Leaf anatomy and water relations of Eucalyptus camaldulensis and E. globulus seedlings

Leaf anatomy and water relations of juvenile leaves varied between Eucalyptus camaldulensis and E. globulus seedlings. Eucalyptus camaldulensis had three or four pairs of opposite and horizontal lower leaves. Above these, the leaves were alternate, pendant, and nearly vertical. Eucalyptus globulus leaves were opposite and sessile. Stomata were more numerous but smaller in E. camaldulensis (amphistomatous) than in E. globulus (hypostamatous). In both species waxes were more abundant on the abaxial than on the adaxial leaf surface. Waxes of E. camaldulensis consisted of plates arranged perpendicular to the leaf surfaces; those of E. globulus were rodlets branching at right angles. In pot experiments (with a restricted soil volume) transpiration rates of the two species varied and depended on how they were expressed. When the soil was well watered, transpiration rate per seedling or per unit of the abaxial leaf surface area was higher in E. camaldulensis than in E. globulus. As the soil in the pots dried, plant water stress increased faster in E. camaldulensis than in E. globulus. During the first 2 weeks dry weight increment of shoots and roots was greater in E. globulus than in E. camaldulensis. Thereafter, growth of roots and shoots accelerated in E. camaldulensis and surpassed that of E. globulus. After a few weeks root growth of both species declined, reflecting the limitation of pot size. The pot experiments were misleading in suggesting that E. globulus was more drought resistant than E. camaldulensis. The higher transpiration rate and extensive rooting of E. camaldulensis in the restricted volume of soil in the pots induced greater water deficits than developed in E. globulus. When plants of both species were grown in long plastic tubes (with an unrestricted soil volume) water stress did not develop faster in E. camaldulensis than in E. globulus. The tube experiments showed that a major factor in greater drought avoidance of E. camaldulensis over E. globulus was capacity of the former to produce a deep and ramifying root system that could absorb water from deep soil layers after the surface soil dried. This advantage of E. camaldulensis over E. globulus was obscured in the pot experiments.