The Formation of Chlorophyll A in Etiolated Oat Seedlings

Abstract

The ratio of chlorophyll to chlorophyll b has been determined for the green tissues of many species of plants. Chlorophyll usually lies between 67% and 78% for the normal green tissues of higher land plants (2, 10, 11). In etiolated tissues, however, chlorophyll has been reported to be formed much more rapidly than b, when the plants are first exposed to light, regardless of the spectral region employed (8). Although not specifically commenting on this interesting phenomenon, Frank (3) has recently published spectrophotometric data indicating that the same thing holds true for completely dark-grown seedlings of Avena byzantium var. satwa, after five-hour exposures to light. It is possible that this early rapid production of chlorophyll may be correlated with the initial presence in dark-grown plants of relatively higher percentage of protochlorophyll a than of protochlorophyll &, condition which reportedly exists in pumpkin seeds (9). In studies on the production of chlorophyll in Avena sativa (5), the authors have observed an almost complete absence of chlorophyll b in chlorophyll extracts obtained from plants irradiated for two or three hours with visible light Husked seeds were laid out on wet absorbent cotton and germinated in complete darkness under controlled conditions of temperature and humidity. After two and half days, some of the seedlings were irradiated for two hours with monochromatic blue light (4358 ?) isolated with Corning polished glass filter #5113, 2 mm. thick, 5-cm. layer of 1.2% CuS04, and 5-cm. layer of 0.4% quinine hydrochloride in N/5 HCl. The transmissions of the liquid filters have been reported elsewhere (1). Radiation intensity at the level of the plants was approximately 5.5 ergs per mm.2 per second. The shoots of the irradiated seedlings and of dark-grown controls were then removed just below the coleoptilar node and ground in pure acetone. The acetone extract was centrifuged and the pigments transferred to ethyl ether in separatory funnel. The acetone was removed by successive washings with distilled water to which trace of calcium carbonate had been added. Extractions were carried out in very dim light and the extracts stored in the dark for two days over solid carbon dioxide, until spectroscopic studies could be made.