Phenolics lie at the centre of functional versatility in the responses of two phytochemically diverse tropical trees to canopy thinning

Abstract

Saplings in the shade of the tropical understorey face the challenge of acquiring sufficient carbon for growth as well as defence against intense pest pressure. A minor increase in light availability via canopy thinning may allow for increased investment in chemical defence against pests, but it may also necessitate additional biochemical investment to prevent light-induced oxidative stress. The shifts in secondary metabolite composition that increased sun exposure may precipitate in such tree species present an ideal milieu for evaluating the potential of a single suite of phenolic secondary metabolites to be used in mitigating both abiotic and biotic stressors. To conduct such an evaluation, we exposed saplings of two unrelated species to a range of light environments and compared changes in their foliar secondary metabolome alongside corresponding changes in the abiotic and biotic activity of their secondary metabolite suites. Among the numerous classes of secondary metabolites found in both species, phenolics accounted for the majority of increases in antioxidant and UV-absorbing properties as well as activity against an invertebrate herbivore and a fungal pathogen. Our results support the hypothesis that phenolics contribute to the capacity of plants to resist co-occurring abiotic and biotic stressors in resource-limited conditions.

Keywords: Abiotic stress; UV radiation; biotic stress; chemical defence; light environment; metabolomics; multiple stressors; secondary metabolites; sunflecks; tropical forest.

Fig. 1.

Richness (total number of compounds) of secondary metabolite classes by species and leaf age. Compounds that were detected in all treatments for a species–leaf age group, which represent the majority of compounds in all cases, are coded as ‘all treatments’. Compounds that were only detected in a given treatment are coded as such. The two USUV treatments are grouped together as no compounds were detected in only one of these treatments. (This figure is available in colour at JXB online.)

Fig. 2.

Ferric reducing antioxidant power (FRAP) of secondary metabolite extracts. Antioxidant power is displayed as the ferric reducing equivalent of the secondary metabolite extract per gram of leaf tissue (DW). Pairwise statistical comparisons (Tukey’s HSD) within leaf age resulting in P≤0.05 are indicated by non-matching letters above whiskers (n=5). Box and whisker specifications are per default boxplots in R statistical software. (This figure is available in colour at JXB online.)

Fig. 3.

UV (220–350 nm) absorbance of secondary metabolite extracts. Values are displayed as the log₁₀-normalized absorbance of a standardized mass of secondary metabolite extract multiplied by the total mass of secondary metabolite extract per 100 mg of dried leaf material. Statistical comparisons are as in Fig. 2, with the addition of a dagger appended to the letters above a uniform shade treatment that exhibits marginal differences (0.05<P≤0.10) from the sunfleck treatment (A. blackiana, n=10. B. utile, mature, n=7; expanding, n=5). (This figure is available in colour at JXB online.)

Fig. 4.

Dry masses of secondary metabolite extracts per 100 mg of dried leaf material. Statistical comparisons are as in Fig. 3. (A. blackiana: mature, n=16; expanding, n=13. B. utile: mature, n=7; expanding, n=5). (This figure is available in colour at JXB online.)

Fig. 5.

Mass spectral molecular networks used for neighbourhood classification of secondary metabolites. The network shown is from B. utile. Each node is a metabolite, and connecting lines (‘edges’) indicate that two compounds are structurally related based on their MS/MS fingerprints. Unconnected nodes and groups of nodes indicate metabolites or groups of metabolites that are structurally unrelated to one another. For both expanding and mature leaves, networks were generated using five samples per treatment for B. utile and two samples per treatment for A. blackiana.

Fig. 6.

Mean abundance shifts of secondary metabolite types. Daggers indicate differences between the uniform shade treatments versus the sunfleck treatment at P≤0.05 for the metabolite type to which they are appended (A. blackiana, mature, n=8; expanding, n=10; B. utile, mature, n=7; expanding, n=5). (This figure is available in colour at JXB online.)

Fig. 7.

Lethality index of secondary metabolite extracts against Artemia shrimp. Higher values on the y-axis indicate increasing lethality. n=5. Statistical comparisons are as in Fig. 3. (This figure is available in colour at JXB online.)

Fig. 8.

Inhibition index of secondary metabolite extracts against fungus hyphal growth. Higher values on the y-axis indicate increasing inhibition (A. blackiana. mature, n=14; expanding, n=12; B. utile, mature, n=7; expanding, n=5). Statistical comparisons are as in Fig. 3. (This figure is available in colour at JXB online.)