Water, not carbon, drives drought-constraints on stem terpene defense against simulated bark beetle attack in Pinus edulis

Drought predisposes forest trees to bark beetle-induced mortality, but the
physiological mechanisms remain unclear. While drought-induced water and
carbon limitations have been implicated in defensive failure and tree
susceptibility, evidence demonstrating how these factors interact is
scarce. We withheld water from mature, potted Pinus
edulis (Engelm.) and subsequently applied a double-stem girdle to inhibit
carbohydrate transport from the crown and roots. Within this isolated
segment we then elicited a defense response by inoculating trees with a
bark beetle-fungal symbiont (Ophiostoma sp.). We quantified local mono-
and sesquiterpenes (MST), non-structural carbohydrates (NSC), and pressure
potential of the inner bark. Both drought-stressed and watered
trees had similar NSC concentrations just prior to inoculation and
depleted NSC similarly following inoculation, yet MST induction (i.e.,
increased concentration and altered composition) was constrained only in
drought-stressed trees. Thus, NSC consumption was largely unrelated
to de novo MST synthesis. Instead, stoichiometric calculations
show that induction originated largely from stored resin. Watered trees
experiencing higher pressure potentials consistently induced higher MST
concentrations. We demonstrate the importance of preformed resin
towards an induced MST response in a semi-arid conifer where
drought-constraints on defense occurred through biophysical limitations
(i.e., reduced turgor hindering resin transport) rather than through
substrate limitation.