Light Dosage and Phototropic Responses of Corn and Oat Coleoptiles

Abstract

For a number of years, it has been assumed that the Bunsen-Roscoe reciprocity law is valid for phototropic curvature (21, 30). This law states that if light intensity and time of exposure are varied in such a way that the product of the two remains constant, the photochemical effect of the light should also remain constant (13). Thus it was thought that if plants were exposed to unilateral light, a uniform curvature response could be obtained, all other conditions being constant, provided only that the product of light intensity and exposure time remained the same. Indeed, Fr6schel (19) and Blaauw (4) showed the reciprocity law to be valid over a wide range of intensities for Lepidium seedlings and Avena coleoptiles, respectively. However, their investigations were limited to the low dosages necessary to elicit the first measurable response. Similarly Fuller and Thuente (20) confirmed the law for a number of plants, again only for low dosages. Haig (23) subsequently confirmed Blaauw's results with Avena coleoptiles using unilateral violet light and low dosages. Numerous other examples are discussed by Du Buy and Nuernbergk (16). Thus the reciprocity law is generally accepted for phototropism (21, 30) despite the fact that confirmation at high dosages has not been obtained. Indeed, Arisz (1) clearly demonstrated that the law is not valid for negative curvatures; he suggested that it is only of limited applicability, and Bremekamp (10) concluded that it was valid neither for negative curvature nor for curvature (see below). There are references, furthermore, indicating that the law is not necessarily valid either if the stimulus is intermittent rather than continuous (14, 22) or if it is administered in anything but a strictly unilateral fashion (1, 10, 16, 28). These various exceptions will be considered in the discussion. In 1934, Du Buy and Nuernbergk (15) suggested a phototropic dosage-response curve for Avena coleoptiles. From their results, Went and Thimann (33) plotted curvature response to unilateral light against the logarithm of the dosage of white light' in meter-candle-seconds (MCS), and obtained the graph reproduced in figure 1. The three response peaks are usually designated first, second, and third positive curvature; and the valley between first and second positive curvature, which is actually negative, is designated first negative curvature. It should be pointed out that Went and Thimann's curve is actually a simplification of the data of Du Buy and Nuernbergk, since reaction times following the beginning of the light stimulus were not indicated, and were not the same for all dosages. Thus the absolute magnitude of curvature is less significant for the present consideration than the direction of curvature with respect to light dosage. Preliminary experiments with corn coleoptiles (11) indicated that the reciprocity law is not valid over the complete range of light dosages indicated in the Du Buy and Nuernbergk curve (fig 1). Above a certain dosage (1,000 MCS) the law fails at high intensities and the magnitude of the curvature response decreases. As the dosage is progressively increased above this value, failure of the law becomes increasingly drastic. The present study wilt describe results of experiments investigating phototropic reciprocity relationships of Avena and Zea coleoptiles over a wide range of light dosages. The bearing of these results on the shape of the phototropic dosageresponse curve will be considered, and experiments suggesting possible reasons for the failure of the reciprocity law at intermediate and high dosages will be discussed. Zea coleoptiles were selected for the majority of the experiments described below since a certain amount of information was already available concerning their auxin relationships during phototropism (12). These coleoptiles are more suitable for auxin diffusion studies than those of Avena since they are larger in cross-sectional area, and consequently easier to manipulate. Phototropic auxin diffusion studies in addition to those mentioned above have been made and will be discussed in detail in another paper.