Relationship of the Cobalt and Light Effects on Expansion of Etiolated Bean Leaf Disks

The expansion of etiolated bean leaf disks in the dark may be promoted by briefly exposing the disks to light. Expansion may also be increased by placing the disks on media containing cobaltous or nickelous salts (1) and the author has suggested a close relationship of the light and cobalt effects. Investigations concerning this relationship are reported herein. A full description of the test utilizing the etiolated leaf disks has been published (1) and is recorded elsewhere (2). It is summarized here. Disks, 5.0 mm. in diameter, are cut with a cork borer from the two simple leaves immediately above the cotyledons of etiolated Burpee Dwarf Stringless Greenpod bean plants. A section of a main lateral vein approximates a diameter in each disk. The plants are used seven to nine days after the sowing of the seeds in flats of sand in a darkroom maintained at a temperature of 25 ± 10 C. All manipulations involving the disks are performed in dim green light. Five milliliters of a test solution are added to a Petri dish containing three 9 cm. sheets of Whatman no. 1 filter paper. Ten disks, lower epidermis up, are placed on the pad of filter paper wetted with the test solution. The basal solution contains 3%c D-glucose by weight and KNO3 at an 0.08 M concentration. The pH of each solution is adjusted to 5.6. The Petri dishes are kept in a dark cabinet at 25 ± 10 C. The diameters, perpendicular to the segments of heavy veins, are remeasured at the desired times. In figure 1, the values for diameters of disks two days after light exposures are plotted against the length of time the disks were exposed to incandescent-filament light at an intensity of 200 foot-candles. The greatest increase per amount of light energy occurs with exposures of five minutes or less. Actually, as found in other experiments, the sharp rise of the curve occurs with exposures of one minute or less. The curve does not level off at a maximum value. It probably would continue upward with longer exposures since expansion of disks exposed to light at the beginning of several experiments was considerably further increased by another exposure the next day. Thus, the limiting condition affected by light is not completely eliminated by a single exposure to light. After all light treatments, expansion was nearly uniform in all directions.