Do the capacity and kinetics for modification of xanthophyll cycle pool size depend on growth irradiance in temperate trees?

The present study investigated the interaction of growth irradiance (Q_int) with leaf capacity for and kinetics of adjustment of the pool size of xanthophyll cycle carotenoids (sum of violaxanthin, antheraxanthin and zeaxanthin; VAZ) and photosynthetic electron transport rate (J_max) after changes in leaf light environment. Individual leaves of lower‐canopy/lower photosynthetic capacity species Tilia cordata Mill. and upper canopy/higher photosynthetic capacity species Populus tremula L. were either illuminated by additional light of 500–800 µmol m^?2 s^?1 for 12 h photoperiod or enclosed in shade bags. The extra irradiance increased the total amount of light intercepted by two‐fold for the upper and 10–15‐fold for the lower canopy leaves, whereas the shade bags transmitted 45% of incident irradiance. In control leaves, VAZ/area, VAZ/Chl and J_max were positively associated with leaf growth irradiance (Q_int). After 11 d extra illumination, VAZ/Chl increased in all cases due to a strong reduction in foliar chlorophyll, but VAZ/area increased in the upper canopy leaves of both species, and remained constant or decreased in the lower canopy leaves of T. cordata. The slope for VAZ/area changes with cumulative extra irradiance was positively associated with Q_int only in T. cordata, but not in P. tremula. Nevertheless, all leaves of P. tremula increased VAZ/area more than the most responsive leaves of T. cordata. Shading reduced VAZ content only in P. tremula, but not in T. cordata, again demonstrating that P. tremula is a more responsive species. Compatible with the hypothesis of the role of VAZ in photoprotection, the rates of photosynthetic electron transport declined less in P. tremula than in T. cordata after the extra irradiance treatment. However, foliar chlorophyll contents of the exposed leaves declined significantly more in the upper canopy of P. tremula, which is not consistent with the suggestion that the leaves with the highest VAZ content are more resistant to photoinhibition. This study demonstrates that previous leaf light environment may significantly affect the adaptation capacity of foliage to altered light environment, and also that species differences in photosynthetic capacity and acclimation potentials importantly alter this interaction.     

Patterns of damage to <Emphasis Type="Italic">Tilia cordata</Emphasis> by <Emphasis Type="Italic">Phyllonorycter issikii</Emphasis> (Lepidoptera,Gracillariidae) in Izhevsk City

Patterns of damage to the small-leaved lime Tilia cordata Mill. by the lime miner Phyllonorycter issikii Kumata (Lepidoptera, Gracillariidae) were monitored in the city of Izhevsk from 1999 to 2002. The connection between the degree of damage done by the miner with that of environmental pollution with heavy metals (zinc, copper, iron, and lead) was shown. Smaller damage to the lime in localities with high content of heavy metals ensures preservation of foliage and creates favorable conditions for successful use of lime trees in urban landscaping.     

Accumulation of particulate matter,heavy metals,and polycyclic aromatic hydrocarbons on the leaves of Tilia cordata Mill. in five Polish cities with different levels of air pollution

Urbanized areas are struggling with the problem of air pollution and as the number of people living in cities is increasing, the situation is likely to deteriorate. One of the most harmful pollutants is particulate matter (PM). Increased levels of PM in the atmosphere are likely to have a negative impact on human health. Phytoremediation technology could be a solution. It involves plants acting as bio-filters by accumulating particles on, and in the leaves, thus removing the particles from the atmosphere. This study investigates the accumulation of PM including heavy metals (HMs) and polycyclic aromatic hydrocarbons (PAHs), on the foliage of small-leaved lime (Tilia cordata Mill.) in five Polish cities. There were significantly different PM amounts found in the trees between the cities which related to the different quantities of PM in the atmosphere at these cities. Significant differences were found between cities for the amounts of the different particulate size fractions, and the HMs and PAHs in leaves. Strong winds reduced the amount of PM on leaves, especially the smallest fractions, but no relationship with precipitation was found. The results suggest that T. cordata improves the air quality in cities and can be used as an effective bioindicator for PM air pollution.     

