Value of the mucilaginous pellicle to seeds of the sand-stabilizing desert woody shrub Artemisia sphaerocephala (Asteraceae)

Abstract

Seeds (achenes) of certain desert species (Artemisia) deposit considerable amounts of fixed carbon into an external polysaccharide pellicle that has a high capacity for holding water and can be hydrated and dehydrated many times. One function is considered to be the adhesion of the seed to sand particles during dew which protects from predation by ants. We have questioned whether the germinating embryo could utilize this pellicle as a nutrient source in the poor desert soil. Embryo extracts from dry imbibing and germinating seeds were assayed for a number of specific endo-glycosylase and exo-glycosylase activities and for the ability of these enzymes to degrade preparations of isolated pellicle. No evidence was found for any enzymic cleavage or hydrolysis of the pellicle or for any products to be available for utilization during germination of the seeds. Of commercial enzymes tested only polygalacturonase released reducing sugars from pellicle preparations after long incubation times indicating the high level of resistance of pellicle to enzymic cleavage. Comparisons of the water holding capacity of seeds with or minus their pellicles showed that periods of hydration were extended after dew deposition if pellicle was present. We suggest that a value of pellicle at low water availability is the provision of an enhanced opportunity for metabolic events in the embryo including those of DNA repair, the maintenance of genomic integrity and sustained viability in the seed bank.