Leaf plasticity to light intensity in Italian cypress (Cupressus sempervirens L.): adaptability of a Mediterranean conifer cultivated in the Alps

Abstract

Italian cypress (Cupressus sempervirens L.) is native to the eastern Mediterranean, an area characterised by hot, dry summers and mild winters. Over the centuries, however, the species has been introduced into more northerly regions, a long way from its native range. The current, generally warmer climatic conditions brought about by global warming have favoured its cultivation in even more northerly areas in the Alps and other European alpine regions. Given that not only temperature, but also light availability are limiting factors for the spread of cypress in these environments, it is important to ascertain how this species copes with low light conditions. The photosynthetic characteristics of cypress leaves collected from different portions of the crown with contrasting light availability were evaluated by several methods. Chlorophyll a (Chl a), chlorophyll b (Chl b) and carotenoid (Car) content was found to be higher in shade leaves than in sun leaves when measured on a fresh mass basis, although enzymatic activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCo) and nitrate reductase were lower in shade leaves. When the efficiency of PSII was measured by chlorophyll fluorescence, a marked reduction in F(m) was found in shade leaves, while F(o) remained unchanged. The use of exogenous electron donors diphenyl carbazide (DPC) and NH(2)OH actually improved the photosynthetic efficiency of shade leaves, and the same effect was found when PSII electron transport activity was measured as O(2) evolution. Altogether, these results seem to indicate lesser photosynthetic efficiency in shade leaves, probably an impairment on the donor side of the PSII. At the same time, analysis by SDS-PAGE revealed differences in the polypeptide composition of the thylakoid membranes of sun and shade leaves: the bands corresponding to 23 kDa, 28-25 kDa and 33 kDa polypeptides were less intense in the thylakoid membranes extracted from shade leaves. These results were further confirmed by an immunological study showing that the content of the 33 kDa protein, corresponding to the extrinsic PSII protein PsbO, was significantly diminished in shade leaves. The high plasticity of cypress leaves appears to be an advantageous trait in the plant's response to variations in environmental conditions, including global change. Implications for the management of this Mediterranean species at the northern edge of its distribution are discussed.