Leaf anatomy enables more equal access to light and CO2 between chloroplasts
New Phytologist, (1999)Abstract
The function of a leaf is photosynthesis, which requires the interception of light and access to atmospheric CO\# while controlling water loss. This paper examines the in¯uence of leaf anatomy on both light capture and CO\# diffusion. As photosynthetic metabolism is spread between many chloroplasts, a leaf faces the challenge of matching light capture by a given chloroplast with the metabolic capacity of that chloroplast. Chloroplasts nearest the leaf surface receive the greatest irradiance and therefore absorb more light per unit chlorophyll than chloroplasts in the centre of a leaf. Electron transport and carbon ®xation capacities per unit of chlorophyll decline with increasing depth in the leaf, to compensate for the decline in light absorbed per unit chlorophyll. Many key photosynthetic protein complexes in chloroplasts have nuclear encoded genetic information. Consequently, all chloroplasts within a given cell have a similar metabolic complement, which limits the potential gradient of photosynthetic capacity per unit chlorophyll across the leaf. A simple model couples light absorption through the leaf (based on the Beer±Lambert law) with the pro®le of chlorophyll through a leaf and the gradient in photosynthetic capacity. It is validated by comparison with "\%CO\# ®xation pro®les through spinach leaves obtained in various studies. The model can account for published "\%C ®xation pro®les obtained with blue, red and green light of different irradiances and white light applied in different combinations to the adaxial and abaxial surfaces of spinach leaves. The model con®rms that spongy mesophyll increases the apparent extinction coefficient of chlorophyll compared to palisade tissue. The palisade tissue nearest the surface which receives light facilitates the penetration of light to a greater depth, while spongy mesophyll promotes scattering to enhance light absorption, thus reducing the gradient in light absorbed per unit chlorophyll through a leaf. CO\# ®xation faces a diffusional limitation, which necessitates Rubisco to be spread evenly across the cell walls exposed to intercellular airspace. Mesophyll cell structure re¯ects the need to have a large cell surface per unit volume exposed to airspaces. The regular array of columnar cells in palisade tissue, or cell lobing in monocot leaves, results in greater exposed surface per unit tissue volume than spongy mesophyll. The exposed surface area per unit leaf area scales with photosynthetic capacity such that the difference in CO\# partial pressure between substomatal cavities and the sites of carboxylation within chloroplasts is, on average, independent of photosynthetic capacity of the leaf. However, Rubisco speci®c activity declines as the Rubisco content per unit leaf area increases due to greater internal diffusional limitations.