We used activated charcoal methods to test for the presence of phytotoxic substances in soils that had received inputs of decomposing Cycas micronesica K.D. Hill leaves and stems that were heavily infested with the armored scale Aulacaspis yasumatsui Takagi. Velocity of Momordica charantia Descourt. and Carica papaya L. seedling emergence was increased by the addition of charcoal to these soils. Furthermore, M. charantia and C. papaya seedling height and dry weight were among the response variables that were increased by the addition of charcoal. Legacy effects of scale-infested C. micronesica plant litter deposited in these soils resulted in phytotoxic compounds that inhibited seedling emergence and plant growth. Scale-infested Cycas leaves should not be used as mulch or in compost until phytotoxic causal mechanisms are more fully understood.
Cycas micronesica leaf and strobili expansion patterns were measured in three locations and seasons on Guam and then were fitted with non-linear models to evaluate the use of the parameters for informing management decisions. All growth curves except for microstrobili height conformed to a negative exponential function. Microstrobili height development could not be fitted with any traditional linear or non-linear function, so spline models were used to smooth the effect of elapsed days. Leaf and leaflet expansion patterns were influenced by habitat and season, indicating development of the vegetative organ is plastic. In contrast, the models that described development of megasporophyll length, microstrobili height, and microstrobili diameter were not influenced by habitat or season. Moreover, seed diameter developmental patterns were only minimally influenced by location. These results indicate developmental patterns of the reproductive structures were primarily constitutive. We have demonstrated two empirical approaches to fitting models of Cycas micronesica organ growth and development and that both methods are useful for determining the influence of spatial or temporal factors in the timing of organ development. This approach may be used to inform horticultural or conservation questions of other rare cycad species.
The profile of nonstructural carbohydrates (NSC) was quantified to determine sugar and starch relationships of megagametophyte tissue during Cycas micronesica K.D. Hill seed ontogeny. Field work occurred in northern Guam where megastrobili were marked and dated as they emerged from stem apices of plants in a natural population. Seeds were harvested beginning 6 months after megastrobili emergence and continuing until 28 months, and gametophyte tissue was separated from the remainder of each seed. Carbohydrates within lyophilized gametophyte tissue were quantified by high-pressure liquid chromatography. The levels of glucose and fructose declined from a high at 6 months to a homeostasis at 11 months, and the levels of sucrose similarly declined from 6 months to a homeostasis at ≈14 months. Starch content exceeded sugar content and increased from 6 months to reach a homeostasis at ≈18 months. Maltose was not detected in any sample. Stoichiometric quotients changed dramatically until ≈14 months, when they became fairly stable until 28 months. Starch concentration was ≈5-fold greater than sugar concentration at 6 months, and increased to ≈15-fold greater than sugar concentration by 28 months. Total NSC in mature megagametophytes was almost 70% on a dry weight basis. Our results are in agreement with the biological function of this haploid tissue, as copious carbohydrate resources are readily available to support embryo and seedling growth.