Photosynthetic acclimation to light in woody and herbaceous species: a comparison of leaf structure, pigment content and chlorophyll fluorescence characteristics measured in the field

Alfalfa Mosaic Virus and White Clover Mosaic Virus Combined Infection Leads to Chloroplast Destruction and Alterations in Photosynthetic Characteristics of Nicotiana benthamiana

Alfalfa mosaic virus (AMV) is one of the most widely distributed viruses; it often exhibits combined infection with white clover mosaic virus (WCMV). Even so, little is known about the effects of co-infection with AMV and WCMV on plants. To determine whether there is a synergistic effect of AMV and WCMV co-infection, virus co-infection was studied by electron microscopy, the double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA), and real-time fluorescence quantitative PCR (RT-qPCR) of AMV and WCMV co-infection in Nicotiana benthamiana. Meanwhile, measurements were carried out on the photosynthetic pigments, photosynthetic gas exchange parameters, and chlorophyll fluorescence parameters. The results showed that the most severe disease development was induced by AMV and WCMV co-infection, and the disease grade was scale 7. N. benthamiana leaves induced mottled yellow-green alternating patterns, leaf wrinkling, and chlorosis, and chloroplasts were observed to be on the verge of disintegration. The relative accumulation of AMV CP and WCMV CP was significantly increased by 15.44-fold and 10.04-fold upon co-infection compared to that with AMV and WCMV single infection at 21 dpi. In addition, chlorophyll a, chlorophyll b, total chlorophyll, the net photosynthetic rate, the water use efficiency, the apparent electron transport rate, the PSII maximum photochemical efficiency, the actual photochemical quantum yield, and photochemical quenching were significantly reduced in leaves co-infected with AMV and WCMV compared to AMV- or WCMV-infected leaves and CK. On the contrary, the carotenoid content, transpiration rate, stomatal conductance, intercellular CO2 concentration, minimal fluorescence value, and non-photochemical quenching were significantly increased. These findings suggest that there was a synergistic effect between AMV and WCMV, and AMV and WCMV co-infection severely impacted the normal function of photosynthesis in N. benthamiana.

Hyperspectral Indices Developed from Fractional-Order Derivative Spectra Improved Estimation of Leaf Chlorophyll Fluorescence Parameters

Chlorophyll fluorescence (ChlF) parameters offer valuable insights into quantifying energy transfer and allocation at the photosystem level. However, tracking their variation based on reflectance spectral information remains challenging for large-scale remote sensing applications and ecological modeling. Spectral preprocessing methods, such as fractional-order derivatives (FODs), have been demonstrated to have advantages in highlighting spectral features. In this study, we developed and assessed the ability of novel spectral indices derived from FOD spectra and other spectral transformations to retrieve the ChlF parameters of various species and leaf groups. The results obtained showed that the empirical spectral indices were of low reliability in estimating the ChlF parameters. In contrast, the indices developed from low-order FOD spectra demonstrated a significant improvement in estimation. Furthermore, the incorporation of species specificity enhanced the tracking of the non-photochemical quenching (NPQ) of sunlit leaves (R2 = 0.61, r = 0.79, RMSE = 0.15, MAE = 0.13), the fraction of PSII open centers (qL) of shaded leaves (R2 = 0.50, r = 0.71, RMSE = 0.09, MAE = 0.08), and the fluorescence quantum yield (ΦF) of shaded leaves (R2 = 0.71, r = 0.85, RMSE = 0.002, MAE = 0.001). Our study demonstrates the potential of FOD spectra in capturing variations in ChlF parameters. Nevertheless, given the complexity and sensitivity of ChlF parameters, it is prudent to exercise caution when utilizing spectral indices for tracking them.

Effects of nitrogen addition and Bothriochloa ischaemum and Lespedeza davurica mixture on plant chlorophyll fluorescence and community production in semi-arid grassland

Background

Grass-legume mixture can effectively improve productivity and stimulate overyielding in artificial grasslands, but may be N-limited in semi-arid regions. This study investigated the effects of N addition on chlorophyll fluorescence and production in the grass-legume mixtures community.

