Effect of aging on flowering of the axillary buds of Pharbitis nil

In Pharbitis seedlings, aging is associated with definite trends in the pattern of flowering of the axillaries: 1) the locus of maximum flowering is displaced continuously upward, 2) there is a continuous loss of responsiveness to induction from the base of the plant, upward, 3) flowering of the axillaries is suppressed in general when the terminal bud fails to flower, and 4) flowering at the distal nodes regresses when the terminal bud fails to flower. Phenomena I and 2 start at the base of the plant and move progressively upward, whereas 3 and 4 are tied to the possibly rhythmic responses of the terminal buds to floral induction, with aging. Buds at axils that form after induction are capable of flowering. Buds at nodes that flower maximally range in development from those not visible with a stereomicroscope to those with enlarging apices ready to form the first leaf primordia at the time of induction. Axillaries that have formed leaf primordia fail to flower in response to one inductive night. OUR PREVIOUS paper (Owens and Paolillo, 1986) was devoted to describing the pattern of flowering of the terminal bud of Pharbitis nil as a function of aging. In the present paper, the patterns of flowering at the axillary meristems are described, at seedling ages beyond those reported in the literature (King and Evans, 1969). The rate of activity in an axillary bud and events taking place there are related to the position of the axillary along the axis (McDaniel and Hsu, 1976; Bernier, Kinet and Sachs, 198 lb), possibly because position determines the stage of development of an axillary at the time a plant is induced to flower (Owens, 1984). The vegetative stage reached upon release from apical dominance can influence the floral patten (Goh, 1975). Axillary buds may be subject to many correlative influences, including "signals" from the terminal bud, leaves and roots (Phillips, 1975; Bernier, Kinet and Sachs, 198 la, b). Translocation of the floral stimulus is intimately associated with movement of assimilates in Pharbitis (King, Evans and Wardlaw, 1968; Ogawa and King, 1979), and dilution of the floral stimulus reaching an apex can inhibit flowering (Zeevaart, Brede and Cetas, 1977; ' Received for publication 18 July 1985; revision accepted 20 November 1985. This research was supported by NSF Grant PCM7718954 and by the Comell University Minority Graduate Fellowship Program. This study represents a portion of a doctoral dissertation submitted to the Field of Botany, Comell University, January 1984 by the senior author. Ogawa and King 1979). The expression of flowering also reflects changing sink activity of the nodes and remobilization of nutrients (Bernier et al., 1981b; Sachs and Hackett, 1983) even if interactions among hormones are involved (Sachs and Thimann, 1964; Phillips, 1975; Wareing, 1979). Pharbitis seedlings commonly respond to aging with a change in the locus of nodes bearing flowers. Flowering is displaced acropetally with aging (Imamura, 1967; King and Evans, 1969). The present report shows that some aspects of the pattern of flowering displayed by the axillaries are related to events taking place at the terminal bud, whereas others are not. MATERIALS AND METHODS-The materials and methods used in this study have been described in a previous paper (Owens and Paolillo, 1986). In this study, the first foliage leaf above the cotyledons is said to be attached at node 1. RESULTS AND DISCUSSION-Figure 1 visualizes the change in responsiveness of axillary buds to induction, as a function of position along the axis and as a function of aging. Four phenomena can be demonstrated directly from the data: 1) The locus of maximum flowering tends to migrate up the axis with aging, 2) there is an upward migration of loss of responsiveness with aging, 3) flowering of axillaries is suppressed when flowering at the terminals is suppressed, 4) flowering at the distal nodes regresses when few or no terminals flower. Phenomena I and 2 start at the base of the plant