Contemporary cycads form a primitive plant group, and research on this group may yield increased understanding of many aspects of plant biology (Brenner et al., 2003). The study of cycad taxonomy has received recent attention, and the result is a description of roughly 300 species (Hill et al., 2007). In contrast, the study of cycad horticulture and physiology has been neglected, although more practical research may shed light on reasons why this plant group has persisted throughout hundreds of millions of years (Norstog and Nicholls, 1997). Furthermore, we have suggested more cycad taxa would become prevalent in the international cycad horticulture trade as a result of more practical research (Marler et al., 2007b).
Guam's native cycad taxa, Cycas micronesica K.D. Hill, provides a striking specimen plant in the urban landscape. Louis de Freycinet led a French scientific expedition on Guam for several months in 1819, and his records indicate that this taxa was among the most commonly planted crop plants in the homesteads of that era (Barratt, 2003). Additionally, phenology of the species controlled much of the scheduling in Guam's early 19th century agricultural calendar. Until recently, its contemporary use in the urban forest was secondary to that of the exotic Cycas revoluta Thunb. However, both species have been decimated from the horticultural scene as a result of the 2003 invasion of the cycad-specific scale Aulacaspis yasumatsui Takagi. This arthropod invasion positioned C. micronesica on the IUCN Red List only 3 years after it occurred (Marler et al., 2006a). For this and other threatened cycad taxa, increased horticultural research may improve the role of horticulture in conservation efforts.
Cycads are slow-growing, dioecious, long-lived perennial plants. The Guam taxa is an arborescent species with stems to 6 m in the sites we have studied. Most female plants are monopodial, and one pseudocone is produced at a time. Ovule and ultimate seed number per pseudocone are highly heterogeneous, and up to 100 or more seeds is common. Seed maturation requires 20 to 24 months, but viable seeds can be harvested from plants as early as 12 months with postharvest embryo development. All cycads are characterized by an abundance of secondary compounds (Marler et al., 2005a). Some cycad compounds are toxins, and the study of these toxins may improve our understanding of health issues in relation to the myriad plant secondary compounds to which humans are exposed (Shaw et al., 2007).
We have been studying the physiology of C. micronesica in relation to a group of sterols and their derived glucosides because of the various issues discussed here. We have chosen these compounds because of the demonstrated epidemiological links between cycad seed consumption on Guam and amyotrophic lateral sclerosis–parkinsonism dementia complex (Borenstein et al., 2007; Whiting, 1963). Natural sterols and steryl glucosides extracted from cycad megagametophyte tissue (Fig. 1) are among the list of cycad compounds that have been studied in relation to toxicity, and they have proven to be the most neurotoxic compounds in various mammalian species (Kim et al., 2008; McDowell et al., 2007; Shaw et al., 2007; Tabata et al., 2008; Valentino et al., 2006). We have determined that spatial variation of concentration or content of these compounds in the cycad seed is strongest among plant locations and within seed tissues, moderate among plants within a location, and least among same-aged seeds within a plant (Marler et al., 2005b, 2007a). Concentration of these compounds is greatest in young seeds, declines linearly after log transformation of both axes as seeds increase in age while attached to plants (Marler et al., 2006b), but remains stable in detached seeds during extended storage (Marler et al., 2007b). Bioaccumulation of these compounds in megagametophytes is linear throughout seed ontogeny, and the glucosides accumulate more rapidly than the free sterols (Marler and Shaw, 2009).
Phenotypic plasticity of various traits is a critical aspect of plant physiology (e.g., van Kleunen and Fischer, 2007), yet we are aware of no studies that have determined the magnitude of phenotypic plasticity of any cycad compound in relation to plant or environmental factors. Thus, our recent focus has been to determine the magnitude of biosynthetic and allocational plasticity of the sterols and steryl glucosides in C. micronesica.
The aims of this study were to determine the association of several allometric factors with mature seed sterol and steryl glucoside concentrations and contents. The study of cycad gametophyte development may illuminate our understanding of evolution of the analogous angiosperm endosperm (Brenner et al., 2003), so our results may have broad implications within a range of disciplines. The specific phenotypic question we asked was, does concentration and total content of free and glycosylated sterols in seed megagametophytes of C. micronesica correlate with (1) somatic tissue size, seed load per plant, and the ratio of somatic tissue size to seed load, (2) mature seed size, or (3) embryo size in mature seeds?
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