THE PANGLOSSIAN PARADIGM AND DELAYED INDUCIBLE ACCUMULATION OF FOLIAR PHENOLICS IN MOUNTAIN BIRCH

We review some hypotheses for outlining how the nutrient status of deciduous woody plants may affect the accumulation of foliar phenolics, and how defoliation may alter the plant carbon/nutrient balance. Earlier experiments suggest that phenolics tend to accumulate in mountain birch leaves after previous defoliation. Reduced leaf size and lower leaf nitrogen content tend to indicate that these delayed inducible responses are mediated by nitrogen deficiency. Several other observations are consistent with this hypothesis. There is frequently a negative correlation between leaf phenolic and nitrogen contents. Fertilization increases leaf nitrogen content and reduces leaf phenolic content, while root damage causes reverse effects. Such carbon/nutrient imbalances may provide a plausible mechanism for delayed inducible accumulation of phenolics, while non-localized rapidly inducible responses most likely require specific elicitors that stimulate secondary metabolism and/or translocation of allelochemicals from the synthesis or storage sites to the target tissues. We speculate that plant inducible responses might have originally evolved as stress responses, and later specific triggering mechanisms could have evolved for both pathogen and herbivore defences.