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Chapter 32: 12-15

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Transportation of Nutrients II

WebLecture Topics

Plant Productivity

Plant cross-pollination resulting in hybrids is a normal part of nature, where plants growing side-by-side can exchange pollen carried by wind, insects, or animals. Where the chromosome patterns are compatible, hybrids may survive to adulthood to reproduce themselves, and where new characteristics enhance survivability, the hybrids may outcompete their parent plants and replace them in the local environment. This is natural genetic engineering, where plant DNA and chromsomes change as a result of natural processes.

Humans have domesticated and interbred plants for perhaps ten thousand years. Archaeological studies by plant geneticists in Mexico indicate a very rapid development of modern maize hybrids from teosinte grass and local grasses to create larger and larger ears. Variations of a species bred for particular characteristics are cultivars. Over time, human-directed hybridization created cultivars with larger fruit, larger yields, disease resistance, and drought resistance.

Genetic Engineering: Making better Plants

The demand for food in a world where the population is doubling about every 50 years has inspired plant biologists to look for new ways to improve plants by increasing individual plant productivity. One of the techniques used now is transgenic hybridization. This controversial method introduces genetic material from another species (bacteria, for example) into the plant's own genetic material. Transgenic crops inherit the bacterial resistance to particular herbicides, which can then be be used to control weeds or other undesirable plants in the crop's fields. Other characteristics introduced this way can include drought resistance, allowing the farmers to grow crops successfully in dryer climates, or disease resistance, reducing the need for expensive pesticides. Some transgenic crops have been developed to include vitamins or minerals not normally found in the species, improving nutrition for areas with limited food variety.

But these crops also bring challenges to traditional farming methods. Transgenic crops are often created by a company so that their seeds are infertile; the seeds produced at the end of the growing season cannot be used to plant crops the next year, forcing farmers to buy seed each year. Pollen from transgenic crops can be carried from one field to another non-transgenic crop, cross-pollenating that crop against the farmer's wishes. There is concern that as transgenic crop growth spreads, no "untainted" cultivars will remain. A business that actually owns the DNA sequence of the transgenic cultivar as an industrial asset can control the availability of the crop.

Symbiosis and Parasitism

Genetics are not the only factors that determine crop yield. We've already discussed organic material, minerals, soil particle size, and water, but soil also needs living organisms to help plants develop properly.

While the atmosphere is roughly 78% nitrogen, this nitrogen is in a triple-bonded gas form, N2, that cannot be broken down by plants, which require nitrogen to form amino acids and proteins, and the enzymes essential for metabolic processes like photosynthesis and cellular respiration. Healthy soil contains ammonifying and nitrifying bacteria strains. The ammonifying bacteria can break down N2 and water into separate elements, then reform the nitrogen and hydrogen to create ammonium ions NH4+ and release oxygen. The nitrifying bacteria can break down the ammonium and use the oxygen to create nitrates, NO3- ions, that the plant can use to make its amino acids. Biologists consider this relationship symbiotic: the plant attracts water that sustains the bacteria, and the bacteria provide ammonia and nitrates that the plant needs to grow. In particular, legumes form nodules or swelling structures along their roots to support nitrogen-fixing bacteria. Different species of beans and peas are associated with different strains of the Rhizobium bacterium species.

Plants have other symbiotic relations. One of the most important is the sybiotic association of plant roots and mycorrhiza, a species of fungus that can absorb minerals from the soil efficiently, and pass these on to the plant. As with the bacterium strains and legumes, plants that depend on mycorrhizae must be matched with the correct species for the relationship to be mutually beneficial.