Cell division was examined during leaf initiation in the
slow-to-green mutant of Lolium temulentum L. to
test the hypothesis that the cell cycle in the leaf primordium is a
key regulator of the well characterized reduction in final leaf
length in the mutant compared with that of the wild type. The cell
doubling time (cdt, by colchicine method) was substantially longer in
the youngest leaf primordium (YLP) of the mutant (107 h) than in the
wild type (43 h) although the duration of the most rapid cell cycle
(cc, by percentage labelled mitoses method) was between 18–20 h
in each. As a consequence, the proportion of rapidly proliferating
cells was only 20% in the mutant compared with 47% in the wild type.
The size of the shoot apical meristem and the plastochron were
similar between genotypes which indicates that the shoot meristem was
largely buffered from the effects of the mutation. Mitotic cell area
was also similar in the YLP of both genotypes. However, as the leaf
elongated, mitotic cell area and interphase cell size were
significantly larger in the mutant compared with the wild type. This
change was coupled with a reduced number of cells per unit length of
leaf in the mutant. The data are consistent in showing that the
proportion of rapidly proliferating cells in the YLP (but not the
rate of cell division) is a key parameter which influences growth of
the leaf.