Defence plasticity in the spiny plant Aralia elata (Miq.) Seem. in response to light and soil fertility

Background and Aims Plants have evolved various defences against herbivores, including direct chemical and structural defences and co-opted biological defences by predatory insects. However, the effects of abiotic habitat conditions on the quantitative expression of defence traits of spiny species have not been elucidated. Methods Here, we investigated whether a spiny deciduous tree, Aralia elata (Miq.) Seem., changes its defence expression across light and nutrient gradients. We measured allocation to spines and C-based secondary metabolites (condensed tannins and total phenols) on A. elata plants growing across light and nutrient gradients in situ in natural landscapes in Japan. Second, we examined the effects of light and soil nutrient condition on allocation to shoot organs, spines and chemical defences of juveniles of two genotypes of the species, respectively spiny (mainland population) and non-spiny (island population), grown in a glasshouse. Key Results In the field investigation, absolute spine mass, spine mass fraction, total phenols and condensed tannins all responded positively to canopy openness. Total phenol content was also negatively related to soil N. In the glasshouse, spiny genotype individuals had less total biomass, had lower stem allocation and were shorter than non-spiny genotype individuals. In spiny genotype trees, both spine mass fraction and total phenols decreased under low light conditions. Nutrient additions had negative effects on spine mass fraction and total phenols, but no effect on absolute spine mass. Conclusions These results suggest that development of spines is costly for A. elata and receives greater allocation when carbohydrate supply is more plentiful. Thus, light is a more important determinant of spine allocation than soil nutrients for A. elata.

Automated summary

This study investigated the expression of defence traits in the spiny plant Aralia elata under different light and nutrient conditions. The results showed that spine mass, spine mass fraction, total phenols and condensed tannins all increased with canopy openness, while total phenol content was negatively related to soil nitrogen. In low light conditions, spiny genotype trees had lower spine mass fraction and total phenols, and nutrient additions had negative effects on spine mass fraction and total phenols.