Light Direction, Absorption, and Photosynthesis

Plant photosynthesis generally increases with irradiance until light saturation occurs. The directional quality of light, however, can affect its penetration and absorption within a leaf. For example, increasing the angle of incidence (from perpendicular) at which light intersects the leaf surface decreases penetration depth and, ultimately, absorption. Although studies at the individual leaf level are few, several lines of evidence suggest that the leaf’s developmental environment underlies internal light absorption and subsequent photosynthetic responses to diffuse versus direct light. Thick, sun-grown leaves show lower photosynthesis under diffuse relative to direct light, whereas thin, shade-grown leaves show no advantage. Despite its potential impact on agricultural and ecosystem productivity, the effect of diffuse light on photosynthesis at the leaf level is not well understood. Earles et al. () have investigated whether or not the spatial distribution of light absorption relative to electron transport capacity in sun- and shade-grown sunflower (Helianthus annuus) leaves underlies its previously observed diffuse light photosynthetic depression. Using a new one-dimensional porous medium finite element gas-exchange model parameterized with light absorption profiles, they found that weaker penetration of diffuse versus direct light into the mesophyll of sun-grown sunflower leaves led to a more heterogenous saturation of electron transport capacity and lowered its CO2 concentration drawdown capacity in the intercellular airspace and chloroplast stroma. This decoupling of light availability from photosynthetic capacity under diffuse light is sufficient to generate an 11% decline in photosynthesis in sun-grown but not shade-grown leaves, primarily because thin shade-grown leaves similarly distribute diffuse and direct light throughout the mesophyll.