Sexual reproduction traits of Holcus lanatus L. and Parnassia palustris L. in response to absolute and relative supply of nitrogen and phosphorus

Plant investment in sexual reproduction is affected by absolute and relative nitrogen (N) and phosphorus (P) supply. Reduced investment in reproductive traits has been suggested as an important adaptation to strong P-limitation. We experimentally tested how absolute and relative nutrient supply affected sexual reproduction traits of two grassland species. Seedlings of a common grass (Holcus lanatus) and an endangered forb (Parnassia palustris) were grown from seed from a dune area with low relative P-availability. Plants were grown in a full factorial experiment with three N:P supply ratios (5, 15, 45) and two absolute supply levels of N and P. After one year, a range of traits was measured as a proxy for investment in sexual reproduction. We found that N:P supply ratio did not affect sexual reproduction at low nutrient supply; at high nutrient supply, investment in sexual reproduction was significantly less at higher N:P supply ratios. For Holcus lanatus, N:P supply ratio 45 restricted the increase in sexual reproduction upon increasing nutrient supply. Parnassia palustris survival and flowering were low, especially at N:P supply ratio 15 (no results were available for P. palustris at N:P supply ratio 45 due to the high mortality). Our results highlight that at low nutrient supply N:P ratio rarely affected investment in sexual reproduction traits but at high supply low relative P-supply restricted plants’ sexual reproduction investment.