Introduction
All terrestrial plants emit biogenic volatile organic compounds (BVOCs), relatively small chain hydrocarbon compounds that have lower boiling points and evaporate easily. These compounds play an important role in atmospheric chemistry [1], plant physiology [2], plant defense and competition [3,4,5,6], and communication between plants and other organisms [7,8,9].
Monoterpenes are the second most dominant group of BVOCs after isoprene, with an estimated global annual emission rate of 107.5 Tg C yr−1 contributing ~12% to the global BVOC budget [10]. Monoterpenes are generally derived from the condensation of two isoprene (C5H8) units and include a variety of well-known compounds including pinene, linalool, and limonene among others [11]. Biosynthesis of monoterpenes is catalyzed by monoterpene synthases (cyclases), which convert the universal precursor geranyl diphosphate (GDP) to the parent structures of the various monoterpene groups. De novo synthesis is light-dependent and can occur in the cytosol through the mevalonic pathway (MEV) or in the chloroplast, through the methylerythritol phosphate (MEP) pathway [11,12].
Different plant species will have different enzymes leading to the formation of specific monoterpenes (monoterpene synthases), leading to a huge diversity of these plant natural products across the plant kingdom [11,12]. For instance, α-pinene and β-pinene make up most monoterpene emissions from oaks and conifers [13,14,15,16], while E-β-ocimene is commonly released by plants of the Salicaceae family [17,18,19]. Although a wide spectrum of monoterpenes is emitted by tree species [2,20], considerable emissions of some compounds (e.g., α-pinene and β-pinene, Δ3-carene, limonene, etc.) are also reported from Poaceae species as well as from rice, maize, bamboo, and other grasses [21,22,23,24,25,26]. Besides the emission of monoterpenes from the tree and grass species, over the last two decades there has been increasing work investigating monoterpene emissions in other crops [27,28], and horticultural species [29,30,31,32,33,34], as well as ornamental plants and invasive alien species [27,28,35,36].
The emission of BVOCs varies in time and intensity in response to abiotic factors such as temperature, light intensity, CO2, O3,and O2 concentrations, but the exact mechanisms behind these responses are not yet fully understood [19,37]. However, monoterpene emissions have been reported to have distinct dependencies on light and temperature compared to those reported for other BVOCs. This is attributed to the ability of some plants to store them and their high solubility in water (such as linalool) [38,39,40]. Terpenes can be produced de novo and released immediately or stored in non-specific internal pools or specialized endogenous and exogenous structures such as resin secretory structures and glandular trichomes [41,42,43].
The emissions of stored monoterpenes are mainly temperature dependent, while the non-stored monoterpenes are believed to be dependent on both temperature and light [41]. Moreover, the stored monoterpene emissions are also influenced by other factors such as humidity, diffusion resistance, cell wall, membrane thickness, and pool storage size [43,44,45,46]. In the next sections, we provide an overview of the abiotic environmental factors affecting monoterpene emissions, including putative mechanisms, and identify knowledge gaps to be addressed by future research.
References
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Journal: Plants Authors: Malik et al
Introduction All terrestrial plants emit biogenic volatile organic compounds (BVOCs), relatively small chain hydrocarbon compounds that have lower boiling points and evaporate easily. These compounds play an important role in atmospheric chemistry [1], plant physiology [2], plant defense and competition [3,4,5,6], and communication between plants and other organisms [7,8,9]. Monoterpenes are the second most dominant group of BVOCs after isoprene, with an estimated global annual emission rate of 107.5 Tg C yr−1 contributing ~12% to the global BVOC budget [10]. Monoterpenes are generally derived from the condensation of two isoprene (C5H8) units and include a variety of well-known compounds including pinene, linalool, and limonene among others [11]. Biosynthesis of monoterpenes is catalyzed by monoterpene synthases (cyclases), which convert the universal precursor geranyl diphosphate (GDP) to the parent structures of the various monoterpene groups. De novo synthesis is light-dependent and can occur in the cytosol through the mevalonic pathway (MEV) or in the chloroplast, through the methylerythritol phosphate (MEP) pathway [11,12]. Different plant species will have different enzymes leading to the formation of specific monoterpenes (monoterpene synthases), leading to a huge diversity of these plant natural products across the plant kingdom [11,12]. For instance, α-pinene and β-pinene make up most monoterpene emissions from oaks and conifers [13,14,15,16], while E-β-ocimene is commonly released by plants of the Salicaceae family [17,18,19]. Although a wide spectrum of monoterpenes is emitted by tree species [2,20], considerable emissions of some compounds (e.g., α-pinene and β-pinene, Δ3-carene, limonene, etc.) are also reported from Poaceae species as well as from rice, maize, bamboo, and other grasses [21,22,23,24,25,26]. Besides the emission of monoterpenes from the tree and grass species, over the last two decades there has been increasing work investigating monoterpene emissions in other crops [27,28], and horticultural species [29,30,31,32,33,34], as well as ornamental plants and invasive alien species [27,28,35,36]. The emission of BVOCs varies in time and intensity in response to abiotic factors such as temperature, light intensity, CO2, O3,and O2 concentrations, but the exact mechanisms behind these responses are not yet fully understood [19,37]. However, monoterpene emissions have been reported to have distinct dependencies on light and temperature compared to those reported for other BVOCs. This is attributed to the ability of some plants to store them and their high solubility in water (such as linalool) [38,39,40]. Terpenes can be produced de novo and released immediately or stored in non-specific internal pools or specialized endogenous and exogenous structures such as resin secretory structures and glandular trichomes [41,42,43]. The emissions of stored monoterpenes are mainly temperature dependent, while the non-stored monoterpenes are believed to be dependent on both temperature and light [41]. Moreover, the stored monoterpene emissions are also influenced by other factors such as humidity, diffusion resistance, cell wall, membrane thickness, and pool storage size [43,44,45,46]. In the next sections, we provide an overview of the abiotic environmental factors affecting monoterpene emissions, including putative mechanisms, and identify knowledge gaps to be addressed by future research.
References