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Closed and semi-closed plant growth chambers have long been used in studies of plant and crop physiology. These studies include the measurement of photosynthesis and transpiration via photosynthetic gas exchange. Unfortunately, other gaseous products of plant metabolism can accumulate in these chambers and cause artifacts in the measurements. The most important of these gaseous byproducts is the plant hormone ethylene (C 2 H 4). In spite of hundreds of manuscripts on ethylene, we still have a limited understanding of the synthesis rates throughout the plant life cycle. We also have a poor understanding of the sensitivity of intact, rapidly growing plants to ethylene. We know ethylene synthesis and sensitivity are infl uenced by both biotic and abiotic stresses, but such whole plant responses have not been accurately quantifi ed. Here we present an overview of basic studies on ethylene synthesis and sensitivity. Ethylene Sensitivity An analysis of ethylene sensitivity should start with a review of ambient levels. The technically correct SI unit for gas concentration in air is the mole fraction, expressed as moles of gas per mole of air (mol•mol-1). One ppm of a gas equals one micromole per mole of air, and one ppb equals one nanomole per mole of air (Table 1). Abeles (1992) cites ethylene levels have been reported as high as 500 nmol•mol-1 (500 ppb) in California and 700 nmol•mol-1 (700 ppb) in Washington D.C., primarily attributed to automobile exhaust. We have been continuously monitoring the ethylene concentration in the air above the Utah State University Research Greenhouse in Logan, Utah, for the past 2 years. Levels are typically below the detectable limit of our gas chromatograph (about 1 nmol•mol-1), but during calm periods with increased traffi c (7:45 to 8 AM) levels can increase to 1 to 2 nmol•mol-1. These measurements suggest that crop plants in rural areas are exposed to levels that average <1 nmol•mol-1 from anthropogenic ethylene emissions. Biogenic emissions of ethylene from intensive agricultural areas could expose crops to much higher levels of ethylene if the rate of synthesis was high and turbulent mixing with the atmosphere was limited. However, atmospheric turbulence on even calm days is suffi cient to keep ethylene levels within about 3 nmol•mol-1 of ambient even during periods of peak ethylene synthesis from stressed crops (assuming a high production rate of 10 nmol per kg per second). Our calculations suggest that the biogenic contribution to ethylene levels in the air around unstressed plants would be less than 0.03 nmol•mol-1 with a slight breeze. Levels from 50 to 100 nmol•mol-1 are common in greenhouses with heating or ventilation problems and have caused a broad range of crop damage in the horticulture industry (Blankenship and Kemble, 1996; Gibson et al., 2000; Mortensen, 1989). North Carolina State University provides helpful information on how to prevent C 2 H 4 problems in greenhouses and a service for checking air samples posted on the web at www.ces.ncsu.edu/depts/hort/greenhou...
Closed and semi-closed plant growth chambers have long been used in studies of plant and crop physiology. These studies include the measurement of photosynthesis and transpiration via photosynthetic gas exchange. Unfortunately, other gaseous products of plant metabolism can accumulate in these chambers and cause artifacts in the measurements. The most important of these gaseous byproducts is the plant hormone ethylene (C 2 H 4). In spite of hundreds of manuscripts on ethylene, we still have a limited understanding of the synthesis rates throughout the plant life cycle. We also have a poor understanding of the sensitivity of intact, rapidly growing plants to ethylene. We know ethylene synthesis and sensitivity are infl uenced by both biotic and abiotic stresses, but such whole plant responses have not been accurately quantifi ed. Here we present an overview of basic studies on ethylene synthesis and sensitivity. Ethylene Sensitivity An analysis of ethylene sensitivity should start with a review of ambient levels. The technically correct SI unit for gas concentration in air is the mole fraction, expressed as moles of gas per mole of air (mol•mol-1). One ppm of a gas equals one micromole per mole of air, and one ppb equals one nanomole per mole of air (Table 1). Abeles (1992) cites ethylene levels have been reported as high as 500 nmol•mol-1 (500 ppb) in California and 700 nmol•mol-1 (700 ppb) in Washington D.C., primarily attributed to automobile exhaust. We have been continuously monitoring the ethylene concentration in the air above the Utah State University Research Greenhouse in Logan, Utah, for the past 2 years. Levels are typically below the detectable limit of our gas chromatograph (about 1 nmol•mol-1), but during calm periods with increased traffi c (7:45 to 8 AM) levels can increase to 1 to 2 nmol•mol-1. These measurements suggest that crop plants in rural areas are exposed to levels that average <1 nmol•mol-1 from anthropogenic ethylene emissions. Biogenic emissions of ethylene from intensive agricultural areas could expose crops to much higher levels of ethylene if the rate of synthesis was high and turbulent mixing with the atmosphere was limited. However, atmospheric turbulence on even calm days is suffi cient to keep ethylene levels within about 3 nmol•mol-1 of ambient even during periods of peak ethylene synthesis from stressed crops (assuming a high production rate of 10 nmol per kg per second). Our calculations suggest that the biogenic contribution to ethylene levels in the air around unstressed plants would be less than 0.03 nmol•mol-1 with a slight breeze. Levels from 50 to 100 nmol•mol-1 are common in greenhouses with heating or ventilation problems and have caused a broad range of crop damage in the horticulture industry (Blankenship and Kemble, 1996; Gibson et al., 2000; Mortensen, 1989). North Carolina State University provides helpful information on how to prevent C 2 H 4 problems in greenhouses and a service for checking air samples posted on the web at www.ces.ncsu.edu/depts/hort/greenhou...
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