Elina Vapaavuori

HERBIVORE RESISTANCE IN BETULA PENDULA: EFFECT OF FERTILIZATION, DEFOLIATION, AND PLANT GENOTYPE

Plant resistance to herbivores is affected both by genetic and environmental factors. The carbon–nutrient balance hypothesis (CNB) explains environmentally induced variation in both constitutive and delayed herbivore-induced resistance (DIR) in terms of variation in soil fertility and light regime. The CNB hypothesis predicts that an increase in the availability of nutrients (e.g., fertilization) decreases both constitutive and induced resistance against herbivores. We tested the relative roles of plant genotype, defoliation, and soil fertility in determining herbivore resistance of cloned silver birch Betula pendula Roth saplings. As indicators of insect and mammalian resistance we conducted bioassays with a geometrid moth, Epirrita autumnata (Borkhausen), and counted the resin droplets on the shoot of the saplings, respectively. In addition, we measured rapid induced resistance (RIR) against the insect herbivore. Finally, we analyzed leaf secondary chemistry to investigate the correlations of secondary chemicals with the level of resistance measured using the performance of E. autumnata. With respect to the constitutive resistance against an insect herbivore, our results support the CNB hypothesis; the larvae of E. autumnata had a higher relative growth rate and pupal mass on fertilized saplings compared to nonfertilized saplings, i.e., the fertilized saplings had a lower resistance level. However, the relative growth rate of E. autumnata was significantly decreased by defoliation only when the larvae were grown on fertilized saplings. The number of resin droplets increased due to fertilization and, in fertilized saplings, following defoliation, but these responses were highly determined by the genotype of the sapling. Altogether, the results on resin droplets are not in accordance with the CNB hypothesis. The concentration of condensed tannins correlated negatively with E. autumnata growth rate and pupal mass in both fertilization levels, whereas the concentration of total nontannin phenolics correlated positively with the E. autumnata growth rate in nonfertilized saplings. In addition, the concentration of myricetin glycosides correlated negatively with the pupal mass of E. autumnata, whereas the correlations between E. autumnata performance indices and other groups of flavonol glycosides were either significantly positive (kaempferol glycosides) or nonsignificant (quercetin glycosides). Further, the concentration of 3,4′-dihydroxypropiophenone 3-glucoside (DHPPG) correlated positively with the magnitude of induction in E. autumnata growth rate and pupal mass in fertilized saplings, where the significant induction in resistance occurred. The correlations of secondary chemistry and E. autumnata performance indices suggest that the constitutive level of resistance of B. pendula against E. autumnata is mainly determined by the concentration of condensed tannins, whereas the induced resistance is determined by the concentration of nontannin phenolics, such as flavonol glycosides and DHPPG. We observed significant differences among the clones in their insect and mammalian resistance (i.e., genetic basis for the resistance), which indicates that resistance can evolve as a response to herbivory. However, fertilization explained a higher proportion of variance in insect performance indices than the genotype of the plant, whereas the opposite was true for the amount of resin droplets, which we used as an indicator of mammalian resistance.

Physiology of carbon allocation in trees

Abstract The influence of mineral nutrients, light, carbon dioxide, ozone, ammonia, water, temperature, soil texture and soil acidity on carbon allocation in trees is reviewed. The growth rhythms of the different plant parts on a seasonal basis are examined as well as the change in source-sink balance caused by plant age and genetic constitution. The exact outcome of all these factors on plant growth and carbon allocation is difficult to predict. However, one distinct pattern with regard to carbon allocation, and important for plant survival, becomes evident from this evaluation. Root growth is always decreased when plants become carbon limited, independently of whether this situation is caused by reduced photosynthesis (O3, low light, or shortage of K, Mg or Mn) or competition between root growth and NH4+ as sinks for carbon skeletons (atmospheric NH3 and root uptake of NH4+). Low soil temperatures as well as competition from intensive shoot growth affect root development in a similar way. Inhibition of mycorrhizal development after exposure to O3, NH3, and low availability of Mg can also be explained by the same mechanisms.

