Physiological, stomatal and ultrastructural ozone responses in birch (Betula pendula Roth.) are modified by water stress

The physiological, stomatal and ultrastructural responses to ozone and drought of ozone-sensitive and more ozone-tolerant birch ( Betula pendula Roth.) clones were studied singly and in combination, in a high-stress chamber experiment and in a low-stress open-field experiment. In the chamber experiment, well watered (WW), moderately watered (MW) or drought-stressed (DS) saplings were exposed for 36 d to 0 or 130 nmol mol^∠1 ozone. In the open-field experiment, well watered or drought-stressed saplings were grown for one growing season in ambient air or exposed to 1·8 × ambient ozone. Drought stress reduced growth rate, stomatal conductance, stomatal density and the proportion of starch and thylakoids in chloroplasts, but stimulated net photosynthesis, Rubisco and chlorophyll quantity at the end of the growing season, and increased the size and density of plastoglobuli. Ozone fumigations caused more variable, clone- and exposure-dependent responses in growth, decreased stomatal conductance and net photosynthesis, an increased number of stomata, visible and ultrastructural chloroplast injuries, and enhanced autumn yellowing of the leaves. Ozone-induced changes in plastoglobuli, starch and thylakoids resembled drought responses. The two experiments revealed that, depending on the experimental conditions and the variable, the response to drought and ozone stress can be independent, additive or interactive. Drought protected the plants from ozone injuries under high-stress conditions in the chamber experiment. In the low-stress, open-field experiment, however, enhanced ozone damage was observed in birch saplings grown under restricted water supply.     

Growth response to step-decrease in nutrient availability in small birch (Betula pendula Roth)

Abstract Changes in the uptake and allocation of carbon and nitrogen, after a step-decrease in nutrient availability, were investigated in small birch (Betula pendula Roth). By demonstrating stable nutrition, before and after the decrease in nutrient supply, it was possible to eliminate the effects of plant size and age. Immediately following the step-decrease in nutrient availability, net nitrogen uptake to leaves and the relative rate of increase in shoot area tended to zero. Although photosynthetic rate per shoot area decreased, carbon uptake remained in excess of that used in structural growth and respiration. More of the excess carbon was accumulated as starch in leaves than in roots. After a lag phase, the relative rates of increase in plant dry matter, starch amount, net nitrogen uptake to leaves and shoot area development equalled that of the reduced rate of nutrient supply. It is concluded that the reduction in plant relative growth rate was much more attributable to the reduced allocation of photosynthate to leaf area growth than to the reduction in photosynthesis per shoot area.     

Growth models of silver birch (Betula pendula Roth.) on two volcanic mountains in the French Massif Central

Silver birch woodlands of two volcanic mountains (altitude 850 m and 1450 m) were studied in order to establish a growth model of birch. Height, radial increment and crown width were measured on both sites taking into account two situations: isolated birch or birch within a stand. For the latter case two categories were sampled considering the competition status of the tree: birch suffering the most severe competition (suppressed tree) were distinguished from trees facing the lowest competition (dominant tree). Measures of competition were also made using plots where each tree was located by its coordinates and its height, trunk circumference and crown width recorded. Examination of radial growth curves showed similar patterns for the two sites for the same category of tree. Radial growth was always inferior for the site located at the highest altitude but tree lifespan was about two-fold longer. Radial increment data were used to calculate circumference as function of tree age. Models predicting circumference with time were then established for each category of tree (dominant, suppressed or isolated) using Chapman–Richards'model. Height and crown width models were produced using circumference as the predictor. Competition indices based on vertical or horizontal angles weighted by the distance were calculated for birches in stands. Indices using vertical angles appeared to be more resistant and robust to characterize local competition. Competition index was then associated with the parameters of Chapman-Richards' growth circumference model for the two sites and models predicting the circumference from the age of the tree and its competition index are proposed.     

