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. |
