Abaxial and adaxial stomatal density, stomatal conductances and water status of bean primary leaves as affected by paclobutrazol

The plant growth retardant, paclobutrazol at 8.5 or 17.0 muM concentrations effectively inhibited the stem elongation and primary leaf expansion of bean seedlings. Although the Tetardant reduced the relative water content in well-watered plants, the water and pressure potentials remained high in the primary leaves. K+, Na+, Mg2+ and Ca2+ contents in the primary leaves of the paclobutrazol-treated plants were not significantly different from those in the control. The stomatal density increased on both surfaces but the length of guard cells was not reduced significantly on the adaxial epidermes of the paclobutrazol-treated primary leaves. The inhibitory effect of paclobutrazol on the abaxial stomatal conductances became more pronounced with time during the light period but the adaxial surfaces displayed similar or slightly higher conductances than those of the control. The transpiration rate on a unit area basis did not change significantly or increased in the treated leaves thus the reduced water loss of paclobutrazol-treated plants was due to the reduced leaf area. Stomatal conductances of the adaxial surfaces responded more intensively to exogenous abscisic acid and the total leaf conductance decreased faster with increasing ABA concentration in the control than in the paclobutrazol-treated leaves. Paclobutrazol, an effective inhibitor of phytosterol biosynthesis, not only amplified the stomatal differentiation but increased the differences between the adaxial and abaxial stomatal conductances of the primary leaves.