Pollen viability as a bio-indicator of air quality

Many air pollutants cause plant deterioration. In this study pollen viability was used as bio-indicator of air quality. The study was carried out in the city of Perugia where road traffic is the most important cause of air pollution. Three areas, with different intensity of road traffic (very high, medium and absent) but all characterized by the presence of the same plant species, were selected. Eight species were studied: Hedera helix L., Convolvulus sepium L., Cynodon dactylon (L.) Pers., Quercus ilex L., Dactylis glomerata L., Parietaria diffusa M. et K., Daucus carota L. and Tilia cordata Miller. The pollen of these species was treated with TTC (2, 3, 5 Tryphenil-Tetrazolium-Chloride) staining solution and viability was then estimated by light microscopy. The results showed that the pollen viability was inversely proportioned with pollution. The highest difference in pollen viability between the areas was registered in Tilia cordata. Quercus ilex instead showed that there was no difference in pollen viability between the three different areas. Parietaria diffusa showed a particular behaviour; the highest pollen viability percentage was in polluted areas. The statistical analysis (ANOVA) showed that the main source of variability of the pollen viability depends on the plant but also the site and the interaction between plant and site were very important with a high significant level (p < 0.0001). This revised version was published online in June 2006 with corrections to the Cover Date.     

Genetic signature of the natural gene pool of Tilia cordata Mill. in Lithuania: Compound evolutionary and anthropogenic effects

Tilia cordata Mill. is a valuable tree species enriching the ecological values of the coniferous‐dominated boreal forests in Europe. Following the historical decline, spreading of Tilia sp. is challenged by the elevated inbreeding and habitat fragmentation. We studied the geographical distribution of genetic diversity of Tilia cordata populations in Lithuania. We used 14 genomic microsatellite markers to genotype 543 individuals from 23 wild‐growing populations. We found that Tilia cordata retained high levels of genetic diversity (population F _IS = 0–0.15, H _o = 0.53–0.69, H _e = 0.56–0.75). AMOVA, Bayesian clustering, and Monmonier''s barrier detection indicate weak but significant differentiation among the populations (F _ST = 0.037***) into geographically interpretable clusters of (a) western Lithuania with high genetic heterogeneity but low genetic diversity, bottleneck effects, (b) relatively higher genetic diversity of Tilia cordata on rich and most soils of midland lowland, and (c) the most differentiated populations on poor soils of the coolest northeastern highland possessing the highest rare allele frequency but elevated inbreeding and bottleneck effects. Weak genetic differentiation among the Tilia cordata populations in Lithuania implies common ancestry, absence of strong adaptive gradients, and effective genetic exchange possible mediated via the riparian networks. A hypothesis on riparian networks as gene flow mediators in Tilia cordata was raised based on results of this study.     

The ecological significance of phenology in four different tree species: effects of light and temperature on bud burst

The process of adaptation is the result of stabilising selection caused by two opposite forces: protection against an unfavourable season (survival adaptation), and effective use of growing resources (capacity adaptation). As plant species have evolved different life strategies based on different trade offs between survival and capacity adaptations, different phenological responses are also expected among species. The aim of this study was to compare budburst responses of two opportunistic species (Betula pubescens, and Salix x smithiana) with that of two long-lived, late successional species (Fagus sylvatica and Tilia cordata) and consider their ecological significance. Thus, we performed a series of experiments whereby temperature and photoperiod were manipulated during dormancy. T. cordata and F. sylvatica showed low rates of budburst, high chilling requirements and responsiveness to light intensity, while B. pubescens and S. x smithiana had high rates of budburst, low chilling requirements and were not affected by light intensity. In addition, budburst in B. pubescens and S. x smithiana was more responsive to high forcing temperatures than in T. cordata and F. sylvatica. These results suggest that the timing of growth onset in B. pubescens and S. x smithiana (opportunistic) is regulated through a less conservative mechanism than in T. cordata and F. sylvatica (long-lived, late successional), and that these species trade a higher risk of frost damage for the opportunity of vigorous growth at the beginning of spring, before canopy closure. This information should be considered when assessing the impacts of climate change on vegetation or developing phenological models.     

