Plant of the Week
English Pea
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
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