HERBIVORE-INDUCED RESISTANCE IN BETULA-PENDULA - THE VOLE OF PLANT VASCULAR ARCHITECTURE

We studied the role of plant vascular architecture in the determinatio n of the spatial extent of herbivore induced responses within Betula p endula Roth saplings. The induced responses were measured in bioassays in terms of the relative growth rate of larvae of a geometrid moth, E pirrita autumnata. We hypothesised that the level of induced resistanc e of a certain leaf would be determined by the degree of vascular conn ectivity between the leaf in question and a damaged leaf, as suggested by recent theoretical and empirical studies. A comparison of the cont rol plants with the damaged plants indicated that damaging one leaf of a sapling was sufficient to induce an increase in the resistance leve l. There were also differences among the leaves within a plant in the resistance level, but these differences could not be explained by the degree of vascular connectivity with the damaged leaf. These results s uggest that the vascular connections have low power as explanations of the spread and spatial extent of the induced resistance in Betula pen dula saplings. Instead, the resistance level of all leaves within a sa pling increased following the damage. We suggest that the pattern of i ncreased resistance observed in this experiment may be beneficial for the young saplings studied. For young saplings at their early stages o f development, it may be beneficial to be able to distribute the induc tion signal to all leaves as fast as possible and thus repel the herbi vore totally. For a young sapling, the capability of repelling the her bivore totally might thus be a feasible strategy whereas an older sapl ing may tolerate localised damage better and compensate for the damage within the undamaged plant parts.