Making Plants, Part 1: Sow What?

By David Harry, Ph.D.

Many of us take for granted the origins of common items we use or consume on a daily basis. Consider food, clothing, and paper products. Many of these items are derived directly from plants, or from animals which in turn depend on plant-based feeds. Clearly, we owe a great deal to plants and agriculture. But what is the origin of these plants we depend on? LettuceWhile many plants are reproduced from seeds, a thorough consideration of this topic is somewhat more complicated. (Or why else would I be writing about it?) As a biologist working to commercialize pongamia trees as an oilseed crop, I expend considerable thought evaluating alternative approaches for propagating and distributing plant materials. Seeds are but one of several options. Furthermore, there is a renewed interest in broader philosophical and socio-political questions around the marketing and patenting of seeds and other types of germplasm (cuttings, tubers, bulbs, plantlets, etc.) in commercial agriculture. Questioning current practices is often an effective way to increase overall awareness, but alternative approaches that superficially appear simple may be impractical or otherwise more complicated than they might seem.

This is the first of two blog postings in which I take a closer look at how crop plants are propagated, disseminated, and ultimately grown. In particular, I’ll compare and contrast the use of sexually produced seeds vs. asexual propagules such as bulbs, cuttings or grafts. How do such approaches differ, and what are their relative advantages and disadvantages? I’ll focus on seeds in this installment, and take up asexual methods next time.

Plants, Seeds, Or?
Many factors can influence choosing one type of propagule over another (seed, starter plant, etc.). Certainly one important factor is operational scale. You likely know someone (perhaps yourself) who enjoys puttering with landscaping in their yard, or perhaps someone whose goal is to produce the perfect juicy tomato. Smaller-scale farmers may take pride in producing a substantive proportion of their own food. Other smaller operators may sell produce in local farmers markets, or perhaps open their farm gates to the public by offering U-pick sweet-corn, berries, or miscellaneous fruits. Larger farming operations might specialize by growing a few crops for sale to co-ops, food processors, or elsewhere. A common thread among all these operations, regardless of their scale, is the need to cultivate plants. Where do all these plants come from?

Larger farms growing annual crops might work exclusively using seeds. Many crops such as corn, beans, soybeans, sorghum, and others, are readily grown from sown seeds BeanSeeds(e.g. drilled, broadcast, or burrowed into rows). Other crops such as lettuce or cucurbits (melons, cucumbers, pumpkins etc.) might be started in flats and later transplanted as individual starts. On the other hand, smaller operations such as hobbyists and backyard gardeners may chose convenience over price and purchase starter plants from a local nursery. Specifics are likely to vary from farm to farm depending on goals, local climate, facilities, and labor.

Most crop plants can be loosely classified as annual, biennial, or perennial, corresponding to plants completing their life cycle in one, two, or multiple years. (Flowering bulbs, like tulips, may seem to blur these boundaries. Because the bulbs are long-lived, such plants are considered perennials.) Many annual and biennial plants are grown from seeds, which for many species are nature’s way of initiating the next generation of plants. Propagation from seeds can be an ideal and economically efficient mechanism in the right circumstances, with radishes, beans, corn and grains serving as good examples.

Seedlings of some plants, like tomatoes, are initially grown in a protected environment before being transplanted into the field. Such extra efforts are often taken to overcome environmental limitations or length of growing season. Tomato seeds and young plants prefer warmer conditions, with transplanted starters providing a head-start for warmer weather later in the season. For hobbyists, it may simply make sense to pay a bit more for a starter plant rather than take on the extra effort of germinating seeds. On the other hand, larger-scale operators may germinate seeds in trays and produce their own starter plants. Plant productivity is another consideration. Healthy tomato plants can be highly productive, so backyard hobbyists may require only few plants to satiate their craving for fresh tomatoes. On the other hand, root plants such as radishes produce only one (typically small) edible unit per plant. I don’t believe I’ve ever encountered starter plants for radishes!

Historical Seed Production
Seeds are the most common vehicle for reproducing annual and biennial crop plants. (Potatoes, vegetatively propagated from tubers, are an exception in that they are grown as annuals, but are in fact perennial). Most seeds are purchased from seed suppliers, and yet all seeds are not equal. Over recent decades the pathway from seed provider to farmer has grown more complex.

Until the first half of the twentieth century, seeds for subsequent crops were typically collected by setting aside selected fruits, allowing them to fully ripen, and finally harvesting and processing mature seeds. Some farmers produced their own seeds, while others purchased or exchanged seeds with neighbors, or perhaps purchased seeds from smaller companies. Larger seed companies, at least as we know them today, did not yet exist. Farmers recognized and utilized different locally adapted strains (i.e. landraces), although they did not understand how the landraces had developed.

The science of plant breeding was in its infancy (see Kingsbury, 2009, Hybrid: The History & Science of Plant Breeding). By the1800’s, a small number of pioneering breeders and scientists were experimenting with interbreeding (crossing) plants by transferring pollen from one plant onto flowers of another. Some workers began stretching the boundaries of interbreeding by experimenting with hybridization, initially using “wide-crosses” between different races or in some cases between individuals of different species. (Luther Burbank, 1849-1926, is among the better known pioneers of hybridization).

