Red and Blue Light Differently Influence Actinidia chinensis Performance and Its Interaction with Pseudomonas syringae pv. Actinidiae.

Light composition modulates plant growth and defenses, thus influencing plant-pathogen interactions. We investigated the effects of different light-emitting diode (LED) red (R) (665 nm) and blue (B) (470 nm) light combinations on Actinidia chinensis performance by evaluating biometric parameters, chlorophyll a fluorescence, gas exchange and photosynthesis-related gene expression. Moreover, the influence of light on the infection by Pseudomonas syringae pv. actinidiae (Psa), the etiological agent of bacterial canker of kiwifruit, was investigated. Our study shows that 50%R-50%B (50R) and 25%R-75%B (25R) lead to the highest PSII efficiency and photosynthetic rate, but are the least effective in controlling the endophytic colonization of the host by Psa. Monochromatic red light severely reduced ΦPSII, ETR, Pn, TSS and photosynthesis-related genes expression, and both monochromatic lights lead to a reduction of DW and pigments content. Monochromatic blue light was the only treatment significantly reducing disease symptoms but did not reduce bacterial endophytic population. Our results suggest that monochromatic blue light reduces infection primarily by modulating Psa virulence more than host plant defenses.

Nitrogen deposition outweighs climatic variability in driving annual growth rate of canopy beech trees: Evidence from long-term growth reconstruction across a geographic gradient.

In this study, we investigated the role of climatic variability and atmospheric nitrogen deposition in driving long-term tree growth in canopy beech trees along a geographic gradient in the montane belt of the Italian peninsula, from the Alps to the southern Apennines. We sampled dominant trees at different developmental stages (from young to mature tree cohorts, with tree ages spanning from 35 to 160 years) and used stem analysis to infer historic reconstruction of tree volume and dominant height. Annual growth volume (GV ) and height (GH ) variability were related to annual variability in model simulated atmospheric nitrogen deposition and site-specific climatic variables, (i.e. mean annual temperature, total annual precipitation, mean growing period temperature, total growing period precipitation, and standard precipitation evapotranspiration index) and atmospheric CO2 concentration, including tree cambial age among growth predictors. Generalized additive models (GAM), linear mixed-effects models (LMM), and Bayesian regression models (BRM) were independently employed to assess explanatory variables. The main results from our study were as follows: (i) tree age was the main explanatory variable for long-term growth variability; (ii) GAM, LMM, and BRM results consistently indicated climatic variables and CO2 effects on GV and GH were weak, therefore evidence of recent climatic variability influence on beech annual growth rates was limited in the montane belt of the Italian peninsula; (iii) instead, significant positive nitrogen deposition (Ndep ) effects were repeatedly observed in GV and GH ; the positive effects of Ndep on canopy height growth rates, which tended to level off at Ndep values greater than approximately 1.0 g m-2  y-1 , were interpreted as positive impacts on forest stand above-ground net productivity at the selected study sites.

A global assessment of forest surface albedo and its relationships with climate and atmospheric nitrogen deposition.

We present a global assessment of the relationships between the short-wave surface albedo of forests, derived from the MODIS satellite instrument product at 0.5° spatial resolution, with simulated atmospheric nitrogen deposition rates (Ndep ), and climatic variables (mean annual temperature Tm and total annual precipitation P), compiled at the same spatial resolution. The analysis was performed on the following five forest plant functional types (PFTs): evergreen needle-leaf forests (ENF); evergreen broad-leaf forests (EBF); deciduous needle-leaf forests (DNF); deciduous broad-leaf forests (DBF); and mixed-forests (MF). Generalized additive models (GAMs) were applied in the exploratory analysis to assess the functional nature of short-wave surface albedo relations to environmental variables. The analysis showed evident correlations of albedo with environmental predictors when data were pooled across PFTs: Tm and Ndep displayed a positive relationship with forest albedo, while a negative relationship was detected with P. These correlations are primarily due to surface albedo differences between conifer and broad-leaf species, and different species geographical distributions. However, the analysis performed within individual PFTs, strengthened by attempts to select 'pure' pixels in terms of species composition, showed significant correlations with annual precipitation and nitrogen deposition, pointing toward the potential effect of environmental variables on forest surface albedo at the ecosystem level. Overall, our global assessment emphasizes the importance of elucidating the ecological mechanisms that link environmental conditions and forest canopy properties for an improved parameterization of surface albedo in climate models.

