The Effect of Shade During Leaf Expansion on Photosynthesis by White Clover Leaves

In three experiments measurements of photosynthesis were made on single leaves of white clover (Trifolium repens L.) on three cultivars grown in a controlled environment. Plants which had grown under an irradiance of 30 J m−2 s−1, or in shade within a simulated mixed sward, produced leaves with photosynthetic capacities some 30 per cent lower than did plants grown at 120 J m−2 s−1 without shade. There were no differences between treatments either in photosynthesis measured at 30 J m−2 s−1, or in respiration rates per unit leaf dry weight. Respiration per unit leaf area was higher in the plants grown at 120 J m−2 s−1, reflecting the lower specific leaf area of these leaves. There were no differences between the three cultivars examined. Leaves which were removed from the shade of a simulated sward shortly after becoming half expanded achieved photosynthetic capacities as high as those which were in full light throughout their development. It is suggested that it is this characteristic which enables clover plants growing in an increasingly dense mixed sward to produce a succession of leaves of high photosynthetic capacity, even though each lamina only reaches the top of the sward at a relatively late stage in its development.