Effects of Fertilization and Competition on Plant Biomass Allocation and Internal Resources: Does Plantago lanceolata Follow the Rules of Economic Theory?

According to economic theory, plants should modify biomass allocation by (1) investing in those organs that obtain the limiting resource (this subset of economic theory is referred to as the optimal partitioning theory - OPT) and by (2) storing easily available resources. Although used often, these predictions are also frequently criticized for contradictory results and methodological problems. We tested the effects of fertilization and competition on biomass allocation and concentrations of internal resources in Plantago lanceolata. To separate the effects of ontogenetic drift from true plasticity, we determined how plant size and plant age influence partitioning of biomass and resources. We also determined how root/shoot ratios are affected when the biomass of generative organs and storage carbohydrates are subtracted from measurements. The results for storage of resources supported economic theory: fertilized plants had higher concentrations of nitrogen than unfertilized plants, and unfertilized plants subjected to competition stored higher concentrations of non-structural carbohydrates than unfertilized plants without competition or fertilized plants with or without competition. Other results, mainly those related to the OPT, were inconsistent with economic theory. Nearly all examined allocations (except for the root/shoot ratio) were affected by both plant size and ontogeny. They also showed true plasticity in their response to competition but usually not in their response to fertilization. Although our experimental plants did not increase leaf biomass when carbon was relatively scarce (in the fertilized treatment), higher foliar nitrogen might have enhanced photosynthesis without biomass investment in leaves. Our results indicate that economic theory and the methodological problems associated with its measurement should be reconsidered.