Single Leaf and Canopy Photosynthesis in a Ryegrass Sward

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Abstract
Swards of S24 perennial ryegrass (Lolium perenne L.) were grown in the field and harvested by repeated cutting. The rate of photosynthesis of the swards and of single leaves of plants taken from them was measured throughout the growing season. Leaf area and light interception of the sward were also measured. In each growth period, sward photosynthesis increased with increasing leaf area and then declined. In the first growth period, during which the crop flowered, the maximum rate of sward photosynthesis was higher and was reached later than in succeeding vegetative growth periods. The photosynthetic capacity of successive youngest expanded leaves was high throughout the first growth period, and only declined due to leaf age when no further leaves were produced after flowering. However, in the succeeding vegetative growth periods, although the first leaves to expand had high photosynthetic capacities, later leaves had progressively lower capacities. The extent to which changes in the photosynthetic capacity of single leaves accounts for the changes in sward photosynthesis observed in the field is examined using a mathematical model.

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