Effects of soil salinity on the expression of direct and indirect defences in wild cotton <i>Gossypium hirsutum</i>

1. Previous studies have frequently reported effects of abiotic factors on herbivore- induced plant defences based on effects on single plant traits. However, plants commonly express multiple defences simultaneously and these traits are often correlated. Thus, a fuller understanding of abiotic-context dependency in plant defence requires measuring multiple traits and addressing their patterns of cor- related expression.
2. We evaluated the effects of soil salinity on the expression of direct (phenolic compounds, gossypol gland density) and indirect (volatile organic compounds, ex- trafloral nectar) defensive traits in wild cotton Gossypium hirsutum. Specifically, we asked whether soil salinity affects the induction of these traits, and whether it shapes trait correlations potentially underlying altered patterns of trait induc- tion. We conducted a factorial experiment with 16 cotton genotypes where we manipulated soil salinity and defence induction by applying artificial leaf damage (25% mechanical damage and caterpillar oral secretions) and measured defence levels at different time points post damage.
3. Leaf damage induced most traits except gossypol gland density, whereas sa- linity did not have a mean effect (across constitutive and induced levels) on any of the measured traits. Nonetheless, salinity prevented the induction of phenolic compounds (condensed and hydrolysable tannins), and also af- fected trait correlations. Specifically, phenolic compounds were negatively associated with nectar production only under salinized conditions, an appar- ent trade-off that could affect the induction of phenolic compounds. In ad- dition, positive correlations between phenolic compounds and gland density and root biomass observed under control conditions were lost under salinized conditions.
4. Synthesis. By investigating the effects of soil salinity on the expression of multiple direct and indirect defensive traits and their underlying correlations, these find- ings build towards a better understanding of how abiotic context dependency shapes plant allocation to and expression of multiple defensive traits.