No abstract

Research seeking to explain intraspecific variations in plant phenolics has focused on two general paradigms, the resource availability (carbon/nutrient balance) and induced-defense models. We experimentally tested both hypotheses to explain changes in phlorotannin concentrations in the brown alga Fucus vesiculosus. Fucus was collected monthly from two estuarine sites (Cape Cod, Massachusetts, USA) differing in nitrogen availability, and analyzed for polyphenolic levels and tissue-nitrogen concentrations. In situ nutrient enrichment experiments were conducted to measure changes in polyphenolic levels related toN availability. Simulated grazing experiments examined the possibility of induced increases in polyphenolic concentrations in Fucus vesiculosus. Paired choice experiments conducted in the laboratory examined feeding selectivity by the snail Littorina littorea for either Fucus population.Field measurements and experiments revealed temporal and site-related changes in tissue constituents. Polyphenolic concentrations were consistently, sometimes two times, higher in Fucus vesiculosus from the !ow-N site compared with algae from the high-N site; tissue-N content was higher in the population from the high-N site. For both populations, tissue N was inversely correlated with polyphenolic concentrations. Only Fucus from the low-N site showed a significant reduction in polyphenolic concentrations under experimental enrichment. Regression analysis revealed a significant inverse relationship (R 2 = 0.71) between phenolic concentrations and growth rates for Fucus at the high-N, but not the !ow-N site. This suggests a growth cost associated with phlorotannin production for this phenolic-poor population due, perhaps, to C-limited growth. Phlorotannin concentrations increased significantly in clipped Fucus at the low-N site in two of three simulated grazing experiments, indicating an inducible response. We did not find a significant inducible response for the high-N, phenolic-poor population, and our choice experiments revealed a clear preference for this population by Littorina littorea. Our results offer support for both the induced-defense and carbon/nutrient balance hypotheses as explanation for variations in phlorotannin concentrations in Fucus vesiculosus. We suggest that withinspecies variation in polyphenolics is due to a complex interaction of environmental (nutrient availability, irradiance levels) and defense-related (grazing activity) factors.

Research seeking to explain the ecological role of polyphenolics (phlorotannins) in plants and brown algae has largely focused on 2 alternative concepts, the carbonhutrient (Cm) balance and the inducible defense models. We tested the hierarchy of effects of both models on phlorotannin production in the brown alga Fucus veslculosus (Fucales) by simultaneously manipulating the N environment and simulating herbivory for 2 oceanic (high and low intertidal) and estuarine populations. We measured phlorotannin levels in algae under control, grazed, N-enriched, and grazed + N-enriched treatments with time (0 to 14 d ) throughout the year to determine onset and duration of the response.We found greater support for the inducible defense model; generally, both grazed and grazed + Nenriched fronds had significantly higher phlorotannin concentrations than control thalli. When we found a n inducible response, it was rapid (within 3 d ) and relatively long term (>2 wk). However, the induced response was minimal for both oceanic populations during March, perhaps d u e to fixed-C limitation, and was absent for the estuarine and high intertidal populations during June, the period of peak phlorotannins at both sites. Although Nenrichment resulted in depressed concentrations of phlorotannins only for the estuarine population, we did measure a significant negative correlation between tissue N and phenolics for the oceanic population, a s predicted by the C/N balance model. Thus, while the induc~ble defense response takes preeminence over resource availability effects (C/N balance hypothesis), this study revealed that phlorotannin production is likely controlled by a complex interaction of environmental, developmental and defense-related factors, emphasizing the applicability of both models in marine systems. (Ragan et al. 1979) and storage of surplus fixed carbon, particularly under conditions of nutrient deficiency (Mattson 1980, Bryant et al. 1983, Gershenzon 1984. Understanding the ecological trade-offs in allocation of C-based resources between primary and secondary metabolic pathways is critical to predicting the success of a producer in its environ-O Inter-Research 1996 Resale of full article not permitted

No abstract

No abstract