Plant responses to variable timing of aboveground clipping and belowground herbivory depend on plant age

Aims
Plants use different types of responses such as tolerance and
induced defense to mitigate the effects of herbivores. The direction
and magnitude of both these plant responses can vary with
plant age. However, most studies have focused on aboveground
herbivory, whereas important feeding occurs belowground. Here,
we tested the hypothesis that plant tolerance and defense following
shoot damage or root herbivory depends on plant age.
Methods
In order to test our hypothesis, we exposed the perennial grass species
Holcus lanatus to defoliation and root nematode inoculation at
three growth stages (young, intermediate and old plants), and examined
responses of plant traits related to tolerance (regrowth following
defoliation) and defense (leaf and root nitrogen and phenolics).
Important Findings
Defoliation overall reduced plant shoot and root biomass as well as
foliar concentrations of phenolics regardless of plant age at defoliation.
In contrast, defoliation increased foliar N concentrations, but
only when defoliation occurred at intermediate and old plant age.
Inoculation with root-feeding nematodes reduced root N concentrations
after a prolonged period of growth, but only when nematodes
had been inoculated when plants were young. The relative
shoot regrowth rate of plants increased immediately after defoliation
but this was independent of the plant age at which defoliation
occurred, i.e. was not stronger in plants that were defoliated at a
more advanced age, as hypothesized. Similarly, relative root growth
rates increased shortly after defoliation, but this was only observed
for plants defoliated when they were young. We conclude that plant
responses to aboveground and belowground herbivory in traits
related to both defense and tolerance are affected by plant age, but
do not generally change with plant age.