Morphological and stomatal responses of Norway spruce foliage to irradiance within a canopy depending on shoot age

Morphological and stomatal responses of Norway spruce (Picea abies) foliage to light availability were studied in respect to shoot age. Needle minor d iameter (D-1, anatomical width), major diameter (D-2, anatomical thickness) , dry weight (M). and tissue density index (I-D) increased. and needle flat ness (Fl) and specific leaf area (SLA) decreased with foliage age, while sh ade foliage demonstrated higher morphological plasticity as compared to sun foliage. Needle minor diameter, dry weight, and the ratio of total to proj ected leaf area increased. acid needle flatness and specific leaf area decr eased with daily average photosynthetic photon flux density (Q(D)). The cur rent-year foliage exhibited the highest variation with irradiance, while th e morphological plasticity decreased with needle ageing. The morphological characteristics of needles were independent of irradiance if Q(D) was above 300 mu mol m(-2) s(-1). D-1 was the only linear needle characteristic whic h significantly changed with light availability within a canopy, and thus d etermined needle flatness. SLA. as well as the ratio of total to projected leaf are,? (TLA:PLA). Needle flatness was a characteristic responding most sensitively to the photosynthetic photon flux density, R-2 was 0.68, 0.44. and 0.49 for the current-year, 1-year-old, and 2-year-old foliage, respecti vely. TLA/PLA ranged from 2.2 to 4.0 depending on D-1. Variation in SLA in response to light availability can be attributed to changes both in needle shape acid tissue density. Stomatal responses to photosynthetic photon flux density (Q(P)) depended on foliage type (sun or shade) and age. Sun needle s demonstrated higher daily maximum leaf conductances to water vapour compa red to shade needles. The shade needles responded more sensitively to chang es in Q(P) at drawn and sunset than the sun needles, while older needles of both foliage types exhibited Faster stomatal responses. The light-saturati on of leaf conductance (g(L)) was achieved by 20 mu mol m (2) s(1) for shad s foliage, and approximately by 50 mu mol m(-2) s(-1) for sun foliage. As a rule. g(L) changed in response to irradiance faster in the evening, i.e, a t decreasing irradiance. Stomata were not usually completely closed in the dark before sunrise and after sunset, the phenomenon being more pronounced in older shoots and sun needles. Nightly water losses from spruce foliage a re attributable primarily to older shoots, and are related to age-dependent changes in stomatal responsiveness. (C) 2001 Elsevier Science B.V. All rig hts reserved.