An evaluation of the costs of making specific secondary metabolites: Does the yield penalty incurred by host plant resistance to insects result from competition for resources?

Abstract

The presumption that the synthesis of ‘defence’ compounds in plants must incur some ‘trade-off’ or penalty in terms of annual crop yields has been used to explain observed inverse correlations between resistance to herbivores and rates of growth or photosynthesis. An analysis of the cost of making secondary compounds suggests that this accounts for only a small part of the overall carbon budget of annual crop plants. Even the highest reported amounts of secondary metabolites found in different crop species (flavonoids, allylisothiocyanates, hydroxamic acids, 2-tridecanone) represent a carbon demand that can be satisfied by less than an hour's photosynthesis. Similar considerations apply to secondary compounds containing nitrogen or sulphur, which are unlikely to represent a major investment compared to the cost of making proteins, the major demand for these elements. Decreases in growth and photosynthesis in response to stress are more likely the result of programmed down-regulation. Observed correlations between yield and low contents of unpalatable or toxic compounds may be the result of parallel selection during the refinement of crop species by humans.

Keywords:

• Yield drag
• host plant resistance
• assimilate partitioning
• secondary compounds
• photosynthesis

Acknowledgement

The authors are indebted to John Picket (Rothamsted Research) and Kirsten Brandt (Newcastle University) for critical reading of the manuscript.