Light-mediated constraints on leaf function correlate with leaf structure among deciduous and evergreen tree species

Leaf structure has been shown to be an important determinant of leaf photosynthetic characteristics, yet the nature of this relationship remains ambiguous. It has been suggested that intra-leaf shading of chloroplasts may explain the negative influence of increasing leaf thickness/density on mass-based photosynthesis. To explore further the importance of light-mediated functional limitations conferred by leaf structure, we examined photosynthetic responses to high unidirectional and bidirectional irradiances among broad- and needle-leaves ranging widely in mass per area (LMA). Except for leaves with the lowest LMA (< 40 g m-2), photosynthesis increased in bidirectional irradiance, and the ratio of unidirectional to bidirectional photosynthesis (Au/Ab) was strongly and negatively related to LMA, approaching 0.5 for the thickest/densest leaves. Bidirectional illumination also increased stomatal conductance, but the magnitude of stomatal response was modest and uncorrelated with LMA. Consequently, the ratio of intercellular CO2 partial pressures during unidirectional versus bidirectional irradiation was positively related to LMA. Hence, it appears that many C3 leaves may not be "light saturated" under high unidirectional illumination. The negative, exponential Au/Ab-LMA relationship in this study supports the notion that, as LMA increases, an increasing fraction of leaf chloroplasts are functioning at subsaturating irradiance under unidirectional light.