Globe amaranth is an important horticultural species native to Central America. It features globe-shaped, clover-like flowerheads, 4 cm long at maturity. The true flowers within the flowerheads are tiny and inconspicuous. As a tropical annual plant, globe amaranth continually blooms throughout summer and early fall. Thus, it has been a favorite landscape plant in annual beds, rock gardens, and garden borders. After being removed from the plant, the papery-textured flowers of globe amaranth can retain their color and shape for a long time. Therefore, globe amaranth has also been popular as a cut flower and a dried flower (Cocozza Talia, 1993; Lourdusamy et al., 2002). Continuous germplasm selection and breeding have led to the production of many cultivars of globe amaranth such as those varying in flower colors that include purple, lavender, magenta, red, orange, and white.
Despite the fact that globe amaranth is an important annual flower plant and its flowers have been the subject of many biological studies, little, if anything, is known about its floral volatile chemistry. Globe amaranth is an outcrossing species. Flowers of globe amaranth are visited by butterflies, bees, and other insects. For plants that are cross-pollinated, floral volatiles often play an important role in their reproductive success by functioning as a cue to attract pollinators (Effmert et al., 2005). In addition, the flowers of globe amaranth have been used as a type of herbal medicine (Niu et al., 2009), implying that they may contain essential oils. Based on the hypothesis that globe amaranth flowers produce and emit volatiles, the first objective of this study was to determine what volatile compounds are emitted from the flowers of globe amaranth.
Another aspect of our interest on floral volatile chemistry of globe amaranth lies in the regulation of floral volatile emission. Emission of floral volatiles of some plants has been shown to exhibit a diurnal pattern (Matile and Altenburger, 1988). For example, emission of methyl benzoate, one of the most abundant scent compounds of bee-pollinated snapdragon (Antirrhinum majus) flowers, occurs in a rhythmic manner with the maximum emission coinciding with the foraging activity of bumblebees (Bombus spp.) (Kolosova et al., 2001). Thus, the second objective of this study was to determine whether the emission of floral volatiles of globe amaranth display a diurnal pattern; and, if so, whether the diurnal pattern is regulated by light and/or the circadian clock.
Ethylene is an important plant hormone involved in many aspects of plant biology, including flower development and senescence. It has been shown to be involved in the regulation of floral volatile emissions of certain plants (Underwood et al., 2005). Because the flowers of globe amaranth senesce at a very slow rate, we are interested in determining whether ethylene is involved in regulating floral volatile emission of this species. In addition to attracting pollinators, some floral volatiles have a defense role to deter herbivorous insects and other unwanted visitors (Raguso, 2004). For example, nicotin emitted from the flowers of tobacco (Nicotiana attenuate) has a defense role (Euler and Baldwin, 1996). The jasmonic acid (JA) pathway has been shown to play a critical role in production and emission of defense plant volatiles from vegetative tissues (Howe, 2004). In addition, salicylic acid (SA) is also involved in the production and release of some defense volatile compounds (Shulaev et al., 1997; Zhao et al., 2010). However, little is known on whether the JA and SA signaling pathways play a role in regulating floral volatiles. The third objective of this study was to determine whether silver thiosulphate (STS), an ethylene inhibitor, JA, and SA can affect the emission of volatiles from the flowers of globe amaranth.
In this study, a dynamic headspace technique was used to collect volatiles released from the flowers of globe amaranth and the volatiles were analyzed using gas chromatography–mass spectrometry (GC-MS). First, four cultivars of globe amaranth, Las Vegas White, Las Vegas Pink, Las Vegas Purple, and Fireworks, that have flower colors of white, pink, purple, and pink, respectively, were analyzed and compared for their floral volatile profiles. Then ‘Fireworks’ was chosen for a detailed study of regulation of floral volatile emission, including the impact of light, STS, JA, and SA. Lastly, biochemical pathways leading to the production of predominant floral volatiles from globe amaranth are proposed and the implications to globe amaranth breeding are discussed.
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