Drought and methyl jasmonate deplete non-structural carbohydrates to similar levels but induce divergent terpene responses in <i>Pinus edulis</i> seedlings

Drought and herbivory are prevalent stressors that often interact to constrain forest regeneration. Drought-induced depletion of nonstructural carbohydrates (NSC) may impair seedling chemical defenses, increasing vulnerability to pests and pathogens. To investigate NSC thresholds influencing defense capacity, we quantified the effects of drought and simulated insect herbivory on NSC (starch, sucrose, glucose, and fructose) and mono- and sesquiterpene (MST) defenses in five-year-old piñon pine (Pinus edulis) seedlings. Seedlings were either well-watered or subjected to drought until stomatal closure before treatment with methyl jasmonate (MeJA) to simulate herbivory. Both drought and MeJA treatments individually reduced NSC content in needles and stems by 50%, with no further decrease observed under combined stressors. Regardless of stressor(s), NSC was depleted to ~0.5% and ~0.7% dry weight in needles and stems, respectively. While drought alone more than doubled MST concentrations in both tissues, total MST concentrations remained unchanged in response to MeJA, suggesting NSC was instead mobilized to support other unidentified metabolic processes. By demonstrating that NSC were depleted to similar lower limits across all stressors and combinations, this study suggests the existence of reserve thresholds below which seedling capacity to respond to subsequent stress may become constrained.