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Defence is an important function of plant secondary metabolism. An array of compounds is produced, which deter herbivore grazing (van Loon & Schoonhoven, 1999; Dyer et al., 2001, Gonzalez et al., 1995; Schmeller et al., 1997). Secondary plant metabolites are highly diverse in structure and abundant throughout all plant genera. Regulation of secondary metabolite production, transformation and allocation in the plant influence defence mechanisms in plants. Secondary metabolite concentration and diversity may vary among locations (Johnson et al., 1985, von Borstel et al., 1989; Ralphs et al., 2000), and within and among seasons (Johnson et al., 1985; O’Dowd & Edgar, 1989; Laus et al., 1997; Hook et al., 1999), and this variation in concentration can have a large impact on resistance to herbivory.
The maintenance of active secondary metabolite concentrations at a certain level may require the production of defence compounds to be very flexible. This should occur independently of the variation in environmental factors such as climate, light, humidity and nutrients. There are also inducible defence compounds, the levels of which are raised when necessary (Karban & Baldwin, 1997). However, our study focuses on the flexibility of a constitutive defence system in response to nutrient supplies. It is generally accepted that increasing nutrient supplies leads to an increased shoot : root ratio. If secondary metabolites production takes place throughout the whole plant, no consequences would be expected from increasing nutrient supply. However, when the production of secondary metabolites is coupled with root growth only, higher nutrient supplies would lead to lower concentrations in shoot and root. We sought to determine whether a plant species is capable of adapting defence levels to increased nutrient supply through different allocation patterns, increased production or both. A plant species was selected which produces secondary metabolites in the roots, allocates them over the whole plant and occurs under a wide range of nutrient conditions. Senecio spp. grow in poor sandy dune soils and at road sites, but are also found as noxious weeds in pastures, where they cause acute and chronic liver damage when ingested by cattle (Mattocks, 1986). The use of Senecio spp. in traditional human medicine could give rise to carcinogenic effects (Steenkamp et al., 2001). These effects are due to pyrrolizidine alkaloids (PAs), which are regarded as part of the defence mechanism of the plant against herbivores (Hartmann & Witte, 1995; Van Dam et al., 1995a). Recently, we showed that PAs influence soil borne fungi (Hol & Van Veen, 2002). PA production in Senecio spp. takes place in the root (Hartmann & Dierich, 1998) and is related to root growth (Hartmann & Toppel, 1987; Sander & Hartmann, 1989). Therefore it is important to understand the response of PA production and transformation in relation to growth biomass and variation in nutrient supplies.
Differences in the distribution of secondary metabolites over a plant can be considerable. Five-fold differences between different plant parts were observed in PA content of S. vulgaris (Hartmann & Zimmer, 1986) and Cynoglossum officinale (Van Dam et al., 1995b). Most data reported on this subject concern detailed studies of above-ground plant parts and it is unknown whether the same variation in secondary metabolite concentration will be found below-ground. This may have considerable effect on the defence of the root tissue against insects, fungi, nematodes and bacteria, which is essential for survival to the flowering stage. The older, basal root parts may also be used for storage of PAs. Optimal defence theory predicts that the level of defence in plant parts is related to the importance of that plant part for fitness (Rhoades, 1979). We therefor expect high concentration in the root tips, which are important for root growth.
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Decreasing PA concentrations in roots and shoots with increasing nutrient-supply appear to result from a diluting effect. Increasing nutrient supplies increased plant biomass, while PA production remained constant. Other studies on the response of plants to nutrient addition found significant changes in concentration of secondary metabolites (Iason & Hester, 1993; Höft et al., 1996; Baricevic et al., 1999; Salmore & Hunter, 2001). However, statistical analyses suggest that changes or differences in concentration of allelochemicals can sometimes be explained by changes in tissue biomass only (Koricheva, 1999). Thus, changes in concentration may not result from changes in production, but from dilution or concentration due to enhanced or reduced growth in the whole plant or plant parts.
It is unclear whether the same effect may be expected in flowering plants. Brown & Molyneux (1996) did not find an effect of nutrient deficiency on PA concentrations. Both flowerhead tissue biomass and total amount of PAs increased with nutrient supply, while concentrations remained constant. However, they only measured PAs in flowers and therefore it is not certain whether this reflects a change in total PA production in the plant, or only a change in allocation to the flowerheads in order to compensate for the dilution effect. In an experiment where resource availability to individual plants was controlled, Vrieling & Van Wijk (1994a) found no effect of N or P limitation on PA concentration.