Methods

An N addition experiment was conducted in the Bothriochloa ischaemum and Lespedeza davurica mixture community, with seven mixture ratios (B0L10, B2L8, B4L6, B5L5, B6L4, B8L2, and B10L0) according to the sowing abundance of B.ischaemum and L.davurica and four N addition levels, N0, N25, N50, and N75 (0,25,50,75kgNhm-2 a-1), respectively. We analyzed the response of chlorophyll fluorescence parameters of the two species, the rapid light-response curves of chlorophyll fluorescence, as well as aboveground biomass (AGB) and overyielding.

Results

Our results showed that the two species showed different photosynthetic strategies, with L.davurica having significantly higher initial fluorescence (Fo), effective photochemical quantum yield of PSII (ΦPSII), and coefficient of photochemical fluorescence quenching (qP) than B. ischaemum, consisting with results of rapid light-response curves. N addition and mixture ratio both had significant effects on chlorophyll fluorescence and AGB (p<0.001). The ΦPSII and qP of L.davurica were significantly lowest in B5L5 and B6L4 under N addition, and the effect of N varied with mixture ratio. The photosynthetic efficiency of B. ischaemum was higher in mixture than in monoculture (B10L0), and ΦPSII was significantly higher in N50 than in N25 and N50 at mixture communities except at B5L5. The community AGB was significantly higher in mixture communities than in two monocultures and highest at B6L4. In the same mixture ratio, the AGB was highest under the N50. The overyielding effects were significantly highest under the N75 and B6L4 treatments, mainly attributed to L.davurica. The partial least squares path models demonstrated that adding N increased soil nutrient content, and complementary utilization by B.ischaemum and L.davurica increased the photosynthetic efficiency. However, as the different photosynthetic strategies of these two species, the effect on AGB was offset, and the mixture ratio's effects were larger than N. Our results proposed the B6L4 and N50 treatments were the optimal combination, with the highest AGB and overyielding, moderate grass-legume ratio, optimal community structure, and forage values.

Impact of invasive species Parectopa robiniella (Gracillariidae) on fluorescence parameters of Robinia pseudoacacia in the conditions of the steppe zone of Ukraine

Robinia pseudoacacia L. is one of the most common and environmentally adaptable introduced tree species which has become an important element of artificial afforestation and landscaping in Ukraine over the past 150 years. Throughout the history of its introduction on the territory of Ukraine, this species was considered resistant because of the absence of dangerous phytophages. At the beginning of the XXI century, the phytosanitary situation changed as the result of the penetration and rapid spread of a number of North American invasive phytophages. The appearance and distribution of the miner Parectopa robiniella (Clemens, 1863) (Lepidoptera, Gracillariidae) feeding on R. pseudoacacia was recognized as the largest invasion in Ukraine. This paper considers the issues of studying the effect of P. robiniella caterpillars feeding on R. pseudoacacia in various forest-growing conditions in the steppe zone of Ukraine. The process of photosynthesis, as the most important physiological parameter, was chosen as indicator of condition. The study was conducted using biosensor technology which made it possible to measure the effect of caterpillar feeding on critical parameters of chlorophyll fluorescence (the Kautsky curve). The research has shown that the initial value of fluorescence induction was within the range of 196–284 RFU, and the maximum value of the background fluorescence parameter was recorded in undamaged leaves and under shading conditions. Both the effect of phytophages and the shading factor caused a significant decrease in the values of fluorescence induction of the “plateau” both in the conditions of an artificially washed sandbar, on the watershed area of a watershed-gully landscape, as well as on natural sandy-loam soil. The maximum values of photosynthetic fluorescence induction under the simultaneous influence of the studied factors had rather high variability. In contrast to the fluorescence induction parameter, the “plateau” of the highest maximum fluorescence induction was reached in the absence of pest damage under conditions of total shading. As revealed by dispersion and regression analyses, the maximum fluorescence index was most dependent on the amount of solar radiation and on the degree of the leaf surface damage by phytophages. Significantly higher values of the steady-state fluorescence induction parameter were determined in the absence of insect damage in both shading and lighting conditions. A statistically significant combined influence of abiotic and biotic factors on the “plateau” fluorescence induction parameter was determined in comparison with the mono-influence of individual factors. A highly significant dependence of the maximum efficiency indicator of primary photosynthesis processes on individual factors of exogenous influence was established, while the combined effect of these factors did not affect this parameter. The obtained data allow one to apply in practice the methods of analyzing chlorophyll fluorescence induction to establish the physiological state of tree flora in forest and garden farms.

