Photosynthetic performance of leaves and twigs of evergreen holly (Ilex aquifolium L.)

Twigs and stems of holly (Ilex aquifolium) contain up to 40% of leaf chloro phyll (ca. 300 mg Chi. m(-2) twig area) when compared on a unit surface are a. Light penetration through the outer layers (epidermis, periderm or outer bark) of the shoot is age-dependent and ranges from roughly 40% of transmi tted sunlight in recent-year's twigs down to 5% of rest light reaching the chlorophyll-containing chlorenchymous bark tissue in Ilex main stems. Oxyge n gas exchange under optimum conditions revealed photosynthetic rates in me chanically isolated twig and branch chlorenchymes being nearly as high as c omparable rates of leaves. Yet, high chlorenchymal photosynthesis was obtai ned only when tissues had been artificially separated. intact tissues had m uch lower rates which can be explained by a highly impermeable Ilex-perider m and an effective stem- and twig-internal carbon cycling.