Nutrition and the ozone sensitivity of birch (Betula pendula)

 Cuttings of a single birch clone (Betula pendula) were grown in field fumigation chambers throughout the growing season in either filtered air (control) or 90/40 nl O₃ l^–1 (day/night). Both regimes were split into plants under high and low nutrient supply (macro- and micronutrients). The stomatal density of leaves was increased by ozone but was lowered at high nutrition, while the inner air space was hardly affected by the treatments. Ozone induced macroscopic leaf injury regardless of nutrition, but leaf shedding was delayed in the low-fertilized plants, despite O₃ uptake being similar to that in high-fertilized plants. The leaf turn-over was enhanced in the O₃-exposed high-fertilized plants, but length growth and leaf formation of stems were not affected by ozone in either nutrient regime. Leaves of high-fertilized plants showed O₃-caused decline in photosynthetic capacity, water-use efficiency, apparent carbon uptake efficiency and quantum yield earlier as compared with low-fertilized plants, whereas chlorophyll fluorescence (F_V/F_M) and leaf nitrogen concentration were rather stable. CO₂ uptake rate and rubisco activity of young leaves compensated for the O₃ injury in the ageing leaves of the low-fertilized plants. In 8-week-old leaves, however, the O₃-induced decline in CO₂ uptake did not differ between the nutrient regimes and was associated with increased dark respiration rather than changed photorespiration. The balance between CO₂ supply and demand was lost, as was stomatal limitation on CO₂ uptake. High nutrition did not help leaves to maintain a high photosynthetic capacity and life span under O₃ stress. Received: 6 July 1996 / Accepted: 4 June 1997     

Responses of selected birch (Betula pendula Roth) clones to ozone change over time

A long-term free air ozone fumigation experiment was conducted to study changes in physiological ozone responses during tree ontogeny and exposure time in ozone sensitive and tolerant clones of European white birch (Betula pendula Roth), originated from south and central Finland. The trees were grown in soil in natural microclimatic conditions under ambient ozone (control) and 1.4-1.7 x ambient (elevated) ozone from May 1996 to October 2001, and were measured for stem and foliage growth, net photosynthesis, stomatal conductance, stomatal density, visible injuries, foliar starch content and bud formation. After 6 years of exposure, the magnitude of ozone-induced growth reductions in the sensitive clone was 12-48% (significant difference), levels similar or greater than those reported earlier for 2- and 3-year-old saplings undergoing shorter exposures. In the tolerant clone, growth of these larger trees was reduced by 1-38% (significant difference in stem volume), although the saplings had previously been unaffected. In both clones, ozone stress led to significantly reduced leaf-level net photosynthesis but significantly increased stomatal conductance rates during the late summer, resulting in a lower carbon gain for bud formation and the onset of visible foliar injuries. Increasing ozone sensitivity with duration of exposure was explained by a change in growth form (relatively reduced foliage mass), a lower photosynthesis to stomatal conductance ratio during the late summer, and deleterious carry-over effects arising from the reduced number of over-wintering buds.     

Response of small birch plants (Betula pendula Roth.) to elevated CO2 and nitrogen supply

Small birch plants were grown for up to 80 d in a climate chamber at varied relative addition rates of nitrogen in culture solution, and at ambient (350 μmol mol^-1) or elevated (700 μmol mol^-1) concentrations of CO₂. The relative addition rate of nitrogen controlled relative growth rate accurately and independently of CO₂ concentration at sub-optimum levels. During free access to nutrients, relative growth rate was higher at elevated CO₂. Higher values of relative growth rate and net assimilation rate were associated with higher values of plant N-concentration. At all N-supply rates, elevated CO₂ resulted in higher values of net assimilation rate, whereas leaf weight ratio was independent of CO₂. Specific leaf area (and leaf area ratio) was less at higher CO₂ and at lower rates of N-supply. Lower values of specific leaf area were partly because of starch accumulation. Nitrogen productivity (growth rate per unit plant nitrogen) was higher at elevated CO₂. At sub-optimal N-supply, the higher net assimilation rate at elevated CO₂ was offset by a lower leaf area ratio. Carbon dioxide did not affect root/shoot ratio, but a higher fraction of plant dry weight was found in roots at lower N-supply. In the treatment with lowest N-supply, five times as much root length was produced per amount of plant nitrogen in comparison with optimum plants. The specific fine root length at all N-supplies was greater at elevated CO₂. These responses of the root system to lower N-supply and elevated CO₂ may have a considerable bearing on the acquisition of nutrients in depleted soils at elevated CO₂. The advantage of maintaining steady-state nutrition in small plants while investigating the effects of elevated CO₂ on growth is emphasized.     

