Response of the Hypocotyl Hook of Bean Seedlings to Radiant Energy and Other Factors

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Abstract
The effect of red radiant energy on the hypocotyl hook of the bean seedling was observed in relation to a series of environmental factors including age, temperature, culture medium, irradiance and indoleacetic acid (IAA) concentration. A highly reproducible bioassay based on the rate of angular opening of the excised hypocotyl hook is described. The photo-sensitivity increased with age, and the rate had a temperature optimum of 30[degree]C in darkness and 25[degree]C when exposed to 0.1 [mu]w/cm2 of red radiant energy (625-700 m[mu]). In darkness at 25[degree]C, no change in angle occurred in the first 20 hours; thereafter, the hook opened slowly. Red radiant energy caused the hook to begin to open rapidly within a few hours. The integrated rate over the first 20 hours was proportional to the logarithm of the incident energy with about 20[degree] opening for each ten-fold increase in continuous irradiance between .001 ([mu]w/cm2 and 10 [mu]w/cm^. Excised hooks in distilled water produced a more uniform and greater response than those in solutions of sugars or any inorganic material tested. IAA caused hooks kept in complete darkness to close and produced negative values of hook opening, thus opposing the effect of the photoreaction. IAA did not alter the slope or linearity of the response to log-energy.