1225 Reynolds Road
Cross Junction, VA 22625-1726
by Don Spoon
Recently, a question was presented to me, “Can one pollen grain fertilize all the ovules in an iris flower?” I will attempt to answer this question and add what I hope will be some interesting facts about iris reproduction.
Highly successful land plants, such as irises, have two different plant forms (sporophyte and gametophyte) that alternate in each generation between a diploid sporophyte organism designed to resist drying out and haploid organisms (pollen or microgametophytes and ovules containing macrogametophytes) that require a moist environment inside the sporophyte where fertilization takes place. Each sporophyte microspore mother cell produces by meiosis (reduction and division, 2N to 1N sets of chromosomes, or in tetraploids 4N to 2N) four microspores that develop into four binucleate pollen grains with tough coats. Irises are seed plants (Angiosperms) with one embryonic seed leaf (Monocotyledons) and leaves with parallel venation. Irises advance the basic lily plan and reduce the flower parts, which are basically modified leaves, from multiples of six to three.
The female part of the iris flower is the pistil composed of the stigma, the style, and the ovary. The male part is the stamen made of the attachment filament and the anther with four long, parallel pollen sacs. The pollen is not “ripe” until the pollen sacs open and the puffy masses of pollen are apparent. If the nourishing filament of the stamen is broken too early when the pollen sacs are immature, the pollen may be of no use. The stigma has a sticky secretion that not only holds the pollen grains, but also contains various nourishing chemicals, such as sugars, that causes the pollen grain organisms to germinate and build the pollen tubes that digest their way with enzymes as they descend down the style tissue to the ovary. The pollen grain is remarkably resistant to drying and has one tiny area where the germinating pollen grain organism will emerge. The ovary contains the ovules, each with two coverings (integuments) that surround the multicellular macrogametophytes in their little “moist pond.” The surrounding integuments that later develop into the two seed coats leave a tiny opening, the micropyle, for the pollen tube to enter. A tall bearded iris can have at most, about one hundred ovules distributed in the three chambers of the ovary. This threefold redundancy may be useful, since a Verbena bud moth larva that may penetrate the developing pod usually does not go from one ovary chamber to the next and destroy all the seeds. The ovules are lined up in rows and held by stalks though which they receive nourishment as they develop into seeds (analogous to multiple fetuses in a mammalian womb). The central region of the ovary that the ovules stalks attach to is called the placenta. If the iris pod is broken from the stalk too early, the small seeds may be underdeveloped and unable to germinate or grow.
When the pollen grains are applied onto the stigmatic lip of the iris flower by the insect or human pollinator, there can be thousands that will germinate and form pollen tubes. From that moment the race is on, and when all ovules are reached, all Johnny-come-lately pollen tubes are essentially lost. There will be just one winning pollen grain’s pollen tube reaching each ovule, so there’s your answer. One pollen grain pollinates one ovule, and only one. The pollen will germinate in minutes on the stigmatic lip. Each pollen grain will take about eight hours to grow a pollen tube down through the style arms to the ovary and finally growing to and into the ovule opening. If the pollen is applied on only one of the three stigmatic lips, the pollen tubes will still go into all three chambers of the ovary and may fertilize all the ovules. Most hybridizers apply pollen to all three stigmatic lips. The fertilized ovary will swell and grow into a pod as the stalk gains strength and resiliency. In about two months the pod will dry, turn tan, and begin to crack at the end. At this point, harvest the pod in a folded paper sandwich bag and place it in a cool place for a couple of weeks while the seeds harden, then they can be separated from the dried, brown pod walls.
If the hybridizer is too rough while daubing pollen, the style arms may be broken, and the pollen tubes in the broken style arms can’t reach the ovary. If there is rain, hail, strong winds, frost, or overly hot conditions, the pollination can fail. You need to make the cross at least three hours before rain occurs, otherwise you can cover the flower with a plastic bag with the top corner cut out so it will not overheat inside the bag. Some hybridizers strip off all three falls and cover the cross with a section of nylon stocking so no interloping insect will enter with unwanted pollen.
