The response of simulated swards of perennial ryegrass and white clover to enriched atmospheric C02: Interaction with nitrogen and photosynthetic photon flux density

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Abstract
This paper reports the growth response to elevated CO2 concentration of artificial swards of clover (Trifolium repens L. cv. Grasslands Pitau), ryegrass (Lolium perenne L. cv. Grasslands Ruanui), and mixed clover - ryegrass. The growth rate of the swards was varied by changing nitrogen nutrition and photosynthetic photon flux density (PPFD). We examined whether the response to elevated CO2 was the same in a dense, established sward as previously reported for isolated plants. Artificial swards of clover and ryegrass were grown separately or together in 15 cm2 pots. The swards were established in a glasshouse and then transferred to 3 controlled environment rooms at 20°C day/15°C night, with a 12 h photoperiod. After 3 weeks in the controlled environment rooms, all combinations of the following treatments were imposed: 340, 600, or 900 cm3 CO2/m3; 185 or 460 µmol/m2 per s PPFD; and 20 or 200 ppm nitrogen. The swards were harvested 4 times at II - 15-day intervals beginning 117 days after sowing. At harvest they were cut to 2.5 cm above the soil surface. Mean growth rate was constant over the final 2 harvests. Over all treatments the growth rate of clover was stimulated by 5211,70 at the elevated CO2 concentrations, ryegrass growth rate by up to 3911,70, and the mixed swards by 32070. Growth rate for the mixed sward was higher (0.231 g/ day per pot) than for the pure swards (0.185 g/day per pot clover; 0.175 glday per pot ryegrass). There were no significant interactions for the final 2 harvests between sward type and CO2 concentration, although the response of clover to elevated CO2 concentrations tended to be greater than that of ryegrass. There were significant interactions of nitrogen nutrition and PPFD with CO2 concentration and these are discussed. These responses to increased CO2 concentration were similar to reported results on spaced plants. The results are interpreted in terms of the balance between the supply of carbohydrate from photosynthesis and the utilisation of that carbohydrate during growth. Some implications for the breeding of grass and clover are discussed.