The human footprint in the carbon cycle of temperate and boreal forests.

Temperate and boreal forests in the Northern Hemisphere cover an area of about 2 x 10(7) square kilometres and act as a substantial carbon sink (0.6-0.7 petagrams of carbon per year). Although forest expansion following agricultural abandonment is certainly responsible for an important fraction of this carbon sink activity, the additional effects on the carbon balance of established forests of increased atmospheric carbon dioxide, increasing temperatures, changes in management practices and nitrogen deposition are difficult to disentangle, despite an extensive network of measurement stations. The relevance of this measurement effort has also been questioned, because spot measurements fail to take into account the role of disturbances, either natural (fire, pests, windstorms) or anthropogenic (forest harvesting). Here we show that the temporal dynamics following stand-replacing disturbances do indeed account for a very large fraction of the overall variability in forest carbon sequestration. After the confounding effects of disturbance have been factored out, however, forest net carbon sequestration is found to be overwhelmingly driven by nitrogen deposition, largely the result of anthropogenic activities. The effect is always positive over the range of nitrogen deposition covered by currently available data sets, casting doubts on the risk of widespread ecosystem nitrogen saturation under natural conditions. The results demonstrate that mankind is ultimately controlling the carbon balance of temperate and boreal forests, either directly (through forest management) or indirectly (through nitrogen deposition).

Assessing the effects of nitrogen deposition and climate on carbon isotope discrimination and intrinsic water-use efficiency of angiosperm and conifer trees under rising CO2 conditions.

The objective of this study is to globally assess the effects of atmospheric nitrogen deposition and climate, associated with rising levels of atmospheric CO2 , on the variability of carbon isotope discrimination (Δ(13) C), and intrinsic water-use efficiency (iWUE) of angiosperm and conifer tree species. Eighty-nine long-term isotope tree-ring chronologies, representing 23 conifer and 13 angiosperm species for 53 sites worldwide, were extracted from the literature, and used to obtain long-term time series of Δ(13) C and iWUE. Δ(13) C and iWUE were related to the increasing concentration of atmospheric CO2 over the industrial period (1850-2000) and to the variation of simulated atmospheric nitrogen deposition and climatic variables over the period 1950-2000. We applied generalized additive models and linear mixed-effects models to predict the effects of climatic variables and nitrogen deposition on Δ(13) C and iWUE. Results showed a declining Δ(13) C trend in the angiosperm and conifer species over the industrial period and a 16.1% increase of iWUE between 1850 and 2000, with no evidence that the increased rate was reduced at higher ambient CO2 values. The temporal variation in Δ(13) C supported the hypothesis of an active plant mechanism that maintains a constant ratio between intercellular and ambient CO2 concentrations. We defined linear mixed-effects models that were effective to describe the variation of Δ(13) C and iWUE as a function of a set of environmental predictors, alternatively including annual rate (Nrate ) and long-term cumulative (Ncum ) nitrogen deposition. No single climatic or atmospheric variable had a clearly predominant effect, however, Δ(13) C and iWUE showed complex dependent interactions between different covariates. A significant association of Nrate with iWUE and Δ(13) C was observed in conifers and in the angiosperms, and Ncum was the only independent term with a significant positive association with iWUE, although a multi-factorial control was evident in conifers.

Effects of simulated nitrogen deposition on the nutritional and physiological status of beech forests at two climatic contrasting sites in Italy.

