Shade Needles Research Articles

Vertical gradient of needle ozone uptake within the canopy of Cryptomeria japonica

Leaf ozone uptake through the stomata is an important index for the ozone risk assessments on trees. Stomatal conductance (gs) and ozone concentration ([O3]), determinants of the leaf ozone uptake, are known to show vertical gradients within a tree canopy. However, less is known about the within-canopy vertical gradient of leaf ozone uptake. This study was aimed to elucidate how the vertical gradient of [O3] and gs affect needle ozone uptake within a canopy of mature Cryptomeria japonica trees in a suburban forest at Tokyo, Japan. For this purpose, a multilayer gas exchange model was applied to estimate the vertical gradient of needle gs and the accumulated ozone uptake during the study period (POD1, Phytotoxic Ozone Dose above a threshold of 1 nmol m−2 s−1). In addition, we also tested several scenarios of vertical gradient of [O3] within the canopy for sensitivity analysis. The POD1 was declined from the top to the bottom of the canopy. This tendency strongly depended on the vertical gradient of gs and was hardly affected by the changes in simulated vertical reductions of the [O3]. We further assessed the photosynthesis of sunlit needles (needles absorbing both direct and diffuse light) and shaded needles (needles only absorbing diffuse light). The photosynthesis of shaded needles in the upper half of the canopy made a great contribution to the entire canopy photosynthesis. In addition, given that their POD1 was lower than that of sunlit needles, ozone may affect sunlit and shaded needles differently. We concluded that these considerations should be incorporated into modeling of the calculation of ozone uptake for mature trees to make accurate predictions of the ozone effects on trees at the canopy scale.

Effect of two organic fertilizers on some anatomical features of Pinus pinea L. seedlings grown in field and lathe house

Effect of two organic fertilizers on some anatomical features of Pinus pinea L. seedlings grown in field and lathe house

Effects of mild drought on the morphology of sun and shade needles in 20-year-old Norway spruce trees

Several studies have looked at how individual environmental factors influence needle morphology in conifer trees, but interacting effects between drought and canopy position have received little attention. In this study, we characterized morphological responses to experimentally induced drought stress in sun exposed and shaded current-year Norway spruce needles. In the drought plot trees were suffering mild drought stress, with an average soil water potential at 50 cm depth of -0.4 MPa. In general, morphological needle traits had greater values in sun needles in the upper canopy than in shaded needles in the lower canopy. Needle morphology 15 months after the onset of drought was determined by canopy position, as only sun needle morphology was affected by drought. Thus, canopy position was a stronger morphogenic factor determining needle structure than was water availability. The largest influence of mild drought was observed for needle length, projected needle area and total needle area, which all were reduced by ~27% relative to control trees. Needle thickness and needle width showed contrasting sensitivity to drought, as drought only affected needle thickness (10% reduction). Needle dry mass, leaf mass per area and needle density were not affected 15 months after the onset of mild drought. Our results highlight the importance of considering canopy position as well as water availability when comparing needle structure or function between conifer species. More knowledge about how different canopy parts of Norway spruce adapt to drought is important to understand forest productivity under changing environmental conditions.

Norway spruce needle size and cross section shape variability induced by irradiance on a macro- and microscale and CO2 concentration

Needle cross-section parameters differ according to needle orientation on a shoot corresponding to irradiance microgradient. Irradiance is a stronger morphogenic factor determining needle size and shape than CO2 concentration. We investigated the effects of irradiance on macroscale and microscale, elevated CO2 concentration [CO2], and their interaction on Norway spruce (Picea abies L. Karst.) needles. The irradiance macroscale was represented by sun and shade shoots from two vertical positions in a crown and the irradiance microscale corresponded to spatial orientation of individual needles on a shoot (upper, side, and lower needles relative to the shoot axis). Determination of needle cross section shape using generalized Procrustes analysis and principal component analysis provided a novel approach for evaluating needle morphometry. As expected, shade needles on the irradiance macrogradient were flatter and had less cross-sectional area and smaller volume than did sun needles. The irradiance microgradient was detected within both sun and shade shoots, being steeper in sun shoots than shade shoots. On the microscale, the irradiance gradient induced changes in needle size and cross section shape according to needles’ orientation on a shoot. Due to a more favourable light environment the traits of the upper needles within both sun and shade shoots resembled more the sun needle traits. The sun needle volume was significantly larger in the case of elevated [CO2] as compared to ambient [CO2]. Irradiance was a stronger morphogenic factor determining needle size and cross section shape compared to CO2 concentration. We demonstrated that generalized Procrustes analysis can be a very powerful tool in ecophysiological studies for evaluating small-scale, subtle leaf shape changes on an intraspecific level caused by environmental factors.

