Pedro J. Aphalo

Effects of solar UV-A and UV-B radiation on gene expression and phenolic accumulation in Betula pendula leaves

Ultraviolet (UV) radiation is an important environmental factor for plant communities; however, plant responses to solar UV are not fully understood. Here, we report differential effects of solar UV-A and UV-B radiation on the expression of flavonoid pathway genes and phenolic accumulation in leaves of Betula pendula Roth (silver birch) seedlings grown outdoors. Plants were exposed for 30 days to six UV treatments created using three types of plastic film. Epidermal flavonoids measured in vivo decreased when UV-B was excluded. In addition, the concentrations of six flavonoids determined by high-performance liquid chromatography-mass spectrometry declined linearly with UV-B exclusion, and transcripts of PAL and HYH measured by quantitative real-time polymerase chain reaction were expressed at lower levels. UV-A linearly regulated the accumulation of quercetin-3-galactoside and quercetin-3-arabinopyranoside and had a quadratic effect on HYH expression. Furthermore, there were strong positive correlations between PAL expression and accumulation of four flavonols under the UV treatments. Our findings in silver birch contribute to a more detailed understanding of plant responses to solar UV radiation at both molecular and metabolite levels.

Nutrient availability and the effect of increasing UV-B radiation on secondary plant compounds in Scots pine

Growth and pigment composition of plants can be affected by both UV-radiation and availability of nutrients. Therefore, the UV-induction in a plant as a protective response against UV-B radiation may depend on the UV-B dose encountered and the nutritional state of a plant. To test this, young Scots pine (Pinus sylvestris) seedlings were grown under two nutrient levels (relative nutrient addition fates, RAR, of 4 and 6%) and exposed to six different daily doses of UV-radiation (UV-A and UV-B) for one growing season. Both the growth and the accumulation of photosynthetic pigments were positively affected by the availability of nutrients, while the accumulation of condensed tannins and its precursor, (+)-catechin decreased significantly under higher nutrient level, which may imply some trade-off between primary and secondary metabolites. However, nutrient level did not markedly affect the total accumulation of flavonols. Since there was no decrease in the growth of the seedlings and the content of photosynthetic pigments in needles under increased UV-B levels, the seedlings had apparently good protection against UV-B radiation. Protection was probably dependent on the UV-induction of flavonoid compounds in needles, which differed between the two nutrient levels used. Under moderate nutrient availability (RAR 4%), the accumulation of flavonols and also, chlorophylls was highest at the ambient or near to ambient levels of UV-radiation, the levels that pines are generally adapted in natural conditions. As the availability of nutrients and thereby, the resources in seedlings increased (RAR 6%), the UV-B levels higher than ambient increased specifically the accumulation of diacylated flavonols when dose responses were detected. Furthermore, the enhancement of UV-A radiation caused an accumulation of the dihydroflavonols, ampelopsin and taxifolin, under high nutrient availability. Since the flavonoid responses were very compound-specific, and in some cases were also dose-dependent and different between the two nutrient levels used, it can be suggested that the induction of secondary compounds in the flavonoid pathway is multi-step regulated by both environmental factors, UV-radiation and nutrients. Already the present variation in the ambient range of UV-B radiation at northern latitudes can influence the chemical composition of Scots pine seedlings and enhancement in nutrients may increase their responsiveness, particularly to high UV-intensities.

Multiple Roles for UV RESISTANCE LOCUS8 in Regulating Gene Expression and Metabolite Accumulation in Arabidopsis under Solar Ultraviolet Radiation

Summary: Under natural sunlight, this study demonstrates multiple and complex roles for the UV-B photoreceptor UV RESISTANCE LOCUS 8 in the acclimation of Arabidopsis plants to UV radiation. Photomorphogenic responses triggered by low fluence rates of ultraviolet B radiation (UV-B; 280–315 nm) are mediated by the UV-B photoreceptor UV RESISTANCE LOCUS8 (UVR8). Beyond our understanding of the molecular mechanisms of UV-B perception by UVR8, there is still limited information on how the UVR8 pathway functions under natural sunlight. Here, wild-type Arabidopsis (Arabidopsis thaliana) and the uvr8-2 mutant were used in an experiment outdoors where UV-A (315–400 nm) and UV-B irradiances were attenuated using plastic films. Gene expression, PYRIDOXINE BIOSYNTHESIS1 (PDX1) accumulation, and leaf metabolite signatures were analyzed. The results show that UVR8 is required for transcript accumulation of genes involved in UV protection, oxidative stress, hormone signal transduction, and defense against herbivores under solar UV. Under natural UV-A irradiance, UVR8 is likely to interact with UV-A/blue light signaling pathways to moderate UV-B-driven transcript and PDX1 accumulation. UVR8 both positively and negatively affects UV-A-regulated gene expression and metabolite accumulation but is required for the UV-B induction of phenolics. Moreover, UVR8-dependent UV-B acclimation during the early stages of plant development may enhance normal growth under long-term exposure to solar UV.