A Comparison of Photoprotective Mechanism in Different Light-Demanding Plants Under Dynamic Light Conditions

Light intensity is highly heterogeneous in nature, and plants have evolved a series of strategies to acclimate to dynamic light due to their immobile lifestyles. However, it is still unknown whether there are differences in photoprotective mechanisms among different light-demanding plants in response to dynamic light, and thus the role of non-photochemical quenching (NPQ), electron transport, and light energy allocation of photosystems in photoprotection needs to be further understood in different light-demanding plants. The activities of photosystem II (PSII) and photosystem I (PSI) in shade-tolerant species Panax notoginseng, intermediate species Polygonatum kingianum, and sun-demanding species Erigeron breviscapus were comparatively measured to elucidate photoprotection mechanisms in different light-demanding plants under dynamic light. The results showed that the NPQ and PSII maximum efficiency (F v'/F m') of E. breviscapus were higher than the other two species under dynamic high light. Meanwhile, cyclic electron flow (CEF) of sun plants is larger under transient high light conditions since the slope of post-illumination, P700 dark reduction rate, and plastoquinone (PQ) pool were greater. NPQ was more active and CEF was initiated more readily in shade plants than the two other species under transient light. Moreover, sun plants processed higher quantum yield of PSII photochemistry (ΦPSII), quantum yield of photochemical energy conversion [Y(I)], and quantum yield of non-photochemical energy dissipation due to acceptor side limitation (Y(NA), while the constitutive thermal dissipation and fluorescence (Φf,d) and quantum yield of non-photochemical energy dissipation due to donor side limitation [Y(ND)] of PSI were higher in shade plants. These results suggest that sun plants had higher NPQ and CEF for photoprotection under transient high light and mainly allocated light energy through ΦPSII and ΦNPQ, while shade plants had a higher Φf,d and a larger heat dissipation efficiency of PSI donor. Overall, it has been demonstrated that the photochemical efficiency and photoprotective capacity are greater in sun plants under transient dynamic light, while shade plants are more sensitive to transient dynamic light.

Seasonal Dynamics of Photochemical Performance of PS II of Terrestrial Mosses from Different Elevations

Mosses are critical components of tropical forest ecosystems and have multiple essential ecological functions. The drying and rehydrating and often hot environments in tropical regions present some of the greatest challenges for their photosynthetic activities. There is limited knowledge available on the physiological responses to the changing environments such as temperature and water pattern changes for terrestrial mosses. We examined the seasonal dynamics of photochemical performance of PS II through the measuring of chlorophyll fluorescence of 12 terrestrial mosses in situ from five different elevations by Photosynthesis Yield Analyzer MINI-PAM-II, along with the seasonal changes of climatic factors (air temperature, dew point, relative humidity and rainfall), which were collected by local weather stations and self-deployed mini weather stations. The results showed a great seasonality during observing periods, which, mainly the changes of rainfall and relative humidity pattern, presented significant impacts on the photochemical performance of PS II of terrestrial mosses. All these tested moss species developed a suitable regulated and non-regulated strategy to avoid the detrimental effect of abiotic stresses. We found that only Hypnum plumaeforme, Pterobryopsis crassicaulis and Pogonatum inflexum were well adapted to the changes of habitat temperature and water patterns, even though they still experienced a lower CO₂ assimilation efficiency in the drier months. The other nine species were susceptible to seasonality, especially during the months of lower rainfall and relative humidity when moss species were under physiologically reduced PS II efficiency. Anomobryum julaceum, Pogonatum neesii, Sematophyllum subhumile, Pseudotaxiphyllum pohliaecarpum and Leucobryum boninense, and especially Brachythecium buchananii, were sensitive to the changes of water patterns, which enable them as ideal ecological indicators of photosynthetic acclimation to stressed environments as a result of climate change.

