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.