1CABI Bioscience Centre, Delémont, Switzerland. 2University of Leiden, The Netherlands.
The seedling stage of a plant species is - besides the seed stage - usually the period in a plant's life-time with the highest risk of mortality. It would therefore seem crucial for young plants to develop the optimal balance between growth and defence. Pyrrolizidine alkaloid content of Senecio jacobaea seedlings starts to accumulate immediately after germination. During the first 30 days after germination, a significant negative correlation was found between shoot:root ratio (s/r ratio) and Pa concentration. Since the s/r ratio is positively correlated with relative growth rate during the rosette stage, we suggest that young S. jacobaea plants with a high s/r ratio and hence a high growth capacity necessarily have lower Pa defence levels than plants with a low s/r ratio. Laboratory experiments with two generalist herbivores, i.e. the slug Deroceas reticulatum and the nematode Helicotylenchus sp., revealed that the level of herbivory was not correlated with Pa concentration in S. jacobaea seedlings. However, resistance to D. reticulatum was significantly correlated with seedling size, ie. the larger the seedling, the less leaf area removed. These results are discussed in the context of the optimal defence theory.