Effect of Flowering on the Photosynthetic Capacity of Ryegrass Leaves Grown With and Without Natural Shading

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Abstract
The photosynthetic capacity of newly expanded leaves of vernalized or non-vernalized plants of S24 perennial ryegrass (Lolium perenne L.), grown in long or short photoperiods, was measured in two experiments. In the first, leaves were protected from shading during development, while in the second, the natural shade of neighbouring tillers in a sward was allowed. In the first experiment there was little effect of vernalization, day length or flowering, and leaves in all treatments had photosynthetic rates at 250 W m−2 of between 28 and 32 mg CO2 dm−2h−1. In the second experiment the photosynthetic rate of successive leaves fell as sward leaf area increased. This downward trend was reversed, however, in flowering tillers in the vernalized long-day treatment, while in the other treatments, which did not flower, photosynthetic capacity continued to fall. It is concluded that the leaves of reproductive tillers have high photosynthetic capacities because stem extension carries them to the top of the canopy where they are well illuminated during expansion.