LEAF STOMATAL DENSITY AND DIFFUSIVE CONDUCTANCE IN THREE AMPHISTOMATOUS HYBRID POPLAR CULTIVARS

Stomatal density and diffusive conductance were characterized for both leaf surfaces in three amphistomatous hybrid cultivars of poplar. Consistent differences in stomatal density were observed between cultivars and between leaf surfaces within a cultivar. Mean stomatal density in the three cultivars ranged from 150 to 330 stomata mm⁻² for abaxial leaf surfaces and from 75 to 100 stomata mm⁻² for adaxial surfaces. The density of stomata on the abaxial versus adaxial surface was related to the spatial orientation of leaves with respect to the horizon and this stomatal ratio ranged from 1–4 to 4–0 in the three clones. Also, stomatal density was greater in leaves at higher rather than lower nodal positions. Differences in diffusive conductance between cultivars and leaf surfaces were observed on intact and detached leaves in the light and dark. Within each cultivar mean abaxial conductance (K_ab) was greater than adaxial conductance (K_ad). Mean conductances in the light for the three cultivars ranged from 0–22 to 0–62 cm s⁻¹ for abaxial, and from 0–15 to 0–17 cm s⁻¹ for adaxial surfaces, and in the dark they were between 0.06 and 0.26 cm s⁻¹ for abaxial, and from 0–04 to 0–06 for adaxial surfaces. The differences in conductance between cultivars and between leaf surfaces were correlated with their respective stomatal densities. Stomatal response to light and to leaf excision also varied between cultivars and between the two leaf surfaces.