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Articles

Growth, Pigment Synthesis, and Ultrastructural Responses of Phaseolus vulgaris L. cv. Blue Lake to Intermittent and Flashing Light ^1,2

Department of Botany, Duke University, Durham, North Carolina 27706

Growing bean plants (Phaseolus vulgaris L. cv. Blue Lake) on cycles of 1 minute light-1 minute dark or 5 minutes light-5 minutes dark, providing an integrated 12 hours light-12 hours dark per day for each set of plants, led to production after 21 days of new leaves low or lacking in chloroplast pigments. Subsequently, dry weight increase was sharply cut. Leaf area was affected by the light regimes after the second week of growth. By the fourth week, plants on the 1 minute light-1 minute dark cycle showed about one-half the leaf area of the controls. Shoot growth was favored over root growth to the greatest degree on the 1 minute light-1 minute dark regimes. Chlorophyll a/b ratios were close to 3.0 in all of the intermittent light regimes, but the total amounts of chlorophyll in milligrams per primary leaf were higher from day 9 to day 23 for the 12 hour light-12 hours dark controls than for other plants.

Although they produced chlorophyll, the plants receiving 1 or 2 milliseconds per second of light continued to lose weight at the same rate as the dark controls; thus, it is assumed there was no net photosynthesis. Plants receiving flashing light allocated significantly more food reserves from the seed to roots than did dark controls. Total chlorophyll formation was significantly accelerated by 2 milliseconds per second light. With 1 millisecond per second light, it took 5 days longer to achieve the same level of chlorophyll. After the 18th day, there was a steady decline in chlorophyll, b degrading more rapidly than a.

It is thought that several light-driven reactions are involved in the observed pigment synthesis, photosynthesis, food allocation, and growth of bean. Some of these reactions may be cyclic and others linear. Collectively, they must reach a harmonic point for normal metabolism and development to occur. Because time courses for each of these reactions are different, the intermittent and flashing light technique offers the possibility of individually studying some of the key light-driven reactions.

² Partial financial support for this research was provided by a Biomedical Research Support Sub-Grant administered by Duke University, and a National Science Foundation Grant DEB77-15845 to Dr. Boyd Strain for operation of the Duke Phytotron.