Density Dependence Influences the Efficacy of Wastewater Remediation by Lemna minor

As part of a circular economy (CE) approach to food production systems, Lemnaceae, i.e., duckweed species, can be used to remediate wastewater due to rapid nutrient assimilation and tolerance of non-optimal growing conditions. Further, given rapid growth rates and high protein content, duckweed species are a valuable biomass. An important consideration for duckweed-mediated remediation is the density at which the plants grow on the surface of the wastewater, i.e., how much of the surface of the medium they cover. Higher duckweed density is known to have a negative effect on duckweed growth, which has implications for the development of duckweed-based remediation systems. In the present study, the effects of density (10-80% plant surface coverage) on Lemna minor growth, chlorophyll fluorescence and nutrient remediation of synthetic dairy processing wastewater were assessed in stationary (100 mL) and re-circulating non-axenic (11.7 L) remediation systems. Overall, L. minor growth, and TN and TP removal rates decreased as density increased. However, in the stationary system, absolute TN and TP removal were greater at higher densities (50-80% coverage). The exact cause of density related growth reduction in duckweed is unclear, especially at densities well below 100% surface coverage. A further experiment comparing duckweed grown at 'low' and 'high' density conditions with the same biomass and media volume conditions, showed that photosynthetic yield, Y(II), is reduced at high density despite the same nutrient availability at both densities, and arguably similar shading. The results demonstrate a negative effect of high density on duckweed growth and nutrient uptake, and point towards signals from neighbouring duckweed colonies as the possible cause.

Effects of Planting Density on Morphological and Photosynthetic Characteristics of Leaves in Different Positions on Cunninghamia lanceolata Saplings

The improvement of the stand yield and economic benefits of Cunninghamialanceolata (Lamb.) Hook, one of the most important plantation trees in China, has always been a concern. An appropriate planting density plays an important role in increasing productivity. To determine the optimum planting density of C. lanceolata, we measured leaf morphology, gas exchange parameters, and photosynthetic pigments in replicate leaves on 4-year-old saplings grown in four canopy positions under a low (D1), medium (D2), and high (D3) planting density (approximately 900, 2505, and 6660 trees·ha−1, respectively). We then ranked trait variations using the coefficient of variation to explore the influence of planting density. Planting density significantly influenced the leaf morphology, gas exchange parameters, and the photosynthetic pigment contents of C. lanceolata. Medium planting density (D2) resulted in a larger leaf area and specific leaf areas, a higher net photosynthesis rate, and higher photosynthetic pigment contents. The responses of leaf gas exchange parameters had the most plastic traits that responded to planting density. An appropriate planting density and leaf position might improve C. lanceolata leaf morphology and physiological attributes, which in turn would facilitate growth.

Habitat loss and canopy openness mediate leaf trait plasticity of an endangered palm in the Brazilian Atlantic Forest

Forest cover and light availability comprise key factors for plant establishment in tropical forests. In the Brazilian Atlantic Forest (AF), Euterpe edulis (Areacaceae) is an endangered and keystone food resource contributing to forest functionality. We investigated the influence of forest loss and light availability on leaf traits and acclimatization of young individuals of E. edulis in AF fragments. We aimed to understand (i) how canopy openness and transmitted light are affected by forest cover at the landscape scale and the individual palm level; and (ii) how local and landscape features, combined and separately, affect key leaf traits widely known to be related to plant growth. The study was carried out in 15 forest fragments, ranging from 16 to 97% of surrounding forest cover. In each fragment, we sampled 10-20 individuals of E. edulis and analyzed nine leaf traits related to morphological, biochemical and chemical aspects. We also took hemispherical photographs to estimate canopy openness on the top of each E. edulis and also within fragment plots. We found that young plants predominantly occurred in more shaded environments. Additionally, E. edulis succeeded to acclimate in six of the nine traits analyzed, with most traits being affected by local and landscape features. It is likely that the lack of variation in traits related to protection against herbivory are limiting the species establishment in highly deforested landscapes. Our results provide novel evidence that both landscape and local contexts affect the leaf traits of E. edulis young plants leading to biochemical, chemical and morphological adjustments.

