Effect of aging on growth and flowering of the terminal bud of Pharbitis nil

The tendency of the terminal bud of Pharbitis nil to flower after one long night was studied in the context of growth changes occurring with aging. Seedlings were grown under constant light and temperature, selected for uniformity and subjected to a 16-hr dark period at ages ranging from 3 to 14 days. A decline in percent terminals flowering was observed after day 6, to zero at day 10. Flowering of the terminal was obtained in some plants, thereafter. Floral index of development scored lower for terminals flowering on days 7-9 and 1 1-14 than on days 3-6. Loss of the tendency to flower was associated with a decline in rate of leaf production in the terminal bud and a change in rate of elongation of the epicotyl axis. The data lend support to a nutrient-diversion hypothesis in the control of flowering in Pharbitis. THE RELATIONSHIP between resource allocation, photosynthesis, and flowering is being reexamined (Wareing, 1979; Bernier, Kinet and Sachs, 1981; Sachs and Hackett, 1983; Bodson, 1984; Kinet and Sachs, 1984). Resource allocation may have a considerable influence on the flowering response (Sachs, 1977; Sachs and Hackett, 1977, 1983) and may be used to explain the general effects of chemicals or the environment on the regulation of developmental processes such as flowering (Sachs and Hackett, 1977; Sachs et al., 1979). According to the nutrient-diversion hypothesis, agents or events which reduce alternative sink activity or competitive demand should behave as promoters of flowering while promoters of alternative sink activity should behave as inhibitors of flowering (Sachs and Hackett, 1983). Of course, the agents causing these changes can be hormones. It has been demonstrated for Pharbitis nil that exogenous growth substances influence flowering by causing a redistribution of assimilates (Ogawa and King, 1979, 1980). Changing rates of axial growth or of production of appendages at the shoot apex may be related to or representative of changes in sink activity at these regions of a plant. Previous reports indicate that the relationship between stem growth and flowering is highly variable for Pharbitis (Zeevaart, 1964; Shinozaki, ' Received for publication 18 July 1985; revision accepted 20 November 1985. This research was supported by NSF Grant PCM7718954 and by the Cornell Univ. Minority Graduate Fellowship Program. This study represents a portion of a doctoral dissertation submitted to the Field of Botany, Cornell University, January 1984 by the senior author. 1972; Ogawa and King, 1980; Suge, 1980; Owens, 1984). But they also suggest that sink activity in the axial tissue may influence flowering (Marushige and Marushige, 1963; King and Evans, 1969). Changing leaf production has been examined as a function of aging in Pharbitis (Marushige and Marushige, 1963; Zeevaart, 1964; Wada, 1967; King and Evans, 1969). But its relationship to stem growth and flowering as a function of aging needs further clarification. In the present study, flowering of the terminal apices of seedlings of Pharbitis nil was compared to changes in growth of the epicotyl axis and to changes in the rate of leaf production, as a function of age. MATERIALS AND METHODS-The experiment described herein was designed after many preliminary investigations to establish methods and conditions. The main findings have been confirmed by a second experiment, with equal sampling intensity. Plants of Pharbitis nil Chois. strain Violet (Japanese Morning Glory), were grown from seeds purchased from the Marutane Seed Company, Ltd., Kyoto, Japan. Methods of scarification and imbibition were modified from Takimoto (1967). A batch of ca. 2,000 seeds was treated 55 min with concentrated H2SO4 followed by 15 min in running tap water and imbibition in aerated water (17 hr). Imbibed seeds were allowed to germinate and grow under constant light (ca. 3 x 103, W/cm2, cool white fluorescent light) and temperature (25 + 1C), at a relative humidity of 80%. The zero hour for germination was taken as the end of soaking in water. The seeds were placed hilum up on a 1 cm layer of soil that