Many factors can affect the respiration rate of an intact plant or of its individual organs. Relevant factors include the species and growth habit of the plant, the type and age of the specific organ, and environmental variables such as the external oxygen concentration, temperature, and nutrient and water supply (see Chapter 25, Web Topic 11.7, and Web Essay 11.5).
Whole-plant respiration rates, particularly when considered on a fresh-weight basis, are generally lower than respiration rates reported for animal tissues. This difference is due in large part to the presence, in plant cells, of a large central vacuole and cell wall compartments, neither of which contains mitochondria. Nonetheless, respiration rates in some plant tissues are as high as those observed in actively respiring animal tissues, so the plant respiratory process is not inherently slower than respiration in animals. In fact, isolated plant mitochondria respire faster than mammalian mitochondria, when expressed on a per mg protein basis.
Even though plants generally have low respiration rates, the contribution of respiration to the overall carbon economy of the plant can be substantial (see Web Topic 11.7). Whereas only green tissues photosynthesize, all tissues respire, and they do so 24 hours a day. Even in photosyn-thetically active tissues, respiration, if integrated over the entire day, can represent a substantial fraction of gross photosynthesis. A survey of several herbaceous species indicated that 30 to 60% of the daily gain in photosynthetic carbon was lost to respiration, although these values tended to decrease in older plants (Lambers 1985).
Young trees lose roughly a third of their daily photo-synthate as respiration, and this loss can double in older trees as the ratio of photosynthetic to nonphotosynthetic tissue decreases. In tropical areas, 70 to 80% of the daily photosynthetic gain can be lost to respiration because of the high dark respiration rates associated with elevated night temperatures.
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