Comparative physiological and transcriptomic analysis of pear leaves under distinct training systems

Canopy architecture is critical in determining the light interception and distribution, and subsequently the photosynthetic efficiency and productivity. However, the physiological responses and molecular mechanisms by which pear canopy architectural traits impact on photosynthesis remain poorly understood. Here, physiological investigations coupled with comparative transcriptomic analyses were performed in pear leaves under distinct training systems. Compared with traditional freestanding system, flat-type trellis system (DP) showed higher net photosynthetic rate (P_N) levels at the most time points throughout the entire monitored period, especially for the interior of the canopy in sunny side. Gene ontology analysis revealed that photosynthesis, carbohydrate derivative catabolic process and fatty acid metabolic process were over-represented in leaves of DP system with open-canopy characteristics. Weighted gene co-expression network analysis uncovered a significant network module positive correlated with P_N value. The hub genes (PpFKF1 and PpPRR5) of the module were enriched in circadian rhythm pathway, suggesting a functional role for circadian clock genes in mediating photosynthetic performance under distinct training systems. These results draw a link between pear photosynthetic response and specific canopy architectural traits, and highlight light harvesting and circadian clock network as potential targets for the input signals from the fluctuating light availability under distinct training systems.

Study on Spectral Response and Estimation of Grassland Plants Dust Retention Based on Hyperspectral Data

Accurate monitoring of plant dust retention can provide a basis for dust pollution control and environmental protection. The aims of this study were to analyze the spectral response features of grassland plants to mining dust and to predict the spatial distribution of dust retention using hyperspectral data. The dust retention content was determined by an electronic analytical balance and a leaf area meter. The leaf reflectance spectrum was measured by a handheld hyperspectral camera, and the airborne hyperspectral data were obtained using an imaging spectrometer. We analyzed the difference between the leaf spectral before and after dust removal. The sensitive spectra of dust retention on the leaf- and the canopy-scale were determined through two-dimensional correlation spectroscopy (2DCOS). The competitive adaptive reweighted sampling (CARS) algorithm was applied to select the feature bands of canopy dust retention. The estimation model of canopy dust retention was built through random forest regression (RFR), and the dust distribution map was obtained based on the airborne hyperspectral image. The results showed that dust retention enhanced the spectral reflectance of leaves in the visible wavelength but weakened the reflectance in the near-infrared wavelength. Caused by the canopy structure and multiple scattering, a slight difference in the sensitive spectra on dust retention existed between the canopy and leaves. Similarly, the sensitive spectra of leaves and the canopy were closely related to dust and plant physiological parameters. The estimation model constructed through 2DCOS-CARS-RFR showed higher precision, compared with genetic algorithm-random forest regression (GA-RFR) and simulated annealing algorithm-random forest regression (SAA-RFR). Spatially, the amount of canopy dust increased and then decreased with increasing distance from the mining area, reaching a maximum within 300–500 m. This study not only demonstrated the importance of extracting feature bands based on the response of plant physical and chemical parameters to dust, but also laid a foundation for the rapid and non-destructive monitoring of grassland plant dust retention.

A combination of morphological and photosynthetic functional traits maintains the vertical distribution of bryophytes in a subtropical cloud forest

PREMISE

The distribution and performance of bryophyte species vary with vertical gradients, as a result of changes in environmental factors, especially light. However, the morphological and physiological drivers of bryophyte distribution along forest vertical gradients are poorly understood.