Micronutrient levels in some temperate European tree species: a comparative field study

Micronutrient concentrations in foliage and stemwood (including bark) of six European tree species (Betula pendula Roth., Quercus robur L., Fraxinus excelsior L., Fagus sylvatica L., Tilia cordata Mill. and Picea abies (L.) Karst.) planted on the same type of soil at six sites in three different countries were studied. Micronutrient concentrations in foliage were considerably higher than in stemwood for all elements and species studied, except for Fe in spruce. Interspecies comparisons revealed significant differences in concentrations both in foliar and stemwood biomass, as well as in stemwood:foliage nutrient ratios. Lime foliage showed a considerably higher concentration of B than all other species, while the stemwood concentration of this element was highest in ash. Mn concentration in both foliar and stemwood biomass of ash was extremely low compared with concentrations in other species. Birch stemwood showed nearly double the level of Zn in other species at all sites. Zn concentrations in the birch foliage were also higher than in other species, with the exception of the Lithuanian sites, which showed lower EDTA-extractable Zn concentrations in the soils. The concentration of Cu was lowest in spruce foliage, while Cu concentrations in stemwood were similar in all species. It was concluded that species-related differences in microelement nutrition must be taken into account when evaluating the nutrient status of common European forest tree species, and when using them as bio-indicators of the effects of environmental pollution.     

Performance index in assessing the physiological state of trees in urban ecosystems

Based on PAM and PEA measurements of fluorescence of bark chloroplasts, we have compared the information capacity of the methods for assessing the physiological state of Tilia cordata Mill. by the maximal quantum efficiency of PS II photochemistry (F _v/F _m) and by the performance index (PI). The measurements were made on annual shoots of linden trees growing in different environs. It was shown that the chlorophyll content in the bark of shoots growing near a busy urban street was twice less than in controls growing out of town. For the trees in the unfavorable environment, a small decrease in (F _v/F _m) was registered, and there was a significant statistical scatter in these values as compared with controls. The PI and its constituent parameters calculated from fluorescence induction curves (PEA method) are more informative and allow recognizing changes in the primary energy conversion processes in PS II when they are still small. Thus, PI can be used as a sensitive, robust, and rapid test to evaluate the physiological state of trees and other plant objects even under minor environmental changes.     

Plant regeneration by somatic embryogenesis from cultured immature embryos of oak (Querem robur L.) and linden (Tilia cordata Mill.)

Embryogenic cultures and somatic embryos were obtained from immature zygotic embryos of oak (Quercus robur L.) cultured on a modified MS medium and WPM containing BAP (1 mg·l^–1) and GA₃ (1 mg·l^–1) or BAP and IBA. Germination and conversion of oak somatic embryos into plantlets was achieved on WPM containing a reduced concentration of cytokinin. Linden (Tilia cordata Mill.) somatic embryos developed in embryogenic tissues initiated from immature zygotic embryos cultured on a modified MS medium supplemented with 2,4-D (0.3-2.0 mg·l^–1). Germination of linden somatic embryos and plantlet formation occurred on MS medium containing a low concentration of IBA. Oak and linden plantlets produced from somatic embryos were successfully established in soil. Somatic embryos and plantlets were also regenerated from embryogenic cultures of Quercus petraea and Tilia platyphyllos.Abbreviations BAP 6-benzyIaminopurine - GA₃ gibberellic acid - IBA indole-3-butyric acid - 2,4-D 2,4-dichlorophenoxyacetic acid - MS Murashige and Skoog (1962) - WPM woody plant medium     

Chlorophyll fluorescence imaging of photosynthetic activity in sun and shade leaves of trees

