THE EFFECT OF COTYLEDON EXCISION ON THE GROWTH OF PEA SEEDLINGS

Pea plants grown under controlled conditions for 28 days were sampled nine times during this growth period. On each sampling day 80 plants were harvested. Cotyledons were excised from 40 plants and the root-shoot axis allowed to continue growth. The other 40 plants were divided into roots, shoots, and cotyledons; the dry weights of alcohol-water extracts and residues from these organs were determined. Shoot lengths, numbers of nodes, buds, flowers, and fruits of 28-day-old plants, whose cotyledons were excised at different times, were determined. These data indicate that cotyledon excision after the 13th day has no apparent effect on seedling growth, whereas excision before the 13th day reduces growth. Glucose-U-C'4 supplied to attached cotyledons of 6and 16-day-old seedlings resulted in no major difference in the distribution pattern of recovered label throughout the plant even though 16-day-old cotyledons were depleted of their normal endogenous food reserve. EXCISION of pea cotyledons has been one approach to studying the interdependency between the cotyledons and the axis tissues. Two areas of investigation have resulted from these studies: the dependency of the axis on the cotyledons in terms of vegetative development and flowering (Fries, 1954; Moore, 1964; Bain and Mercer, 1966b); and the dependency of the cotyledons on the axis in terms of enzymatic activity and subcellular organization (Varner, Balce, and Huang, 1963). Fries (1954) has examiined the effect of various culture media on decotyledonized pea seedlings grown in the dark. Baiin and Mercer (1966b) have discussed the dependency of the axis on the cotyledons of Pisum sativum L. 'Victory Freezer' in terms of three developmental phases and concluded that the young seedling continues to draw upon the reserve material in the cotyledon long after morphological development of the axis would indicate. Bain and Mercer (1966b), investigating the control which the axis exerts over changes in the cotyledons, observed that the presence of the axis was essential during the first 48 hr of germination for the development of the subcellular organization, apparent in cotyledonary cells still attached to axis tissue. Varner et al. (1963) observed that the mitochondria in cotyledons excised from the axis tissue before imbibition lose their phosphorylative function after 2 days and their oxygen absorption capacity after 3 days. Varner (1962) suggested that the funletional integrity of the mitochondria depends on a heatstable organic factor that moves into the cotyleI Received for publication 26 February 1968. 2 Present address: Radiation Biology Laboratorv, Smithsonian Institute, Washington, D.C. dons from the root-shoot axis sometime during the first 24 hr of germination. Once the signal has been received by the cotyledons the respiratory mechanism and the development of certain enzymatic activities proceed. If a small amount of the hypocotyl is left attached to the cotyledon when the axis tissue is dissected away from the dry cotyledons, the cotyledons retain their normal activities (Varner et al., 1963). Reported here are the effects cotyledon excision has on the growth of pea seedlings. Cotyledons were excised from seedlings of different ages. Regardless of when cotyledon excision occurred all plants were grown for 28 days. The effects of cotyledon excision on dry weight, shoot height, number of nodes, and degree of flowering were observed. Uniformly labelled glucose-CI4 was supplied to cotyledons of intact plants at two different ages, to determine the extent of utilization and of transport from cotyledon to axis. M\ATERIALS AND METHODS Pea seeds (Pisum sativum L. 'Alaska') obtained from the W. A. Burpee Co. were soaked 4 hr in distilled water at 20 C and germinated between sheets of moist paper towelling. On the 3rd day, seedlings with radicles 2 to 3 cm long were selected and transplanted inito flats containing washed quartz sand. The flats were placed in growth chambers (Percival #PGC 78) 60 cm from the light bank where the seedlings received 2,000 ft-c during a 16-hr photoperiod. The temperature was maintained at 20 C i 1 C. The seedlings were watered daily or as needed with a nutrient solution (Steward, Lyndon, and Barber, 1965). Sampling-Seedlings were harvested on the