Agronomy as a manipulative tool

Summary

The general ecological mechanisms that determine the interactions between availability of re-sources (nutrients, water) and synthesis of defence related secondary metabolites in the plants will be presented. In short, under normal conditions in nature, where growth is limited by a relatively constant availability of nitrogen at a moderate level, plants strike a balance between defence and growth. They use a fraction of resources such as carbohydrate, N and S to reach a genetically determined normal level of defence mechanisms, which will prevent or quickly overcome most types of infections. If the local availability of nitrogen happens to increase (e.g. from decomposition of animal faeces), the growth rate increases, and the balance shifts. The concentration of defence compounds in the plant decreases, and the plant now relies more on compensatory growth and less on resistance as response to infections (Stamp 2003 Q. Rev. Biol. 78, 23-55).
While most of this information comes from investigations of wild plants in nature, mainly in the context of understanding the consequences of pollution with nutrients, the same mechanisms appear to operate in agricultural settings. Most of these data are about antioxidants, where high concentrations are required to induce a biological response in human cells. Compounds such as polyacetylenes, alkaloids, furanocoumarins etc. have higher biological impact per molecule, and are more important for the plant defence. In cases where the bioavailability of these defence compounds is similar to or higher than for the antioxidants, a given concentration is likely to result in a stronger impact on human health (Brandt et al. 2004, TIFS 15, 384-393).
If a farmer wants to maximise the concentration of a bioactive compound in the crop (or to optimise it, if too high levels are detrimental for food quality or human health), is it therefore important to reduce the fertilisation intensity, in particular to avoid periods with high excess of nutrients such as N. Other ways of manipulating the balance is by partial drying and other methods that impose controlled levels and types of stress on the plants. Existing models relate to bioactive compounds that defend the plant against diseases and pests, including glucosinolates and tannins. Compounds with other physiological roles, such as sunscreens or involved in photosynthesis or signalling (colour, scent), will act differently, so their highest levels may correspond to a different range of resource availability than for the defence compounds (Brandt & Mølgaard 2001, J.Sci. Food Agric. 81, 924-931). This would be the case for most flavonols and carotenoids. Compounds that store minerals, such as phytate, will increase with the input of the mineral up to quite high levels.
This implies that existing defined types of agronomic practice, such as those used in organic farming, will have consistent and predictable consequences for concentrations of bioactive compounds in the crop (when the variation due to genotype and climate is taken into consideration). Even though neither system requires the use of a particular amount or timing of nutrients, experienced farmers cultivating plants with or without pesticides will learn how much fertiliser to use and how for their system, creating consistently higher levels of defence compounds in organic fresh plant foods (typically 10-50% more than corresponding conventional) (Brandt & Mølgaard 2001, J.Sci. Food Agric. 81, 924-931).
To predict the effect of novel practices or new technologies, the most important consid-eration is therefore how they will affect the resources available to the plant. Based on epidemiol-ogical data, the increase in life expectancy by a doubling of the vegetable intake has been esti-mated to 1-2 years (van’t Veer et al. 2000 Pub. Health Nutr. 3, 103–107). So if the bioactive compounds are responsible for this effect, an increase of 10-50% will increase the life span by 1-12 months. The decrease in yield and thus increase in the cost of raw material is around 30%. If multiplied with the number of people potentially affected, this benefit/cost ratio is much better than for some if the existing food safety measures, e.g. against BSE and trichinosis.