The differences in pigment levels, photosynthetic activity and the chlorophyll fluorescence decrease ratio R _Fd (as indicator of photosynthetic rates) of green sun and shade leaves of three broadleaf trees (Platanus acerifolia Willd., Populus alba L., Tilia cordata Mill.) were compared. Sun leaves were characterized by higher levels of total chlorophylls a + b and total carotenoids x + c as well as higher values for the weight ratio chlorophyll (Chl) a/b (sun leaves 3.23–3.45; shade leaves: 2.74–2.81), and lower values for the ratio chlorophylls to carotenoids (a + b)/(x + c) (with 4.44–4.70 in sun leaves and 5.04–5.72 in shade leaves). Sun leaves exhibited higher photosynthetic rates P _N on a leaf area basis (mean of 9.1–10.1 μmol CO₂ m⁻² s⁻¹) and Chl basis, which correlated well with the higher values of stomatal conductance G _s (range 105–180 mmol m⁻² s⁻¹), as compared to shade leaves (G _s range 25–77 mmol m⁻² s⁻¹; P _N: 3.2–3.7 μmol CO₂ m⁻² s⁻¹). The higher photosynthetic rates could also be detected via imaging the Chl fluorescence decrease ratio R _Fd, which possessed higher values in sun leaves (2.8–3.0) as compared to shade leaves (1.4–1.8). In addition, via R _Fd images it was shown that the photosynthetic activity of the leaves of all trees exhibits a large heterogeneity across the leaf area, and in general to a higher extent in sun leaves than in shade leaves.     

Adjustment of leaf photosynthesis to shade in a natural canopy: rate parameters

The present study was performed to investigate the adjustment of the rate parameters of the light and dark reactions of photosynthesis to the natural growth light in leaves of an overstorey species, Betula pendula Roth, a subcanopy species, Tilia cordata P. Mill., and a herb, Solidago virgaurea L., growing in a natural plant community in Järvselja, Estonia. Shoots were collected from the site and individual leaves were measured in a laboratory applying a standardized routine of kinetic gas exchange, Chl fluorescence and 820 nm transmittance measurements. These measurements enabled the calculations of the quantum yield of photosynthesis and rate constants of excitation capture by photochemical and non-photochemical quenchers, rate constant for P700⁺ reduction via the cytochrome b₆f complex with and without photosynthetic control, actual maximum and potential (uncoupled) electron transport rate, stomatal and mesophyll resistances for CO₂ transport, K_m(CO₂) and V_m of ribulose-bisphosphate carboxylase-oxygenase (Rubisco) in vivo. In parallel, N, Chl and Rubisco contents were measured from the same leaves. No adjustment toward higher quantum yield in shade compared with sun leaves was observed, although relatively more N was partitioned to the light-harvesting machinery in shade leaves ( H. Eichelmann et al., 2004 ). The electron transport rate through the Cyt b₆f complex was strongly down-regulated under saturating light compared with darkness, and this was observed under atmospheric, as well as saturating CO₂ concentration. In vivo V_m measurements of Rubisco were lower than corresponding reported measurements in vitro, and the k_cat per reaction site varied widely between leaves and growth sites. The correlation between Rubisco V_m and the photosystem I density was stronger than between V_m and the density of Rubisco active sites. The results showed that the capacity of the photosynthetic machinery decreases in shade-adjusted leaves, but it still remains in excess of the actual photosynthetic rate. The photosynthetic control systems that are targeted to adjust the photosynthetic rate to meet the plant's needs and to balance the partial reactions of photosynthesis, down-regulate partial processes of photosynthesis: excess harvested light is quenched non-photochemically; excess electron transport capacity of Cyt b₆f is down-regulated by ΔpH-dependent photosynthetic control; Rubisco is synthesized in excess, and the number of activated Rubisco molecules is controlled by photosystem I-related processes. Consequently, the nitrogen contained in the components of the photosynthetic machinery is not used at full efficiency. The strong correlation between leaf nitrogen and photosynthetic performance is not due to the nitrogen requirements of the photosynthetic apparatus, but because a certain amount of energy must be captured through photosynthesis to maintain this nitrogen within a leaf.     

