Preliminary Studies on the Photosynthetic Structures of Trifolium alpinum L. as Related to Productivity

Abstract

Trifolium alpinum L. is a high-quality alpine forage plant growing spontaneously from 1900 to 2800 m above sea level and is widely distributed in Piedmont and the Valle d'Aosta (Italy), where it can reach population frequencies of 90 per cent. Yields were determined on forage harvested in the Valle dell'Orco (Piedmont) and were comparable to cultivated clovers from higher latitudes; yields decreased progressively as the elevation increased. The chemical and nutritional characteristics of the forage, though comparable to clovers cultivated in the Po valley (Italy), were, however, more constant. The structure of the leaf lamina as related to elevation was investigated using light microscopy, TEM and SEM. This is complemented by data on chlorophyll concentration, succulence, specific leaf weight and area. At all elevations T. alpinum lacks, apart from bundle sheath cell chloroplasts in a centrifugal arrangement, the structural characteristics of C4 plants. The chlorophyll a:b ratio (less than four) is typical of a C2 plant. Succulence indices (S and Sm) were very low, making CAM pathway photosynthesis unlikely. Unusual and difficult to interpret structures included: small functional chloroplasts in both the epidermises, stomata present almost exclusively in the upper epidermis and mitochondria enveloped (or enclosed) by chloroplasts. It was observed that, as the elevation increases, populations are selected which are well-adapted for gas exchange (increase in specific leaf area, stomatal density and intercellular spaces) and characterized by a decrease in the grana thylacoid:integrana thylacoid ratio (consistent with the increase in the chlorophyll a:b ratio), the per cent water, Sm and the specific leaf weight.