Many environmental factors can alter the operation of metabolic pathways and respiratory rates. Here we will examine the roles of environmental oxygen (O2), temperature, and carbon dioxide (CO2).
Oxygen. Oxygen can affect plant respiration because of its role as a substrate in the overall process. At 25°C, the equilibrium concentration of O2 in an air-saturated (21% O2), aqueous solution is about 250 |M. The Km value for oxygen in the reaction catalyzed by cytochrome c oxidase is well below 1 | M, so there should be no apparent dependence of the respiration rate on external O2 concentrations (see Chapter 2 on the web site for a discussion of Km). However, respiration rates decrease if the atmospheric oxygen concentration is below 5% for whole tissues or below 2 to 3% for tissue slices. These findings show that oxygen diffusion through the aqueous phase in the tissue imposes a limitation on plant respiration.
The diffusion limitation imposed by an aqueous phase emphasizes the importance of the intercellular air spaces found in plant tissues for oxygen availability in the mitochondria. If there were no gaseous diffusion pathway throughout the plant, the cellular respiration rates of many plants would be limited by an insufficient oxygen supply (see Web Essay 11.3).
Water saturation/low O2. Diffusion limitation is even more significant when plant organs are growing in an aqueous medium. When plants are grown hydroponically, the solutions must be aerated vigorously to keep oxygen levels high in the vicinity of the roots. The problem of oxygen supply also arises with plants growing in very wet or flooded soils (see Chapter 25).
Some plants, particularly trees, have a restricted geographic distribution because of the need to maintain a supply of oxygen to their roots. For instance, dogwood and tulip tree poplar can survive only in well-drained, aerated soils because their roots cannot tolerate more than a limited exposure to a flooded condition. On the other hand, many plant species are adapted to grow in flooded soils. Herbaceous species such as rice and sunflower often rely on a network of intercellular air spaces (aerenchyma) running from the leaves to the roots to provide a continuous, gaseous pathway for the movement of oxygen to the flooded roots.
Limitation in oxygen supply can be more severe for trees having very deep roots that grow in wet soils. Such roots must survive on anaerobic (fermentative) metabolism or develop structures that facilitate the movement of oxygen to the roots. Examples of such structures are outgrowths of the roots, called pneumatophores, that protrude out of the water and provide a gaseous pathway for oxygen diffusion into the roots. Pneumatophores are found in Avicennia and Rhizophora, trees that grow in mangrove swamps under continuously flooded conditions.
Temperature. Respiration typically increases with temperature (see, however, Web Essay 11.3). Between 0 and 30°C, the increase in respiration rate for every 10°C increase in ambient temperature (commonly referred to as the dimensionless, temperature coefficient, Q^) is about 2. Above 30°C the respiration rate often increases more slowly, reaches a plateau at 40 to 50°C and decreases at even higher temperatures. High night temperatures are thought to account for the high respiratory rates of tropical plants.
Low temperatures are utilized to retard postharvest respiration rates during the storage of fruits and vegetables. However, complications may arise from such storage. For instance, when potato tubers are stored at temperatures above 10°C, respiration and ancillary metabolic activities are sufficient to allow sprouting. Below 5°C, respiration rates and sprouting are reduced in most tissues, but the breakdown of stored starch and its conversion to sucrose impart an unwanted sweetness to the tubers. As a compromise, potatoes are stored at 7 to 9°C, which prevents the breakdown of starch while minimizing respiration and germination.
CO2 concentration. It is common practice in the commercial storage of fruits to take advantage of the effects of atmospheric oxygen and temperature on respiration, and to store fruits at low temperatures under 2 to 3% oxygen and 3 to 5% CO2. The reduced temperature lowers the respiration rate, as does the reduced oxygen. Low levels of oxygen are used instead of anoxic conditions to avoid lowering tissue oxygen tensions to the point that stimulates fermentative metabolism.
Carbon dioxide has a limited direct inhibitory effect on the respiration rate at a concentration of 3 to 5%, which is well in excess of the 0.036% (360 ppm) normally found in the atmosphere. The atmospheric CO2 concentration is increasing rapidly as a result of human activities, and it is projected to double, to 700 ppm, before the end of the twenty-first century (see Chapter 9).
Compared to plants grown at 350 ppm CO2, plants grown at 700 ppm CO2 have been reported to have a 15 to 20% slower dark respiration rate (on a dry-weight basis) (Drake et al. 1999), but this has been questioned (Jahnke 2001; Bruhn et al. 2002). The number of mitochondria per unit cell area actually doubles in the high CO2 environment. These data imply that the respiratory activity in the light instead may increase at higher ambient CO2 concentrations (Griffin et al. 2001). Thus it is presently a matter of debate how plants growing at an increased CO2 concentration will contribute to the global carbon cycle.
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