The preeminent moment of iris reproduction occurs when the pollen tube enters the ovule. But before entering, led by the pollen tube nucleus, the generative nucleus following behind divides by mitosis to produce two haploid nuclei. The pollen tube, relative to its thickness, is an extremely long filament filled with cytoplasm, yet it contains no proplastids that will develop into chloroplasts, thus the chloroplasts and their DNA only come from the pod parent and the egg cell of the ovule microgametophyte. Since the chloroplasts power the plant’s photosynthesis, it is easy to understand why it is always best to use as the pod parent the most vigorous parent. If you want to obtain variegated foliage, you must have the trait in the pod parent as Allan Ensminger discovered. The gene that produces the variegated foliage trait is most likely in the chloroplast DNA.
Once the two sperm nuclei enter the iris ovule a most remarkable thing happens; there is not one, but two fertilizations. One haploid sperm nucleus unites with the macrogametophyte cell that contains the haploid egg nucleus to produce the diploid sporophyte zygote. The other sperm nucleus produces the triploid endosperm by uniting with the diploid nucleus of the central cell formed from fusing two cells of the macrogametophyte. The other five cells, of the total of eight that made up the macrogametophyte produced from three divisions of one megaspore, are transient and play minor accessory roles. Then, both zygote cell and the endosperm cell increase by cell division (mitosis) which eventually ceases as the seed coat dries and hardens, and the enclosed embryo with its attached endosperm becomes dormant. By analogy, the endosperm is to the iris embryo what an egg yolk sac is to the embryo of a fish, amphibian, reptile, bird, or egg-laying monotreme mammal like a Duckbilled Platypus.
In the iris seed coat are special plant hormones like abscissic acid that prevent the embryo from breaking dormancy. These germination inhibitors have to slowly leach out in the wetting cycles of the soil so that only a percentage of the seeds germinate each year for several years. Iris seeds can remain dormant and viable in the soil up to 5 years and if stored cool and dry for thirty years or more. You can remove much of these germination inhibitors by soaking the seeds for about ten days with daily changes of water taking care to keep them cool as in a basement. Then, if the seeds are planted you will have a much higher percentage (90 to 100%) to germinate the first year.
You may elect to obtain the needed ninety days of stratification, cold temperature under 50 degrees F, in a refrigerator rather than in the garden. However, if your refrigerator shuts down and the seeds dry out or it gets too cold and freezes where you placed the seeds, you may lose them all. This is the hybridizer’s worst nightmare, the loss of a year’s hybridizing work, and it has occurred. If they germinate early in the refrigerator and the roots get too long, you may lose some when they are planted. I prefer to plant the seeds in soil in pots for later transplanting with the pots sitting on the ground in sand or mulch, or I plant them directly in the seedbed soil 4 inches apart and ½ inch deep by November 1. The seedlings that germinate in the second or third year should be transferred to new beds, as they cannot compete with the large, first year seedlings. We give each seed bed four years before digging it up, as we are looking for rebloom, winter hardiness, and disease resistance that take longer to evaluate. As soon as possible after you have evaluated a seedling as worthy of saving, move it to a select bed with the seedlings at least 9 inches apart. Otherwise, it may not bloom the next year and even get misplaced. Next the seedling is lined out in a reselect bed for final evaluation before consideration for introduction. Then you will need another couple of years to build up stock, send it to regional, national and international competitions and get feedback from garden judges. Thus, in a minimum of six years you may go from the cross to introduction and sale.
The most important rule for planting iris seeds or transplanting seedlings is to plant them in the very best prepared bed you can, so they can reach their fullest potential. If you can easily insert a rod at least one foot deep into your seed bed then you have prepared an adequate root run. Of course, never walk on a seed bed or you will compact the soil and press out the essential air content. Give them the best space, soil preparation, fertilizer, and care possible and they will reward your efforts. If they are planted in a poorly prepared bed, or overcrowded, they may never grow well, and you may never know how good they were.
A seed pod developing on a bearded iris. What is unique about this seed pod is that it is variegated, and so is the foliage. This is exciting to hybridizers, and one of the things that makes hybridizing even more fun.
We like to get everyone involved in hybridizing in our local Iris Society. Pictured above are two of our youth members, Kaylee and Nicole, enjoying hybridizing at Winterberry in May.
1225 Reynolds Road
Cross Junction, VA 22625-1726