Floral scent
Floral scent, or flower scent, is composed of all the volatile organic compounds (VOCs), or aroma compounds, emitted by floral tissue (e.g. flower petals). Other names for floral scent include, aroma, fragrance, floral odour or perfume. Flower scent of most flowering plant species encompass a diversity of VOCs, sometimes up to several hundred different compounds. The primary functions of floral scent are to deter herbivores and especially folivorous insects (see Plant defense against herbivory), and to attract pollinators. Floral scent is one of the most important communication channels mediating plant-pollinator interactions, along with visual cues (flower color , shape, etc.). Biotic interactions. Perception by flower visitors. Flower visitors such as insects and bats detect floral scents thanks to chemoreceptors of variable specificity to a specific VOC. The fixation of a VOC on a chemoreceptor triggers the activation of an antennal glomerulus, further projecting on an olfactory receptor neuron and finally triggering a behavioral response after processing the information (see also Olfaction, Insect olfaction). The simultaneous perception of various VOCs may cause the activation of several glomeruli, but the output signal may not be additive due to synergistic or antagonistic mechanisms linked with inter-neuronal activity. Therefore, the perception  of a VOC within a floral blend may trigger a different behavioral response than when perceived isolated. Similarly, the output signal is not proportional to the amount of VOCs, with some VOCs in low amounts in the floral blend having major effects on pollinator behavior. A good characterization of floral scent, both qualitative and quantitative, is necessary to understand and potentially predict flower visitors' behavior. Flower visitors use floral scents to detect, recognize and locate their host species and even discriminate among flowers of the same plant. This is made possible by the high specificity of floral scent, where both  diversity of VOCs and their relative amount may characterize the flowering species, an individual plant, a flower of the plant, and the distance of the plume from the source. To make the best use of this specific information, flower visitors rely on long-term and short-term memory that allows them to efficiently choose their flowers. They learn to associate the floral scent of a plant with a reward such as nectar and pollen, and have different behavioral responses to known scents versus unknown ones. They are also able to react similarly to slightly different odor blends. Mediated biotic interactions . A primary function of floral scent is to attract pollinators and ensure the reproduction of animal-pollinated plants. Some families of VOCs presented in floral scents have likely evolved as herbivore repellents. However, these plant defenses are also used by herbivores themselves to locate a plant resource, similar to pollinators attracted by the floral scent. Therefore, flower traits can be subject to antagonistic selection pressures (positive selection by pollinators and negative selection by herbivores). Plant-plant communications. Floral scents are the only types of volatile cues that are used to inform other plants about the mating environment. Plants that sense  floral scents emitted by other plants can adapt their floral phenotypic traits that affect pollination and mating. For instance, in sexually deceptive orchids, floral scents emitted after pollination reduce the flower's attractiveness to pollinators. This mechanism acts as a signal to pollinators to visit unpollinated flowers within an inflorescence. Biosynthesis of floral VOCs. Most floral VOCs belong to three main chemical classes. VOCs in the same chemical class are synthesized from a shared precursor, but the biochemical pathway is specific for each VOC and often varies from one plant species another. Terpenoids (or isoprenoids) are derived from isoprene and  synthesized via the mevalonate pathway or the erythritol phosphate pathway. They represent the majority of floral VOCs and are often the most abundant compounds in floral scent blends. The second chemical class is composed of the fatty acid derivatives synthesized from acetyl-CoA, of which most of them are known as green leaf volatiles, because they are also emitted by vegetative parts (ie: leaves and stems) of plants, and sometimes higher abundance than from floral tissue. The third chemical class is composed of benzenoids/phenylpropanoids, also known as aromatic compounds, they are synthesized from phenylalanine. Regulation of emissions. Floral scent  emissions of most flowering plants vary predictably throughout the day, following a circadian rhythm. This variation is controlled by light intensity. Maximal emissions coincide with peaks of the highest activity of visiting pollinators. For instance, snapdragon flowers, mostly pollinated by bees, have the highest emissions at noon, whereas nocturnally-visited tobacco plants have the highest emissions at night. Floral scent emissions also vary along with floral development, with the highest emissions at anthesis, i.e. when the flower is fecund, and reduced emissions after pollination, probably due to mechanisms linked with fecundation. In tropical orchids, floral scent emission is terminated immediately  following pollination, primarily to reduce the expenditure of energy on fragrance production. In petunia flowers, ethylene is released to stop the synthesis of benzenoid floral volatiles after successful pollination. Abiotic factors, such as temperature, atmospheric CO2 concentration, hydric stress, and soil nutrient status also impact the regulation of floral scent. For instance, increased temperatures in the environment can increase the emission of VOCs in flowers, potentially altering communication between plants and pollinators. Finally, biotic interactions may also affect the floral scent. Plant leaves attacked by herbivores emit new VOCs in response to the attack, the so-called herbivore-induced plant  volatiles (HIPVs). Similarly, damaged flowers have a modified floral scent compared to undamaged ones. Micro-organisms present in nectar may alter floral scent emissions as well. Measurement. Measuring floral scent both qualitatively (identification of VOCs) and quantitatively (absolute and/or relative emission of VOCs) requires the use of analytical chemistry techniques. It requires collecting floral VOCs, and then analyzing them. VOCs sampling. The most popular methods rely on adsorbing floral VOCs on an adsorbent material such as SPME fibers or cartridges by pumping air sampled around inflorescences through the adsorbent material. It is also possible to extract chemicals stocked in petals  by immersing them in a solvent and then analyze the liquid residue. This is more adapted to the study of heavier organic compounds, and/or VOCs that are stored in floral tissue before being emitted into air. Sample analysis. Separation. Gas chromatography (GC) is ideal to separate volatilized VOCs due to their low molecular weight. VOCs are carried by a gas vector (helium) through a chromatographic column (the solid phase) on which they have different affinities, which allows to separate them. Liquid chromatography may be used for liquid extractions of floral tissue. Detection and identification. Separation systems are coupled with a  detector, that allows the detection and identification of VOCs based on their molecular weight and chemical properties. The most used system for the analysis of floral scent samples is GC-MS (gas chromatography coupled with mass spectrometry). Quantification. Quantification of VOCs is based on the peak area measured on the chromatogram and compared to the peak area of a chemical standard: Specificity of floral scent analysis. Floral scent is often composed of hundreds of VOCs, in very variable proportions. The method used is a tradeoff between accurately detecting quantifying minor compounds and avoiding detector saturation by major compounds. For most  analysis methods routinely used, the detection threshold of many VOCs is still higher than the perception threshold of insects, which reduces our capacity to understand plant-insect interactions mediated by floral scent. Further, the chemical diversity in floral scent samples is challenging. The time of analysis is proportional to the range in molecular weight of VOCs present in the sample, hence a high diversity will increase analysis time. Floral scent may also be composed of very similar molecules, such as isomers and especially enantiomers, which tend to co-elute and then to be very hardly separated. Unambiguously detecting and quantifying  them is of importance though, as enantiomers may trigger very different responses in pollinators.