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Relation between Mesophyll Surface Area, Photosynthetic Rate, and Illumination Level during Development for Leaves of Plectranthus parviflorus Henckel ¹

^a Department of Biology, University of California, Los Angeles, California 90024

The influence of illumination level during leaf development on the mesophyll cell surface area per unit leaf area (A^mes/A), CO₂ resistances, and the photosynthetic rate was determined for leaves of Plectranthus parviflorus Henckel. The relative importance of A^mes/A versus CO₂ resistances in accounting for observed changes in photosynthesis was quantitatively evaluated using equations based on analogies to electrical circuits.

When the illumination during development was raised from 900 to 42,000 lux, the leaves more than tripled in thickness as the mesophyll cells increased in size and frequency, which caused A^mes/A to go from 11 to 50. The net rate of photosynthesis at light saturation concomitantly increased 4-fold, reflecting a corresponding decrease in the total resistance for CO₂ movement per unit leaf area. However, the CO₂ resistance per unit area of mesophyll cells remained about 580 seconds per centimeter for leaves grown under 900 to 42,000 lux. Thus, for P. parviflorus, the increased photosynthetic rate for leaves developing under higher illuminations resulted from a higher A^mes/A, not from changes in the CO₂ resistances within individual mesophyll cells, expressed per unit area of cell surface. Results are discussed in terms of previously observed increases in thickness, internal leaf area, and photosynthetic rates for sun versus shade leaves on various plant species.

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