Shape of leaf photosynthetic electron transport versus temperature response curve is not constant along canopy light gradients in temperate deciduous trees

Responses of foliar light-saturated net assimilation rate (A_max), capacity for photosynthetic electron transport (J_max) and mitochondrial respiration rate (R_d) to long-term canopy light and temperature environment were investigated in a temperate deciduous canopy composed of Populus tremula L. in the upper (17–28 m) and of Tilia cordata Mill. in the lower canopy layer (4–17 m). Climatic measurements indicated that seasonal average daily maximum air temperature (T_max) was 5·5 °C (range 0·7–10·5 °C) higher in the top than in the bottom of the canopy, and strong positive correlations were observed between T_max and seasonal average integrated quantum flux density (Q_int), as well as between seasonal average daily mean temperature and Q_int. Because of changes in leaf dry mass and nitrogen per unit area, A_max, J_max, and R_d scaled positively with Q_int in both species at a common leaf temperature (T). According to J_max versus T response curves and dark chlorophyll fluorescence transients, photosynthetic electron transport was less heat resistant in P. tremula with optimum temperature of J_max, T_opt, of 33·5 ± 0·6 °C than in T. cordata with T_opt of 40·7 ± 0·6 °C. This difference was suggested to manifest evolutionary adaptation of photosynthetic electron transport to cooler environments in P. tremula, the range of which extends farther north than that in T. cordata. Possibly because of acclimation to long-term canopy temperature environment, T_opt was positively related to Q_int in P. tremula, foliage of which was also exposed to higher irradiances and temperatures, but not in T. cordata, in the canopy of which quantum flux densities and temperatures were lower, and gradients in the environmental factors less pronounced. Parallel to changes in T_opt, the activation energy for photosynthetic electron transport decreased with increasing Q_int in P. tremula, indicating that J_max of leaves acclimated to colder environment was more responsive to T in lower temperatures than that of high T acclimated leaves. Similar alterations in the activation energy for mitochondrial respiration rate were also observed, indicating that acclimation to temperature of mitochondrial and chloroplastic electron transport proceeds in a co-ordinated manner, and possibly involves long-term changes in membrane fluidity properties. We conclude that, because of correlations between temperature and light, the shapes of J_max versus T, and R_d versus T response curves vary within tree canopies, and this needs to be taken account in modelling whole canopy photosynthesis.     

Vegetation structure and diversity of an ancient temperate deciduous forest in SW Denmark

Bolderslev Skov (113 ha) is the largest contiguous ancient forest remnant in Denmark. The forest has been preserved since 1998 as a strict non-intervention forest reserve. We studied vegetation structure, floristic gradients, and diversity of the forest in 50 plots of 100 m² placed according to a restricted random sampling design. Dominant tree species were Fagus sylvatica, Fraxinus excelsior, Tilia cordata and Quercus robur, which in most parts of the forest form a mixed canopy. Most stands appeared to be of moderate age, 55-80 years old, and large old trees were rare. pH in association with light and thickness of the litter layer were the most important factors in explaining floristic gradients in the forest. Soil moisture (dry to mesic) was not strongly correlated with DCA axes for neither tree nor field layer, but had a significant effect on the distribution of a number of herb species. Forest structure was not important in explaining species distributions. Field layer species richness showed a positive relationship with the pH-gradient. At the scale of 1m² plots we also found a highly significant negative relationship between field layer species richness and the plot-wise Ellenberg indicator value for nitrogen availability. Structure of the tree layer had little effect on field layer species richness. The mixed composition, long continuity, and presence of a high proportion of the regional species pool assigns a high conservation value to Bolderslev Skov and makes it an important site for future studies of the dynamic properties, niche preferences, and inter-specific competition of temperate deciduous forest species. This revised version was published online in June 2006 with corrections to the Cover Date.     

