Influence of light availability on leaf structure and growth of two Eucalyptus globulus ssp globulus provenances

Light availability strongly affects leaf structure of the distinctive ontog enetic leaf forms of Eucalyptus globulus Labill. ssp. globulus. Late-maturi ng plants from St. Marys, Tasmania and early maturing plants from Wilsons P romontory, Victoria thereafter referred to as Wilsons From.) were grown for 9 months in 100, 50 or 10% sunlight. Growth, biomass and leaf area were si gnificantly reduced when plants were grown in 10% sunlight. Provenance diff erences were minimal despite retention of the juvenile leaf form by the Tas manian plants throughout the study. The time taken for initiation of vegeta tive phase change by the Wilsons From. saplings increased with decreasing l ight availability, but the nodal position of change on the main stem remain ed the same. Both juvenile and adult leaves remained horizontal in low Ligh t conditions, but became vertical with high irradiance. Leaf dimensions cha nged with ontogenetic development, but were unaffected by light availabilit y. Juvenile leaves retained a dorsiventral anatomy and adult Wilsons From. leaves retained an isobilateral structure despite a tenfold difference in l ight availability. Stomatal density and distribution showed ontogenetic and treatment differences. At all irradiances, juvenile leaves produced the sm allest stomata and adult leaves the largest stomata. Amphistomy decreased w ith decreasing irradiance. Detrended, correspondence analysis ordination hi ghlighted the structural changes influenced by ontogenetic development and light availability. Adult leaves had characteristics similar to the xeromor phic, sun-leaf type found in arid, high-light conditions. Although juvenile leaves had characteristics typical of mesomorphic leaves, several structur al features suggest that these leaves are more sun-adapted than adult leave s.