K.M. Kelly

Seed coat structure and dormancy

An understanding of dormancy mechanisms is of ecological and economic importance. Identification of the level at which dormancy is imposed appears to be species specific. The variation brought about by this therefore requires that developmental studies be included in seed coat dormancy experiments. In most cases, a site of permeability can be identified during the developmental process, and this information can be utilized later to remove dormancy. Under natural conditions, the removal of seed coat dormancy requires the interaction of a number of ecological and physiological dormancy-breaking cues.

Effect of acid scarification on seed coat structure, germination and seedling vigour of Aspalathus linearis

Summary Sulphuric acid scarification of Aspalathus linearis seed reduced their impermeability and increased their germination by 100%. Treatments of 120 minutes altered the seed coat structure, extensively damaging cuticle, macrosclereid, osteosclereid, hilar, strophiolar and cotyledon layers. Seedling vigour was adversely affected by scarification. This resulted in a decrease in seedling fresh mass and an increase in seedling abnormalities which increased their mortality.

The Lens as the Site of Permeability in the Papilionoid Seed, Aspalathus linearis

Summary An impaction treatment which involved shaking impermeable seeds of Aspalathus linearis vigorously, rendered seeds permeable at the lens. The macrosclereid cells at the lens separated and became raised. Impaction did not alter the structure of the hilum. Physiologically, imbibition of impacted seeds was controlled and 100 percent germination was recorded after seeds had been impacted for 12 hours. Seedling vigour in terms of fresh seedling mass and seedling morphology was not adversely affected by the impaction treatment. A “mild” dormancy-breaking treatment such as impaction is recommended for breaking dormancy in this case, in preference to the more commonly used concentrated sulphuric acid scarification.

The role of natural agents in the removal of coat-imposed dormancy in Dichrostachys cinerea (L.) Wight et Arn. seeds

AbstractsDichrostachys cinerea seeds are impermeable to water and do not germinate readily. The plant, however, contributes to a bush encroachment problem, minimising visibility in wildlife reserves and reducing the area available for grazing. Natural dormancy-breaking conditions must therefore exist, promoting the germinability of these normally dormant seeds. Diurnal temperature fluctuations (50°C/15°C), especially when combined with moisture were found to break dormancy. High temperatures, such as those generated by fire, destroyed seeds. Seeds ingested by herbivorous browsers such as nyala, became permeable. Seed burial for 48 weeks, was a more effective treatment than storage in promoting permeability. Distribution of D. cinerea will therefore be restricted to areas in which these conditions or factors occur. Seeds germinated optimally at 30δC in the laboratory.

The Transport and Metabolism of Sucrose and Acetate in Guayule (Parthenium argentatum Gray) in Summer

Summary Radiocarbon applied to single leaves of summer grown guayule plants as 14 C-sucrose and 14 C-acetate was subsequently exported to the untreated plant components. The distribution of 14 C indicated that sucrose was more rapidly exported and metabolized than acetate. The rate of incorporation of 14 C into the various fractions analyzed led to the conclusion that sucrose is the major translocatable product in these rubber producing plants. The applied 14 C-sucrose was rapidly incorporated into the organic acid fraction. As the radioactivity in this fraction diminished, that in the resin (acetone) fraction of the stems increased followed subsequently by an increase in the rubber (petroleum ether) fraction. 14 C from acetate was incorporated fairly rapidly into both resin and rubber within the stems. It is concluded that in the case of Parthenium argentatum sucrose serves as a major source of carbon for rubber production. It is readily transported to the stems where it is converted to acetate which serves as the primary precursor for resin and rubber production. The relationship between resin and rubber warrants further attention as the presence of 14 C in the resin fraction decreased as that in the rubber fraction increased.

A preliminary study of the carbohydrate metabolism in Parthenium argentatum

Abstract The decline in starch observed in guayule at the end of summer cannot be attributed to a decrease in temperature, but rather that starch can, upon hydrolysis, provide carbon for the synthesis of fructans. In this plant, fructans are important winter reserves arising from a changed ratio of carbon utilization to carbon fixation, and are seen as important in the maintenance of turgor during stress. Evidence suggests that bud break, leaf initiation, and flowering are times of major seasonal metabolic shifts with reallocation of carbohydrate reserves. It is concluded that growth has priority on carbon allocation, while rubber is seen as a secondary product and the result of metabolic ‘overspill’. The amount of rubber produced is the result of an interaction between enzyme and substrate availability and the compartmentation of cells. These findings have important implications in studies aimed at increasing rubber biomass.

In vitro propagation of Gladiolus flanaganii

The successful in vitro propagation of Gladiolus flanaganii, a rare summer-flowering species from the Natal Drakensberg is described. Kinetin was able to stimulate axillary bud growth and ensured that the corms did not enter dormancy during the tissue culture stage. The incorporation of NAA in the hardening-off medium had no deleterious effect on corm growth and stimulated root production.

A Comparison of Transport and Metabolism of [14C] Sucrose and [14C] Fructose in Summer and Winter in Guayule (Parthenium argentatum Gray)

Summary The metabolism of carbohydrates in guayule has not been considered with respect to its role in cis-polyisoprene synthesis. Application of radioactive precursors ([ 14 C] sucrose and [ 14 C] fructose), to the leaves of guayule plants in summer and winter showed that the fate of the carbohydrate depends on the chemical structure of the substrate and the time of application. [ 14 C] Sucrose was incorporated into resins during summer and into rubber in winter. [ 14 C] Fructose was incorporated into resins rather than rubber in winter. This would imply that the plastids utilize fructose for resin synthesis, thus availing cytosolic sucrose for the synthesis of rubber in the winter, and growth in the summer. The amount of translocated radioactivity was similar in winter and summer. The loss of leaves during winter reduced the photosynthetic area, while the loss of carbon from leaves in summer, decreased the available photosynthates. These two phenomena did not disadvantage the plant as far as the allocation of carbon was concerned. No plant components acted as preferential sinks in winter when the carbohydrate was applied as [ 14 C] sucrose. However, the radioactivity from [ 14 C] fructose was retained in the stems during the winter. The pith of the root crown incorporated the most radioactivity in summer.

Carbohydrate production in guayule

Guayule produces both ethanol and water-soluble fructans. The water-soluble fructans act as a ‘true’ carbohydrate reserve. When the demand for carbon increases greatly, the water-soluble fructans are readily depolymerized. The ethanol-soluble fructans are quickly metabolized providing a readily accessible carbohydrate source. They are extensively depolymerized at budbreak and flowering. The polymerization of water-soluble fructans occurs in November/December and during the winter months. This is indicative of the availability of excess fructose at these times. The mature stem and roots appear to be primary sites of fructan storage, with the supply in the roots being the least accessible. Starch, which occurs primarily in the leaves, is apparently involved in sucrose storage.

Chemical composition of pods and seeds of Dichrostachys cinerea as a basis of seed dispersal

The occurrence of very few seeds of Dichrostachys cinerea in the soil seed bank prompted the present study as to why animal browsers and seed predators should select the infructescence of this plant. The preferential and extensive browsing of this indehiscent legume is apparently not due to any exceptional nutritive value of the pods and seeds. However, it is probably the strong rich aroma of the infructescence which initially attracts animals to this food source. The structure of the infructescence, ensures that upon location a large number of pods and seeds are consumed. Ingestion of the pods will provide fibre, lipid, fatty acids, carbohydrates, amino acids, nitrogen and protein.