Solar UV-B radiation affects leaf quality and insect herbivory in the southern beech tree Nothofagus antarctica.

We examined the effects of solar ultraviolet-B (UV-B) radiation on plant-insect interactions in Tierra del Fuego (55°S), Argentina, an area strongly affected by ozone depletion because of its proximity to Antarctica. Solar UV-B under Nothofagus antarctica branches was manipulated using a polyester plastic film to attenuate UV-B (uvb−) and an Aclar film to provide near-ambient UV-B (uvb+). The plastic films were placed on both north-facing (i.e., high solar radiation in the Southern Hemisphere) and south-facing branches. Insects consumed 40% less leaf area from north- than from south-facing branches, and at least 30% less area from uvb+ branches than from uvb− branches. The reduced herbivory on leaves from uvb+ branches occurred for both branch orientations. Leaf mass per area increased and relative water content decreased on north- versus south-facing branches, while no differences were apparent between the UV-B treatments. Solar UV-B did lead to lower gallic acid concentration and higher flavonoid aglycone concentration in uvb+ leaves relative to uvb− leaves. Both the flavonoid aglycone and quercetin-3-arabinopyranoside were higher on north-facing branches. In laboratory preference experiments, larvae of the dominant insect in the natural community, Geometridae “Brown” (Lepidoptera), consumed less area from field-grown uvb+ leaves than from uvb− leaves in 1996–97, but not in 1997–98. Correlation analyses suggested that the reduction in insect herbivory in the field under solar UV-B may be mediated in part by the UV-B effects on gallic acid and flavonoid aglycone.

Field performance of silver-birch planting-stock grown at different spacing and in containers of different volume

Silver birch seedlings were grown in two types of trays differing incell volume. For each kind of tray, three different spacings were achieved bygrowing seedlings in all, half or one quarter of the cells. The planting-stockso produced was out-planted at two forest sites and growth followed for fiveyears. Container size had a large effect on seedling morphology and on fieldperformance. Density also had a large effect on seedling morphology, but littleinfluence on performance. The tightest relationship of field performance was tothe mean dry weight of the stems of the planting stock (R2 = 0.94).Mean height and ‘sturdiness’ index, defined as diameter ÷height, were not correlated with performance. Of the non-destructivemeasurements, mean root-collar diameter was the most closely correlated withperformance.

Clonal differences in growth and phenolics of willows exposed to elevated ultraviolet-B radiation