Although root biomass was not significantly influenced by nutrient supply, concentrations of PAs in the roots decreased as nutrients increased (Table 1). This might be the consequence of a larger shoot resulting in a larger PA sink in the above-ground parts. The correlation between root biomass and PA production was positive and stronger than the simulated correlation. This is what is expected, since PAs in Senecio spp. are produced in the roots (Hartmann & Toppel, 1987). However, this reduces flexibility in PA production. If PA production per root is not flexible, and independent of resources like N, then changes in biomass may have a large impact on PA concentrations and thus on the defence of the plant.
Often the negative correlation between plant biomass and secondary metabolite concentration is regarded as evidence for a trade-off. The effect of nutrients on concentrations of secondary metabolites can be explained by two mechanisms. Nutrients, in particular N, are integral components of secondary compounds. Nutrient addition may therefore increase the concentration of PAs. Alternatively, when nutrients are not limiting for the production of PAs, they control to a large degree plant biomass production and secondary metabolites may be diluted when nutrients are supplied at larger rates, which is reflected in a lower concentration.
The resource availability hypothesis (Coley et al., 1985) predicts that under conditions of slow growth, plants should invest more in defence against herbivores. Although large and more nutritious plants may attract more herbivores, there is no need for more defence as long as the growth is faster than the removal of mass by herbivory. This would imply that under conditions of increased plant growth, due to increased availability of nutrients, dilution of secondary plant metabolites is acceptable for a plant explaining the present observations.
Concentrations of individual PAs consistently decreased with one exception. Jacobine remained constant in the shoot and increased in the roots. This might suggest that jacobine is of more importance for the defence of the plant than the other PAs are under the prevalent conditions. Jacobine is unusually toxic to guinea pigs (Cavia porcellus), which are generally resistant to PAs (Chung & Buhler, 1995). However, jacobine concentrations in the roots were very low in comparison to the other PAs and therefor would have to be very toxic. We could not find any study in which effects of jacobine on herbivorous insects or pathogens were compared with the effects of other PAs. By contrast, Vrieling & de Boer (1999) found that the jacobine and erucifoline chemotype of S. jacobaea are equally attractive for preference and performance of the specialist herbivore Tyria jacobaeae.
PA concentrations in root tips were on average lower than in the other root parts. High concentrations were expected in root tips, which are extremely important for the plant with regard to water and nutrient uptake and at the same time vulnerable to attack by herbivores and pathogens. Optimal defence theory (Rhoades, 1979) argues that the plant should protect the most valuable parts best, implying high concentrations of defence compounds for instance in the flowers, seeds, young leaves and below-ground in fine roots.
Sander & Hartmann (1989) found the root apex to be the site of enhanced biosynthesis of PAs, although production was found in all root parts. In the present experiment PAs could hardly be detected at these production sites, which may indicate a fast transport to storage sites, such as the cortex of the main roots. This is opposite to observations by Hartmann & Toppel (1987), who found significant differences in total PA concentration of root parts in root cultures of the annual S. vulgaris. The fine roots contained twice as much PA as the coarse roots. In root cultures there is no transport to above-ground plant parts and also re-allocation in the root cultures may be different from natural circumstances. In this study we used whole plants from the vegetative stage of the monocarpic perennial S. jacobaea. Mature roots often contained higher concentrations of alkaloids than the root tips, like hyoscyamine accumulation in Atropa belladonna (Falk & Doran, 1996). Also in lupines epidermal cells are preferred sites of storage for quinolizidine alkaloids (Wink et al., 1984). The more mature parts of roots are important for regrowth. Defoliation will not kill Senecio spp. (Obeso & Grubb, 1994; Vrieling et al., 1996), but Longitarsus jacobaeae larvae, which feed on the roots can be deleterious to S. jacobaea (Windig, 1993). Although L. jacobaeae is a specialist herbivore, Vrieling & Wijk (1994b) found a negative correlation between PA concentration and herbivory by both adults and larvae. Thus an important part of the root system seems to have a defence in the form of relative high concentrations of PAs, but at present it is unknown against which herbivores or pathogens.