Changes in stomatal characteristics and photochemical efficiency during leaf development in six species of Sorbus

Measurements of Sorbus stomata size and density, maximal photochemical efficiency of photosystem II (F-v/F-m), variable-to-initial fluorescence ratio (F-v/F-0) and potential electron acceptor capacity ('area') were performed during leaf development in four parental diploid species, S. aria, S. aucuparia, S. chamaemespilus, S. torminalis, and two hybrid species, S. hazslinszkyana and S. intermedia. In fully expanded mature leaves, stomata lengths and densities were significantly larger in the shrub S. chamaemespilus than in the five tree species. The best performance of both the F-v/F-m and the F-v/F-0 ratio was recorded in S. intermedia, whereas S. chamaemespilus had the highest value of 'area'. From a physiological point of view, the results of this study showed that the photosystem II reaction centers remained intact functionally through all phenological stages of leaf expansion for all examined species of Sorbus.