Two Mallotus species of different life histories adopt different defense strategies in relation to leaf age

Plants defend their leaves using multiple defense traits that change functions with leaf age. We examined the effects of leaf age on the development of multiple defense traits in two related Mallotus (Euphorbiaceae) species: young plants of the fast-growing Mallotus japonicus (Spreng.) Mull. Arg. and the slow-growing Mallotus philippensis (Lam.) Mull. Arg. Sequential leaves of the two species were measured for their leaf area, leaf mass/area, densities of trichomes and pellucid dots, extrafloral nectar volume, and the numbers of extrafloral nectaries and pearl bodies. Mallotus japonicus shifted its defense tactics from direct defense using trichomes and pellucid dots in young leaves to biotic defense using extrafloral nectar and pearl bodies in middle-aged leaves. In contrast, M. philippensis used direct, chemical defense throughout all leaf ages, together with the shift from indirect, biotic defense using extrafloral nectar in young leaves to direct, physical defense using leaf toughness in middle-aged leaves. These results strongly suggest that, in relation to life history, plants can alter optimal combinations of multiple defense traits with leaf age.