Résumé
We have investigated the spatial
distributions of expansion and cell cycle in sunflower
(Helianthus annuus L.) leaves located at two positions
on the stem, from leaf initiation to the end of expansion. Relative
expansion rate (RER) was analyzed by following the
deformation of a grid drawn on the lamina; relative division rate
(RDR) and flow-cytometry data were obtained in four
zones perpendicular to the midrib. Calculations for determining in situ
durations of the cell cycle and of S-G2-M in the epidermis are
proposed. Area and cell number of a given leaf zone increased
exponentially during the first two-thirds of the development duration.
RER and RDR were constant and similar in
all zones of a leaf and in all studied leaves during this period.
Reduction in RER occurred afterward with a tip-to-base
gradient and lagged behind that of RDR by 4 to 5 d
in all zones. After a long period of constancy, cell-cycle duration
increased rapidly and simultaneously within a leaf zone, with cells
blocked in the G0-G1 phase of the cycle. Cells that began their cycle
after the end of the period with exponential increase in cell number
could not finish it, suggesting that they abruptly lost 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.