© 2001 Heron Publishing—Victoria, Canada
Foliar morphological and physiological plasticity in Picea abies and Abies alba saplings along a natural light gradient
Giacomo Grassi (1) and Umberto Bagnaresi (1)
1. Dipartimento di Colture Arboree, Univ. di Bologna, via Filippo Re 6, I-40126 Bologna, Italy / Received August 18, 2000
Summary
The role of morphological versus physiological foliar plasticity in the capacity for, and mechanisms of, photosynthetic acclimation
was assessed in Picea abies (L.) Karst. and Abies alba Mill. saplings in a forest gap–understory light gradient (relative irradiance, RI, ranging from 0.02 to 0.32). The species
investigated showed a similar foliar morphological plasticity along the light gradient, at both the needle level (through
alteration in leaf dry mass per area) and the shoot level (through alteration in the silhouette area ratio, e.g., shoot silhouette
to projected needle area ratio). In both species chlorophyll (Chl) concentration on a mass basis decreased at increasing RI,
but was independent of RI when expressed on an area basis. In contrast, leaf N concentration on a mass basis was independent
of RI, but was positively influenced by RI when expressed on an area basis. The parameters describing photosynthetic performance
at low light (dark respiration rate, apparent quantum yield and light compensation point) suggest that Abies alba was better suited to maintain a positive carbon balance in shaded conditions. By contrast, parameters describing biochemical
capacity at high light (maximum electron transport rate, Jmax and maximum ribulose-1,5-biphosphate carboxylation capacity, Vcmax) indicate that only Picea abies was capable of acclimating physiologically to high photosynthetic photon flux densities (PPFDs) by increasing nitrogen partitioning
to Rubisco and Vcmax/mass by increasing RI. These results support the hypothesis that interspecific differences in nitrogen partitioning within
the photosynthetic apparatus may provide a mechanistic basis for species separation along a light gradient. The differences
in photosynthetic plasticity observed are likely to influence regeneration patterns and habitat breadth of the species investigated.
The limited ability of Abies alba saplings to exploit high-light conditions may be a competitive disadvantage in large canopy gaps and thus limit recruitment
of this species to small gaps.
Keywords:
A/Ci curves, leaf mass per area, light acclimation, nitrogen partitioning, silhouette area ratio.