Anthropogenic activities have resulted in a significant increase of reactive nitrogen (N) compounds in the atmosphere and a rise in N deposition on forest ecosystems. Increasing N loads impact forest productivity and health, altering tree physiological status and nutrient balance with possible beneficial and detrimental consequences. The impact of N deposition has received considerable attention by scientific research, covering medium and high latitudes, while experimental studies in the Mediterranean area are almost absent. The present study used a manipulative approach, through replicated N additions (background deposition, 30, 60 kg N ha-1yr-1) to simulate the cumulative effects of N deposition in two beech (Fagus sylvaticaL.) forests located in contrasting climatic regions of Italy. Leaf nutrients and photosynthetic pigments were tested as monitoring indicators after four years of N fertilization. Foliar N and pigment concentrations indicated not limiting N conditions at both forest sites, although changes in chlorophylls and carotenoids showed an early response of the canopy to N additions. N-to-phosphorus (P) and sulfur (S) ratios increased under elevated N fertilization, which could be partly related to the relative enhancement of foliar N concentration, and partly associated with the reduction of foliar P and S. The two eutrophic beech forests monitored were not severely affected by chronic N addition, not showing critical nutritional and physiological impairments over the short to medium period. However, the modifications in leaf nutrient and pigment compositions showed an incipient stress response and accentuated the differences induced by climatic and soil characteristics at the two sites. The potential use of nutrients and photosynthetic pigments in monitoring forest N deposition under contrasting climatic conditions and the eventual limits of manipulative experiments are discussed.

Sanio's laws revisited. Size-dependent changes in the xylem architecture of trees.

Early observations led Sanio [Wissen. Bot., 8, (1872) 401] to state that xylem conduit diameters and lengths in a coniferous tree increase from the apex down to a height below which they begin to decrease towards the tree base. Sanio's law of vertical tapering has been repeatedly tested with contradictory results and the debate over the scaling of conduit diameters with distance from the apex has not been settled. The debate has recently acquired new vigour, as an accurate knowledge of the vertical changes in wood anatomy has been shown to be crucial to scaling metabolic properties to plant and ecosystem levels. Contrary to Sanio's hypothesis, a well known model (MST, metabolic scaling theory) assumes that xylem conduits monotonically increase in diameter with distance from the apex following a power law. This has been proposed to explain the three-fourth power scaling between size and metabolism seen across plants. Here, we (i) summarized available data on conduit tapering in trees and (ii) propose a new numerical model that could explain the observed patterns. Data from 101 datasets grouped into 48 independent profiles supported the notions that phylogenetic group (angiosperms versus gymnosperms) and tree size strongly affected the vertical tapering of conduit diameter. For both angiosperms and gymnosperms, within-tree tapering also varied with distance from the apex. The model (based on the concept that optimal conduit tapering occurs when the difference between photosynthetic gains and wall construction costs is maximal) successfully predicted all three major empirical patterns. Our results are consistent with Sanio's law only for large trees and reject the MST assumptions that vertical tapering in conduit diameter is universal and independent of rank number.

Seasonal and interannual variability of photosynthetic capacity in relation to leaf nitrogen in a deciduous forest plantation in northern Italy.

Gas exchange was measured in a forest plantation dominated by Fraxinus angustifolia Vahl. and Quercus robur L. in northern Italy, over three growing seasons that differed in water availability (2001, 2002 and 2003). The objectives were to: (1) determine variability in the photosynthetic parameters V(cmax) (maximum carboxylation capacity) and J(max) (maximum rate of electron transport) in relation to species, leaf ontogeny and drought; and (2) assess the potential of the photosynthesis-nitrogen relationship for estimating leaf photosynthetic capacity. Marked seasonal and interannual variability in photosynthetic capacity was observed, primarily caused by changes in leaf ontogeny and water stress. Relatively small differences were apparent between species. In the absence of water stress (year 2002), the seasonal patterns of V(cmax) and J(max) were characterized by a rapid increase during spring, a relatively steady state during summer and a rapid decline during autumn. In years with a moderate (year 2001) or a severe (year 2003) water stress, photosynthetic capacity decreased during the summer in proportion to drought intensity, without a parallel decline in leaf nitrogen content. The V(cmax)-nitrogen relationship was significantly affected by both leaf ontogeny and drought. As a consequence, the use of a single annual regression to predict V(cmax) from leaf nitrogen yielded good estimates only during the summer and in the absence of water stress. Irrespective of the mechanisms by which photosynthetic capacity is affected by water stress, its large seasonal and interannual variability is of great relevance for modeling the forest carbon cycle.

