Sexual reproduction of Lolium perenne L. and Trifolium repens L. under free air CO2 enrichment (FACE) at two levels of nitrogen application

Field experiments in managed grassland have shown that the response of vege tative growth to elevated CO2 is nitrogen-dependent in grasses, but indepen dent in N-2-fixing legumes. In the present study, we tested whether this is also true for reproduction. We evaluated reproductive growth, flowering ph enology, seed development, reproductive success and seed germination in the grass Lolium perenne L. and the legume Trifolium repens L., growing in mon ocultures in a free air carbon dioxide enrichment (FACE) system at ambient (35 Pa) and elevated (60 Pa) partial pressure of CO2 and two levels of nitr ogen fertilization (14 and 56 g N m(-2) a(-1)). In both species, elevated C O2 had no significant effect on sexual reproduction. In L. perenne, reprodu ction was mainly nitrogen-dependent. The weak interactions between CO2 and mineral N supply (13% more flowers and 8% more grains per spike at high N, 7% less flowers and 8% less grains at low N) were not significant. Under el evated CO2, grain maturation was slightly enhanced and grain weight tended to decrease. No influence could be ascertained in the date of anthesis, the temporal pattern of grain growth, the rate of grain abortion and germinati on. Trifolium repens, grown under CO2 enrichment at both levels of N fertil ization, flowered 10 d earlier, tended to form more infloresences per groun d area and more flowers (8-12%) and seeds (> 18%) per inflorescence than at ambient CO2. The temporal pattern of seed growth was about the same in all treatments; embryo development, however, was accelerated in fumigated plan ts. The number of aborted seeds per pod, seed size, thousand-seed weight an d germinability did not show any influence Of CO2. Fumigated plants at high N were attacked slightly more frequently by seed-eating weevils, which low ered the seed output per pod. In summary, the reproductive response of L. p erenne and T repens to CO2 enrichment on the flower and inflorescence level was far weaker than expected from the results on vegetative growth.