Stereological study of ultrastructural changes in the shoot apical meristem of Nicotiana tabacum during floral induction

Shoot apices of a short-day sensitive line of Nicotiana tabacum L. cv. NCTG-22 have been examined by electron microscopy for ultrastructural changes which occurred in the central zone over a 17-day period during the transition from vegetative to reproductive growth. Plants were grown in controlled-environment chambers ofthe NCSU Phytotron and exposed to an inductive photoperiod after a 6-wk juvenile phase. Ultrastructural changes were investigated from photomicrographs using a semi-automatic stereological procedure and a microcomputer. After exposure to only one inductive cycle cell and nuclear cross sectional areas in short-day plants were significantly larger than in long-day controls. Subsequently, under short days cross sectional areas of cells, nuclei, vacuoles and proplastids decreased, while mitochondrial cross sectional area and relative volume increased during the course of the induction period. In induced apices as cross sectional areas and relative volumes of vacuoles and proplastids decreased, their profile numbers increased. The reduction in cross sectional areas of cells and most organelles was associated with an increase in rate of leaf initiation and size of the apical dome. The demand for sufficient energy input to maintain the surge in growth and activity preceding floral initiation was reflected by the significant increases in cross sectional area, profile numbers and density of the mitochondria population. Even though the transition period is quite long for Nicotiana, cells and organelles in the central zone were observed to progress through similar changes in morphology that are known to occur in Sinapis and Xanthium which exhibit a more rapid and absolute response to photo-induction. IN PLANTS of Nicotiana tabacum, the time of floral initiation is of considerable economic importance since leaf number becomes limited to the number present at the time of evocation (Bonnand, 1956, 1959; Hopkinson and Hannam, 1969; Thomas et al., 1975). It has been shown for some species that quantitative information on ultrastructural changes in cellular and organelle composition of the shoot apex as it progresses from the vegetative to reproductive stage of growth is useful in the development of a better understanding of the flowering process (Gifford and Stewart, 1965; Havelange and Bemier, 1974; Havelange, Bernier, and Jacqmard, 1974; Lin and Gifford, 1976; Auderset and Greppin, 1977; Havelange, 1980). Furthermore, such evidence is essential for advances in ability to manipulate I Received for publication 15 October 1985; revision accepted 8 July 1986. Paper No. 10152 of the Journal series of the North Carolina Agricultural Research Service, Raleigh, NC. This investigation was supported, in part, by a grant from the North Carolina Tobacco Foundation, and from the Center for Electron Microscopy of the School of Agriculture and Life Sciences, North Carolina State Univ. We thank Dr. E. A. Wernsman for providing the seeds used in this investigation, and Dr. C. G. Van Dyke for assistance in the SEM preparations. 2 Address correspondence to this author. this event (Bemier, Kinet and Sachs, 1981). Most of the studies on flowering physiology have typically used absolute and rapidly transforming photoperiodic species, e.g., Sinapis alba (Bernier, Bronchart and Kinet, 1970; Bernier et al., 1974; Havelange et al., 1974; Bodson, 1975), Xanthium strumarium (Jacqmard et al., 1976; Havelange, 1980), Chenopodium album (Gifford and Tepper, 1962; Gifford and Stewart, 1965; Gifford and Corson, 1971). The flue-cured cultivars of Nicotiana tabacum, however, exhibit a quantitative response to photoperiod, and for these plants the floral transition is not as rapid as in absolute species (Waterkeyn, Martens and Nitsch, 1965; Hopkinson and Hannam, 1969; Thomas et al., 1975). Stereological methods are currently available for accurate measurement of differences in the ultrastructural composition of cells in different parts of the shoot apex (Mauseth and Niklas, 1979; Mauseth, 198 1a, b, 1982). These methods permit the calculation of the areal fraction of a section occupied by the individual cells and organelles seen on photomicrographs, the fraction of the tissue volume occupied by those organelles, and also the numbers of organelle profiles per unit of cross sectional area or volume (Weibel, Kistler and Scherle, 1966;