A batch of lemon balm-lemon verbena syrup reminds Jeanne of the multiple evolutionary origins of lemon flavor.
The citrus lemon itself is only one of many plant species that lends its namesake flavor or fragrance to our food and drinks. Lemon flavor primarily comes from a few terpenoid essential oils: citral (also called geranial, neral, or lemonal), linalool, limonene, geraniol, and citronellal. The production of one or more of these essential oils has independently evolved multiple times in species on widely separated branches of the plant phylogeny (see figure).
I was reminded of this extraordinary evolution of lemoniness today when I made a syrup (recipe below) from lemon balm (Melissa officinalis), a mint (family Lamiaceae), and lemon verbena (Aloysia citrodora), in the verbena family (Verbenaceae). The mints and the verbenas are closely related families within the order Lamiales, however, so combining them together did not produce a particularly phylogenetically extreme lemony syrup. Many other species in the mint family produce lemony essential oils, such as lemon basil (Ocimum spp.), lemon thyme (Thymus spp.), and lemon mint (Monarda spp.). And the sand verbenas (Abronia spp.) growing on the sand dunes on the California coast have a lemony aroma.
Moving away from the Lamiales, lemony essential oils are most spectacular, of course, in the citruses (family Rutaceae, order Sapindales). Several of the Australian Myrtaceae species (order Myrtales) are lemony and used as a spice, including lemon myrtle (Backhousia citriodora), lemon gum (Corymbia citriodora), and lemon tea tree (Leptospermum polygalifolium). The lemony leaves and fruit of Litsea cubeba, in the bay family (Lauraceae, order Laurales), are locally popular as a spice in its native tropical Asia. Popular now the world over, however, are other tropical Asian natives the lemon grasses, some 55 species of grasses in the genus Cymbopogon (family Poaceae, order Poales). Cymbopogon citratus is the most popular culinary species. Commercial citronella essential oil (the real stuff in natural insect repellent) is distilled from C. nardus and C. winterianus. In all of these diverse taxa, the lemony essential oils serve as defense compounds against pathogens and herbivores. This is not to say that these plants all taste the same, which of course they don’t. They all have their own unique flavor compounds as well, and the relative proportions of the lemony terpenoids vary.
While the lemony essential oil compounds themselves have independently evolved numerous times, the structures that plants use to store and deploy them against marauders are highly variable across the plant tree of life. Citrus leaves and fruit concentrate their essential oils in oil glands. Specialized cells cluster together, synthesize the oils, and secrete them into the space between the cells, forming a chamber (gland) filled with oil (Thomson et al. 1976). The oil glands are so large that you can see them as translucent spots if you hold a citrus leaf up to the light.
The oil glands in citrus are found in almost all plant structures but are most abundant in the outer fruit rind (zest). Many species in the Myrtaceae also have oil and resin canals or specialized storage cells, although these are structurally different from those in citruses. The essential oils in lemongrass leaves are stored in specialized cells nestled between the vascular bundles in the spongy mesophyll, beneath the photosynthetic tissue. The cell walls of these lemongrass oil cells are lignified (woody), perhaps contributing physical as well as chemical defense against marauding herbivores (Lewinsohn et al. 1998).
The verbenas and the mints in the Lamiales have my favorite essential oil delivery system—trichomes. Trichomes are hair-like growths on the outside surface of the leaf. Like epicuticular wax, trichomes take on particular shapes and forms in different plant lineages and species. The hooked trichomes on the leaves of bean plants (Fabaceae), for example, feel like sandpaper if you rub your hand over them. These hooks may serve the plant in part by slowing insect herbivores down, if their efficacy at catching bedbugs is any indication. The leaves of mints and verbenas have two kinds of trichomes. The first kind is hair-like, with either no branches, as in lemon balm, or with multiple branches, as in lavender ( Lavandula spp.). These trichomes may help cool the leaf by deflecting excess solar radiation. The other trichomes on these leaves are glandular hairs. These trichomes fill with essential oil and sit like squat little water balloons on the surface of the leaf. The flowers, too, are covered in glandular hairs. See great scanning electron microscope pictures of the two kinds of trichomes in mint family species here.
When you rub a verbena or mint-family leaf between your fingers, you rupture the glandular trichomes and release some of the most fragrant substances this planet has to offer. They may be repellent to bugs, but hardly to us.
Fresh herb syrup
There are two different ways I make a sweet syrup out of fresh herbs. In the first, I start by making a strong infusion (“tea”) out of the herbs. I usually barely cover them with water in a pot, bring it to a boil, then turn the heat off and let it steep for however long it needs. The steeping is definitely up to you, but you should start tasting it after one minute. Steeping leaves like lemon balm and lemon verbena for more than 5 minutes or so will start to draw out some of the bitter and dark tannins from the leaf (remember the essential oil is on the surface), which may be fine with you, but you should just keep checking and tasting. Then, I strain out the leaves and add an equal volume of sugar to the remaining liquid in a saucepan and stir to dissolve the sugar while I bring it to a boil. The second method is to measure or guess how much liquid I’ll need to cover the herbs packed into the bottom of a saucepan, then add an equal volume of sugar to that amount of warm/hot water, stir it to dissolve the sugar, then pour it over the herbs, bring it back to a boil, turn off the heat, let it steep, then strain. Either way, stirring in about a tablespoon of vodka or brandy per cup of syrup will help preserve it for about a month in the fridge if you’re not going to can it or freeze it. I like stirring a little of this syrup into club soda to make a grown-up soda, and it’s also good incorporated into or onto ice creams or sorbets or drizzled over an almond cake.
Gattuso, S., C. M. van Baren, A. Gil, A. Bandoni, G. Ferraro, and M. Gattuso. 2008. Morpho-histological and quantitative parameters in the characterization of lemon verbena (Aloysia citriodora palau) from Argentina. Boletín Latinoamericano y del Caribe de Plantas Medicinales y Aromáticas 7: 190-198.
Lewinsohn, E., N. Dudai, Y. Tadmor, I. Katzir, U. Ravid, E. Putievsky, and D. M. Joel. 1998. Histochemical localization of citral accumulation in lemongrass leaves (Cymbopogon citratus (DC.) Stapf., Poaceae). Annals of Botany 81: 35-39.
Stewart, A. 2013. The drunken botanist: the plants that create the world’s great drinks. Algonquin Books of Chapel Hill, Chapel Hill, N.C.
Svoboda KP and RI Greenway. 2003. Lemon scented plants. International Journal of Aromatherapy 13: 23-32.
Thompson, W.W., K. A. Platt-Aloia, and A. G. Endress. 1976. Ultrastructure of oil gland development in the leaf of Citrus sinensis L. Botanical Gazette 137: 330-340.).