© 1999 Heron Publishing—Victoria, Canada
Differences in chemical composition relative to functional differentiation between petioles and laminas of Fraxinus excelsior
Ülo Niinemets (1)
1. Estonian Institute of Ecology, Riia 181, Tartu EE 2400, Estonia (ylo@zbi.ee) / Received March 6, 1997
Summary
Differences in structural and nonstructural carbohydrates, lignin and chlorophyll, and Rubisco (ribulose-1,5-bisphosphate
carboxylase/oxygenase) activity between petioles and leaflets were studied along a canopy light gradient in Fraxinus excelsior L., which has pinnate compound leaves and up to 20% of foliar biomass invested in petioles. Long-term light conditions at
the sampling locations were characterized by values of seasonal mean integrated quantum flux density (Qint, mol m–2 day–1) estimated by combining data from hemispherical photographs at the sampling locations with measurements of global solar radiation
above the canopy during the growing season. The contribution of petioles to leaf carbon assimilation was disproportionally
lower than that of leaf laminas. Though the light relationships of assimilative compounds—foliar chlorophyll concentration
increasing with decreasing Qint to improve leaf absorptance, foliar N concentration and Rubisco activity being relatively constant along the light gradient—were
similar for both petioles and leaflets, petiole nitrogen and chlorophyll concentrations were only 30% and 10%, respectively,
of those of leaflets. Nonstructural carbohydrate concentration was about 20% higher in petioles than in leaf laminas, indicating
that petioles also serve as storage tissues for photosynthates. Relationships between foliar structural carbon components
and irradiance—increasing lignin (L) and decreasing structural polysaccharide (SP) concentrations with increasing Qint—were qualitatively similar for petioles and leaflets. However, petioles had lower L, but higher SP and total investment in structural compounds (L + SP) than leaflets. Greater lignification at high irradiances in leaflets than in petioles was attributed to greater water
stresses at high light, and to more variable water contents of actively transpiring leaflets. Low lignin concentration in
combination with high osmotically active carbohydrate concentrations in petioles suggest that turgor plays an important role
in the mechanical properties of petioles. As a result of lower lignin and protein concentrations, the glucose cost of petiole
construction (g glucose per g dry mass) was about 5% lower than that of leaf laminas.
Keywords:
carbon, construction costs, irradiance, leaf structure, lignin, nonstructural carbohydrates, structural carbohydrates.