Ozone exposure results in various carry-over effects and prolonged reduction in biomass in birch (Betula pendula Roth)

In the first experiment, saplings of ozone-sensitive and a more tolerant clone of Betula pendula Roth were exposed to ambient ozone (control treatment, accumulated exposure over a threshold 40 nmol mol ^{? 1} (AOT40) exposure of 1·0 μmol mol ^{? 1} h) and 1·5 × ambient ozone (elevated-ozone treatment, AOT40 of 17·3 μmol mol ^{? 1} h) over one growing season, 1996. After over-wintering, the dormant elevated-ozone saplings were transferred to the control blocks and assessed for short-term carry-over effects during the following growing season. In the second experiment, three sensitive, four intermediate and three tolerant clones were grown under ambient ozone (control treatment, AOT40 of 0·5–0·8 μmol mol ^{? 1} h per growing season) and 1·6–1·7 × ambient ozone (elevated-ozone treatment, AOT40 of 18·3–18·6 μmol mol ^{? 1} h per growing season) from May 1994 until May 1996, and were assessed for long-term carry-over effects during growing season 1997, after a 12–16 months recovery period. Deleterious short-term carry-over effects of ozone exposure included reduced contents of Rubisco, chlorophyll, carotenoids, starch and nutrients in leaves, lower stomatal conductance, and decreased new shoot growth and net assimilation rate, followed by a 7·5% (shoot dry weight (DW)), 15·2% (root DW) and 23·2% (foliage area) decreased biomass accumulation and yield over the long term, including a reduced root : shoot ratio. However, a slow recovery of relative growth rates during the following two seasons without elevated ozone was apparent. Several long-lasting structural, biochemical and stomatal acclimation, stress-defence and compensation reactions were observed in the ozone-tolerant clone, whereas in the sensitive clone allocation shifted from growth towards defensive phenolics such as chlorogenic acid. The results provide evidence of persistent deleterious effects of ozone which remain long after the ozone episode.     

Somatic embryogenesis in cell cultures of birch (Betula pendula Roth.)

Somatic embryogenesis was induced in cell cultures of birch (Betula pendula Roth.) derived from juvenile tissue of seed embryos and from mature leaf tissue. Embryos were formed in liquid and on solidified medium containing 2,4-dichlorophenoxyacetic acid (2,4-D) and 6-furfurylaminopurine (kinetin). Sometimes somatic embryos formed only after transfer to medium devoid of growth regulators. The embryos germinated on hormone-free medium and were potted in soil and grown in the greenhouse.Finnish Forest Research InstituteUniversity of Helsinki, School of Pharmacy;     

Dependence of starch storage on nutrient availability and photon flux density in small birch Betula pendula Roth)

Abstract Small birch plants (Betula pendula Roth) were grown in a climate chamber at different levels of nutrient availability and at two photon flux densities. The extent to which starch storage was dependent upon nutrient availability and photon flux density was investigated. Acclimated values of starch concentration in leaves were highest at low nutrient availability and high photon flux density. Starch storage in roots was only found at the lowest nutrient availability. However, the relative rate of starch storage (starch stored per unit plant dry weight and time) was higher in plants with good nutrition. The data suggest that, at sub-optimal nutrient availability, the momentary rate of net shoot photosynthesis is unlikely to limit the structural (as opposed to carbon storage) growth of the plant. Although photosynthetic rate per unit leaf area (as measured at the growth climate) was slightly lower in plants with poor nutrient availability, photosynthetic rate per unit leaf nitrogen was higher. These data suggest a priority of leaf nitrogen usage in photosynthesis, with limiting amounts of leaf nitrogen (and possibly other nutrients) for subsequent growth processes. This argument is consistent with the higher concentrations of starch found in plants with poor nutrient availability.     

Effects of elevated carbon dioxide concentration on photosynthesis and growth of small birch plants (Betula pendula Roth.) at optimal nutrition

Small birch plants (Betula pendula Roth.) were grown from seed for periods of up to 70d in a climate chamber at optimal nutrition and at present (350 μmol mol^?1) or elevated (700 μmol mol^?1) concentrations of atmospheric CO₂. Nutrients were sprayed over the roots in Ingestad-type units. Relative growth rate and net assimilation rate were slightly higher at elevated CO₂, whereas leaf area ratio was slightly lower. Smaller leaf area ratio was associated with lower values of specific leaf area. Leaves grown at elevated CO₂ had higher starch concentrations (dry weight basis) than leaves grown at present levels of CO₂. Biomass allocation showed no change with CO₂, and no large effects on stem height, number of side shoots and number of leaves were found. However, the specific root length of fine roots was higher at elevated CO₂. No large difference in the response of carbon assimilation to intercellular CO₂ concentration (A/C_i curves) were found between CO₂ treatments. When measured at the growth environments, the rates of photosynthesis were higher in plants grown at elevated CO₂ than in plants grown at present CO₂. Water use efficiency of single leaves was higher in the elevated treatment. This was mainly attributable to higher carbon assimilation rate at elevated CO₂. The difference in water use efficiency diminished with leaf age. The small treatment difference in relative growth rate was maintained throughout the experiment, which meant that the difference in plant size became progressively greater. Thus, where plant nutrition is sufficient to maintain maximum growth, small birch plants may potentially increase in size more rapidly at elevated CO₂.     

