Plant of the month (©Pierre Laszlo, all rights reserved)
Trifolium, genus (Fabaceae)
They are small annual, biennial, or short-lived perennial herbaceous plants. The common name, clover, has a deceptive apparent etymology. Perhaps counterintuitively, the name is not related to ‟cloves,‟ derived from the French word for nails, clous. It is derived instead from West Germanic *klaiwaz- “sticky pap, » which might point to a cognate of glue and clay.
The genus is believed to have originated in the Medierranean in the early Miocene, i.e., 16-23 million years ago. Trifolium repens, one of the 245 species comprising the genus, spread together with migrating animals before recorded history throughout Europe and Western Asia. Domestication began four centuries ago in the Netherlands.
At the same time that clover was domesticated, it’s symbiotic partner, a bacterium, Rhizobium, was also domesticated. In this partnership, both vastly benefit from it.
The benefit to clover is nutritional. Due to an enzyme known as nitrogenase, Rhizobium has the outstanding capability to assimilate nitrogen directly from the air. The nitrogen molecule is made of two nitrogen atoms. These atoms are linked by one of the strongest bonds in chemistry. Hence, the nitrogen molecule is extremely stable and unreactive. Therefore, were it not for nitrogenase, it would be unassimilable by clover.
Nitrogenase converts this air nitrogen into much wieldier ammonium ions, that the plant feeds upon. Hence, clover has no need for fertilizer as an ammonium source. It serves as its own fertilizer, a feature it shares with all other plants in the same legume family.
Numbers are eloquent on this particular point. Fertilizer, i.e., ammonium nitrate has been produced industrially since the 1910s by the Haber-Bosch process. This industrial nitrogen fixation converts 80 million tons of air nitrogen per year, whereas the biological process converts 100 million tons annually!
Allow me more numbers: 1% of the world's energy supply is consumed in the manufacturing of ammonium nitrate fertilizer. That fertilizer is responsible for sustaining 40% of the Earth's population.
Why does mankind need nitrogen in such huge amounts? Because proteins contain on the average about 15 % of nitrogen.
What does each symbiotic partner, the legume and the bacterium, gain from their association? This symbiosis started 60 million years ago, i.e., making it a recent occurrence in evolutionary terms. By its newly gained ability at nitrogen fixation, the plant is able to synthesize proteins, stored in its leaves and seeds. Proteins from legume plants more generally feed both animals and humans. They reportedly amount to 35 % of proteins eaten worldwide by mmankind. As for the benefit to the bacterium, it draws its energy from sugars, provided by the host plant. Those sugar molecules are products of photosynthesis, i.e., the harvesting of sunlight by clover leaves.
Symbiosis of the Rhizobium aerobic bacterium and the Trifolium plant occurs within nodules, inhabited by the micro-organism on the roots of the plants. Another illustration, at a yet smaller, nanoscopic scale, is the partnership between two different protein molecules, that act jointly to bind and transform nitrogen.
Leghemoglobin, a protein whose structure resembles that of hemoglobin in our blood, is essential to the clover-bacterium partnership. Its presence is necessary to operation of nitrogenase, the enzyme responsible for turning atmospheric nitrogen into ammonia. Leghemoglobin scavenges oxygen. Its role is indeed essential, since otherwise the presence of oxygen — Rhizobium is an aerobic organis — blocks the enzyme. The other component in nitrogenase is a metalloprotein, with the two metals molybdnenum and iron. It uses energy, provided by the plant’s sugars, to convert nitrogen into ammonia.
Clover, together with the other legume plants, due to nitrogen fixation, is important to agriculture. Wheat, for instance, is a net consumer of nitrogen. Crop rotation is the ploy by which mankind has taken advantage of the Rhizobium-legumes symbiosis.
Middle Eastern farmers have practiced crop rotation since about 6000 BC, alternately planting legumes and cereals. A four-field rotation was popularised by the British politician and agriculturist Charles Townshend (1674-1738). The system (wheat, turnips, barley and clover), provided both a fodder crop and a grazing crop allowing livestock to be bred year-round. Townshend thus deserved his nickname as Turnip Townshend! This four-field crop rotation was a key development in the British Agricultural Revolution.
Trifolium, the Latin name for the genus, means ‟three-leaves.‟ However, there exists the occasional four-leaved specimen. As everyone knows, it is a symbol of good luck. I cherish this memory of my father: whenever he came upon a patch of clover, he would stand looking at it for an instant, then quickly bend down, pick and hold a four-leaved specimen. Good eyesight, surely. Also, a quality of intense concentration. Although I do not have his uncanny ability at identifying four-leaved clover, I may have inherited it in other pursuits
If indeed clover means ‟sticky,‟ it could not have been better named from the viewpoint of bees. They have served as its pollinators for many millions of years. It is yet another symbiotic relationship. From the plant nectar, the insect gets food for the hive.
Pollen from white clover has high nutritional value to bumblebees and bees, it is rich in proteins, with a content of about 25 %. The plant gets increased genetic diversity, reproducing better and spreading more widely than depending only on the wind.
Arguably, more honey is obtained from this plant than from any other in the world.
Honey producers tend to locate their hives in Trifolium repens (white clover) growing areas in the spring, to collect the high quality clover honey.
The last word belongs to the poet. Emily Dickinson wrote a riddle, with clover as the answer. Its first four lines read:
THERE is a flower that bees prefer,
And butterflies desire;
To gain the purple democrat
The humming-birds aspire.