The science of genetics, describing the inheritance of genes from parents to offspring, was born in the early 1900’s and quickly lead to the development and early commercialization of hybrid corn, beginning in the 1920s. The rate of adoption of hybrid corn EGE-USDA-Corn-Hybrid-2was astounding, growing from less than 10% of Iowa corn in 1935 to over 90% in 1939 (Crow, 1998). While enormously successful, the development of hybrid varieties of an outbreeding (or cross pollinating) crop like corn first requires creating inbred lines—a major undertaking of time and resources. Without going into specifics, hybrid varieties offer several advantages over non-hybrids: increased uniformity and year-to-year repeatability. But these advantages come at a cost to farmers—hybrids do not breed true, meaning that seeds from hybrid plants grow into offspring that are highly variable and therefore commercially undesirable. In short, each crop of hybrid plants must be replenished by sowing a freshly created generation of hybrid seeds. As farmers fully recognize, seed-saving from hybrid crops is not practical and farmers must purchase new hybrid seeds each year.

Modern Seed Production
Plant breeding, the development of new plant varieties, and the commercial distribution of seeds have changed substantially since hybrid corn was first commercialized in the 1920s. The success of hybrid corn prompted the development of hybrid varieties in many other crops. As technologies evolved, structural changes within the plant breeding industry also took place. Today’s breeding industry has undergone substantial consolidation as smaller seed companies were acquired by larger players such as Monsanto, DuPont Pioneer, and Syngenta. Only some of these changes were driven by the implementation of genetic engineering (GMO) technology, since to-date relatively few GMO crops have been commercialized (e.g. corn, soybeans, cotton, alfalfa, sugar beets and papaya). Large seed companies command a substantial market share among large farming operations, but there nevertheless remains a smaller (and growing) market for seeds targeted to smaller farmers and home gardeners. For example, along with growth of farmers markets and a renewed interest in locally-produced food, there has been a resurgence of interest in open-pollinated and heirloom vegetables, particularly among people favoring more traditional approaches to agriculture.

Not all seeds are equal. The following list illustrates the types of descriptions someone might encounter while flipping through their favorite seed catalog.

  • Hybrid (or F1): a variety produced by intercrossing two different parental varieties. These are often inbred lines of the same species, but in some instances could be from different species. Hybrids are often more vigorous and uniform than their non-hybrid counterparts. Hybrid varieties must be produced afresh each generation because hybrid parents do not “breed true,” their offspring are highly variable.
  • Open pollinated: This designation has no strict definition except that it encompasses plants that were pollinated naturally, without any assistance by people. Wind, insects, bees, hummingbirds, etc., are all natural pollinators. Plants that are open pollinated often include among their offspring a mix of individuals from self-pollinations (both male and female gametes originated from the seed parent) and others from outcrosses (female gamete contributed by the seed parent whereas the pollen originated from a second individual). Because of the genetic mixing that can occur within open pollinated varieties, they tend to exhibit less uniformity than other types of varieties. Furthermore, in order for open pollinated varieties to reproduce faithfully (remaining true-to-type), they should be isolated from related varieties to minimize the movement of stray pollen. Further complicating the make-up of this group, naturally selfing plants such as beans and peas are often included (see below).
  • Selfing variety: Selfing varieties include plants whose offspring are usually produced by self-pollinations, in which male gametes from the seed parent effect fertilization. Often, such pollinations occur within the same flower, sometimes before it even opens. Selfing varieties tend to “breed true,” meaning their offspring share many if not all attributes of the parental generation, even when planted near other selfing varieties. Pollen movement is generally of little concern. For a hobbyist gardener wishing to produce their own seeds for subsequent crops, selfing plants are ideal. Selfing varieties are typically not labeled as such (and are often unlabeled), but may be grouped in the broad category of open pollinated varieties.
  • Heirloom variety: These tend to be older varieties with historical significance or reputations. They are not hybrids, and instead are reproduced as open pollinated or selfed varieties. Over time, heirloom varieties from various producers may gradually drift apart, and in some instances, may receive a new designation (e.g. brandywine red vs. brandywine pink).

Concluding Remarks
Not all seeds are the same, and some types of seeds are better suited for certain purposes than others. A hobbyist gardener may give these differences little if any thought if their goal is simply to grow some fresh vegetables in the summer months. Nevertheless seed producers, represented as larger commercial producers, heirloom producers, or backyard seed savers, should all be aware of the differences and manage their seed crops accordingly. Differences in reproductive biology among plants and plant varieties can have major impacts on how seedling-derived offspring grow and yield. Perennial plants are subject to similar constraints and concerns, but their longer life-span (and often larger size) make it feasible to consider alternative propagation strategies. This will be the subject of my next installment. Stay tuned!

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