Local adaptation through genetic differentiation in highly fragmented Tilia cordata populations

We assessed the level of geographic differentiation of Tilia cordata in Denmark based on tests of 91 trees selected from 12 isolated populations. We used quantitative analysis of spring phenology and population genetic analysis based on SSR markers to infer the likely historical genetic processes within and among populations. High genetic variation within and among populations was observed in spring phenology, which correlated with spring temperatures at the origin of the tested T. cordata trees. The population genetic analysis revealed significant differentiation among the populations, but with no clear sign of isolation by distance. We infer the findings as indications of ongoing fine scale selection in favor of local growth conditions made possible by limited gene flow among the small and fragmented populations. This hypothesis fits well with reports of limited fruiting in the investigated Danish T. cordata populations, while the species is known for its ability to propagate vegetatively by root suckers. Our results suggest that both divergent selection and genetic drift may have played important roles in forming the genetic patterns of T. cordata at its northern distribution limit. However, we also speculate that epigenetic mechanism arising from the original population environment could have created similar patterns in regulating the spring phenology.     

Floral sources used by the orchid bee Euglossa cordata (Linnaeus, 1758) (Apidae: Euglossini) in an urban area of south-eastern Brazil

Although orchid bees (Apidae: Euglossini) are known as key pollinators in tropical ecosystems, knowledge of their floral sources is still scarce, especially for those species commonly found in urban environments. We aimed to identify the pollen, nectar and resin sources used by the widespread species Euglossa cordata in an urban area in south-eastern Brazil. The residual pollen from 81 brood cells of nine nests reactivated between October 2013 and December 2014 was acetolysed and analysed. A total of 50 pollen types belonging to 20 botanical families were identified in the samples. Pollen sources included species from seven families; five of them were plants with poricidal anthers (Bixaceae, Commelinaceae, Fabaceae, Melastomataceae and Solanaceae). The mass-flowering trees Handroanthus chrysotrichus and H. heptaphyllus (Bignoniaceae) are firstly reported as important pollen sources to an orchid bee species. Nectar was collected primarily from plants with long, tubular corolla as Acanthaceae, Apocynaceae, Bignoniaceae and Convolvulaceae. The vine species Dalechampia stipulacea (Euphorbiaceae) acted as a floral resin source. All pollen sources consisted of native plants whereas some exotic plant species were visited for nectar collection. Although nesting in an area encompassed by a high proportion of invasive plant species, Euglossa cordata females preferred to visit native plants to gather floral resources.     

Within-canopy variation in the rate of development of photosynthetic capacity is proportional to integrated quantum flux density in temperate deciduous trees

The present study was undertaken to test for the hypothesis that the rate of development in the capacity for photosynthetic electron transport per unit area (J_max;A), and maximum carboxylase activity of Rubisco (V_cmax;A) is proportional to average integrated daily quantum flux density (Q_int) in a mixed deciduous forest dominated by the shade‐intolerant species Populus tremula L., and the shade‐tolerant species Tilia cordata Mill. We distinguished between the age‐dependent changes in net assimilation rates due to modifications in leaf dry mass per unit area (M_A), foliar nitrogen content per unit dry mass (N_M), and fractional partitioning of foliar nitrogen in the proteins of photosynthetic electron transport (F_B), Rubisco (F_R) and in light‐harvesting chlorophyll‐protein complexes (V_cmax;A ∝ M_AN_MF_R; J_max;A ∝ M_AN_MF_B). In both species, increases in J_max;A and V_cmax;A during leaf development were primarily determined by nitrogen allocation to growing leaves, increases in leaf nitrogen partitioning in photosynthetic machinery, and increases in M_A. Canopy differences in the rate of development of leaf photosynthetic capacity were mainly controlled by the rate of change in M_A. There was only small within‐canopy variation in the initial rate of biomass accumulation per unit Q_int (slope of M_A versus leaf age relationship per unit Q_int), suggesting that canopy differences in the rate of development of J_max;A and V_cmax;A are directly proportional to Q_int. Nevertheless, M_A, nitrogen, J_max;A and V_cmax;A of mature leaves were not proportional to Q_int because of a finite M_A in leaves immediately after bud‐burst (light‐independent component of M_A). M_A, leaf chlorophyll contents and chlorophyll : N ratio of mature leaves were best correlated with the integrated average quantum flux density during leaf development, suggesting that foliar photosynthetic apparatus, once developed, is not affected by day‐to‐day fluctuations in Q_int. However, for the upper canopy leaves of P. tremula and for the entire canopy of T. cordata, there was a continuous decline in N contents per unit dry mass in mature non‐senescent leaves on the order of 15–20% for a change of leaf age from 40 to 120 d, possibly manifesting nitrogen reallocation to bud formation. The decline in N contents led to similar decreases in leaf photosynthetic capacity and foliar chlorophyll contents. These data demonstrate that light‐dependent variation in the rate of developmental changes in M_A determines canopy differences in photosynthetic capacity, whereas foliar photosynthetic apparatus is essentially constant in fully developed leaves.     

