THE PHOTOINHIBITION OF GROWTH IN ETIOLATED STEM SEGMENTS. II. GROWTH CAUSED BY COBALT IN PISUM

BERTSCH, WALTER F., and WILLIAM S. HILLMAN. (Yale U., New Haven, Conn.) The photoinhibition of growth in etiolated stem segments. I. Growth caused by sugars in Pisum. Amer. Jour. Bot. 48(6): 504-511. Illus. 1961.-Etiolated pea stem segments were used to study the red, far-red reversible photoinhibition of stem growth. The growth of immature, apical tissue was insensitive to light after excision from the seedling, unless a growth-promoting sugar (sucrose, glucose, fructose) was included in the medium. Red light specifically inhibited the growth caused by the sugars. Growth promotions of the same magnitude caused by indoleacetic acid or gibberellic acid were not reduced by irradiation of the tissue. The effects of various other sugars were tested. Arabinose and galactose, which do not promote growth, did not cause photosensitive growth in the isolated tissue. Mannose and xylose promoted growth at low concentrations, and this small promotion was inhibited by irradiation. Photoinhibition occurred even at inhibitory concentrations of mannose and xylose. The distinctness of light-sensitive growth is discussed, and the photoinhibition of a growth process which is limited by availability of sugars is suggested. THE PHOTOCONTROL of etiolation in higher plants is exerted at least partly through the red, far-red reversible photoreaction (Downs, 1955; Withrow, Klein and Elstad, 1957; Fujii, 1957; Thomson, 1959). Experimental systems involving excised tissues should be valuable for studying the processes that link the photoreaction itself to the final effects on growth. Most such studies have been conducted with tissues in which red light promotes growth (e.g., Liverman, Johnson and Starr, 1955; Klein, 1959); the results below, however, were obtained with etiolated pea stem segments in which red light inhibits growth under suitable conditions (Hillman, 1959). They indicate that the photosensitivity of the growth of such tissue depends on the composition of the medium used and that it is not the same if different substances are used to promote the same amount of growth. The photosensitivity of growth caused by sugars is the subject of the present paper; subsequent papers (Bertsch, in preparation) will deal in detail with the effects of metal ions and of indoleacetic and gibberellic acids in the same system. MATERIALS AND METHODS-Plant growth conditions-Seeds of Pisum sativum L., variety 'Alaska,' were obtained from Asgrow Inc., of New Haven, 1Received for publication January 10, 1961. 2Represents a portion of a dissertation presented by W.F.B. to the Faculty of the Graduate School of Yale University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. W.F.B. was supported by pre-doctoral fellowship CF-9318 of the National Cancer Institute, U. S. Public Health Service; by a Yale University fellowship; and by funds provided to Yale University by the American Cancer Society Institutional Grant. W.S.H. was supported by National Science Foundation grant G-4433 to A. W. Galston. The authors are indebted to Professor Galston for his advice and criticism. 3Present address: Laboratoire du Phytotron, Gif-surYvette (Seine-et-Oise), France. Connecticut. They were soaked for 3-4 hr in tap water, sown in washed vermiculite (Mica-Gro Type B, supplied by Platt Seed Co., Branford, Connecticut), and grown in plastic flats in darkness at 2628 C. The developing seedlings were protected from the darkroom safe-lights by black cloth, and exposed only when the cloth was lifted to inspect or remove the plants. All manipulations were performed under dim-green safe-lights consisting of green fluorescent tubes wrapped with 4 layers of dark-green and 6 layers of amber cellophane. Selection of seedlings and preparation of tissueSeedlings were allowed to develop until a majority of the third internodes were about 30 mm long (7-8 days). Pea seedlings of the variety 'Alaska' obtained from the Asgrow Company developed a recurved hook when grown in darkness at temperatures between 26 and 28 C. In order to keep the material as homogeneous as possible, sections were harvested only from seedlings which had an apical curvature of 220?-280?, and third internodes from 15 mm to 65 mm long. Figure 1 is a picture comparing "acceptable" and "unacceptable" seedlings which were grown in the same flat. Seedlings were harvested by cutting above the vermiculite with a razor blade. A 5-mm segment was cut just below the apical hook of those seedlings which met the above requirements, and placed in buffer (0.02 M P04; KH2PO4-Na2HPO4, pH 6.0) in a shaded Petri dish. The device used to cut the segments was a lucite block with removable razor blades placed 5 mm apart. In certain experiments, tissues other than the apical 5 mm were used, and in these instances the particular tissue will be described. After enough segments had been cut, they were randomized by mixing in buffer. Appropriate ir-