Effect of Temperature and Nitrogen Supply on the Growth of Perennial Ryegrass and White Clover. 1. Carbon and Nitrogen Economies of Mixed Swards at Low Temperature

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Abstract
Simulated mixed swards of perennial ryegrass (Lolium perenne L. cv. S23) and white clover (Trifolium repens L. cv. S100) were grown from seed under a constant 10°C day/8°C night temperature regime and their growth, and carbon and nitrogen economies examined. The swards received a nutrient solution, every second day, which contained either high (220 μg g−1) or low (40 μg g−1) nitrate N. The High-N swards had rates of canopy photosynthesis and dry matter production (over the linear phase of growth) similar to those previously shown by mixed swards at high temperature. The Low-N swards grew more slowly; canopy photosynthesis, at a given LAI, was similar to that at High-N but lower LAI's were sustained. Clover increased its contribution to total carbon uptake and total dry weight throughout the period in the Low-N treatment and, despite the fact that grass took up most of the available nitrate, clover maintained a consistently higher N content by virtue of N2-fixation. At High-N, grass dominated throughout the measurement period. Earlier, when plants grew as spaced individuals, clover grew less well than grass, but once the canopy was closed it had a similar relative growth rate and thus maintained a steady proportion of total sward dry weight. It is proposed that early in the development of the crop, leaf area production is the limiting factor for growth, and that in this respect clover is adversely affected by low temperature relative to grass. Later, as the LAI of the crop builds up, and the canopy becomes fully light intercepting, net canopy photosynthesis plays a more dominant role and here the higher photosynthetic rate per unit leaf area of the clover is crucial.