Northern environment predisposes birches to ozone damage.

Ozone sensitivity of silver birch ( BETULA PENDULA Roth) has been thoroughly investigated since early 1990's in Finland. In our long-term open-field experiments the annual percentage reduction in basal diameter and stem volume increment were the best non-destructive growth indicators for ozone impact when plotted against AOTX. Remarkable differences in defence strategies, stomatal conductance, and defence compounds (phenolics), clearly indicate that external exposure indices are ineffective for accurate risk assessment for birch. For flux-based approaches, site-specific values for G(max) and G(dark) are necessary, and determinants for detoxification capacity, ageing of leaves, and cumulative ozone impact would be needed for further model development. Increasing CO(2) seems to counteract negative ozone responses in birch, whereas exposure to spring time frost may seriously exacerbate ozone damage in northern conditions. Therefore, we need to proceed towards incorporating the most important climate change factors in any attempts for ozone risk assessment.

Carbon assimilation and nitrogen in needles of fertilized and unfertilized field-grown Scots pine at natural and elevated concentrations of CO2.

Effects of elevated CO2 concentration ([CO2]) on carbon assimilation and needle biochemistry of fertilized and unfertilized 25-30-year-old Scots pine (Pinus sylvestris L.) trees were studied in a branch bag experiment set up in a naturally regenerated stand. In each tree, one branch was enclosed in a bag supplied with ambient [CO2] (360 micromol mol(-1)), a second branch was enclosed in a bag supplied with elevated [CO2] (680 micromol(-1)) and a control branch was left unbagged. The CO2 treatments were applied from April 15 to September 15, starting in 1993 for unfertilized trees and in 1994 for fertilized trees, which were treated with N in June 1994. Net photosynthesis, amount and activity of Rubisco, N, starch, C:N ratio and SLA of needles were measured during the growing season of 1995. Light-saturated net photosynthetic rates of 1-year-old and current-year shoots measured at ambient [CO2] were not affected by growth [CO2] or N fertilization. Elevated [CO2] reduced the amount and activity of Rubisco, and the relative proportion of Rubisco to soluble proteins and N in needles of unfertilized trees. Elevated [CO2] also reduced the chlorophyll concentration (fresh weight basis) of needles of unfertilized trees. Soluble protein concentration of needles was not affected by growth [CO2]. Elevated [CO2] decreased the Rubisco:chlorophyll ratio in unfertilized and fertilized trees. Starch concentration was significantly increased at elevated [CO2] only in 1-year-old needles of fertilized trees. Elevated [CO2] reduced needle N concentration on a dry weight or structural basis (dry weight minus starch) in unfertilized trees, resulting in an increase in needle C:N ratio. Fertilization had no effect on soluble protein, chlorophyll, Rubisco or N concentration of needles. The decrease in the relative proportions of Rubisco and N concentration in needles of unfertilized trees at elevated [CO2] indicates reallocation of N resources away from Rubisco to nonphotosynthetic processes in other plant parts. Acclimation occurred in a single branch exposed to high [CO2], despite the large sink of the tree. The responses of 1-year-old and current-year needles to elevation of growth [CO2] were similar.

Responses of Scots pine (Pinus sylvestris) seedlings grown in different nutrient regimes to changing root zone temperature in spring.

We examined effects of nutrient availability and changing root zone temperature (RZT) on growth, gas exchange and plasma membrane H(+)-ATPase (PM-ATPase) activity of roots of 1-year-old Scots pine (Pinus sylvestris L.) seedlings during spring flushing. The 6-week growth-chamber experiment was carried out in hydroponic cultures that supplied the seedlings with low (0.5 mM N) or high (3 mM N) nutrient concentration and two rates of increase in RZT were simulated: slow warming (SW-treatment) and fast warming (FW-treatment). Air temperature, humidity, and light conditions were similar in all treatments. Growth of roots and shoots was retarded at low RZT, and fresh mass increment of roots was closely correlated with RZT sum. High nutrient availability increased nitrogen concentrations of needles and stems, but only at RZTs >13 degrees C. Low RZT and low availability of nutrients suppressed gas exchange of the seedlings. Real PM-ATPase activity was highly dependent on RZT. At high RZTs, real PM-ATPase activity was affected by nutrient availability but this effect was related to root growth. We conclude that, under conditions of high nutrient availability, Scots pine seedlings can compensate for the suppressive effects of long-term exposure to low RZT by rapidly accelerating growth, gas exchange and root metabolism, but only when RZT has increased above a threshold value, which was 13 degrees C in this study.

Effects of root temperature on growth and photosynthesis in conifer seedlings during shoot elongation.

