THE EFFECTS OF LEAF ORIENTATION ON PHOTOSYNTHESIS, TRANSPIRATION AND DIFFUSIVE CONDUCTANCE OF LEAVES OF CONTRASTING FESTUCA SPECIES

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Abstract
Summary: A comparative study was made of the effect of leaf orientation on photosynthesis, transpiration and diffusive conductance of leaves of Festuca gigantea, F. pratensis, F. arundinacea, F. arundinacea var. atlantigena and F. marei. Conductance was also examined in F. drymeja and F. scariosa. Adaxial leaf surfaces ranged from the relatively flat (F. drymeja and F. gigantea) to the heavily corrugated (F. marei and F. scariosa). Stomatal numbers were greatest on the adaxial surface of all species, whilst the frequency on abaxial surfaces ranged from zero in F. gigantea and F. scariosa to more than 60 mm−2 in F. marei.The photosynthesis/transpiration ratio was slightly greater with the abaxial surface uppermost and greater in those species with relatively flat adaxial surfaces. The small change in the ratio with change in leaf orientation was due to the relatively greater reduction in transpiration than in photosynthesis. Species with heavily corrugated leaves transpired faster than those with flat leaf surfaces. Change in leaf orientation did not affect the ranking of the species in terms of transpiration or of leaf conductance but, because of the contrasting responses of F. gigantea in two separate experiments, ranking was not the same. There was no consistent effect of orientation on total leaf conductance but large differences were found in the relative responses of the two surfaces in some species. In those with very few abaxial stomata the adaxial surface always had greatest conductance, whichever surface was uppermost. Where both leaf surfaces had appreciable numbers of stomata, the upper surface always exhibited the greater conductance, irrespective of whether it was the adaxial or abaxial. F. arundinacea showed the greatest change in the ratio of adaxial leaf conductance in response to change in leaf orientation.