PANSIES: Today's Flowers Are Not Like Grandma's
Posted on Wednesday, December 3, 2008
Pansies. The word brings to mind a classic shape of flower with five petals of white, yellow, blue, purple, orange-brown, or splashed colors in any mixture of the previous listed colors, or even flowers having two petals colored alike in any of these combinations and three petals in some completely different combination. In fact, there are hundredsofcolorsandcolorpatterns in pansies from which you canchooseforyourbeds. Lookat the photo and you can see some of the variety available, or go to a garden center or nursery center and see a larger variety.
If you recall, grandmother needed help in clipping off those pansy flowers which had started to fade and were starting to produce seed pods, and when we asked, “ WHY ? ”, she simply announced that “ too many old flowers or seed pods make the plant stop making any more. ” If you were like me, then you helped mother or grandma. It was not much work if they had planted only a few, but if they had planted several dozen or more, it became quite a chore. Then you had to do the job again the next day or every two or three days, then again and again. I began to dislike pansies with a passion.
Though grandma didn’t know what she was doing physiologically to the plant and couldn’t really explain WHY, we now know that certain stimuli, like day length, or cold, or food reserves, or other factors depending on the species, induce the plant to produce a flowering hormone (FH ) that promotes flowering. When these flowers are pollinated, this induces the plant to produce another hormone (called “ auxin ” or plant growth hormone ) that causes the pollinated flower ovary to grow. Inside this developing fruit, the sperm from the pollen fertilizes the egg starting the seed to develop. As the seed reach a certain size, this induces the plant to produce a third hormone (the ripening hormone - RH ) causing the fruit to mature and ripen. Both the auxin hormone and the ripening hormone have been isolated and identified, but the FH has not, though it has been shown to occur.
It is very interesting that a new hormone causes the plant to stop production of a former hormone. If we could devise a way to stop the flowers from being pollinated, NO growth hormone would be produced. The plant would continue to produce only the flowering hormone and would flower indefinitely. Plant breeders have found a way to do this with the pansy.
The procedure is something like the following but not quite so simple to do. First, two different kinds of pansy plants are chosen that have the color and size of flowers desired. One plant is treated with a chemical like colchicine which in the pansy plant causes the plant cells to double the number of chromosomes making the plant have FOUR chromosomes of each type (a tretraploid ) instead of two (a diploid ). This plant will be one of the parents in the genetic breeding program. The second plant remains a diploid and is treated with a different chemical that causes a rearrangement of the linear sequence of genes on its chromosomes to make its sequence different from that of the tetraploid. This preliminary work takes time but is necessary.
Pollen from the diploid plant is used to pollinate the tetraploid one. A seed capsule will develop containing seeds that when planted will develop into a triploid plant in which each cell will have three sets of chromosomes. This plant will grow normally, produce FH and flower normally but the triploid plant cannot produce eggs or pollen because the gene sequence of the chromosomes are different; the plant is sterile. None of the other hormones are produced so the plant will flower continuously as long as it receives water and fertilizer. No seed will be produced because the triploid is sterile, thus there are no seed pods that have to be removed to keep the plant flowering.
Many of the varieties of pansy plants available are such triploids but not all. I just visited a nursery to see if packs of pansies are still available this fall and there are some, but in examining the old or faded flowers, I found developing seed pods on almost all of them. Some seed pods were so mature that the seed inside the pods were already turning brown, the natural color of mature pansy seed. These plants were obviously normal diploids, the seed of which is much cheaper. Visits to several display beds at businesses in Bentonville showed pansies in full bloom with no seed pods. These were obviously triploids, the seed of which is much higher.
Does the triploid condition convey any other traits than sterility ? Yes, several. With the pansy, the flower diameter can be 50 % or more larger, the flowers do not age as fast so larger numbers of flowers can be present and the color intensity can be much greater. These examples are assuming that only one pair of genes determines the trait in a diploid, which sometimes is true but for some traits more than one pair of genes are involved.
Is triploidy of value in plants other than pansies ? Yes. Seed is available from all seed supply companies that will produce seedless watermelons. Examination of the packaged seed will show that there are two sizes or colors of seed present. One will produce the seedless watermelon plants which will flower normally with no eggs or pollen of its own. The other will produce normal diploid watermelon plants which flower and produce both eggs and pollen. Insects will carry the pollen from diploid flowers to both diploid flowers and tetraploid flowers which are fully capable of being pollinated and forming melons but as no eggs are in the tetraploid to be fertilized no seeds will be formed. As a result, more flowers are produced and more watermelons with no seeds are produced.
As a side thought, the grower gets his main crop as seedless watermelons AND a valuable bonus crop of “ Sugarbaby melons ” for that variety is probably the most common pollinator for seedless watermelons. Sugarbaby melons are dark green in color and small in size so the farmer has no trouble telling which is which during harvest. Production of seedless watermelons is a multimillion dollar farming crop based on this genetic dreaming that became a reality.
Editor’s Note: Dr. Lane, who lives at rural Gravette, is a retired professor from the University of Arkansas Department of Biological Sciences.
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