Trade-offs in phenolic metabolism of silver birch: Effects of fertilization, defoliation, and genotype

We examined the chemical responses of 10 silver birch (Betula pendula) clones to fertilization and defoliation in a field experiment. In defoliation, every second leaf was removed from the saplings. Three days later, two undamaged short-shoot leaves were collected, air-dried, and analyzed for condensed tannins and 34 nontannin phenolic compounds by high-performance liquid chromatography. The clones showed substantial variation in phenolic composition of the leaves and in chemical responses to fertilization and defoliation. A cluster analysis by UPGMA indicated that the phenolic profiles of birch leaves were affected more by genotype than fertilization or defoliation, and the clones could thus be distinguished from each other. In addition, on the basis of their overall phenolic composition, the clones were clustered loosely in three clone groups. The leaves of fertilized saplings contained lower levels of condensed tannins than controls, as predicted by carbon/nutrient balance (CNB) hypothesis. However, fertilization had no effect on the total amount of nontannin phenolics. The concentrations of (+)-catechin, 3,4′-dihydroxypropiophenone 3-glucoside (DHPPG), 3-cinnamoylquinic acids, and flavone aglycones were lower in fertilized saplings, whereas the opposite was true for 5-cinnamoylquinic acids and the total amount of flavonol glycosides. Although our results provide support for the CNB hypothesis, they also show that the accumulation of phenolic compounds in birch leaves is strongly coordinated. Different branches of the biosynthetic pathway of phenolic compounds may compete for substrates, and such internal metabolic trade-offs may explain the differential accumulation of the compounds. In fertilized saplings, the concentration of condensed tannins was also negatively correlated with the amount of triterpenoid resin droplets measured from the same saplings. We suggest that a linkage via malonyl-CoA between the biosynthetic routes to terpenoids and flavonoid derivatives, such as condensed tannins, may explain the different responses to fertilization reported for terpenoids and phenolics. Undamaged leaves of partially defoliated saplings contained more DHPPG and flavone aglycones and less cinnamic acid derivatives and (+)-catechin than did leaves of control saplings. The induction of DHPPG and flavonoid aglycones was significantly and negatively correlated with the concentration of myricetin glycosides in fertilized saplings, which may indicate a trade-off between induced and constitutive defense. Moreover, in fertilized saplings, the three clone groups formed by UPGMA clustering differed significantly in the magnitude of induction of DHPPG and flavone aglycones. Different birch genotypes may thus have different modes of chemical defense, and the magnitude of chemical response of a genotype may partly depend on resource availability. In general, our results show that new insights in the theory of chemical defense can be gained by accomplishing studies on plant–herbivore interaction with high chemical resolution.

Chemical factors affecting the brown-rot decay resistance of Scots pine heartwood

The cell wall chemistry (amount of hemicellulose, α-cellulose, and total lignin) and the concentration of extractives (total acetone-soluble extractives, resin acids, pinosylvins and the total phenolics quantified as tannin acid equivalents) were studied in brown-rot resistant and susceptible juvenile heartwood of Scots pine (Pinus sylvestris L.). The study material consisted of a total of 18 trees from two 34-year-old progeny trials at Korpilahti and Kerimäki. The trees were selected from among 783 trees whose decay rate had previously been screened in a laboratory test using a brown-rot fungus, Coniophora puteana. Samples from neither location showed any significant difference in the concentration (mg/cm3) of hemicellulose, α-cellulose and total lignin between the decay resistant and susceptible trees. At both locations only the concentration of total phenolics was higher in the decay-resistant heartwood than in the decay-susceptible heartwood. At Korpilahti, the amount of acetone-soluble extractives and the concentration of pinosylvin and its derivatives were higher in the resistant than in the susceptible trees.

Effects of long-term nutrient optimisation on stem wood chemistry in Picea abies

Abstract. The aim of the present study was to determine how long-term nutrient optimisation of Norway spruce stands affects the chemical composition of stem wood. Material for the study was collected from Flakaliden (Sweden) where Norway spruce [Picea abies (L.) Karst.] stands have been grown either without fertilisation or under nutrient optimisation treatment, by supplying a complete nutrient mix in the irrigation water every 2nd day during the growing season. The experiment was established in 1987 and in the autumn of 1998, 12 trees were harvested both in control (no fertilisation) and irrigated-fertilised (IL) stands. The increased growth rate caused by the IL treatment affected the chemical composition of the stem wood. The most pronounced effect was a 7% increase in lignin concentration caused by the IL treatment. Increases in concentrations of acid-soluble lignin (1.1-fold), extractives (1.2-fold), soluble sugars (1.3-fold), sterols (1.3-fold) and dehydroabietic acid (1.6-fold) as well as a decrease in the proportional quantity of terpinolene were also found. These results demonstrate that nutrient optimisation affected the chemical composition of Norway spruce wood, which may influence the suitability of such wood for specific end-use purposes.