Effects of prolonged drought on the anatomy of sun and shade needles in young Norway spruce trees.

Predicted increases in the frequency and duration of drought are expected to negatively affect tree vitality, but we know little about how water shortage will influence needle anatomy and thereby the trees' photosynthetic and hydraulic capacity. In this study, we evaluated anatomical changes in sun and shade needles of 20-year-old Norway spruce trees exposed to artificial drought stress. Canopy position was found to be important for needle structure, as sun needles had significantly higher values than shade needles for all anatomical traits (i.e., cross-sectional needle area, number of tracheids in needle, needle hydraulic conductivity, and tracheid lumen area), except proportion of xylem area per cross-sectional needle area. In sun needles, drought reduced all trait values by 10-40%, whereas in shade needles, only tracheid maximum diameter was reduced by drought. Due to the relatively weaker response of shade needles than sun needles in drought-stressed trees, the difference between the two needle types was reduced by 25% in the drought-stressed trees compared to the control trees. The observed changes in needle anatomy provide new understanding of how Norway spruce adapts to drought stress and may improve predictions of how forests will respond to global climate change.

Diversity in needle morphology and genetic markers in a marginal Abies cephalonica (Pinaceae) population

Differences in needle traits of coniferous tree species are consid- ered as the combined result of direct environmental pressure and specific genetic adaptations. In this study, diversity and differentiation within and among four Abies cephalonica subpopulations of a marginal population on Mt. Parnitha - Greece, were estimated using needle morphological traits and gene markers. We tested the connection of morphological variability patterns of light and shade needles with possible adaptation strategies and genetic diversity. Six morphological characteristics were used for the de- scription of both light and shade needles at 100 trees, describing needle size and shape, stomatal density and needle position on the twigs. Additionally, six RAPD and three ISSR markers were applied on DNA from the same trees. Light needles were significantly different than shade needles, in all traits measured, apparently following a different light harvesting strategy. All four subpopulations exhibited high genetic diversity and the differen- tiation among them was relatively low. Differences among populations in light needles seemed to depend on light exposure and aspect. In shade nee- dles, the four subpopulations seemed to deviate stronger from each other and express a rather geographic pattern, similarly to the genetic markers. Two of the subpopulations studied were lost during a wildfire, two years after sampling. Although the subpopulations burnt were most diverse and most differentiated, we expect a large part of the total genetic diversity of the burnt trees to still exist in the surviving subpopulations, since gene flow must have been effective in keeping all subpopulations connected. Keywords Abies cephalonica, conifer, needle morphology, genetic diversity, adaptation.

Within-canopy variation in needle morphology and anatomy of vascular tissues in a sparse Scots pine forest