Acclimation to high irradiance in temperate deciduous trees in the field: changes in xanthophyll cycle pool size and in photosynthetic capacity along a canopy light gradient

To test the hypothesis that in temperate deciduous trees acclimation to potentially damaging high irradiances occurs via long-term adjustments in foliar photosynthetic capacity, and short-term changes in xanthophyll cycle pool size in response to weather fluctuations, nitrogen concentration and pigment composition were examined along a canopy light gradient in three species –Betula pendula, Populus tremula and Tilia cordata (from most shade intolerant to tolerant), and foliage photosynthetic potentials in P. tremula and T. cordata. Integrated quantum flux density (Q_i) incident on leaves was estimated with a method combining hemispherical photography and light measurements with quantum sensors made over the growing season. Long- and short-term light indices – average total seasonal daily integrated quantum flux density (T_s, mol m^–2 d^–1) and that of the 3 d preceding foliage sampling (T_3d) – were calculated for each sampled leaf. In addition to total integrated quantum flux density, the part of Q_i attributable to direct flux was also computed. Strong linear relationships between the capacity for photosynthetic electron transport per area (J^a_max), estimated from in situ measurements of effective quantum yield of photosystem II (PS II), and Q_i averaged over the season and over the preceding 3 d were found for all studied species. However, the major determinant of J^a_max, the product of electron transport capacity per leaf dry mass (J^m_max) and leaf dry mass per area (M_A), was M_A rather than J^m_max, which was relatively constant along the light gradient. There was evidence that J^a_max is more tightly related to T_s, which characterizes the light climate during foliar development, than to short-term integrated light, possibly because there is little flexibility in adjustments in M_A after the completion of foliar growth. Leaf chlorophyll concentrations and the investment of leaf nitrogen in chlorophyll (Chl/N) were negatively related to Q_i– an investment pattern which improves light harvesting in low light. Xanthophyll cycle pool size (VAZ, violaxanthin + antheraxanthin + zeaxanthin) either expressed per unit chlorophyll (VAZ/Chl) or as a fraction of total carotenoids (VAZ/Car) increased with increasing Q_i in all species. However, contrary to J^a_max, it tended to correlate more strongly with short-term than with long-term average integrated light. There were few interspecific differences in J^a_max, Chl/N, VAZ/Chl and VAZ/Car when the variability in light level incident to the leaves was accounted for, indicating that the foliage of both shade-intolerant and -tolerant temperate tree species possesses considerable phenotypic flexibility. Collectively these results support the view that rapid adjustment of the xanthophyll cycle pool size provides an important means for acclimation to light fluctuations in a time scale of days, during which the potential for photosynthetic quenching of excitation energy is not likely to change appreciably.     

Adjustment of foliage structure and function to a canopy light gradient in two co-existing deciduous trees. Variability in leaf inclination angles in relation to petiole morphology

 Foliar inclination angles, petiole morphology and dry matter partitioning between assimilative and support biomass were studied in shade-intolerant Populus tremula L. and shade-tolerant Tilia cordata Mill. along a natural light gradient across the canopy. The leaves of sub-canopy species T. cordata were on average exposed to lower irradiances, and they were also more horizontal with greater blade inclination angles (ϕ_B, defined as the angle between the leaf fall-line and the horizon; ϕ_B was positive for the leaves inclined upwards, and negative for the leaves inclined downwards) than those in P. tremula. Seasonal average daily integrated quantum flux density (Q _int, mol m^–2 day^–1) and ϕ_B were not related in T. cordata, and only a weak negative effect of Q _int on ϕ_B was detected in P. tremula. Nevertheless, when both species were pooled, there was a strong negative relationship between Q _int and ϕ_B, implying that the leaves became progressively vertical with increasing height in the canopy. Interspecific differences in foliage inclination were mainly related to petiole morphology, in particular to petiole length, rather than to contrasting biomass investment patterns between assimilative and support tissues within the leaf. It was suggested that more horizontal leaves, resulting from the species-specific structure of petioles, partly explain the superior performance of shade-tolerant T. cordata in the understory and the sub-canopy. Received: 13 November 1997 / Accepted: 6 March 1998     