Gene expression responses of paper birch (Betula papyrifera) to elevated CO2 and O3 during leaf maturation and senescence

Gene expression responses of paper birch (Betula papyrifera) leaves to elevated concentrations of CO(2) and O(3) were studied with microarray analyses from three time points during the summer of 2004 at Aspen FACE. Microarray data were analyzed with clustering techniques, self-organizing maps, K-means clustering and Sammons mappings, to detect similar gene expression patterns within sampling times and treatments. Most of the alterations in gene expression were caused by O(3), alone or in combination with CO(2). O(3) induced defensive reactions to oxidative stress and earlier leaf senescence, seen as decreased expression of photosynthesis- and carbon fixation-related genes, and increased expression of senescence-associated genes. The effects of elevated CO(2) reflected surplus of carbon that was directed to synthesis of secondary compounds. The combined CO(2)+O(3) treatment resulted in differential gene expression than with individual gas treatments or in changes similar to O(3) treatment, indicating that CO(2) cannot totally alleviate the harmful effects of O(3).

Effects of decadal exposure to interacting elevated CO2 and/or O3 on paper birch (Betula papyrifera) reproduction

We studied the effects of long-term exposure (nine years) of birch (Betula papyrifera) trees to elevated CO(2) and/or O(3) on reproduction and seedling development at the Aspen FACE (Free-Air Carbon Dioxide Enrichment) site in Rhinelander, WI. We found that elevated CO(2) increased both the number of trees that flowered and the quantity of flowers (260% increase in male flower production), increased seed weight, germination rate, and seedling vigor. Elevated O(3) also increased flowering but decreased seed weight and germination rate. In the combination treatment (elevated CO(2)+O(3)) seed weight is decreased (20% reduction) while germination rate was unaffected. The evidence from this study indicates that elevated CO(2) may have a largely positive impact on forest tree reproduction and regeneration while elevated O(3) will likely have a negative impact.

Urbanization-related changes in European aspen (Populus tremula L.): Leaf traits and litter decomposition

We investigated foliar and litter responses of European aspen (Populus tremula L.) to urbanization, including factors such as increased temperature, moisture stress and nitrogen (N) deposition. Leaf samples were collected in 2006-2008 from three urban and three rural forest stands in the Helsinki Metropolitan Area, southern Finland, and reciprocal litter transplantations were established between urban and rural sites. Urban leaves exhibited a higher amount of epicuticular waxes and N concentration, and a lower C:N ratio than rural ones, but there was no difference in specific leaf area. Urban litter had a slightly higher N concentration, lower concentrations of lignin and total phenolics, and was more palatable to a macrofaunal decomposer. Moreover, litter decay was faster at the urban site and for urban litter. Urbanization thus resulted in foliar acclimatization in terms of increased amount of epicuticular waxes, as well as in accelerated decomposition of the N-richer leaf litter.

Costs of herbivore resistance in clonal saplings of Betula pendula

Abstract. Several studies have found genetic variation in plant resistance to herbivory. One of the explanations suggested for the observed intermediate levels of resistance are the costs of resistance, i.e., negative genetic correlations between resistance and other fitness components that may constrain the evolution of resistance. We studied the cost of herbivore resistance by investigating the genetic correlations between resistance traits and plant growth traits, and between resistance to insect and mammalian herbivores in cloned saplings of silver birch, Betula pendula. We used the performance of a geometrid moth, Epirrita autumnata, as an indicator of insect resistance. The numbers of resin droplets at the base and at the tip of the saplings correlate with mammalian resistance, and were thus used here as indicators of vole and hare resistance, respectively. We have previously observed genetic variation in these resistance traits. Further, we examined the correlations between several groups of secondary chemicals and plant growth traits. Finally, to reveal the effect of environmental factors on the trade-offs mentioned above, we investigated the correlations in saplings that were grown at two nutrient levels. We found significant negative correlations between indices of constitutive insect resistance and relative height growth in non-fertilized saplings, indicating cost of constitutive insect resistance. The two groups of secondary chemicals that have been shown to correlate strongly with constitutive insect resistance, i.e., condensed tannins and flavonol glycosides (especially myricetin glycosides), had different genetic correlations with plant traits; the concentration of condensed tannins did not correlate negatively with any of the plant traits, whereas the concentration of flavonol glycosides correlated negatively with plant height. Insect and mammalian resistance did not correlate negatively, indicating no ecological trade-offs.

Relationship between net photosynthesis and nitrogen in Scots pine: Seasonal variation in seedlings and shoots

The relationship between light saturated net photosynthesis (Amax) and nitrogen concentration (N) was studied in needles of both Scots pine seedlings, grown at three relative growth rates (2,6 and 8%) controlled by nutrient addition rate, and Scots pine shoots collected from four sites with different fertility. In the seedlings, Amax was measured on 14 different dates starting at the beginning of the second growing season and ending when growth of the new shoot and the secondary needles had finished. In shoots from the natural stands Amax of the previous-year shoots was measured on 6 dates throughout the growing season.