Key message Azimuthal sides of the canopy affected neither needle morphology nor vascular anatomy. However, a significance was found to be associated with canopy height. The needle morphological and anato- mical parameters of vascular tissues were scaled up to the forest stand level. Abstract In conifers the needle is the key organ for photosynthesis and transpiration, and these two processes are influenced by the morphological and anatomical structure of the needle. Although many factors are involved in needle development, long-term irradiance gradients through the canopy as well as across the forest are among the main drivers. The present study had two principal ob- jectives: (1) to obtain a better knowledge of the morpho- logical and anatomical parameters of sun, transient and shade needles taken from different azimuthal orientations of a sparse Scots pine (Pinus sylvestris L.) stand; and (2) to scale up these needle data to the forest stand level. One- year-old needles were collected from mature Scots pine trees from branches on the south- and the north-facing azimuthal sides of the canopy at three different canopy heights. Needle structural parameters were measured on cross sections at the needle base. Azimuthal sides of the canopy had no effect on the needle morphology and anatomy since the irradiation was similar on both canopy sides due to the low leaf area index (1.31) of the sparse Scots pine forest. However, sampling height (i.e. sun ver- sus shade needles) had a significant effect on the studied parameters, the largest differences being needle stele area, xylem area, phloem area and number of tracheids. Tracheid frequency, leaf-specific hydraulic conductivity and needle density were the only parameters which were not influ- enced by canopy height. Our measurements revealed that in a given hectare of pine forest stand, water was transported through approximately 40 9 10 9 tracheids situated in 380 9 10 6 needles, whose xylem area was 4.34 m 2 .T his type of data could be helpful for modelling and could provide a better understanding of the forest stand envi- ronment, of tree hydraulic systems or of eddy covariance flux measurements of this stand, an established ecosystem site within the European ICOS network.

Optical properties of leaves and needles for boreal tree species in Europe

Reliable information on the optical properties of leaves and needles is needed for parameterization of radiative transfer models and interpretation of remotely sensed data. The optical properties also convey information about the structure and biochemical constituents of the leaf or needle tissues, and can be linked to the photosynthetic processes of plants. Currently, very little is known about the optical properties of tree species in the European boreal zone. To bridge this gap, we measured directional-hemispherical reflectance and transmittance factors of the three most common tree species in this zone: Scots pine, Norway spruce and Silver birch. The measurements covered a wide spectral range from 350 to 2500 nm with a high spectral resolution of 3–10 nm. To explore the driving factors of the observed leaf-level optical properties, supplementary measurements of structural and biochemical traits of leaves and needles were made. The results showed that the transmittance of pine and spruce needles is clearly lower than reflectance, whereas for birch reflectance and transmittance were similar. In conifers, exposed needles had higher albedo than the shaded needles. Also, the spectra of needles were more dependent on canopy position than the spectra of birch leaves. The relationships between narrowband reflectances and chlorophyll and nitrogen percentage concentrations and specific leaf area were similar for all species, but the strongest correlations were observed for birch.

Recovery kinetics of photochemical efficiency in winter stressed conifers: the effects of growth light environment, extent of the season and species

Evergreens undergo reductions in maximal photochemical efficiency (F(v)/F(m)) during winter due to increases in sustained thermal energy dissipation. Upon removing winter stressed leaves to room temperature and low light, F(v)/F(m) recovers and can include both a rapid and a slow phase. The goal of this study was to determine whether the rapid component to recovery exists in winter stressed conifers at any point during the season in a seasonally extreme environment. Additional goals were to compare the effects of species, growth light environment and the extent of the winter season on recovery kinetics in conifers. Four species (sun and shade needle) were monitored during the winter of 2007/2008: eastern white pine (Pinus strobus), balsam fir (Abies balsamea), Taxus cuspidata and white spruce (Picea glauca). F(v)/F(m) was measured in the field, and then monitored indoors at room temperature and low light for 6 days. The results showed that all species showed a rapid component to recovery in early winter that disappeared later in the season in sun needles but was present in shade needles on most days monitored during winter. There were differences among species in the recovery kinetics across the season, with pine recovering the most slowly and spruce the most quickly. The results suggest an important role for the rapidly reversible form of energy dissipation in early winter, as well as important differences between species in their rate of recovery in late winter/early spring which may have implications for spring onset of photosynthesis.

The impact of long-term CO2 enrichment on sun and shade needles of Norway spruce (Picea abies): Photosynthetic performance, needle anatomy and phenolics accumulation

