Leaf anatomy of alpine plants as related to altitudinal stress

Alopecurus gerardi, Poa alpina, and Carex curvula are spontaneous, perennial forage plants distributed in the high elevation (2300–3200 m) pasture lands of Piedmont and Valle d'Aosta (Italy). Sedum atratum is an annual succulent which grows at elevations up to 3200 m. The three monocotyledons have, in comparison with corresponding plants from the low-land, peculiar organographic and anatomic structures such as curling leaf lamina, vascular bundle sheath layer with chloroplasts arranged in a centrifugal fashion, low stomatal density (stomata number/cm2: upper epidermis x̄= 0–11.9; lower epidermis x̄= 7.66–11.55), thick cuticles, as well as higher values of S (H2O; g/dm2; x̄= 0.6–0.32), Sm (H2O g/mg Chi.; x̄= 0.11–0.16), SLW (g f. wt/dm2; x̄= 0.86–1.36), but lower SLA values (cm2/g f. wt; x̄= 75.07–116.77). All these data are correlated to water stress. Even though the leaf lamina anatomy possesses some features typical of C4 plants, the presence of starch grains in the mesophyll chloroplasts indicates that these plants are probably C3 ones. In spite of high values of thylacoid grana/thylacoid intergrana ratios, typical of shade plants (mesophyll chloroplasts: x̄ up to 3.81; bundle sheath chloroplasts: x̄ up to 5.3), and Chi a/Chl b ratios (x̄ up to 4.23 in C. curvula), the apparent absence of peroxisomes seem to indicate a very efficient dark phase of photosynthesis. S. atratum, in comparison with the typical CAM succulents, which live in dryer and warmer habitats, has a higher values of stomatal density (upper epidermis, x̄= 2.59; lower epidermis, x̄= 3.15) and of SLA (x̄= 24.98), but lower S (x̄= 3.83), Sm (x̄= 1.19) and SLW (x̄= 4.15).