Response of a mature Pinus laricio plantation to a three-year restriction of water supply: structural and functional acclimation to drought.

The response of mature forest stands to a reduction in water availability has received little attention. In particular, the extent to which a short-term reduction in gas exchange can be alleviated in the long-term by acclimation processes is not well understood. We studied the impact of a severe reduction in water availability on the water relations and growth of 35-year-old Pinus laricio Poiret. trees in a replicated experiment. Sapwood and needle increments, soil and tree water status, stand transpiration, xylem embolism and plant hydraulic architecture were monitored over a 3-year period in control and drought-treated plots. Needle length was reduced in drought-treated trees by 30, 19 and 29%, and sapwood increments by 50, 27 and 24% over the 3 years. Drought did not result in tree mortality or in extensive xylem embolism or foliage dieback. On the contrary, a conservative water-use strategy was observed, because minimum leaf water potentials did not differ between treatments or over the season. Plant hydraulic resistance was also unaffected by restricted water availability. Stand transpiration was strongly reduced by drought treatment over the summer, but not during the winter, despite significant differences in leaf area between control and drought-treated trees, implying higher transpiration rates per unit leaf area in the droughted plants. This suggests that water transport capacity, rather than the amount of leaf area, controlled stand transpiration, which is at variance with expectations based on experiments with seedlings and short-term experiments with mature trees.

Variability of stomatal conductance in a small and isolated population of silver fir (Abies alba Mill.).

We analyzed the response to drought of 420 individuals from eight half-sib families from a small and isolated population of silver fir (Abies alba Mill.): 105 of them were kept in well-watered conditions as control while the remaining 315 were exposed to drought for 27 days. A model describing stomatal behavior derived from Monteith and developed in beech by Leonardi et al. was fitted to experimental transpiration data obtained simply from the difference between two daily pot weighings. The estimated parameters were maximum stomatal conductance, maximum transpiration in well-watered conditions and sensitivity to soil water deficit. The model worked well: convergence for all but four individuals and concordance between experimental and fitted data were good (R(2)=0.86). Inter-individual variability for all three estimated parameters was high and two of them (maximum stomatal conductance and sensitivity to soil water deficit) were significantly different among families, suggesting genetic control. Our results validate the simplified method used to evaluate individual stomatal parameters. We also show that in the small and isolated population of our study substantial adaptive variability remains, a crucial prerequisite to endure environmental conditions determined by climatic change foreseen for the next decades.

A simple general method to evaluate intra-specific transpiration parameters within and among seedling families.

A method to evaluate the genetic control of plant response to increasing soil water deficit is proposed. A description of single tree transpiration behavior was obtained considering parameters independent from air and soil conditions. We removed environmental effects by using two approaches: the normalization of drought data to control (watered) plants and the fitting of a process model. We analyzed the transpiration of 475 4-year-old European beech seedlings, belonging to eight full-sib families. Approximately, one-third of the seedlings were kept in well-watered conditions while the others were exposed to drought for 14 days. Daily plant transpiration was estimated as the difference between two subsequent gravimetric measurements. A mechanistic model was fitted to transpiration data separately for each tree. In the model, the relationship of transpiration with vapor pressure deficit and soil water deficit of each tree is modulated by three parameters: maximum leaf conductance (gM1), maximum transpiration in well-watered soil conditions E(M0)1 and a parameter describing stomatal sensitivity to soil water deficit (c). The model successfully fitted most single tree data and a distribution of estimates for the three parameters (gM1, E(M0)1 and c) was obtained. Predicted transpiration values were in good agreement with observed data (R (2) = 0.86). The model approach produced parameters significantly correlated with those of the "normalization to control" approach. Estimated parameters vary considerably among trees, suggesting the presence of individual differences in stomatal behavior and response to drought. In spite of a large among tree (within family) variation, the among families component for gM1, E(M0)1 and c explained 9.5, 3.3 and 0.1% of total parameters variation suggesting a significant genetic control of transpiration processes.