Abstract In this study, the effects of elevated ultraviolet-B (UV-B, 280–320 nm) radiation on growth and leaf phenolics were evaluated in clones of dark-leaved willow ( Salix myrsinifolia Salisb.) and tea-leaved willow ( Salix phylicifolia L.). Willows were raised for one growing season in an irradiation field, where they were exposed either to a constant 50% increase in UV-B CIE radiation simulating 20–25% ozone depletion or to a small increase in UV-A radiation (320–400 nm). Control willows were grown in the irradiation field under solar radiation (ambient control). Despite the high constitutive concentrations of a UV-absorbing leaf flavonoid, dihydromyricetin, UV-treatments clearly reduced the biomass and height growth of the shoots of one tea-leaved willow clone. In contrast, the growth of three other tea-leaved willow clones showed no significant sensitivity to UV-radiation. Under elevated UV-B radiation, the leaves of these clones were able to accumulate some of the UV-B-absorbing quercetins, myricetins or luteolins. In dark-leaved willows, biomass production and growth were not decreased by UV-exposures, although the concentrations of leaf flavonoids were clearly lower than those in tea-leaved willows; however, in all the dark-leaved willow clones, elevated UV-radiation increased the concentrations of certain quercetins, dihydromyricetin and phenolic acids. Other willow leaf phenolics, i.e. salicylates, condensed tannins and gallic acid derivatives, were either decreased by the UV-treatments or were unaffected. The results indicate that (1) the constitutive level and quality of secondary chemicals in native willow species or clones does not predict their sensitivity to elevated UV-radiation, (2) secondary chemical responses to UV-radiation in willows are more clone-specific than species-specific and (3) the leaves of field-grown willows treated with UV-B radiation accumulate only those phenolics that screen UV-B efficiently. In dieser Studie wurden die Effekte einer erhohten Ultraviolett-B-Strahlung (UV-B, 280–320 nm) auf das Wachstum und die Blattphenole von Klonen der dunkelblattrigen Weide ( Salix myrsinifolia Salisb.) und der teeblattrigen Weide ( Salix phylicifolia L.) bewertet. Die Weiden wurden fur eine Saison auf Bestrahlungfeldern gezogen, wo sie entweder zur Simulation eines 20–25-prozentigen Ozonverlustes einer konstant um 50% erhohten UV-B CIE -Strahlung oder einer geringfugig erhohten UV-A-Strahlung (320–400 nm) ausgesetzt waren. Die Kontrollweiden wurden in den Bestrahlungsfeldern unter Sonneneinstrahlung gezogen (Ausenkontrolle). Trotz der hohen konstitutiven Konzentrationen des Blattflavonoids Dihydromyricetin verringerten die UV-Behandlungen die Biomasse und das Grosenwachstum der Schosslinge eines teeblattrigen Weidenklons deutlich. Im Kontrast dazu zeigte das Grosenwachstum von drei anderen teeblattrigen Weidenklonen keine signifikante Empfindlichkeit gegenuber UV-Strahlung. Bei erhohter UV-B-Strahlung konnten die Blatter dieser Klone einige der UV-B-absorbierenden Quercetine, Myricetine und Luteoline akkumulieren. Bei den dunkelblattrigen Weiden war die Biomassenproduktion und das Wachstum nicht verringert, obwohl die Konzentrationen der Blattflavonoide deutlich geringer als bei teeblattrigen Weiden waren. Bei allen dunkelblattrigen Weidenklonen steigerte jedoch die erhohte UV-Strahlung die Konzentrationen bestimmter Quercetine, von Dihydromyricetin und phenolischer Sauren. Andere Weidenblattphenole, z. B. Salicylate, kondensierte Tannine und Gallsaurenderivate wurden entweder durch die UV-Behandlung verringert oder nicht beeinflusst. Die Ergebnisse deuten darauf hin, dass (1) der konstitutive Gehalt und die Qualitat der sekundaren Pflanzenstoffe einheimischer Weidenarten oder Klone keine Vorhersage uber die Empfindlichkeit gegenaber erhohter UV-Strahlung erlaubt, (2) die Reaktion sekundarer Pflanzenstoffe auf UV-Strahlung bei Weiden eher klonspezifisch als artspezifisch ist und (3) die Blatter von Weiden, die auf dem Feld unter UV-B-Strahlung gezogen wurden, nur die Phenole akkumulieren, die UV-B effektiv abschirmen.

Effects of far-red light on the growth, mycorrhizas and mineral nutrition of Scots pine seedlings

The effects of supplementary far-red sidelight on the formation of mycorrhizas and on the accumulation and allocation of dry weight and mineral nutrients were studied in Scots pine (Pinus sylvestris L.) seedlings. Starting one week after germination the seedlings were subjected to two different light quality regimes: control and simulated sparse-canopy conditions (FR+). In the FR+ regime, light reflected by neighbouring plants was simulated by means of supplementary far-red light sources, which reduced the horizontal red/far-red photon ratio (R:FR) without affecting PAR. Seedlings were harvested after three months of treatment. FR+ increased stem height and decreased the total dry weight of seedlings. Dry weight allocation to needles was not affected, whereas dry weight allocation to roots was reduced and that to stems was increased in FR+ treated seedlings. The total number of short root tips and developing mycorrhizas per seedling were lower in FR+ than in control plants. Most short roots were developing mycorrhizas, while non-mycorrhizal short roots and mycorrhizas with mantle or external mycelium were very scarce. Changes in the allocation of nutrients in general followed the changes in dry weight allocation, and changes in nutrient content followed those in total dry weight. However, mismatches among these changes resulted in significant changes in nutrient concentrations in some organs: the concentrations of nitrogen and potassium in needles and the concentration of nitrogen in stems were higher in FR+ than in control seedlings. Changes in biomass and nutrient allocation under low R:FR may promote rapid height growth during early development in stands of Scots pine seedlings, but concomitant reductions in growth of the root system and mycorrhizas may negatively affect tree performance over the long term.

