Internal leaf anatomy and photosynthetic resource-use efficiency: interspecific and intraspecific comparisons.

Leaf mass per unit area (LMA) and internal leaf anatomy often affect net gas exchange because of their effects on internal CO2 conductance to the site of carboxylation, internal shading, competition for CO2 among carboxylation sites, nitrogen concentration and its partitioning. To evaluate effects of LMA and leaf anatomy on CO2 assimilation, water-use efficiency (WUE) and nitrogen-use efficiency (NUE), we measured LMA, leaf thickness, the thickness of mesophyll components, and gas exchange rates at ambient CO2 concentration in leaves of six woody deciduous and evergreen species with different leaf life spans. In two species, CO2 assimilation was also estimated at saturating CO2 concentrations. There were interspecific differences in all morphological variables studied. Long-lived leaves had higher LMA and were thicker than short-lived leaves. Species with high LMA had low assimilation rates and NUE, both in ambient and saturating CO2 concentrations. Thus, in species with high LMA, assimilation was reduced by non-stomatal limitations, possibly because of a lower allocation of N to the photosynthetic machinery than in species with low LMA. Within a species, thicker leaves tended to have a lower tissue density. In intraspecific comparisons under field conditions, increasing internal air volume had positive effects on WUE, probably because of enhanced internal CO2 conductance to the site of carboxylation. We conclude that, in interspecific comparisons, different patterns of N partitioning strongly influence NUE, whereas in intraspecific comparisons, internal leaf anatomy is a key factor regulating resource-use efficiency.