METHODS

For 18 species of mosses and liverworts distributed among three vertical microhabitats (ground, tree trunk, and branch, variance in 28 morphological and photosynthetic functional traits was comparatively analyzed among the microhabitats and bryophyte life-forms in a subtropical cloud forest in Ailao Mountain, Yunnan, southwestern China. Principal component analysis (PCA) was used to summarize trait differences among bryophyte species.

RESULTS

In contrast to trunk and ground dwellers, branch dwellers tended to reduce light interception (smaller leaf and cell sizes, lower chlorophyll content), protect against damage from intense irradiation (higher ratios of carotenoids to chlorophyll), raise light energy use (higher photosynthetic capacity), and cope with lower environmental moisture (pendant life-forms, thicker cell walls). The PCA showed that ecological strategies of bryophytes in response to levels of irradiation were specialized in branch dwellers, although those of ground and trunk dwellers were less distinct.

CONCLUSIONS

Environmental filtering shaped the combination of functional traits and the spatial distribution of bryophytes along the vertical gradients. Bryophyte species from the upper canopy of cloud forests show narrow variation in functional traits in high-light intensity, whereas species in the lower vertical strata associated with low-light intensity used contrasting, but more diverse ecological strategies.

Reflectance Properties of Hemiboreal Mixed Forest Canopies with Focus on Red Edge and Near Infrared Spectral Regions

This study present the results of airborne top-of-canopy measurements of reflectance spectra in the spectral domain of 350–1050 nm over the hemiboreal mixed forest. We investigated spectral transformations that were originally designed for utilization at very different spectral resolutions. We found that the estimates of red edge inflection point by two methods—the linear four-point interpolation approach (S2REP) and searching the maximum of the first derivative spectrum ( D m a x ) according to the mathematical definition of red edge inflection point—were well related to each other but S2REP produced a continuously shifting location of red edge inflection point while D m a x resulted in a discrete variable with peak jumps between fixed locations around 717 nm and 727 nm for forest canopy (the third maximum at 700 nm appeared only in clearcut areas). We found that, with medium high spectral resolution (bandwidth 10 nm, spectral step 3.3 nm), the in-filling of the O 2 -A Fraunhofer line ( F a r e a ) was very strongly related to single band reflectance factor in NIR spectral region ( ρ = 0.91, p < 0.001) and not related to Photochemical Reflectance Index (PRI). Stemwood volume, basal area and tree height of dominant layer were negatively correlated with reflectance factors at both visible and NIR spectral region due to the increase in roughness of canopy surface and the amount of shade. Forest age was best related to single band reflectance at NIR region ( ρ = −0.48, p < 0.001) and the best predictor for allometric LAI was the single band reflectance at red spectral region ( ρ = −0.52, p < 0.001) outperforming all studied vegetation indices. It suggests that Sentinel-2 MSI bands with higher spatial resolution (10 m pixel size) could be more beneficial than increased spectral resolution for monitoring forest LAI and age. The new index R 751 /R 736 originally developed for leaf chlorophyll content estimation, also performed well at the canopy level and was mainly influenced by the location of red edge inflection point ( ρ = 0.99, p < 0.001) providing similar info in a simpler mathematical form and using a narrow spectral region very close to the O 2 -A Fraunhofer line.

Seasonal variation in non-structural carbohydrates, sucrolytic activity and secondary metabolites in deciduous and perennial Diospyros species sampled in Western Mexico

