Functional and structural characteristics of corticular photosynthesis of sun-exposed bark of olive tree (Olea europaea L.) were examined. Stomata are only sporadically present during stem primary growth. Light transmission through the phellem was age dependent, decreasing rapidly in stems older than five years of age. Light transmission was also low in pubescent 1-year-old stems. Light transmission was about 50% higher in wet phellem than in dry phellem. Photosynthetic capacity on a unit area basis (measured with an oxygen disc electrode at 27 degrees C and about 5% CO(2) on chlorophyllous tissue discs isolated from the stem) was higher in 1-, 20- and 30-year-old stems compared with 2-10-year-old stems. Low chlorophyll a/b ratio and light compensation points were recorded in olive stems with low phellem light transmission, in accordance with the shade acclimation hypothesis. The intrinsic photochemical efficiency of photosystem II of all stems, especially young stems, was less than that of the leaves. Our results show that olive tree bark possesses an efficient photosynthetic mechanism that may significantly contribute not only to the reduction in concentrations of CO(2) in the inner bark, but also to whole-tree carbon balance.