Virpi Virjamo

Mycorrhizal symbiosis increases nutrient uptake of the host plant, and it can also improve their stress tolerance. Roots are considered the most frost-sensitive plant parts. However, the freezing tolerance of mycorrhizas, and especially arbuscular mycorrhizas (AM), is poorly understood. Here, we studied the freezing tolerance and recovery of AM and non-mycorrhizal (NM) Thuja occidentalis (L.). After a simulated summer and autumn, whole-plant freezing tests were done using seven exposure temperatures from 5 °C to −45 °C. Then freezing damage of needles was assessed by relative electrolyte leakage. The seedlings were kept for 2 weeks in long-day recovery conditions with day temperature either 10 °C or 22 °C, and then visual damage, shoot and root mass, nutrient concentrations and mycorrhizal colonization were analyzed. Before the frost exposure, AM plants had higher phosphorus (P) concentrations and similar growth to NM plants. Needle freezing tolerance was −23 °C (corresponding to lethal temperature for 50% of specimens) and was not affected by AM. Visual investigation after the recovery period showed a similar result. Lower foliar nitrogen (N) concentration and root mass in seedlings exposed to −18 °C in both AM and NM plants suggests that fine-root damage had taken place already before −18 °C. Recovery in 22 °C increased nutrient uptake and growth only in seedlings exposed to +5 °C and −5 °C, but specific root length increased also after −18 °C. The AM plant shoots grew less than NM ones during the recovery period. Arbuscular mycorrhizas increased foliar N during recovery in all non-lethally exposed seedlings, and P concentrations in seedlings exposed to +5 °C and −5 °C. This was due to a concentration effect by the lower dry mass increment. These results suggest that the functioning of AM fungi can be limited by short growing seasons or in cold soil conditions, which may affect their distribution in cold regions.

We studied responses of needle terpene concentrations and resin canal characteristics to warming in Scots pine ( Pinus sylvestris) and Norway spruce ( Picea abies) seedlings grown in a controlled field set-up in eastern Finland. Warming was simulated during the growing seasons using infrared heaters that increased air temperature in study 1 (2016–2019) by 1 °C and in study 2 (2019–2020) by 2 and 4 °C, compared to ambient conditions. Terpenes were sampled in study 1 from non-matured current year and matured previous year needles in June 2019, and study 2 from mature current year needles in August 2020. In study 1, we also studied resin canal anatomy. We found that 1 °C elevation of temperature caused two-fold increase in concentrations of total terpenes, oxygenated monoterpenes, and sesquiterpenes of non-matured current year needles of Norway spruce. Further, it caused 1.3–1.8-fold increases in sesquiterpene concentrations both in unmatured and matured needles of Scots pine. It also decreased resin canal diameter of mature needles in Norway spruce. In study 2, the stronger warming treatments did not affect terpene concentrations of matured current year needles in either species. Based on our findings, even minor elevation of temperature may affect terpene concentrations of non-mature needles in boreal conifers.

Accumulation of certain phenolics is a well‐known response of plants to enhanced UVB radiation (280–315 nm), but few experiments have compared the relative importance of different phenolic groups for UVB resilience. To study how an altered phenolic profile affects the responses and resilience of silver birch (Betula pendula) to enhanced UVB, we used RNA interference (RNAi) targeting dihydroflavonol reductase (DFR), anthocyanidin synthase (ANS), or anthocyanidin reductase (ANR) to change the accumulation of phenolics. The unmodified control line and RNAi‐modified plants were grown for 51 days under ambient or +32% enhanced UVB dose in a greenhouse. RNAi greatly affected phenolic profile and plant growth. There were no interactive effects of RNAi and UVB on growth or photosynthesis, which indicates that the RNAi and unmodified control plants were equally resilient. UVB enhancement led to an accumulation of foliar flavonoids and condensed tannins, and an increase in the density of stem glands and glandular trichomes on upper leaf surfaces in both the control and RNAi‐modified plants. Our results do not indicate a photoprotective role for condensed tannins. However, decreased growth of high‐flavonoid low‐tannin DFRi and ANRi plants implies that the balance of flavonoids and condensed tannins might be important for normal plant growth.

