SPECTRAL SENSITIVITIES FOR LEAF AND STEM GROWTH OF ETIOLATED PEA SEEDLINGS AND THEIR SIMILARITY TO ACTION SPECTRA FOR PHOTOPERIODISM

LIGHT TAKES part in a number of distinct physiological reactions in plants. Among these are chlorophyll formation, photosynthesis, photoperiodism, phototropism, and cell elongation. Pigments are necessarily involved in the initial absorption of light in each process. Measurement of the effectiveness of different wave lengths of light gives information about the pigment and possible basic similarity of the initial steps in different physiological processes. This method is here applied to establish a relationship between growth of etiolated seedlings and floral initiation in photoperiodically responsive plants. Spectral sensitivities or action curves have been obtained for floral initiation in both longand short-day plants by Parker et al. (1946, 1948). The response to light shows that the same pigment or same class of pigments is effective in both tvpes of plants and that the most effective absorption is in the red portion of the spectrum. Effects of light on stem and leaf growth of etiolated pea seedlings have been measured by Went (1941). Here too the red portion of the visible spectrum is most effective, the response being an increase in leaf area and suppression of internode elongation, while the blue is relatively ineffective. The spectrographic arrangement of Parker et al. (1946), which affords high dispersion, large irradiation area, and high intensity of a given wave length region but with low intensities of other wave lengths as impurity, meets the necessary conditions for use in a detailed study of the action curve for etiolated peas. MATERIALS AND METHODS.-Spectrograph apparatus.-The description of the spectrograph which has previously been given in detail (Parker et al., 1946) is repeated here for immediate reference and to indicate required modifications. The energy source is the crater of a high-intensity arc with rotating cathode (fig. 1). A diaphragm essentially serving as the slit of the spectrograph is placed in the convergent beam from the condensing lens. The essential focusing element is a front-aluminized concave spherical mirror of 2-m. focal length, conjugate foci of which are the slit and focal plane. Two large glass prisms, set for minimum deviation at 5780 A, are placed in the convergent beam from the mirror and the emergent light is reflected from a flat front-silvered mirror that can be rotated by a tangent screw. In experiments with etiolated pea seedlings the instrument was used essentially as a monochromator to throw a narrow wave length region on a slit in the focal plane. This narrow band of the spectrum 1 Received for publication September 8, 1948. had to be spread out to permit radiatinig the seedlings in the 35 cm. boxes and to give variation of intensity. This was accomplished by use of an achromatic lens as shown in the diagram. The F7 lens had previously been used for collimation on a large Steinheil Spectrograph and had a focal length of 195 mm. A 5.5-m. I beam was used as an optical bench in the diverging beam from the achromatic lens. Boxes of peas could be placed along this beam at predetermined distances from the lens corresponding to given magnifications. The bench was housed in black cloth to eliminate all light except that entering through the lens. A side opening in the housing led to the dark room in which the peas were held before treatment and to which they were returned. Intensity could be reduced as much as 1000-fold by the divergence and transmission of the lens (68 per cent). It was further reduced when desired by introducing neutral screens (blackened wire gauze) of known transmissions between the slit and the concave mirror, a more than 100-fold factor being obtainable in this way. Wave length calibration was carried out by use of a low-pressure mercury arc to illuminate the slit. Energy values for definite wave bands were measured with a ten-junction thermopile that was standardized with an N. B. S. certified lamp. Spectrographic procedure.-An action curve can be considered as expressing the energy required to give a definite effect as a function of wave length. This can be obtained in several ways for response of etiolated pea seedlings. The method adopted required two different types of treatment. It avoided investigation of variation of response with duration of illuminations which is intrinsically of interest but not immediately pertinent to the objective. In the first instance the variation of response was measured at different energies given in constant time. A 4-min. daily interval for illumination on 4 successive days was adopted on the basis of previous work by Went (1941). Energies were varied by approximately 20,000-fold for a given wave length band. Sufficient points were taken to give the response, change in leaf length, as a function of energy. These measurements were made for five regions varying from the blue to the red portion of the spectrum. Experiments, were performed as follows. Three stations were established on the optical bench in the diverging radiation from the lens in the focal plane to give relative energies of 18.5, 4.30, and 1.0. Two boxes of peas were irradiated at each of these stations. Neutral screens were then placed in the light beams to reduce the intensity to 1/80 of