Three populations of each of Lolium perenne and Lolium multiflorum were grown at four temperatures at each of three light intensities. Concurrent studies were made on the response of leaf photosynthetic activity and leaf structure of newly-expanded leaves of 7-week old seedlings of each of the populations to the contrasting growth environments.

Photochemical efficiencies, leaf conductances, photorespiration constants and apparent activation energies for photosynthesis were calculated for each population in each of the twelve growth environments.

Photosynthetic rates at light saturation were expressed on the basis of leaf area, of leaf volume and of leaf mesophyll tissue volume. Photosynthetic rates were also measured under different assay conditions.

By expressing photosynthetic rates on a leaf volume basis, variation between populations and growth environments resulting from leaf structural changes were partially removed.

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