Photosynthesis during winter in ryegrass/white clover mixtures in the field

Measurements of canopy gas exchange of mixed communities of ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) in the field during winter showed that instantaneous rates of gross photosynthesis were usually little less than those of canopies of similar leaf area at similar irradiance in summer. Respiration rates per unit dry weight were on average 25% less than those of comparable swards in summer. The daily totals of net photosynthesis, however, were as little as a tenth of summer values, because of the short photoperiods and low average irradiance. Clover leaf area tended to be positioned lower in the canopy than grass leaf area, especially towards the end of winter, and clover had a lower rate of photosynthesis per unit leaf area in situ than grass.