Résumé
We analyzed the effect of short-term
water deficits at different periods of sunflower (Helianthus
annuus L.) leaf development on the spatial and temporal
patterns of tissue expansion and epidermal cell division. Six
water-deficit periods were imposed with similar and constant values of
soil water content, predawn leaf water potential and [ABA] in the
xylem sap, and with negligible reduction of the rate of photosynthesis.
Water deficit did not affect the duration of expansion and division.
Regardless of their timing, deficits reduced relative expansion rate by
36% and relative cell division rate by 39% (cells blocked at the
G0-G1 phase) in all positions within the leaf. However, reductions in
final leaf area and cell number in a given zone of the leaf largely
differed with the timing of deficit, with a maximum effect for earliest
deficits. Individual cell area was only affected during the periods
when division slowed down. These behaviors could be simulated in all
leaf zones and for all timings by assuming that water deficit affects
relative cell division rate and relative expansion rate independently,
and that leaf development in each zone follows a stable three-phase
pattern in which duration of each phase is stable if expressed in
thermal time (C. Granier and F. Tardieu [1998b] Plant Cell Environ
21: 695–703).