This study was performed to test the working hypothesis that the primary determinants influencing seasonal driven modifications in carbon mobilization and other key biochemical parameters in leaves of poorly known Diospyros digyna (Ddg; semi-domesticated; perennial) and D. rekoi (Dre; undomesticated; deciduous) trees are determined by environmental growing conditions, agronomic management and physiological plasticity. Thus, biochemical changes in leaves of both trees were recorded seasonally during two successive fruiting years. Trees were randomly sampled in Western Mexico habitats with differing soil quality, climatic conditions, luminosity, and cultivation practices. Leaves of Ddg had consistently higher total chlorophyll contents (CT) that, unexpectedly, peaked in the winter of 2015. In Dre, the highest leaf CT values recorded in the summer of 2015 inversely correlated with low average luminosity and high Chl a/ Chlb ratios. The seasonal CT variations in Dre were congruent with varying luminosity, whereas those in Ddg were probably affected by other factors, such as fluctuating leaf protein contents and the funneling of light energy to foliar non-structural carbohydrates (NSCs) accumulation, which were consistently higher than those detected in Dre leaves. Seasonal foliar NSC fluctuations in both species were in agreement with the carbon (C) demands of flowering, fruiting and/ or leaf regrowth. Seasonal changes in foliar hexose to sucrose (Hex/ Suc) ratios coincided with cell wall invertase activity in both species. In Dre, high Hex/ Suc ratios in spring leaves possibly allowed an accumulation of phenolic acids, not observed in Ddg. The above results supported the hypothesis proposed by showing that leaf responses to changing environmental conditions differ in perennial and deciduous Diospyros trees, including a dynamic adjustment of NSCs to supply the C demands imposed by reproduction, leaf regrowth and, possibly, stress.

Taxonomic and ecological relevance of the chlorophyll a fluorescence signature of tree species in mixed European forests

The variability of chlorophyll a fluorescence (ChlF) parameters of forest tree species was investigated in 209 stands belonging to six European forests, from Mediterranean to boreal regions. The modifying role of environmental factors, forest structure and tree diversity (species richness and composition) on ChlF signature was analysed. At the European level, conifers showed higher potential performance than broadleaf species. Forests in central Europe performed better than those in Mediterranean and boreal regions. At the site level, homogeneous clusters of tree species were identified by means of a principal component analysis (PCA) of ChlF parameters. The discrimination of the clusters of species was influenced by their taxonomic position and ecological characteristics. The species richness influenced the tree ChlF properties in different ways depending on tree species and site. Tree species and site also affected the relationships between ChlF parameters and other plant functional traits (specific leaf area, leaf nitrogen content, light-saturated photosynthesis, wood density, leaf carbon isotope composition). The assessment of the photosynthetic properties of tree species, by means of ChlF parameters, in relation to their functional traits, is a relevant issue for studies in forest ecology. The connections of data from field surveys with remotely assessed parameters must be carefully explored.

Why does needle photosynthesis decline with tree height in Norway spruce?

Needle morphological, chemical and physiological characteristics of Norway spruce were studied in a forest chronosequence in Järvselja Experimental Forest, Estonia. Current-year shoots were sampled from upper canopy positions in five stands, ranging in height from 1.8 to 33.0 m (corresponding age range was 10-85 years). A/C(i) curves were determined to obtain maximum carboxylation rates (V(cmax)) and maximum rates of electron transport (J(max)). Needle nitrogen (N) partitioning into photosynthetic functions was calculated from the values of V(cmax), J(max) and leaf chlorophyll concentration. All needle size parameters (length, width, thickness, volume and cross-sectional areas of mesophyll and xylem) increased significantly with tree height. The needles of taller trees had lower mass-based N and chlorophyll concentrations (21% and 43% difference between shortest and tallest stands, respectively), but higher dry mass per area (35%), dry mass per volume (18%), number of cells per mesophyll cross-section area (40%) and partitioning of N into non-photosynthetic functions (12%). Light saturated net assimilation rate, V(cmax), J(max) and stomatal conductance decreased with tree age (35%, 16%, 12% and 29% difference, respectively). A path analysis model describing tree age-related reduction of photosynthetic capacity as a result of sink limitation provided the best fit to our data. However, since the path model corresponding to source limitation, where photosynthetic reduction derives from changes in needle structure and chemistry was not rejected, we conclude that the decline in photosynthesis with tree age results from several mechanisms (limited sink strength, stomatal and N limitation) operating simultaneously and sequentially.