Differences in chemical composition relative to functional differentiationbetween petioles and laminas of Fraxinus excelsior

Differences in structural and nonstructural carbohydrates, lignin and chlor ophyll, and Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) activ ity between petioles and leaflets were studied along a canopy light gradien t 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 (Q(int), mol m(-2) day(-1)) estimated by combining da ta from hemispherical photographs at the sampling locations with measuremen ts of global solar radiation above the canopy during the growing season. Th e contribution of petioles to leaf carbon assimilation was disproportionall y lower than that of leaf laminas. Though the light relationships of assimi lative compounds-foliar chlorophyll concentration increasing with decreasin g Q(int) to improve leaf absorptance, foliar N concentration and Rubisco ac tivity 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 c arbohydrate concentration was about 20% higher in petioles than in leaf lam inas, indicating that petioles also serve as storage tissues for photosynth ates. Relationships between foliar structural carbon components and irradia nce-increasing lignin (L) and decreasing structural polysaccharide (SP) con centrations with increasing Q(int)-were qualitatively similar for petioles and leaflets. However, petioles had lower L, but higher SP and total invest ment 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 active ly transpiring leaflets. Low lignin concentration in combination with high osmotically active carbohydrate concentrations in petioles suggest that tur gor plays an important role in the mechanical properties of petioles. As a result of lower lignin and protein concentrations, the glucose cost of peti ole construction (g glucose per g dry mass) was about 5% lower than that of leaf laminas.