Defense tradeoffs in fleshy fruits: effects of resource variation on growth, reproduction, and fruit secondary chemistry in Solanum carolinense.

A set of clones of 10 maternal plants was grown for three successive years (1998-2000) under two nitrogen treatments and two water treatments. Path analysis revealed strong direct and indirect effects of nitrogen treatment on growth and reproduction, but fruit morphological and chemical variables were not strongly affected. Fruit pulp chemistry varied only slightly across treatments despite the large differences in growth and reproduction associated with resource variation. Leaf and ripe fruit chemical contents were not significantly correlated across treatments, and maternal plants, and leaf chemical variables did not help explain fruit chemical variation when included as covariates in ANCOVA analyses, suggesting no physiological constraints of leaf chemistry on ripe fruit chemistry. Results suggest that, while maternal plants may vary somewhat in fruit chemistry, and fruit chemistry may vary somewhat depending upon environmental conditions, levels of primary and secondary metabolites within fruits are not best explained by supply-side hypotheses. Ripe fruit chemistry remained relatively constant in the face of drastically changing resource levels, suggesting an adaptive function and supporting the Defense Tradeoff hypothesis. Fruit quality, both in terms of nutritional make-up and putative defensive properties, was maintained despite strong effects on plant growth and reproduction. Because glycoalkaloids are general defense compounds, we conclude that ripe fruit chemistry most likely reflects a balance between selection for attraction of seed dispersers and defense against pests and pathogens.