Growth and gas exchange characteristics were studied in pine (Pinus sylvestris L.) and spruce (Picea abies Karst.) seedlings grown in hydroponic culture in the presence of N (50 mg l(-1)) and transferred at the start of their second growing season to tap water at 5, 8, 12, 16 or 20 degrees C (air temperature between 18-20 degrees C) for 3 weeks (pine) or 5 weeks (spruce). Root growth of both species was completely inhibited at root temperatures of 5 and 8 degrees C, but increased almost exponentially as root temperature increased. Shoot growth was maximal at 12 degrees C in both pine and spruce and decreased at low root temperatures. In both species, CO(2) uptake was decreased at low root temperatures and appeared to be influenced by the pattern of nitrogen retranslocation. In pine seedlings, as root temperature increased, an increasing proportion of the total nitrogen pool was retranslocated to the new shoot, whereas in spruce seedlings nitrogen was retranslocated to the roots. Differences in the retranslocation of nitrogen in the two species were reflected in the amount of soluble protein in needles, which at the end of the experiment increased with increasing root temperature in pine, but decreased in spruce. Our data suggest that in spruce, but not pine, CO(2) uptake was limited by the amount of Rubisco.

Seasonal variation in the photosynthetic capacity of a willow (Salix cv. Aquatica gigantea) canopy. 1 Changes in the activity and amount of ribulose 1,5-bisphosphate carboxylase-oxygenase and the content of nitrogen and chlorophyll at different levels in the canopy.

The role of ribulose 1,5-bisphosphate carboxylase-oxygenase (Rubisco) in the regulation of photosynthesis in the field was assessed by following changes in the amount and activity of Rubisco and in the amounts of chlorophyll and total nitrogen in a willow (Salix cv. Aquatica gigantea) canopy during two growing seasons. An additional aim of this study was to determine how nitrogen is partitioned in the canopy and whether Rubisco and chlorophyll serve as sources of nitrogen. During both growing seasons, the total activity and the amount of Rubisco were high in young, sun-exposed leaves and decreased as the leaves aged and became shaded. In 1985, the specific activity was low (on average 0.8 micromol mm(-1) mg(-1) Rubisco protein) compared with the values obtained in 1986 (on average 1.8 micromol min(-1) mg(-1) Rubisco protein). These differences in the specific activity of Rubisco between the two years might be associated with changes in partitioning of nitrogen. During the cool summer of 1985, about 20% of the total nitrogen was partitioned to Rubisco compared with about 13% during the warm summer of 1986. Rubisco comprised a high proportion (about 60%) of the total soluble protein throughout the 1985 growing season, whereas during the 1986 growing season, the proportion of Rubisco in the total soluble protein fraction was lower and decreased markedly as the leaves became shaded. Chlorophyll did not serve as a source of nitrogen in the canopy. On the contrary, the proportion of total nitrogen in chlorophyll increased with time, which indicates acclimation of the thylakoid membranes to decreasing irradiance. The degree of activation of Rubisco in samples collected in 1986 was high in young, sun-exposed leaves, which suggests that in these leaves, Rubisco may limit the rate of CO(2) uptake. In the shaded leaves of the lower canopy, the degree of activation of Rubisco was low, which indicates that in these leaves, photosynthesis is limited by factors other than Rubisco.

Seasonal variation in the photosynthetic capacity of a willow (Salix cv. Aquatica gigantea) canopy. 2 Comparison of the structure and function of chloroplasts at different levels in the canopy.

Development of leaf chloroplast ultrastructure at five levels in a willow (Salix cv. Aquatica gigantea) canopy was followed during one growing season in the field. Changes in chloroplast ultrastructure were compared with the rate of CO(2) uptake of the same leaves. The highest rates of CO(2) uptake were recorded in young leaves exposed to full available sunlight. In these leaves, the area of the grana stacks was less than 20% of the total chloroplast area and the degree of thylakoid stacking was less than 1.5. The chloroplasts of these leaves contained large amounts of starch and small amounts of plastoglobuli. As the canopy grew and the leaves in the lower parts of the canopy became shaded, the structure of the chloroplast thylakoids gradually changed. In leaves at the two lowest levels of the canopy, the degree of stacking at the end of the growing season was close to 2 and correspondingly the rate of CO(2) uptake was low. The areas of grana stacks and plastoglobuli in these chloroplasts increased and were about 30 and 10% of the chloroplast area, respectively, by the end of the growing season. The increase in the degree of thylakoid stacking was caused by increased biosynthesis of grana lamellae, which in general were thinner than the lamellae of young leaves. The length of the stroma lamellae did not change with leaf age. Morphometric measurements showed that the structure of the chloroplasts in leaves 160 cm above ground was dynamic and responsive to environmental conditions so that photosynthetic capacity remained high for 7 weeks despite an increase in leaf shading.