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Vol. 10 No. 3 - July 2004

Evergreen Plant Surfaces as

Targets Under Changing Climate

By: Satu Huttunen

All botanists know how variable the aerial green surfaces are. Surface structures (cuticles) vary from thin films to thick waxy surfaces. The protective features are not related to thickness of the cuticle, but mainly to the chemistry and morphology of the aerial surfaces. Some surfaces are water-repellent and some others are easily wettable.

Epicuticular wax chemistry and morphology has attracted scientists as early as sufficient magnifications of microscopes have existed to see few microns ultrastructures. Simultaneously, the complicated stomatal structures with special epicuticular wax structures have been of interest to researchers. The observation, that environmental signal  on developing waxes, affects the development of stomata, is quite recent.

Many leaves and needles are evergreen with a considerably long life span. Evergreen plant surfaces and leaves have considerable morphological, ultrastructural and chemical diversity, and variation in their responses to environmental factors. Wax morphology and chemistry are important in plant systematics, but waxes respond to environment in quality and quantity.

Increasing spring and early summer UV-radiation, fluctuating temperatures, drought or wetness as part of the climate change and air-borne pollutants represent a risk for plants. Wintertime accumulation of air-borne pollutants on evergreen surfaces and into leaves under subarctic conditions was observed in 1970s. Late winter and early spring at high-latitude or high-altitude ecosystems are characterized by high radiation levels and fluctuations between freezing and thawing temperatures. Many evergreens are important key species in arctic and alpine ecosystems. In subarctic or alpine ecosystems, the UV dose received by evergreens in the late winter and early spring is high due to reflectance from the long-lasting snow cover.

Are there especially sensitive or inert wax surfaces in plants? A chemical definition of a wax is the ester of a long-chain acid and a long-chain alcohol. Waxes constitute a large array of different chemical class hydrocarbons, wax esters, ketones, aldehydes and sterols. The second feature is that the chain length range from C2 to C 62.

Future climate in subarctic

Most climate models predict that the maximum temperature increase due to the future change in climate will occur at the high altitudes, and that the relative elevation of tropospheric ozone levels will also be remarkable. A new evaluation of the responses in the Arctic tundra and subarctic forest ecosystems will be available later this year in the form of an ACIA Scientific Report (Meeting in Reykjavik).

The relative ozone depletion and the relative increase in UV-B radiation are now greater in the subarctic than at lower altitudes), and this may be more important than the absolute radiation levels. The most marked increases in UV radiation occur in the springtime, and the warming-related declines in snow and ice cover increase exposure. The total impact is greater than the sum of its parts, such as chemical contaminants, UV and climate warming.

In the arctic tundra and subarctic ecosystems, the snow cover normally persists long into the spring even after air temperatures and light have increased to levels suitable for photosynthesis. Bryophytes and lichens photosynthetize under snow, and so do also evergreen vascular plants. The probable result during spring conditions there will be a prolonged snow-free period, which will affect the early active evergreens.

Responses in surface structure and chemistry

Outdoor UV-B supplementation studies of higher plants involving modulated lamp banks have revealed some significant responses, but plant responses to UV-B generally seem to be more subtle than those based on exclusion studies. The most consistent response in higher plants was an increase in the concentrations of soluble leaf UV-B-absorbing compounds. Phenylpropanoids, e.g. hydroxycinnamic acid, cinnamoyl esters, and flavonoids, including flavones and flavonols, and  anthocyanins provide a UV-A and UV-B screen in higher plants. The flavonoids responsible for UV screening vary from species to species, and most plants synthesize a range of compounds to provide more effective screening. So far, most of the studies have been made with summer-green species.

The studies with evergreens have shown that, in warm years, the production of soluble phenolics is higher compared to cold years. UV-B radiation and altitude alter the foliar flavonoid composition in forest tree species, such as Scots and ponderosa pine.  The responses may be transient or long-lasting.  Phenolics increase with needle age in Scots pine, black pine and ponderosa pine Enhanced UV-B radiation increased Scots pine needle cutinization and wall-bound phenolics as well as flavonoids, , which are important during the late winter and early spring.

