The year 1900 provides an important but deceptively simple chronological milestone for analyzing the emergence of plant breeding science. Clearly it was an important year because three European biologists, Karl Correns, Hugo de Vries, and Erich von Tschermak, published papers that resurrected a study on hybridization in peas done over thirty years earlier by Gregor Mendel.
A casual glance at any textbook in plant breeding written after 1900 shows that Mendel's concepts now overwhelmingly provide the major framework for understanding plants and their behavior. Of particular importance were his notions of particulate factors governing inheritance, dominance and recessiveness, segregation of alleles, independent assortment of factors, and diagnosis of the existence of factors based on ratios of progeny classes in precise records of their occurrence by generation. To this Mendelian framework are appended a variety of other methods drawn from plant physiology, biometrics, soil science, plant anatomy, plant pathology, and other disciplines.
Mendel's contributions were crucial to the consolidation of what we now call "plant-breeding science." Indeed, it would be impossible to recognize the discipline in its modern form if Mendelian ideas were surgically removed from the tomes that instruct new students in the art and science of plant breeding. However, it is deceptively simplistic to focus too intensively on Mendel's "rediscovery" in 1900. Several lines of reasoning argue for caution lest we attribute overwhelming importance to Mendel.
First, a vigorous and successful group of people worked throughout the nineteenth century on the creation of new varieties by hybridization, as well as by the older method of selecting "good" cultivars from a heterogeneous mix grown in farmers' fields. Plant breeders also were active in moving seeds around the globe to test which cultivars did well in new locations. In sum, even without Mendelian concepts and methods, plant breeders saw themselves as a small but identifiable group of scientists with a mission.
The second set of reasons for moving cautiously in trying to understand the conceptual framework of plant breeding centers on what should be called the "silent" conceptual framework for plant breeding compared with the "overt" framework. Overtly, plant-breeding science is now based more than anything else on Mendelian concepts. Beneath the surface, however, is the silent or frequently unspoken concept: yield.
Yield is complex. It consists first and foremost of quantity. How much useful produce can be obtained per hectare per year from a piece of ground? Yield also must consider quality, however. To what uses can the produce be put? For wheat, is the grain useful for leavened bread, for pastries, or for cattle feed? For rice, is the grain sticky? Aromatic? For maize, are the seeds sweet or starchy? How hard is it? What color? Would you use it for tortillas, for cornbread, or for feeding chickens?
Closely related to yield quantity and quality is the concept of yield reliability. Given the climate, soil fertility, and presence of pest organisms, is it likely that the yields will be stable from year to year? A cultivar that gives yields that fluctuate greatly from one year to the next is likely to be considered unsuitable in terms of yield.
Contemporary plant-breeding scientists, when asked, are quite forward about the importance of yield to their discipline. In fact, a question about why yield is important is likely to bring the puzzled response of, "What else is there?" However, in contrast to clearly giving homage to Mendel and continually showing their direct intellectual debt to him, plant breeders are far less interested in explaining the origins of the centrality of yield in their discipline. Yet just as plant-breeding science in its current form would be unrecognizable if Mendelism were excised from its body, so, too, would the field as we know it cease to be if the presumption of yield as the primary objective were removed.
Reconstructing the roots of plant-breeding science before 1900, therefore, requires following two different threads and how they came together. First, we must be able to speak to the primacy of yields and its significance for the science. Second, we must understand the vigor, successes, and limitations of plant breeding in the first years of modern agriculture. Chapter 3 addresses when and how Mendelian ideas formed the overt conceptual framework of plant breeding and then examines the eclectic addition of other bits and pieces of scientific lore that plant breeders adopted into the heart of their knowledge.
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