Morphological, anatomical, and physiological leaf traits of Corylus avellana plants growing in different light conditions within the natural reserve "Siro Negri" (Italy) were analyzed. The results highlighted the capability of C. avellana to grow both in sun and shade conditions throughout several adaptations at leaf level. In particular, the more than 100% higher specific leaf area in shade is associated to a 44% lower palisade to spongy parenchyma thickness ratio compared with that in sun. Moreover, the chlorophyll (Chl) a to Chl b ratio decreased in response to the 97% decrease in photosynthetic photon flux density. The results highlighted the decrease in the ratio of Chl to carotenoid content, the maximum PSII photochemical efficiency, and the actual PSII photochemical efficiency (ΦPSII) associated with the increase in the ratio of photorespiration to net photosynthesis (P N ) in sun. Chl a/b ratio was the most significant variable explaining P N variations in shade. In sun, P N was most influenced by the ratio between the fraction of electron transport rate (ETR) used for CO 2 assimilation and ETR used for photorespiration, by Φ PSII , nitrogen content per leaf area, and by total Chl content per leaf area. The high phenotypic plasticity of C. avellana (PI = 0.33) shows its responsiveness to light variations. In particular, a greater plasticity of morphological (PI m = 0.41) than of physiological (PI p = 0.36) and anatomical traits (PI a = 0.24) attests to the shade tolerance of the species.

The relationship between net photosynthetic (P(N)) and leaf respiration ( R) rates of Quercus ilex, Phillyrea latifolia, Myrtus communis, Arbutus unedo, and Cistus incanus was monitored in the period February 2006 to February 2007. The species investigated had low R and P(N) during winter, increasing from March to May, when mean air temperature reached 19.2 degrees C. During the favourable period, C. incanus and A. unedo had a higher mean P(N) ( 16.4 +/- 2.4 mu mol m(-2) s(-1)) than P. latifolia, Q. ilex, and M. communis ( 10.0 +/- 1.3 mu mol m(-2) s(-1)). The highest R ( 1.89 +/- 0.30 mu mol m(-2) s(-1), mean of the species), associated to a significant P(N) decrease ( 62 % of the maximum, mean value of the species), was measured in July ( mean R/P(N) ratio 0.447 +/- 0.091). Q(10), indicating the respiration sensitivity to short-term temperature increase, was in the range 1.49 to 2.21. Global change might modify R/P(N) determining differences in dry matter accumulation among the species, and Q. ilex and P. latifolia might be the most favoured species by their ability to maintain sufficiently higher P(N) and lower R during stress periods

Cistus salvifolius L. is the most widely spread Cistus species around the Mediterranean basin. It colonizes a wide range of habitats growing from sea level to 1,800 m a.s.l., on silicolous and calcicolous soils, in sun areas as well as in the understory of wooded areas. Nevertheless, this species has been mainly investigated in term of its responsiveness to drought. Our aim was to understand which leaf traits allow C. salvifolius to cope with low-light environments. We questioned if biochemical and physiological leaf trait variations in response to a reduced photosynthetic photon flux density were related to leaf morphological plasticity, expressed by variations of specific leaf area (SLA) and its anatomical components (leaf tissue density and thickness). C. salvifolius shrubs growing along the Latium coast (41°43ʹN,12°18ʹE, 14 m a.s.l., Italy) in the open and in the understory of a Pinus pinea forest, were selected and the relationships between anatomical, gas exchange, chlorophyll (Chl) fluorescence, and biochemical parameters with SLA and PFD variations were tested. The obtained results suggested long-term acclimation of the selected shrubs to contrasting light environments. In high-light conditions, leaf nitrogen and Chl contents per leaf area unit, leaf thickness, and Chl a/b ratio increased, thus maximizing net photosynthesis, while in shade photosynthesis was downregulated by a significant reduction in the electron transport rate. Nevertheless, the increased pigment-protein complexes and the decreased Chl a/b in shade drove to an increased lightharvesting capacity (i.e. higher actual quantum efficiency of PSII). Moreover, the measured vitality index highlighted the photosynthetic acclimation of C. salvifolius to contrasting light environments. Overall, our results demonstrated the morphological, anatomical, and physiological acclimation of C. salvifolius to a reduced light environment

BackgroundLimitations to plant growth imposed by the Mediterranean climate are mainly due to carbon balance in response to stress factors. In particular, water stress associated to high air temperature and irradiance in summer causes a marked decrease in CO2 assimilation. Air temperature sensitivity of photosynthesis (PN) differs from that of leaf respiration (RD). PN often decreases sharply at temperature above its optimum while RD increases exponentially over short term rises in temperature. Nevertheless, the impact of water deficit on RD is still far from clear with reports in literature including decreases, maintenance or increases in its rates. The ratio RD/PN can be considered a simple approach to leaf carbon balance because it indicates the percentage of photosynthates that is respired.ResultsThe results underline different morphological, anatomical and physiological traits of the evergreen species co-occurring in the Mediterranean maquis which are indicative of their adaptive capability to Mediterranean stress factors. The ratio RD/PN varies from 0.15 ± 0.04 in autumn, 0.24 ± 0.05 in spring through 0.29 ± 0.15 in winter to 0.46 ± 0.11 in summer. The lower RD/PN in autumn and spring underlines the highest PN rates during the favorable periods when resources are not limited and leaves take in roughly three to five times more CO2 than they lose by respiration. On the contrary, the highest RD/PN ratio in summer underlines the lowest sensitivity of respiration to drought. Among the considered species, Quercus ilex and Pistacia lentiscus have the largest tolerance to low winter temperatures while Phillyrea latifolia and Myrtus communis to drought, and Phillyrea latifolia the highest recovery capability after the first rainfall following drought.ConclusionsThe Mediterranean evergreen specie shows a different tolerance to Mediterranean climate stress factors. The predicted global warming might differently affect carbon balance of the considered species, with a possible change in Mediterranean shrublands composition in the long-term. Understanding the carbon balance of plants in water limited environments is crucial in order to make informed land management decisions. Moreover, our results underline the importance of including seasonal variations of photosynthesis and respiration in carbon balance models.Electronic supplementary materialThe online version of this article (doi:10.1186/1999-3110-54-35) contains supplementary material, which is available to authorized users.

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