Sex-related differences in growth and carbon allocation to defence in Populus tremula as explained by current plant defence theories

Plant defence theories have recently evolved in such a way that not only the quantity but also the quality of mineral nutrients is expected to influence plant constitutive defence. Recently, an extended prediction derived from the protein competition model (PCM) suggested that nitrogen (N) limitation is more important for the production of phenolic compounds than phosphorus (P). We aimed at studying sexual differences in the patterns of carbon allocation to growth and constitutive defence in relation to N and P availability in Populus tremula L. seedlings. We compared the gender responses in photosynthesis, growth and whole-plant allocation to phenolic compounds at different combination levels of N and P, and studied how they are explained by the main plant defence theories. We found no sexual differences in phenolic concentrations, but interestingly, slow-growing females had higher leaf N concentration than did males, and genders differed in their allocation priority. There was a trade-off between growth and the production of flavonoid-derived phenylpropanoids on one hand, and between the production of salicylates and flavonoid-derived phenylpropanoids on the other. Under limited nutrient conditions, females prioritized mineral nutrient acquisition, flavonoid and condensed tannin (CT) production, while males invested more in above-ground biomass. Salicylate accumulation followed the growth differentiation balance hypothesis as low N mainly decreased the production of leaf and stem salicylate content while the combination of both low N and low P increased the amount of flavonoids and CTs allocated to leaves and to a lesser extent stems, which agrees with the PCM. We suggest that such a discrepancy in the responses of salicylates and flavonoid-derived CTs is linked to their clearly distinct biosynthetic origins and/or their metabolic costs.

Assessment of UV Biological Spectral Weighting Functions for Phenolic Metabolites and Growth Responses in Silver Birch Seedlings

In research concerning stratospheric ozone depletion, action spectra are used as biological spectral weighting functions (BSWFs) for describing the effects of UV radiation on plant responses. Our aim was to evaluate the appropriateness of six frequently used BSWFs that differ in effectiveness with increasing wavelength. The evaluation of action spectra was based on calculating the effective UV radiation doses according to 1–2) two formulations of the generalized plant action spectrum, 3) a spectrum for ultraviolet induced erythema in human skin, 4) a spectrum for the accumulation of a flavonol in Mesembryanthemum crystallinum, 5) a spectrum for DNA damage in alfalfa seedlings and 6) the plant growth action spectrum. We monitored effects of UV radiation on the concentration of individual UV absorbing metabolites and chlorophyll concentrations in leaves and growth responses of silver birch (Betula pendula) seedlings. Experiments were conducted outdoors using plastic films attenuating different parts of the UV spectrum. Chlorophyll concentrations and growth were not affected by the UV treatments. The response to UV radiation varied between and within groups of phenolics. In general, the observed responses of phenolic groups and individual flavonoids were best predicted by action spectra extending into the UV‐A region with moderate effectiveness.

Temporal variation in epidermal flavonoids due to altered solar UV radiation is moderated by the leaf position in Betula pendula.

The physiological mechanisms controlling plant responses to dynamic changes in ambient solar ultraviolet (UV) radiation are not fully understood: this information is important to further comprehend plant adaptation to their natural habitats. We used the fluorimeter Dualex to estimate in vivo the epidermal flavonoid contents by measuring epidermal UV absorbance (A(375) ) in Betula pendula Roth (silver birch) leaves of different ages under altered UV. Seedlings were grown in a greenhouse for 15 days without UV and transferred outdoors under three UV treatments (UV-0, UV-A and UV-A+B) created by three types of plastic film. After 7 and 13 days, Dualex measurements were taken at adaxial and abaxial epidermis of the first three leaves (L1, L2 and L3) of the seedlings. After 14 days, some of the seedlings were reciprocally swapped amongst the treatments to study the accumulation of epidermal flavonoids in the youngest unfolded leaves (L3) during leaf expansion under changing solar UV environments. A(375) of the leaves responded differently to the UV treatment depending on their position. UV-B increased the A(375) in the leaves independently of leaf position. L3 quickly adjusted A(375) in their epidermis according to the UV they received and these adjustments were affected by previous UV exposure. The initial absence of UV-A+B or UV-A, followed by exposure to UV-A+B, particularly enhanced leaf A(375) . Silver birch leaves modulate their protective pigments in response to changes in the UV environment during their expansion, and their previous UV exposure history affects the epidermal-absorbance achieved during later UV exposure.