Norway spruce (Picea abies L. Karst) grown under ambient (365–377μmol(CO2)mol−1; AC) and elevated (700μmol(CO2)mol−1; EC) CO2 concentrations within glass domes with automatically adjustable windows and on an open-air control site were studied after 8 years of treatment. The effect of EC on photosynthesis, mesophyll structure and phenolics accumulation in sun and shade needles was examined. Photosynthetic assimilation and dark respiration rates were measured gasometrically; the structural parameters of mesophyll were determined using confocal microscopy and stereological methods. The contents of total soluble phenolics and lignin were assessed spectrophotometrically, and localizations of different phenolic groups were detected histochemically on needle cross-sections.EC enhanced the light-saturated CO2 assimilation rate and reduced dark respiration in the current-year needles. No effects of CO2 enrichment on mesophyll structural parameters were observed. Similarly, the accumulation and localization of phenolics and lignin remained unaffected by EC treatment. Needles differentiated into sun and shade ecotypes in the same manner and to the same extent irrespective of CO2 treatment. Based on these results, it is apparent that the EC-induced enhancement of photosynthesis is not related to changes in the examined structural parameters of mesophyll and accumulation of phenolic compounds.

Effects of different light conditions on the xylem structure of Norway spruce needles

Conifer needles are extraordinarily variable and much of this diversity is linked to the water transport capacity of the xylem and to xylem conduit properties. However, we still know little about how anatomical characteristics influence the hydraulic efficiency of needle xylem in different parts of the crown. In this study we evaluated needle function and anatomy in Norway spruce families exposed to different light conditions. We measured tracheid and needle characteristics of sun-exposed and shaded current-year needles in two experimental plots: a control plot and a thinned plot with 50% reduction in stand density. Sun-exposed needles had a larger tracheid lumen area than shaded needles, and this was caused by a larger maximum tracheid lumen diameter, while the minimum lumen diameter was less plastic. Sun-exposed needles had also higher theoretical hydraulic conductivity than shaded needles. Thinning leads to increased radiation to the lower branches, and presumably exposes the upper branches to stronger water stress than before thinning. Thinning affected several needle parameters both in sun-exposed and shaded needles; tracheid lumens were more circular and minimum tracheid lumen diameter was larger in the thinned plot, whereas maximum tracheid lumen diameter was less plastic on both plots. This study demonstrates that needle xylem structure in Norway spruce is clearly influenced by the light gradient within the tree crown.

Recovery kinetics of winter stressed conifers: The effects of growth light environment, extent of the season, and species.

AbstractEvergreens undergo a dramatic reduction in their maximal photochemical efficiency (measured as Fv/Fm) during winter, which is largely due to increases in a sustained form of thermal energy dissipation. Upon removing winter-stressed leaves to room temperature and low light, Fv/Fm recovers over several days and can include both a rapid phase (reversing in minutes) and a slow phase (reversing over days). Both phases are associated with reversal of sustained energy dissipation. Preliminary examination of recovery of evergreens monitored in January in Minnesota showed an absence of the rapid component to recovery. Our goal was to monitor recovery kinetics of sun and shade evergreens in Minnesota throughout winter in order to assess whether the rapid phase of recovery exists early in the winter and converts to the slowly reversible form as winter progresses. Four species of conifers (sun and shade needles) were monitored during the winter of 2007/08: eastern white pine (_Pinus strobus_ L.), balsam fir [Abies balsamea (L.) P. Mill], Taxus cuspidata (L.) and blue spruce (Picea pungens Engelm.). Fv/Fm was measured on dark acclimated needles in the field, twigs were collected, brought indoors and maintained at room temperature and low light where Fv/Fm was monitored for six days.The results demonstrated that all species, and both sun and shade needles, showed a rapidly reversible component to recovery in early winter (November). In the sun needles this component was rarely present later in the season, while in the shade needles it was present (although only a small fraction of the total sustained energy dissipation) on most days monitored during winter. The slowly reversible component to sustained energy dissipation was present in both sun and shade needles of all species beginning in November. In all cases, shade needles recovered significantly faster than sun needles. There was a significant slowing of recovery (the slowly reversible component) as winter progressed in both sun and shade needles, and significant differences between species in their recovery response. The results indicate a relatively small contribution of the rapidly reversible component of sustained energy dissipation compared with earlier studies on evergreens growing in the milder winter conditions of Colorado. The results also provide evidence that the rapid component to recovery diminishes as the season progresses, particularly in needles growing in full sun where the slowly reversible component of sustained energy dissipation accounts for most or all of the observed sustained energy dissipation.

