Effect of defoliation on the patterns of allocation of a hydroxamic acid in rye (Secale cereale)

Hydroxamic acids are a class of secondary metabolites typical of grasses which play a role in resistance to herbivores. The effects of defoliation (simulated herbivory) on the allocation patterns of a hydroxamic acid (DIBOA) in rye seedlings were evaluated. Defoliated plants were compared with undefoliated plants 5, 7, 9 and 11 days after defoliation and germination, respectively. Shoot DIBOA of defoliated and undefoliated seedlings decreased with increasing age, but DIBOA concentration in defoliated seedlings was generally higher than in undefoliated ones. On the other hand, DIBOA concentration of the primary leaf was not affected by defoliation. The contribution of the primary leaf to the DIBOA content of the whole shoot as well as to the biomass of the whole shoot decreased with time for both defoliation treatments. As age increased the primary leaf of undefoliated seedlings decreased in relative DIBOA content at a greater rate than in relative biomass. The primary leaf of defoliated seedlings showed a similar rate of decrease for both variables. This pattern as well as that for DIBOA concentration in defoliated vs. undefoliated seedlings was interpreted as consistent with predictions of the Optimal Defense theory. In addition, the increased level of DIBOA in the shoot and, to a lesser extent, the unchanged DIBOA level in primary leaf of defoliated seedlings supported an extension of the 'reversion to juvenile phase' phenomenon described for woody plant-mammal interactions. (C) 1997 Elsevier Science B.V.