Photosynthetic parameters of birch (Betula pendula Roth) leaves growing in normal and in CO2- and O3- enriched atmospheres

Two silver birch (Betula pendula Roth) clones K1659 and V5952 were grown in open‐top chambers over 3 years (age 7–9 years). The treatments were increased CO₂ concentration (+CO₂, 72 Pa), increased O₃ concentration (+O₃, 2 × ambient O₃ with seasonal AOT40 up to 28 p.p.m. h) and in combination (+CO₂ + O₃). Thirty‐seven photosynthetic parameters were measured in the laboratory immediately after excising leaves using a computer‐operated routine of gas exchange and optical measurements. In control leaves the photosynthetic parameters were close to the values widely used in a model (Farquhar, von Caemmerer and Berry, Planta 149, 78–90, 1980). The distribution of chlorophyll between photosystem II and photosystem I, intrinsic quantum yield of electron transport, uncoupled turnover rate of Cyt b₆f, Rubisco specificity and K_m (CO₂) were not influenced by treatments. Net photosynthetic rate responded to +CO₂ with a mean increase of 17% in both clones. Dry weight of leaves increased, whereas protein, especially Rubisco content and the related photosynthetic parameters decreased. Averaged over 3 years, eight and 17 mechanistically independent parameters were significantly influenced by the elevated CO₂ in clones K1659 and V5952, respectively. The elevated O₃ caused a significant decrease in the average photosynthetic rate of clone V5952, but not of clone K1659. The treatment caused changes in one parameter of clone K1659 and in 11 parameters of clone V5952. Results of the combined treatment indicated that +O₃ had less effect in the presence of +CO₂ than alone. Interestingly, changes in the same photosynthetic parameters were observed in chamberless grown trees of clone V5952 as under +O₃ treatment in chambers, but this was not observed for clone K1659. These results suggest that during chronic fumigation, at concentrations below the threshold of visible leaf injuries, ozone influenced the photosynthetic parameters as a general stress factor, in a similar manner to weather conditions that were more stressful outside the chambers. According to this hypothesis, the sensitivity of a species or a clone to ozone is expected to depend on the growth conditions: the plant is less sensitive to ozone if the conditions are close to optimal and it is more sensitive to ozone under conditions of stress.     

Characterization of a birch (Betula pendula Roth.) embryogenic gene,BP8

We have isolated the birch homologue (BP8) for the carrot embryogenic gene DC8 by heterologous hybridization. The birch BP8 gene encodes a putative protein of 53 kDa, showing 52% sequence identity with the DC8 gene at the amino acid level. The putative BP8 protein contains 20 repeats of 11 amino acids and thus belongs to the group of LEA proteins isolated from such plants as carrot, cotton and wheat. Northern hybridization of mRNA isolated from birch cells representing different stages of somatic embryogenesis and non-embryogenetic material with a PB8 probe gave no signals, suggesting a low expression level of the BP8 gene.     

Pollen-tube growth rate and seed-siring success among Betula pendula clones

The aim of this study was to investigate whether genetically different pollen donors ( Betula pendula clones) differed in pollen-tube growth rate across 11 maternal plants and in vitro , and whether the differences between the donors were consistent across the recipients. To compare the seed-siring success of competing pollen donors, a two-donor hand-pollination experiment with six donors and six recipients was conducted. The experiments were performed at a plastic-house seed orchard. The donors showed significant variation in pollen-tube growth rate on all the 11 recipients. The rankings of the pollen donors were statistically consistent across different maternal plants. A significant positive correlation between pollen tube growth in vivo and in vitro was found. The seed-siring success of two competing pollen donors was unequal in 20 of 29 cases and there was a significant positive correlation between seed-siring success and pollen-tube growth rate in vivo and in vitro . The results show that fertilizations are not random and pollen competition operates in a B. pendula seed orchard population.