Does chemical defence decline more in developing leaves that become strong and tough at maturity?

Abstract

Leaves that are expanding cannot be very tough or strong because of the constraints of cell expansion, and therefore are particularly vulnerable to being eaten. We predicted that expanding leaves would be better defended chemically than mature leaves, and that this difference would be most pronounced in species whose leaves are tougher and stronger at maturity, i.e. that chemical defence declines as the leaf increases its mechanical defences. We tested this hypothesis by comparing leaf mechanical properties and total phenolics in three species with relatively tough and strong mature leaves (Doryphora sassafras Endl., Acmena smithii (Poir.) Merr. & Perry and Nothofagus moorei (F.Muell.) Krasser) and two species with soft mature leaves (Eucryphia moorei F.Muell. and Toona ciliata M.Roem.).

A significantly higher concentration of total phenolics per leaf dry weight was recorded in expanding leaves than in mature leaves in D. sassafras, N. moorei and A. smithii, but not in the two soft-leaved species. However, the decline in total phenolics in mature leaves of D. sassafras, N. moorei and A. smithii was entirely explained by dilution caused by increased concentration of cell wall. The absence of any difference in 'protoplasmic' concentrations of phenolics is consistent with the view of polyphenols as 'immobile' defences. High protoplasmic concentrations of polyphenols in expanding leaves may be less likely in species whose leaves become toughened and strengthened at maturity if they cannot quickly reclaim the chemical defence as its defence value declines. Instead, these species may be predicted to exhibit higher levels of mobile defences in expanding leaves. Despite the higher concentrations of phenolics in expanding leaves of D. sassafras and N. moorei they were eaten more by a generalist herbivore, Epiphyas postvittana, than were mature leaves, consistent with the higher nitrogen concentration in leaves that required less force and energy to chew.