Seasonal acclimation of photosystem II in Pinus sylvestris. II. Using the rate constants of sustained thermal energy dissipation and photochemistry to study the effect of the light environment

Photosynthesis in evergreen conifers is characterized by down-regulation in autumn and rapid up-regulation in spring. This seasonal pattern is largely driven by temperature, but the light environment also plays a role. In overwintering Scots pine (Pinus sylvestris L.) trees, PSII is less down-regulated and recovers faster from winter stress in shaded needles than in needles exposed to full sunlight. Because the effect of light on the seasonal acclimation of PSII has not been quantitatively studied under field conditions, we used the rate constants for sustained thermal energy dissipation and photochemistry to investigate the dynamics and kinetics of the seasonal acclimation of PSII in needles exposed to different light environments. We monitored chlorophyll fluorescence and needle pigment concentration during the winter and spring in Scots pine seedlings growing in the field in different shading treatments, and within the crowns of mature trees. The results indicated that differences in acclimation of PSII in overwintering Scots pine among needles exposed to different light environments can be chiefly attributed to sustained thermal dissipation. We also present field evidence that zeaxanthin-facilitated thermal dissipation and aggregation of thylakoid membrane proteins are key mechanisms in the regulation of sustained thermal dissipation in Scots pine trees in the field.

Shade effect alters leaf pigments and photosynthetic responses in Norway spruce (Picea abies L.) grown under field conditions

The contents of chlorophyll (Chl) and carotenoids (Car) per fresh mass were lower in shade needles than in sun needles. Ribulose-1,5-bisphosphate carboxylase (RuBPC) activity and contents of soluble proteins were also significantly lower in shade needles. In isolated thylakoids, a marked lower rate of whole chain and photosystem (PS) 2 activities were observed in shade needles. Smaller lower rate of PS1 activity was also observed in shade needles. The artificial exogenous electron donors, diphenyl carbazide (DPC) and NH2OH, significantly restored the loss of PS2 activity in shade needles. Similar results were obtained when Fv/Fm was evaluated by Chl fluorescence measurements. The marked lower rate of PS2 activity in shade needles was due to the lower contents of 47, 33, 28-25, 23, and 17 kDa polypeptides. This conclusion was confirmed by immunological studies showing that the content of the 33 kDa protein of the watersplitting complex was diminished significantly in shade needles.

Seasonal changes in leaf antioxidant systems and xanthophyll cycle characteristics in Taxus x media growing in sun and shade environments

We examined differences between summer and winter in xanthophyll cycle‐dependent energy dissipation and leaf antioxidant systems in needles of the overwintering evergreen Taxus x media cv. Tauntonii (Taunton yew) growing in both sun and shade environments in Saint Paul, Minnesota. During the winter, both sun and shade plants exhibited increases in the capacity for, and utilization of, xanthophyll cycle‐dependent thermal energy dissipation. Winter needles showed decreases (sun needles) or no change (shade needles) in superoxide dismutase activity (EC 1.15.1.1), no change in ascorbate peroxidase activity (EC 1.11.1.11) and no change (sun needles) or increases (shade needles) in reduced ascorbate levels. Both sun and shade needles showed large increases in glutathione reductase activity (EC 1.6.4.2) and total glutathione levels during the winter, in addition to increases in levels of α‐tocopherol. These results suggest an important photoprotective role during the winter for xanthophyll cycle‐dependent energy dissipation and for the antioxidants glutathione and α‐tocopherol. They suggest a less important photoprotective function of the enzyme‐based water–water cycle in winter acclimation in the seasonally very cold environment of Minnesota.