Knowledge about the defensive chemistry of coniferous trees has increased in recent years regarding a number of alkaloid compounds; in addition to phenolics and terpenes. Here, we show that Norway spruce (Picea abies (L.) H. Karst.), an important boreal zone tree species; accumulates 1,6-dehydropinidine (2-methyl-6-(2-propenyl)-1,6-piperideine) in its needles and bark. We reanalyzed previously published GC-MS data to obtain a full picture of 1,6-dehydropinidine in P. abies. 1,6-dehydropinidine appeared to especially accumulate in developing spring shoots. We used solid-phase partitioning to collect the alkaloid fraction of the sprouts and thin-layer chromatography to purify 1,6-dehydropinidine. The antibacterial properties of the 1,6-dehydropinidine fraction were tested using a broth microdilution method; with Streptococcus equi subsp. equi as a model organism. Based on our results 1,6-dehydropinidine is common in alkaloid extractions from P. abies (0.4 ± 0.03 mg g⁻¹ dw in mature needles) and it is especially abundant in young spruce shoots (2.7 ± 0.5 mg g⁻¹ dw). Moreover; 1,6-dehydropinidine extracted from P. abies sprouts showed mild antibacterial potential against Streptococcus equi subsp. equi (MIC 55 µg mL⁻¹). The antibacterial activity of a plant compound thought of as an intermediate rather than an end-product of biosynthesis calls for more detailed studies regarding the biological function of these coniferous alkaloids

We studied the growth responses of boreal Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies L. Karst.) and silver birch (Betula pendula Roth) seedlings to simulated climate warming of an average of 1.3 °C over the growing season in a controlled field experiment in central Finland. We had six replicate plots for elevated and ambient temperature for each tree species. The warming treatment lasted for the conifers for three growing seasons and for the birch two growing seasons. We measured the height and diameter growth of all the seedlings weekly during the growing season. The shoot and root biomass and their ratios were measured annually in one-third of seedlings harvested from each plot in autumn. After two growing seasons, the height, diameter and shoot biomass were 45%, 19% and 41% larger in silver birch seedlings under the warming treatment, but the root biomass was clearly less affected. After three growing seasons, the height, diameter, shoot and root biomass were under a warming treatment 39, 47, 189 and 113% greater in Scots pine, but the root:shoot ratio 29% lower, respectively. The corresponding responses of Norway spruce to warming were clearly smaller (e.g., shoot biomass 46% higher under a warming treatment). As a comparison, the relative response of height growth in silver birch was after two growing seasons equal to that measured in Scots pine after three growing seasons. Based on our findings, especially silver birch seedlings, but also Scots pine seedlings benefitted from warming, which should be taken into account in forest regeneration in the future.

Introduction: In ecological experiments, storage of plant material is often needed between harvesting and laboratory analyses when the number of samples is too large for immediate, fresh analyses. Thus, accuracy and comparability of the results call for pre‐treatment methods where the chemical composition remains unaltered and large number of samples can be treated efficiently.Objective: To study if a fast dry‐air drying provides an efficient pre‐treatment method for quantitative analyses of phenolics.Methodology: Dry‐air drying of mature leaves was done in a drying room equipped with dehumifier (10% relative humidity, room temperature) and results were compared to freeze‐drying or freeze‐drying after pre‐freezing in liquid nitrogen. The quantities of methanol‐soluble phenolics of Betula pendula Roth, Betula pubescens Ehrh., Salix myrsinifolia Salisb., Picea abies L. Karsten and Pinus sylvestris L. were analysed with HPLC and condensed tannins were analysed using the acid‐butanol test.Results: In deciduous tree leaves (Betula, Salix), the yield of most of the phenolic compounds was equal or higher in samples dried in dry‐air room than the yield from freeze‐dried samples. In Picea abies needles, however, dry‐air drying caused severe reductions in picein, stilbenes, condensed tannin and (+)‐catechin concentrations compared to freeze‐drying. In Pinus sylvestris highest yields of neolignans but lowest yields of acetylated flavonoids were obtained from samples freeze‐dried after pre‐freezing.Conclusion: Results show that dry‐air drying provides effective pre‐treatment method for quantifying the soluble phenolics for deciduous tree leaves, but when analysing coniferous species, the different responses between structural classes of phenolics should be taken into account.

Simultaneously with warming climate, other climatic and environmental factors are also changing. Here, we investigated for the first time the effects of elevated temperature, increased ultraviolet‐B (UVB) radiation, fertilization and all combinations of these on the growth, secondary chemistry and needle structure of 1‐year‐old Norway spruce (Picea abies (L.) Karst.) seedlings in an outdoor experiment. After one growing season, elevated temperature increased root : shoot ratio and concentrations of needle piperidine alkaloids, while concentrations of needle catechins and acetophenones and bark flavonoids decreased compared with ambient temperature seedlings. UVB‐radiation increased concentrations of bark condensed tannins, while fertilization increased total biomass and concentrations of needle catechins. In addition to the main effects, concentrations of some individual phenolic compounds showed UV × temperature or UV × temperature × fertilization interactions, and fertilization modified temperature response on root : shoot ratio. All the treatments described here affected the defence chemistry profiles of the seedlings, which may imply some changes in plant‐herbivore interactions in connection with changing climate. The interactions between treatments indicate a need for further experiments involving several simultaneously affecting environmental changes.