© 1997 Heron Publishing—Victoria, Canada
Acclimation to low irradiance in Picea abies: influences of past and present light climate on foliage structure and function
Ülo Niinemets
Institute of Ecology, Estonian Academy of Sciences, Riia 181, Tartu EE 2400, Estonia / Received May 2, 1996
Summary
Leaf retention time increases with decreasing irradiance, providing an effective way of amortizing the costs of foliage construction
over time. To elucidate the physiological mechanisms underlying this dependence, I studied needle life span, morphology, and
concentrations of carbon, nitrogen and nonstructural carbohydrates along a gradient of relative irradiance in understory trees
of Picea abies (L.) Karst. Maximum needle life span was greater in shaded trees than in sun-exposed trees. However, irrespective of irradiance,
needles with maximum longevity were situated in the middle rather than the bottom of the canopy, suggesting that needle life
span is determined by the irradiance to which needles are exposed during their primary growth. Morphology and chemistry of
current-year needles were adapted to prevailing light conditions. Current-year needles exposed to high irradiances had greater
packing of foliar biomass per unit area than shaded needles, whereas shaded needles maximized foliar area to capture more
light. Nitrogen concentrations were higher in shaded needles than in sun-exposed needles. This nitrogen distribution pattern
was related to the high nitrogen cost of light interception and was assumed to improve light absorptance per needle mass of
shaded needles. In contrast, in both 1- and 2-year-old needles, morphology was independent of prevailing light conditions;
however, needle nitrogen concentrations were adjusted toward more effective light interception in 2-year-old foliage but not
in 1-year-old foliage, indicating that acclimation of sun-adapted needles to shading takes more than one year. At the same
time, needle aging was accompanied by accumulation of nonstructural carbohydrates (NSC), and increasing concentrations of
needle carbon, suggesting a shift in the balance between photosynthesis and photosynthate export. The accumulation of NSC
and carbon resulted in a dilution of the concentrations of other needle chemicals and explained the decline in needle nitrogen
concentrations with increasing age. Thus, although morphological inadequacy to low light availabilities may partly be compensated
for by modifications in needle chemistry, age-related changes in needle stoichiometric composition progressively lessen the
potential for acclimation to low irradiance. A conceptual model, advanced to explain how environmental factors and age-related
changes in the activities of needle xylem and phloem transport affect needle longevity, predicted that adaptation of needle
morphology to irradiance during the primary growth period largely determines the fate of needles during subsequent tree growth
and development.
Keywords:
aging, carbon balance, needle longevity, needle morphology, nitrogen content, nonstructural carbohydrates, Norway spruce,
shade tolerance.
