ABSTRACT
Evidence exists that a number of extrinsic factors do influence secondary metabolism. In agriculture, addition of nutrients, particularly nitrogen, is often employed to increase biomass production. However, all plants do not react equally to an increased availability of nutrients; species genetically adapted to low-nutrient soils often fail to respond. Substrate for secondary metabolism originates primarily from carbon fixation or, where nitrogen, sulfur, phosphorus, or halides are involved, from soil and/or waterborne nutrients. Damage caused by wounding or pathogen invasion often leads to a biochemical response by the plant that reduces acceptability of the organ or the whole organism to future attackers. There is substantial evidence that extrinsic factors influence levels of production of secondary metabolites and that these in turn can and do influence palatability of plant materials to herbivores. A number of stressors influence metabolism through messenger intermediates, either hormonal or elicitor, stimulating gene transcription.