Growth, photosynthesis, and needle structure of silver fir ( Abies alba Mill.) seedlings under different canopies

As part of fir reintroduction program in the Karkonosze Mountains (S. Poland) a study was undertaken to investigate growth and ecophysiological performance of young firs ( Abies alba) under varying degrees of natural shade. Three-year-old fir seedlings were planted in five forest stands dominated by Picea abies, Fagus sylvatica, Betula pendula, Pinus sylvestris or Larix decidua. Over four growing seasons, annual apical increments became progressively greater under the light permeable canopy of Larix, but remained stunned under the deep shade of Picea, while intermediate irradiance at other sites resulted in intermediate growth responses. Similarly, apical dominance ratio was positively influenced by irradiance. These two contrasting sites were subsequently used for ecophysiological comparisons. Fir seedlings under the Larix canopy had consistently higher photosynthetic rates throughout the day compared to those under Picea, but in both stands light adapted quantum yield of PSII remained mostly above 0.8 indicating lack of photoinhibition. Quantum yield declined transiently only during sunflecks, which were more frequent under Larix. Light dependence of apparent electron transport rates in both sites was linear showing lack of light saturation, while slope differences between sites were only slight. In contrast, depression of PSII quantum yield by light was less pronounced under Larix, showing an acclimation to higher irradiance. Fir needles produced under Larix were thicker and contained more chlorophyll, carotenoid and nitrogen on area basis (but, except for nitrogen, not on dry mass basis) suggesting that structural modification is a major component of acclimation to light. On anatomical levels, needles produced under Larix had the traits typical for sun-acclimated ones, while those under Picea for shade-acclimated. The magnitude of modifications was, however, less than that reported in literature for sun and shade needles. Young seedlings were able to acclimate to higher irradiance under Larix without showing light stress symptoms. This suggests that success of stand regeneration may be enhanced by adjusting light penetration through the overstorey. Fir seedlings found best growth conditions under the relatively light permeable crowns of light-demanding tree species ( Larix decidua, Pinus sylvestris) suggesting that stands dominated by these trees are more suitable for transformation using silver fir seedlings than those with shade-tolerant tree overstoreys.

Seasonal changes in the xanthophyll cycle and antioxidants in sun-exposed and shaded parts of the crown of Cryptomeria japonica in relation to rhodoxanthin accumulation during cold acclimation.

Xanthophyll rhodoxanthin, which is present in sun-exposed needles of certain gymnosperms in winter, may have a photoprotective role during long-term cold acclimation. To examine how cold acclimation processes vary within tree crowns and to examine putative correlations between xanthophyll cycle pigments (VAZ), rhodoxanthin and the water-water cycle in photoprotection, we monitored seasonal changes in the activities of two key antioxidant enzymes (ascorbate peroxidase (APX) and glutathione reductase (GR)), pigment composition and chlorophyll fluorescence parameters in sun and shade needles of crowns of the gymnosperm Cryptomeria japonica D. Don. Although APX and GR activities in both sun and shade needles were higher in winter than in summer when assayed at 20 degrees C, differences between seasons were less pronounced when enzymatic activities in summer and winter were assayed at 20 and 5 degrees C, respectively. These results suggest that increases in the potential activity of antioxidant enzymes in winter is an adaptation that helps counterbalance reductions in absolute enzyme activity caused by low temperature, and thus allows the photoprotective capacity of the water-water cycle in C. japonica to be maintained at a roughly constant value throughout the year. In shade needles, the concentration of VAZ increased in winter, but no rhodoxanthin accumulated. Photosynthetic activity was maintained in winter. In sun needles, however, the electron transport rate (ETR) and photochemical quenching (q(P)) decreased to their lowest values in December, just before the accumulation of rhodoxanthin, which coincided with the highest amount of VAZ. Changes in rhodoxanthin concentration mirrored changes in VAZ concentration from January to March. Winter values of ETR and q(P) were comparable with summer values after accumulation of rhodoxanthin, indicating that rhodoxanthin may play a more important role than the VAZ cycle in protecting the photosynthetic apparatus from photodamage in winter. Photosynthetic activity may be modulated, as a result of the interception of light by rhodoxanthin, to match the extent to which absorbed light energy can be utilized in winter when the VAZ cycle is unable to operate effectively because of low temperatures.