Genetic and clonal structures of the tree species Tilia cordata mill. in remnants of ancient forests in Denmark

The insect-pollinated forest tree Tilia cordata Mill. grows today in small fragmented populations in Denmark and other western European countries but was, in prehistoric times, a dominating species and is considered an indicator species for ancient forest. The species is known to propagate both sexually and vegetatively, forming clonal groups. Few studies have been made on the species' population genetics and on how clonality affects the population structure. The aim of this study is to evaluate the Danish gene pool by estimating genetic diversity and differentiation, as well as through exhaustive sampling describe clonal structures in some of the populations. Genetic analysis was carried out using nine nuclear microsatellite markers in nine populations, of which four were exhaustively or partly exhaustively sampled. The markers showed a high degree of genetic diversity but low differentiation between populations, with no geographic-related structure. Clonal structures were found in eight out of the nine populations. In the exhaustively sampled populations, recruitment strategies included both sexual and clonal reproduction with indications that clonality may be enhanced by management and other disturbances.     

Adjustment of leaf photosynthesis to shade in a natural canopy: reallocation of nitrogen

The present study was performed to investigate the adjustment of the constituents of the light and dark reactions of photosynthesis to the natural growth irradiance in the leaves of an overstorey species, Betula pendula Roth, a subcanopy species Tilia cordata P. Mill., and a herb Solidago virgaurea L. growing in a natural plant community in Järvselja, Estonia. Shoots were collected from the site and properties of individual leaves were measured in a laboratory, by applying a routine of kinetic gas exchange and optical measurements that revealed photosystem II (PSII), photosystem I (PSI), and cytochrome b₆f densities per leaf area and the distribution of excitation (or chlorophyll, Chl) between the two photosystems. In parallel, N, Chl and ribulose-bisphosphate carboxylase-oxygenase (Rubisco) content was measured from the same leaves. The amount of N in photosynthetic proteins was calculated from the measured contents of the components of the photosynthetic machinery. Non-photosynthetic N was found as the residual of the budget. Growth in shade resulted in the decrease of leaf dry mass to a half of the DW in sun leaves in each species, but the total variation, from the top to the bottom of the canopy, was larger. Through the whole cross-section of the canopy, leaf dry weight (DW) and Rubisco content per area decreased by a factor of four, N content by a factor of three, but Chl content only by a factor of 1.7. PSII density decreased by a factor of 1.9, but PSI density by a factor of 3.2. The density of PSI adjusted to shade to a greater extent than the density of PSII. In shade, the distribution of N between the components of the photosynthetic machinery was shifted toward light-harvesting proteins at the expense of Rubisco. Non-photosynthetic N decreased the most substantially, from 54% in the sun leaves of B. pendula to 11% in the shade leaves of T. cordata. It is concluded that the redistribution of N toward light-harvesting Chl proteins in shade is not sufficient to keep the excitation rate of a PSII centre invariant. Contrary to PSII, the density of PSI – the photosystem that is in immediate contact with the carbon assimilation system – shade-adjusts almost proportionally with the latter, whereas its Chl antenna correspondingly increases. Even under N deficiency, a likely condition in the natural plant community, a substantial part of N is stored in non-photosynthetic proteins under abundant irradiation, but much less under limiting irradiation. At least in trees the general sequence of down-regulation due to shade adjustment is the following: (1) non-protein cell structures and non-photosynthetic proteins; (2) carbon assimilation proteins; (3) light reaction centre proteins, first PSI; and (4) chlorophyll-binding proteins.