Plant of the Week
Latin: Pisum sativum
As we continue our exploration of how new plants are developed, we must consider the English pea. Its role in understanding how traits are passed from one generation to the next is pivotal in the story, thanks to the scientific genius of Gregor Mendel (1822 - 1884), an Austrian monk who chose it as his test plant in studies he conducted in the middle years of the 19th century. In scientific terms, Mendel ranks on par with Galileo and Darwin, for his observations fundamentally changed our understanding of the world. His beautifully simple but elegant experiments explained with mathematical precision how traits are passed from one generation to the next. Application of his work lead to the advances in plant breeding that spurred a revolution in agriculture that remains with us today.
The English or garden pea (Pisum sativum) was a good choice for Mendel’s project. Because it was close at hand, an important vegetable, and it had a series of readily identifiable traits that were found in a number of different cultivar, it was ideal for following the progress of his experiments.
Peas are one of the oldest cultivated food crops, so its origin is guesswork amongst experts. Most likely it originated in Central Asia, but the oldest known find of peas have been carbon dated to 9750 years old from a cave on the Burma (Myanmar)-Thailand border, well outside of the range of proposed botanical origin. They were originally eaten as a dried pea, but over the centuries have been improved to the sweet tasting, edible pod forms we enjoy today.
English peas are cool weather vegetables that grow as annual vines to six feet tall with a glaucous, gray look and compound leaves, the terminal leaflet of which is modified into a tendril for climbing. The flowers are white, borne in the axils of the leaves and produce pods three to four inches long.
Mendel began his pea breeding experiments in 1856. The beauty of his study was that he simplified things. Instead of looking at the whole plant, he observed how traits passed from generation to generation as a set of seven matched pairs of characteristics, including such traits as smooth or wrinkled seed, color of the pea (yellow or green), the color of the seed coat (gray or white) or the plant form (dwarf or vining).
After studying the then-available pea strains, he selected 22 kinds for use in his breeding experiments. His first crosses, all made by hand on flowers that had had their stamens removed to prevent self pollination, involved the smooth and wrinkled seed characteristic. He made 287 crosses on 70 plants. When he harvested the seeds, all were smooth-seeded types. He chose to call the characteristic that prevailed a "dominant" trait; the one that disappeared a "recessive" trait.
The next spring he planted the seeds from his first season and allowed them to pollinate amongst themselves as the bees and nature saw fit. From this second crop of peas he harvested 7324 pea seeds. Of these, 5474 were smooth seeded and 1850 were wrinkled. A man blessed with an innate mathematical ability, Mendel spotted the 3 to 1 relationship between the two traits.
To further understand the mathematical relationships, Mendel started observing two pairs of characteristics, seed color and the wrinkled characteristic. From the 556 peas produced in his double-characteristic crosses, he saw that the population broke into a 9:3:3:1 ratio. Smooth yellow seeds (both dominant traits) were most common, with the wrinkled green seeds (both recessive traits) the least common. Intermediate forms, wrinkled yellow and round green had one dominant gene and one recessive gene.
This must have been one of those eureka moments, because from his analysis of the populations he published a paper in 1866 that laid out how traits were transmitted from generation to generation. The now familiar designation of "A" and "a" to represent dominant and recessive traits were clearly explained as was his law of segregation.
Mendel’s choice of peas was a lucky one for the pea is usually self pollinated with relatively uniform characteristics within a given cultivar. The traits he studied responded nicely to the Mendelian Law of Inheritance, as it later became known. But nature is messy, and as Mendel later learned when he attempted to work with hawkweed and beans, it doesn’t always follow all of the rules one might develop.
Mendel never lived to see his work recognized. In fact it was completely ignored until three scientists independently recognized its significance in 1900 and applied it to the work they were doing. This famous "ah ha" moment hit the scientific community like a California brush fire, sweeping away all sorts of previously held, but incorrect, notions.
When selecting garden peas for the garden, the wrinkled pea types are the way to go because wrinkled selections such as Sugar Snap and Little Marvel have high sugar content while the smooth seeded types are starchy in taste. Peas need to be planted when the soil temperature is around 45 degrees, so earliness is a key to success. A friend tells me she plants her peas in her garden in mid February between bales of hay, making a kind of insulated mini-greenhouse.
Peas grown for shelling usually reach maturity (from seeding) in about 60 days. But, because plants flower over an extended period, they must be harvested every couple days to make sure the seeds don’t get over mature and turn starchy. If peas are picked for green pods, pick them 5 to 7 days after flowering while the seeds are about the size of a BB.
By: Gerald Klingaman, retired
Extension Horticulturist - Ornamentals
Extension News - July 30, 2004
The University of Arkansas Division of Agriculture does not maintain lists of retail outlets where these plants can be purchased. Please check your local nursery or other retail outlets to ask about the availability of these plants for your growing area.