Foliar chemistry of balsam fir and red spruce in relation to elevation and the canopy light gradient in the mountains of the northeastern United States

Red spruce (Picea rubens Sarg.) and balsam fir (Abies balsamea[L.] Mill) are the dominant conifer species at treeline in the mountains of the northeastern United States. The objective of this study was to investigate changes in foliar chemistry of these species along both elevational (below, at, and above treeline) and canopy light (sun vs. shade leaves) gradients. Nutrient concentrations (mass basis) did not show any significant (all P>0.05) differences among elevations, although mean concentrations of all macronutrients (N, P, K, Ca, Mg) tended to be higher at low elevation sites compared to high elevation. This result contradicts the traditional view that plants in cold growth environments are adapted to maintain high foliar nutrient concentrations, but it also gives only weak support for the hypothesis that nutrient limitation plays a role in determining treeline location. Foliar concentrations (mass basis) of lignin (both sun and shade needles) and cellulose (sun needles only) decreased sharply and significantly with increasing elevation, but foliar concentrations of hemicellulose did not change with elevation. These results are consistent with the hypothesis that as a result of carbon limitation at high elevation, synthesis of the most expensive fiber constituent (i.e. lignin) is reduced more than that of the least expensive fiber constituent (i.e. hemicellulose). The reduced lignin concentration at high elevation may have implications for nutrient cycling in this ecosystem where cold temperatures limit decomposition rates.

Photosynthesis and Photoprotection in Overwintering Plants

Abstract: Seasonal differences in the capacity of photosynthetic electron transport, leaf pigment composition, xanthophyll cycle characteristics and chlorophyll fluorescence emission were investigated in two biennial mesophytes (Malva neglecta and Verbascum thapsus) that grow in full sunlight, and in leaves/needles of sun and shade populations of several broad‐leafed evergreens and conifers (Vinca minor, Euonymus kiautschovicus, Mahonia repens, Pseudotsuga menziesii [Douglas fir], and Pinus ponderosa). Both mesophytic species maintained or upregulated photosynthetic capacity in the winter and exhibited no upregulation of photoprotection. In contrast, photosynthetic capacity was downregulated in sun leaves/needles of V. minor, Douglas fir, and Ponderosa pine, and even in shade needles of Douglas fir. Interestingly, photosynthetic capacity was upregulated during the winter in shade leaves/needles of V. minor, Ponderosa pine and Euonymus kiautschovicus. Nocturnal retention of zeaxanthin and antheraxanthin, and their sustained engagement in a state primed for energy dissipation, were observed largely in the leaves/needles of sun‐exposed evergreen species during winter. Factors that may contribute to these differing responses to winter stress, including chloroplast redox state, the relative levels of source and sink activity at the whole plant level, and apoplastic versus symplastic phloem loading, are discussed.

Different Responses of Norway Spruce Needles from Shaded and Exposed Crown Layers to the Prolonged Exposure to Elevated CO<sub>2</sub> Studied by Various Chlorophyll a Fluorescence Techniques

The acclimation depression of capacity of photon utilisation in photochemical reactions of photosystem 2 (PS2) can develop already after three months of cultivation of the Norway spruces (Picea abies [L.] Karst.) under elevated concentrations of CO2 (i.e., ambient, AC, + 350 µmol(CO2) mol-1 = EC) in glass domes with adjustable windows. To examine the role that duration of EC plays in acclimation response, we determined pigment contents, rate of photosynthesis, and parameters of chlorophyll a fluorescence for sun and shade needles after three seasons of EC exposure. We found responses of shaded and exposed needles to EC. Whereas the shaded needles still profited from the EC and revealed stimulated electron transport, for the exposed needles the stimulation of both electron transport activity and irradiance saturated rate of CO2 assimilation (PNmax) under EC already disappeared. No signs of the PS2 impairment were observed as judged from high values of potential quantum yield of PS2 photochemistry (FV/FM) and uniform kinetics of QA reoxidation for all variants. Therefore, the long-term acclimation of the sun-exposed needles to EC is not necessarily accompanied with the damage to the PS2 reaction centres. The eco-physiological significance of the reported differentiation between the responses of shaded and sun exposed needles to prolonged EC may be in changed contribution of the upper and lower crown layers to the production activity of the tree. Whereas for the AC spruces, PNmax of shaded needles was only less than 25 % compared to exposed ones, for the EC spruces the PNmax of shaded needles reached nearly 40 % of that estimated for the exposed ones. Thus, the lower shaded part of the crown may become an effective consumer of CO2.