SPATIAL AND TEMPORAL ANALYSES OF EXPANSION AND CELL-CYCLE IN SUNFLOWER LEAVES - A COMMON PATTERN OF DEVELOPMENT FOR ALL ZONES OF A LEAF ANDDIFFERENT LEAVES OF A PLANT

We have investigated the spatial distributions of expansion and cell c ycle in sunflower (Helianthos annuus L.) leaves located at two positio ns on the stem, from leaf initiation to the end of expansion. Relative expansion rate (RER) was analyzed by following the deformation of a g rid drawn on the lamina; relative division rate (RDR) and flow-cytomet ry data were obtained in four zones perpendicular to the midrib. Calcu lations for determining in situ durations of the cell cycle and of S-C Z-M in the epidermis are proposed. Area and cell number of a given lea f zone increased exponentially during the first two-thirds of the deve lopment duration. RER and RDR were constant and similar in all zones o f a leaf and in all studied leaves during this period. Reduction in RE R occurred afterward with a tip-to-base gradient and lagged behind tha t of RDR by 4 to 5 d in all zones. After a long period of constancy, c ell-cycle duration increased rapidly and simultaneously within a leaf zone, with cells blocked in the G0-G1 phase of the cycle. Cells that b egan their cycle after the end of the period with exponential increase in cell number could not finish it, suggesting that they abruptly los t their competence to cross a critical step of the cycle. Differences in area and in cell number among zones of a leaf and among leaves of a plant essentially depended on the timing of two events, cessation of exponential expansion and of exponential division.