to supplement wild populations (Cerabolini et al., 2004), given that propagation from seeds is critical for maintaining genetic variability. Understanding seed germination will also provide insight into Narcissus life history ecology and habitat requirements to improve management of natural populations (Ikeda and Itoh, 2001; Navarro and Guitian, 2003; Giménez-Benavides et al. …