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Photosynthetic Efficiencies and Photorespiration in Calvin Cycle and C₄-Dicarboxylic Acid Plants¹

The efficiency of photosynthetic utilization of light in Calvin cycle plants was lower than that found in plants with the C₄-dicarboxylic acid path of CO₂ fixation when measured in air. Inhibition of photorespiration by O₂ depletion indicated that potential photosynthetic efficiencies of both groups were similar. The ability of C₄ plants to utilize higher light intensities than Calvin cycle plants was associated with a greater affinity for CO₂ at the carboxylation site.

Photorespiration was detected in a leaf attached to an 18 month old sugarcane plant. The leaf was significantly less efficient than similar leaves from a young plant and displayed a greater resistance to CO₂ transport. An explanation is advanced in terms of end-product feedback from assimilate stored in the stalk.

Key Words: Leaf diffusive resistance • Feedback regulation • Saccharum • Transpiration • Temperate and tropical species

² Agricultural Research Officer.

Received for publication April 3, 1969.