In woody species, potential mechanisms to compensate for tissue loss to herbivory and diseases have been related to post-event shifts in growth, biomass and internal resource allocation patterns, as modulated by external resource limitations. We examined the interactive effects of belowground resource limitations by varying nutrient and water availability, and aboveground carbon limitation imposed by a single defoliation event (40% leaf removal) on stem growth, whole-tree and within-tree resource allocation patterns (total non-structural carbohydrate and nitrogen) and below- and aboveground biomass allocation patterns in 8-month-old, field-grown Eucalyptus globulus Labill. saplings. Two months after treatments were imposed, the direction of the stem growth response to defoliation depended on the abiotic treatment. Five months after defoliation, however, we found little evidence that resource availability constrained the expression of tolerance to defoliation. With the exception of the combined low-nutrient and low-water supply treatment, saplings grown with (1) adequate water and nutrient supplies and even with (2) low-water supply or (3) low-nutrient supply were able to compensate for the 40% foliage loss. The observed compensatory responses were attributed to the activation of several short- and longer-term physiological mechanisms including reduced biomass allocation to coarse roots, mobilization of carbohydrate reserves, robust internal N dynamics and increased ratio of foliage to wood dry mass.

• The mechanisms and conditions affecting expression of systemic induced resistance (SIR) in pine are not clearly understood. Two hypotheses were tested here: that SIR against a pathogen induced by either a pathogen or an insect involves coordinated shifts in phloem secondary metabolism; and that fertility affects the production of these compounds. • To test these hypotheses, a tripartite system was used comprising Austrian pine (Pinus nigra) grown under three different fertility regimes, the fungal pathogen Diplodia pinea, and the defoliator Neodiprion sertifer. • Fungal induction led to systemic accumulation of lignin, phenolic glycosides and stilbenes, whereas insect defoliation led to an increase in germacrene D concentration in branch phloem. Fertility affected the concentrations of only the phenolic glycosides. Multivariate analyses showed coregulation of compounds within at least three consistent groupings: phenolic glycosides, stilbenes and monoterpenes. As groups and as individual compounds, accumulation of phenolic glycosides and stilbenes was negatively correlated with disease susceptibility. • The experimental manipulation of the phenolics and terpenoids metabolic networks achieved in this study by biotic induction and changes in nutrient availability suggests that lignin, phenolic glycosides and stilbenes are important biochemical factors in the expression of SIR against the pathogen in this system.

Systemic induced resistance (SIR) is a well-known host defense mechanism against pathogen attack in herbaceous plants, but SIR has only recently been documented in conifers. We tested if inoculation of Austrian pine (Pinus nigraArnold) with Sphaeropsis sapinea (Fr.:Fr.) Dyko and Sutton or Diplodia scrobiculata de Wet, Slippers and Wingfield results in SIR or systemic induced susceptibility (SIS) to subsequent colonization by S. sapinea. Induction at the stem base resulted in significant (P < 0.01) SIR in the upper stem, and induction in the upper stem resulted in significant (P < 0.05) SIR at the stem base, indicating that SIR is bidirectional in Austrian pine. However, inoculation at the stem base resulted in significant (P < 0.01) SIS in shoot tips, demonstrating that, in the same host species, the expression of resistance can be organ-dependent, resulting in either SIR or SIS depending on the site of challenge infection. Systemic induced resistance in the stem was associated with induced lignification, supporting a potential role for this defense mechanism in disease resistance. Systemic induced susceptibility has been documented before, but this is the first demonstration of organ-dependent expression of both SIR and SIS in a tree or any other plant.