Source and Sink Relations Mediate Depletion of Intrinsic Cycad Seed Carbohydrates by Aulacaspis yasumatsui Infestation

Abstract

Ubiquitous Aulacaspsis yasumatsui Takagi infestations of Guam’s Cycas micronesica K.D. Hill trees cause direct herbivory of most exposed organ surfaces, including developing naked ovules and seeds. The nonstructural carbohydrates of infested vs. noninfested seeds were quantified to understand more fully the influences on seed quality for propagation purposes. Two studies compared seeds from healthy trees with those of unhealthy trees suffering from whole-tree A. yasumatsui infestations. The sugars fructose, glucose, maltose, and sucrose were in greater concentrations and herbivory reduced these free sugars by a greater percentage in sarcotesta tissue than in sclerotesta and gametophyte tissues. Starch concentration was greatest in gametophyte tissue, but herbivory reduced starch by a greater percentage in sarcotesta tissue. A third study was used to manipulate seeds of unhealthy infested trees such that some seeds were uninfested and some seeds were infested and revealed the nonstructural carbohydrates of the uninfested seeds were greater than those of the infested seeds in patterns that were similar to those when the entire tree was protected from herbivory. The combined results indicated that both source and sink relations were involved in the reductions of seed carbohydrates by A. yasumatsui herbivory. The reduction in seed resource pool by the herbivore feeding may be one of the mechanisms that results in reduced germination percentage and increased seedling mortality.