Changes in stomatal frequency, stomatal conductance and cuticle thickness during leaf expansion in the broad-leaved evergreen species, Eucalyptus regnans

Aspects of leaf anatomical and physiological development were investigated in the broad-leaved evergreen species, Eucalyptus regnans F.Muell. Newly emergent leaves were tagged in the field and measured for stomatal conductance while a subset was collected every 14 days for the measurement of stomata and cuticle over a 113-day period. Cuticle thickness increased during leaf expansion, the increase following a sigmoid curve. Stomatal frequency (no. mm(-2)) decreased from 56 to 113 days after leaf emergence. The frequency of both immature and intermediate developmental stages of stomata also decreased over this time, but the total number of stomata per leaf remained relatively constant. Stomatal conductance (g(s)) of young expanding leaves increased during expansion, and was significantly linearly correlated with stomatal frequency (excluding immature stomata), and with cuticle thickness. The progressive increase in gs in young developing leaves was contrary to the observed changes in structural characteristics (increased cuticle thickness and decreased stomatal frequency). This increase in gs with development may be related to the progressive increase in number of mature stomata with larger apertures and, therefore, a higher total pore area in fully expanded leaves.