1 INTRODUCTION
Jasmines are one of the oldest propagated flowering species cultivated mainly because of their unique sweet fragrance note. These horticulturally important flowers have many commercial applications in perfumery and natural cosmetic product formulation. Jasmines are utilized for producing floral wax, flavoured tea and essential oils (Ito et al., 2002; Edris et al., 2008). Around 200 species of Jasminumare known to be distributed throughout the tropical and sub-tropical regions of the globe. Many of the intensely scented species such as,Jasminum sambac , J. grandiflorum , J. auriculatumand J. ajonicum are cultivated extensively in many Eurasian countries primarily for producing attars and other perfumery products (Gupta and Chandra, 1957). Another jasmine species namely, J. auriculatum is also propagated at commercial scale since ages for its fragrant white flowers with maximum floral flush in the summer. This species is native to India producing scented flowers with entirely different olfactory scent note utilized widely in manufacturing concretes and absolutes (Joulain and Laurent., 1995) and also exploited in producing jasmine tea (Ito et al., 2002).
Floral fragrance comprises of an amalgamation of many micro chemicals which are highly volatile in nature. These specialized metabolites are mostly non-polar, have low boiling point and have high vapor pressure. These powerful molecules are produced by the plants mainly to deter herbivores, acts as repellents and helps in pollination. Volatile molecules may not always be released from the floral tissue immediately after biosynthesis. These compounds are often retained in floral tissue in soluble phase and are emitted as scent upon vaporization (Oyama-Okubo et al., 2005). Several floral volatile compounds are stored in vacuoles as water soluble glycosides (Watanabe et al., 1993; Barman and Mitra, 2019), upon physiological necessity, these compounds are cleaved by hydrolytic enzymes and subsequently released as fragrant molecules from floral tissue (Reuveni et al., 1999). In the past decades, a range of scent compounds were identified in many scented flowers such as,Rosa, (Helsper et al., 1998; Hendel-Rahmanim et al., 2007),Petunia (Schuurink et al., 2006), Antirrhinum (Kolosoba et al., 2001), Matricaria (Irmisch et al., 2012), Clarkia(Raguso and Pichersky, 1995), Murraya (Paul et al., 2019). Further, essential oils extracted from various scented flowers were reported to possess many pharmacological values (Kuroda et al., 2005; Wei and Shibamoto., 2007).
Floral scent compounds are broadly categorized into three major classes depending upon their biosynthetic routes viz. fatty acid derivatives, phenylpropanoids/benzenoids and terpenoids compounds (Pickersky et al., 2006). Volatiles metabolism at molecular levels in many scented flowers was studied throughout floral lifespan (Pichersky et al., 1994; Hendel-Rahmanim et al., 2007; Muhlemann et al., 2014). These include expression analysis of linalool synthesis in Clarkia breweri(Pichersky et al., 1994), J. grandiflorum (Pragadheesh et al., 2017), acetyl-CoA-benzylalcohol acetyltransferase (BEAT) in C. breweri (Dudareva et al., 1998), alcohol acetyl tranferases (AAT) inRosa hybrida (Shalit et al., 2003). In addition, transcription factors belonging to MYB R2R3 superfamily were also studied in connection with the upregulation of genes for benzenoid volatiles formation (Schuurink et al., 2006; Liu et al., 2015).
The fragrance emission from flowers is regulated by both metabolic and vaporization processes (Sagae et al., 2008). The biosynthesis and emission of floral fragrance are modulated not only by the floral developmental stages of maturity and circadian rhythm but also by other environmental factors such as light and temperature (Dudareva et al., 2013; Cheng et al., 2016). Among the environmental factors, air temperature was known to play a major role in the biosynthesis and emission of floral fragrance (Cheng et al., 2016; Hu et al., 2013). Earlier work on scented flowers (such as Osmanthus fragrans andPetunia hybrida ) reported variations in the composition of scent volatiles with gradual increase in air temperature (Fu et al., 2017; Sagae et al., 2008). In Lilium sp. the amount of release of floral volatiles was found to be highest at 30°C while least emission recorded at 10°C in the lower range and 40 °C at the upper range (Hu et al., 2013). In Trifolium repens , higher content of emitted floral volatiles was reported when plants are grown at 20°C (Jakobsen and Olsen, 1994). Phenylalanine ammonia-lyase (PAL), the entry point enzyme of phenylpropanoid/benzenoid pathway is known to influence the synthesis of phenylpropanoid/benzenoid floral volatiles. PAL activity can be negatively affected by increase in the atmospheric temperature (Shaked-Sachray et al., 2002; Cna’ani et al., 2015). The transcript accumulation of certain upstream and downstream structural genes involved in the phenylpropanoid/benzenoid scent production inPetunia flowers are also affected by variations in air temperature conditions (Cna’ani et al., 2015). Terpene emission was also known to be primarily influenced by temperature and humidity. The monoterpene emission is also dependent on the surface area of oil which primarily indicates the presence of volatiles as liquid endogenous form within the plant tissue (Dement et al., 1975).
Previous studies on flowers of Jasminum species mainly focused on the scent volatiles composition (Bera et al., 2015; Barman and Mitra, 2019). Earlier research from the authors’ laboratory has provided some relevant information related to the enzymes involved in floral scent production in J. sambac (Bera et al., 2017), one of the most commercially utilized Jasminum species. Subsequently we have also proposed a hypothesis that most of the floral scent compounds inJasminum spp. are biosynthesis and stored in the form of either glycosylated or free endogenous volatile compounds, and these volatiles are emitted following a certain rhythmic pattern (Barman and Mitra, 2019). No information, however, is available on the biosynthetic pathway involving the role of structural genes and enzymes responsible for scent production in other scented Jasminum species. While J. sambac is grown in many tropical and subtropical countries, J. auriculatum cultivation is mostly restricted in Indian subcontinent during summer months. It is therefore important to assess the role of temperature in the biosynthesis of floral volatiles in J. auriculatum more specifically to understand if fluctuations in the air temperature affect the scent volatiles accumulation in this species. Outcome of such study can pave new ways for cultivation of J. auriculatum in climate resilient areas of the tropics. Since jasmines grow mainly in countries under tropical climate, an understanding on the role of air temperature during blooming phase for maximum accumulation of scent volatiles shall be beneficial to farmers and floriculture traders.
In this manuscript, we first studied the physiology of scent volatiles synthesis and emission in J. auriculatum at different stages of floral maturation throughout lifespan under in situ condition. Upon identification of the appropriate flower maturation stage where maximum scent emission occurred, we then studied at that time point under controlled conditions the influence of a range of air temperatures in modulating scent volatiles synthesis and emission reflecting the climate of tropics. More specifically, we measured the amount of active enzymes (in terms of their in vitroactivities) of phenylalanine ammonia-lyase (PAL), phenylacetaldehyde reductase (PAR), acetyl-CoA-benzylalcohol acetyltransferase (BEAT), monoterpene synthase (MTS) and β-glucosidase involved in the biosynthesis of benzenoid and monoterpenoid volatiles in J. auriculatum grown in situand also at different air temperature regime. The expression levels of the structural genes and transcription factor involved in the biosynthesis of benzenoid and terpenoid volatiles were also studied to draw a correlation between the scent related gene expression and volatile emission patterns. Finally, the concentrations of non-volatile metabolites were also assessed from mature buds to understand the role of air temperature in modulating the upstream precursors of scent compounds.