Acclimation of leaf cohorts expanded under light and water stresses: an adaptive mechanism of Eucryphia cordifolia to face changes in climatic conditions?

Eucryphia cordifolia Cav. is a long-lived evergreen tree species, commonly found as a canopy emergent tree in the Chilean temperate rain forest. This species displays successive leaf cohorts throughout the entire growing season. Thus, full leaf expansion occurs under different environmental conditions during growing such as air temperature, vapor pressure deficit and the progress of moderate water stress (WS). These climate variations can be reflected as differences in anatomical and physiological characteristics among leaf cohorts. Thus, we investigated the potential adaptive role of different co-existing leaf cohorts in seedlings grown under shade, drought stress or a combination of the two. Photosynthetic and anatomical traits were measured in the first displayed leaf cohort and in a subsequent leaf cohort generated during the mid-season. Although most anatomical and photosynthetic pigments did not vary between cohorts, photosynthetic acclimation did occur in the leaf cohort and was mainly driven by biochemical processes such as leaf nitrogen content, Rubisco carboxylation capacity and maximal Photosystem II electron transport rather than CO2 diffusion conductance. Cohort acclimation could be relevant in the context of climate change, as this temperate rainforest will likely face some degree of summer WS even under low light conditions. We suggest that the acclimation of the photosynthetic capacity among current leaf cohorts represents a well-tuned mechanism helping E. cordifolia seedlings to face a single stress like shade or drought stress, but is insufficient to cope with simultaneous stresses.

Mesophyll diffusion conductance to CO2: an unappreciated central player in photosynthesis.

Mesophyll diffusion conductance to CO(2) is a key photosynthetic trait that has been studied intensively in the past years. The intention of the present review is to update knowledge of g(m), and highlight the important unknown and controversial aspects that require future work. The photosynthetic limitation imposed by mesophyll conductance is large, and under certain conditions can be the most significant photosynthetic limitation. New evidence shows that anatomical traits, such as cell wall thickness and chloroplast distribution are amongst the stronger determinants of mesophyll conductance, although rapid variations in response to environmental changes might be regulated by other factors such as aquaporin conductance. Gaps in knowledge that should be research priorities for the near future include: how different is mesophyll conductance among phylogenetically distant groups and how has it evolved? Can mesophyll conductance be uncoupled from regulation of the water path? What are the main drivers of mesophyll conductance? The need for mechanistic and phenomenological models of mesophyll conductance and its incorporation in process-based photosynthesis models is also highlighted.

Variability of water relations and photosynthesis in Eucryphia cordifolia Cav. (Cunoniaceae) over the range of its latitudinal and altitudinal distribution in Chile.

The aim of this study was to investigate, during the summer of the year 2008, the variation in leaf water and photosynthetic characteristics of Eucryphia cordifolia Cav. (Cunoniaceae) along its broad latitudinal distribution in central south Chile (36 degrees to 42 degrees S). The latitudinal variation in water potential (Psi(w)), water potential at saturation (Psipi(sat)), water potential at the turgor lost point (Psipi(tlp)), stomatal density of the leaves, leaf nitrogen concentrations and photosynthetic light response were studied in eight populations. The populations located in the northern region of the distribution of E. cordifolia had the lowest leaf water potential. Osmotic potential at full turgor was highest in the two southernmost populations and gradually decreased towards the northernmost points. Similarly, osmotic potential at zero turgor was the lowest in the northern population. On the contrary, the symplastic water content was lower in the two southernmost populations. The highest net photosynthesis rate was recorded for plants in the populations of intermediate distributions, and it was dependent on the precipitation and temperature gradient. The northern populations, which are subject to the lowest precipitations, showed the lowest stomatal densities, which were tightly linked with stomatal conductance variation. Therefore, the variability of A(max) was independent of stomatal density and conductance, so that the northern populations, subject to environments with less water availability, presented higher photosynthetic water use efficiency. Photosynthesis variations were also not associated with changes in leaf water potential or in nitrogen content in E. cordifolia leaves, which did not show any consistent latitudinal or altitudinal trend. In conclusion, the results support our hypothesis that the leaf water relations, stomatal characteristics and photosynthetic rates of the leaf would vary along its latitudinal gradient, helping to explain the ability of E. cordifolia trees to inhabit a broad latitudinal and altitudinal range throughout the central south Chile.

Changes during early development in photosynthetic light acclimation capacity explain the shade to sun transition in Nothofagus nitida.

Nothofagus nitida (Phil.) Krasser, an emergent tree of the Chilean evergreen forest, regenerates under the canopy. Nonetheless, it is common to find older saplings in clear areas. We hypothesized that this transition from shade to sun during the early developmental stages is made possible by an ontogenetic increase in the light acclimation capacity of photosynthesis. To test our hypothesis, we studied photosynthetic performance and photoprotection in N. nitida saplings at different developmental stages corresponding with three different height classes (short: 16.2 cm; medium-height: 48.0 cm; and tall: 73.7 cm) grown under contrasting light conditions (photosynthetic photon flux (PPF) of 20, 300 or 600 micromol m(-2) s(-1)) until newly expanded leaves had developed. Light-saturated CO(2) assimilation rate and light compensation and saturation points increased with increasing PPF. Medium-height and tall saplings acclimated to high light had higher electron transport rates and higher proportions of open Photosystem II reaction centers than shorter plants acclimated to high light. Short saplings showed higher thermal dissipation and contents of xanthophylls than taller saplings. Only medium-height and tall saplings acclimated to high light recovered after photoinhibition. State transitions were higher in short plants growing in low light, and decreased with plant height and growth irradiance. Thus, during development, N. nitida changes the balance of light energy utilization and photoprotective mechanisms, supporting a phenotypic transition from shade to sun during its early ontogeny.