Hybridization: Controlling Seed Production

Eco-Farm: An Acres U.S.A. Primer — Lesson 2: The Forgiveness of Nature, Part 4

This is an excerpt from Charles Walters’ Eco-Farm — An Acres U.S.A. Primer, available from the Acres U.S.A. bookstore at bookstore.acresusa.corn. Read more excerpts from this book using the category “Eco-Farm” (https://members.acresusa.corn/magazine-features/eco-farm/).

16. Hybridization 

Control of seeds has long been an economic objective. Thomas Jefferson once risked the death penalty to smuggle an upland rice seed out of northern Italy. Then as now someone wanted to protect a monopoly. History tells us that the trigger mechanism for Mexico’s revolt from Spain was sprung when a provincial padre was told he couldn’t plant grapes for altar wine, this also to protect the monopoly in Spain. 

Of all the techniques ever used to control seed production, none have rivaled hybridization. 

Hybridization has to do with combination of desirable characteristics found in two plants. The hybrid can be a cross between varieties, and sometimes between species. The details need not detain us. Suffice it to say that to produce purebred strains of, say, corn, by inbreeding, the breeder selects plants with known characteristics. These are heavy yielding plants with strong roots, stiff stalks, good ears, wide leaves and other desired qualities. These plants are inbred—in other words, forced to self fertilize. 

The silk of an ear cannot be fertilized by pollen until it grows out through the husk. To inbreed, ears are covert by a paper bag. This prevents wind pollination, sometimes called open pollination, nature’s process whereby pollen from many corn plants is carried by air to settle down on receptive silks. There it germinates, sending down a pollen tube through which the sperm reaches the egg—or fertilization! Each kernel of corn has its own thread of silk and will not develop unless this thread has been pollinated. 

In the inbreeding process pollen from the tassel of the same plant which bears the ear to be pollinated is shaken into another paper bag. The bag covering the ear is then quickly replaced with the bag containing the pollen. The corn is thus self-pollinated. 

Hybrid seed corn is a first generation cross or hybrid between two or more inbred strains of corn. 

The USDA diagram presented here illustrates the method used in crossing four separate inbred plants to obtain two single crosses. Note how the single crosses are crossed to produce a double cross hybrid seed. Once a desirable hybrid combination has been determined, it can be reproduced repeatedly, simply by maintaining and recrossing the same inbred lines which have already been inbred to the point where each reproduction will give the same results. 

Of all the commercial farm crops, corn is the least difficult to hybridize. Usually six rows of the seed-bearing parent, a ratio of one pollen parent to three seed bearing plants is the general rue. High school and college kids are usually employed to remove tassels of the six rows. As a consequence, the seed set will be hybrid. 

Today corn, celery, carrots and onions are grown with hybrid seeds, corn and onions easily leading the way. There is a reason for this. Many hybrids are produced by hand crossing. This means emasculation, or removal of the anthers of flowers before pollen has spread, and at the same time protecting these flowers from insect and wind-spread pollen. Not many plants can stand the ordeal. As a consequence, a lot of emasculation and pollination is carried on in a greenhouse. Sometimes flowerbuds—cabbage, for instance—are opened artificially and pollen applied to the stigma. Bud pollination is used on self-compatible radishes. 

There is always a question about how pure a seed crop can be kept. Commercial plants that are easily cross-pollinated by insects and wind are heterozygous for some characteristics. They can’t be put into sealed cages, at least when grown on a large scale. 

The world has listened long and hard to science telling about hybrid vigors, about dwarf wheats to feed the world, about the much desired male sterility in plants—it prevents unwanted pollination—about those extra chromosomes in potatoes—tetraploid, meaning four, diploid, meaning two—about control of disease via genetics, about green beans with less pod fiber. There is even research afloat to produce a gasless bean. (There is such a bean, of course, one evolved by nature, but it yields poorly, and so far consumers have elected to endure pain in the intestines more than pain in the pocketbook.) In general, however, hybrids simply have less of a burden to carry—that is, reproduction of their species in the seed. This has accounted for more bins and bushels, often with greater protein and trace mineral deficiencies. 

Nevertheless, the hybrid concept has swept most of the agricultural scene. When D. F. Jones of Connecticut Agricultural Experiment Station developed the double cross corn concept, the door flew open. From that moment on seed breeders worked long and hard with high-yielding, single cross parents. In 1933 only 0.3% of the American corn acreage was planted in hybrid seeds. By 1960 this figure had jumped to 95%. It is probably in excess of 98% at this writing. 

Cytoplasmic male sterility hasn’t been limited to corn alone. It has reached out and touched pearl millet. Hybrid sorghum has largely replaced open pollinated varieties. Soybeans have largely replaced cowpeas for hay and green manure purposes, and hybrids became the name of the game during the 1930s—hybridization and backcrossing (followed by selection). 

Even the buffalo grass has not escaped genetic engineering. The plant has staminate and pistillate on separate plants. This specialty has been used to produce the hybrid Mesa. In other words, seed fields are interplanted with selected clones of two dioecious parents. 

All this is to simply illustrate that crop potential is determined largely before a farmer puts the first tool to the soil, even before he starts his seed bed and rootbed preparations. Many farmers hone close to low-priced seeds, seeds without the genetic potential needed to deliver a healthy, abundant crop. This is being penny wise and pound foolish, to use that folklore saying. 

Seed selection has to be made while the production year is still on the drawing board. Climate figures, so does land. Germination time must be understood. Temperatures that can be endured, and water requirements—all must be made part of the decision process. 

The decision is further complicated by the fact that most seeds can handle and germinate in a wide range of temperatures—usually slowly at the lower range, more rapidly as the soil comes alive at higher temperatures. Temperatures that run too high, on the other hand, may impede good germination.