Plant growth and development are based on the capture and transformation of electromagnetic radiation by chlorophyll. The efficiency of photosynthesis depends upon the presence of sufficient amounts of water, mineral nutrients, carbon dioxide, and light; and it can be regulated by any factor which interferes with the availability or mixture of those items. Plant damage by pollutants, water stress, or pathogens usually results in a reduced rate of photosynthesis. If damage is severe, visible symptoms may be evident on the leaves or stems. However, a decreased photosynthetic rate may not be visually evident but can result in reduced production that may be recognized only at harvest or when ameliorative measures no longer help. Measurement of the depression of photosynthetic rates is, therefore, an important criterion in the evaluation of pollutant effects, water sufficiency, and other stress conditions on plants.