The natural UV-screening mechanisms in evergreens have been shown to include UV light screening via reflectance of UV/violet light by the epidermis, UV light screening via reduction of transmission by special anatomical arrangement of epidermal cells as well as light-reflecting hyaline hypodermal cells, conversion of UV light via fluorescence and UV light screening by UV-screening substances in cell walls and on surfaces. In higher plants, anthocyanins and flavones increase in response to high visible light levels, and UV irradiation induces flavonoids, sinapate esters, isoflavonoids and psoralens, and in evergreens, diacylated flavonol monoglycoside induction, for example, has been detected and p-coumaric acid, ferulic acid and astragalins have been identified as UV-B-absorbing substances.

There is a growing body of evidence to suggest that plants respond to biotic and abiotic stress factors by increasing their capacity to scavenge reactive oxygen species via the phenylpropanoid pathway, and that the production of epicuticular waxes increases , and changes in wax biosynthesis and chemistry  occur. On the other hand, harsh climatic factors (winter abrasion) and air-borne particles and pollutants erode epicuticular waxes. Signal transfer from plant surfaces has indicated the role of cuticular waxes in the environmental control of stomatal development, and environmental factors may further affect the function of plant leaves. Surface structures and epicuticular waxes differ in their composition, water repellency, wettability and structural climatic factors, including snow cover and pollution. Also, evergreen shrub responses to elevated temperatures have been studied, but only a few studies have so far been conducted on activity aspects related to enhanced UV and the effects of lengthened spring.

Anthocyanins have been reported to occur in the mesophyll layers of some evergreen species, e.g. Mahonia, Viburnum and Rhododendron. Prenylpropanoid and flavonoid compounds usually accumulate in the central vacuoles of guard cells and epidermal cells as well as the sub epidermal cells of leaves and shoots. Furthermore, some compounds seem to be covalently linked to plant cell walls. In red mosses, anthocyanins are so firmly wall-bound that they have been recommended for use as cytological stains, but their importance for the species is not fully understood. The changes in light climate and hydrology may affect the spectral behaviour of peatland  canopies and further complicate the interpretation of spectral images.

Anthocyanins absorb blue light and reflect red wavelengths, and theoretically, anthocyanins in the upper epidermis or mesophyll of leaves could compete with light harvesting by chlorophyll and carotenoids. Neill and Gould  from New Zealand observed that anthocyanin production enhanced the absorbance of green-yellow wavelengths in proportion to the pigment concentration. The reflectance of red light was independent of the leaf anthocyanin content.

Photoinduction of anthocyanin biosynthesis by wavelengths in the UV, visible and far-red regions, cold temperature and osmotic induction are the best-known cases of anthocyanin appearance. Other induction factors, e.g. nutrient deficiency and plant hormonal relations, have also been discovered.

New approaches on the study of evergreen plant surfaces will provide basic functional and structural knowledge of the responsiveness and acclimation of subarctic  plants. It will also add to our knowledge of cuticular and wax evolution in extant plants and thus provide an important link with plant evolution.  Protective structural features, e.g. epicuticular waxes and light screening compounds, both surface- and cell wall-associated, and their correlations with function and species diversity have not yet been studied earlier e.g. in subarctic or arctic mosses.

New studies will give new information on the diversity of the light and climate responses of arctic and subarctic evergreens and help to predict the long-term responses in key species and to understand the subtle changes due to microclimate or season. Reconstructive methods (herbarium specimens, environmental specimen banks) may be useful in the evaluation of future trends.

Prof.  Satu Huttunen is Professor in Plant Ecophysiology, Botany Division, Department of Biology, University of Oulu, Finland.

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.

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