Leaf expansion



Rana Munns


Increases in leaf area over time can be a more useful basis for measuring plant growth rates than biomass increases, particularly as non-destructive techniques for measuring leaf area are now available. Plant growth rate can be measured as the relative increase in leaf area over time, by substituting total plant leaf area for total biomass in the conventional RGR equation.

RLER = (ln LA2 – ln LA1 / (t2 – t1)

where RLER is relative leaf expansion rate, LA is total leaf area and t is time at two time intervals, t1 and t2, preferably 2-3 days apart.

Leaf growth is anatomically different in monocotyledonous and dicotyledonous plants. As shoot growth proceeds, dicotyledonous primordia undergo extensive cycles of cell division (peak doubling time about 0.5 days) emerging as recognisable leaves with petioles and lamina that unfold and expand. Lamina expansion follows a coordinated pattern of cell division and cell enlargement that is under genetic control but modi ed by the environment. Final leaf size and shape vary accordingly.

Primordia of monocotyledonous species bear a super cial resemblance to those of dicotyledonous species, but in grass leaves, all phases of cell growth occur in the unexposed lower part of leaves (the sheath) and is not exposed to the environment. Cell division is con ned to basal meristems which give rise to a zone of cell expansion. Cell division and cell enlargement proceed concurrently, but are separated spatially. The emerged blade therefore is composed of cells that are fully expanded, and the elongation of that leaf takes place by addition of fully expanded cells from below.

Measurements of leaf growth of a single leaf are of limited use in growth analysis. Whole plant growth should include all leaves including lateral shoots and tillers.

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