Effects of soil salinity on the expression of direct and indirect defences in wild cotton (Gossypium hirsutum)

Previous studies have 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 correlated expression.

We evaluated the effects of soil salinity on the induction of direct (phenolic compounds, gossypol gland density) and indirect (volatile organic compounds, extrafloral nectar) defensive traits in wild cotton (Gossypium hirsutum). We asked whether soil salinity affects the induction of these traits, and whether it shapes trait correlations potentially underlying salinity effects on trait induction. 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.

Leaf damage induced most traits except gossypol gland density, whereas salinity 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 affected trait correlations. Specifically, phenolic compounds were negatively associated with nectar production only under salinized conditions, an apparent trade-off that presumably affects the induced levels of phenolic compounds. In addition, positive correlations between phenolic compounds and gland density and root biomass observed under control conditions were lost under salinized conditions.

By investigating the effects of soil salinity on the expression of multiple direct and indirect defensive traits and their underlying correlations, these findings build toward a better understanding of how abiotic context-dependency shapes plant allocation to and expression of multiple defensive traits.

Methods
Leaf defensive traits were measured on wild cotton plants found under greenhouse conditions and subjected to soil salinization and (subsequent) artificial leaf herbivory. Extrafloral nectar traits and volatile compounds were measured shortly after leaf damage (24-48 h), and phenolic compounds were measured at two time points, namely: one week and three weeks after damage. Pigment gland density was also measured during this latter time point.  At the end of the experiment, we collected plants to measure root, stem, and new leaf biomass.