M.G. Gilliland

Ultrastructure of dry viable and non-viable Protea compacta embryos

Summary An investigation of Protea compacta seed revealed that a number of ultrastructural differences exist between viable and non-viable embryos. The most conspicuous difference is that the lipid bodies of non-viable embryos coalesced, probably as a result of the rupturing of the «membranes» that enclosed them. On the basis of the size of the globoids present, three different types of protein bodies could be distinguished both in viable and non-viable embryos. Protein bodies containing a large number of small globoids were present mainly in the vicinity of the embryonic root tip while those with a few large globoids were found predominantly in the cotyledonary cells.

Detection of Rubber in Guayule (Parthenium argentatum Gray) at the Ultrastructural Level

Summary Guayule tissue from stems of one-year-old plants was fixed with osmium tetroxide and glutaraldehyde and embedded in araldite resin to trace the deposition of rubber with the electron microscope. The rubber appeared as electron opaque membrane bound particles. They were most abundant in the meristematic cells of bud and shoot primordia and in the epithelial cells of the resin canals which also may be very active metabolically. Smaller vesicles filled with rubber were apparent in the parenchyma cells of the cortex, pith and vascular rays. There was usually no central vacuole, and as the rubber particles increased the organelles became disorganised. These findings suggest that rubber particles are part of the cytoplasm and must be regarded as inclusions in the same sense as protein bodies. Chloroplasts were sparse in the stem tissue where most of the rubber occurs.

CO2 Fixation and Rubber Deposition in the Bark and Leaves of Guayule during Winter

Summary The chloroplasts found in the stem tissue of Parthenium argentatum (guayule) which had passed through one winter fixed a considerable amount of CO 2 and a relatively large proportion was incorporated into rubber. In this region of the stem the cells were not completely filled with rubber and new cells were still generated by the vascular and cork cambia. While the cells in the oldest part of the two-year-old plants appeared to be filled to capacity with rubber, 14 C was still incorporated into cis -polyisoprene. This indicated that despite the presence of copious amounts of rubber some of these cells still had the ability to synthesize rubber. In the parts of the stem produced by the current seasons growth most of the 14 C was incorporated into the aqueous extract and not into rubber which suggests that it was used for general metabolism. At present it is not known which precursors, if any, are obtained directly from the chloroplasts and are used as substrate for rubber production. This aspect is currently receiving attention.

Cyclic patterns of growth and rubber deposition in guayule Parthenium argentatum. Suggestions for a management programme

As the natural pattern of development and rubber synthesis in guayule (Parthenium argentatum) is cyclic any treatments to improve growth and rubber production should be synchronized with the normal sequence of events pertaining to growth and metabolism. Starting with 25-week-old seedlings an attempt has been made to acquire information about rubber deposition, growth and defoliation. Observations were made in the field as well as at the light and electron microscope level. Rubber is deposited mainly in the bark during winter. The highest deposits were found in the secondary stem tissue. S. Afr. J. Plant Soil 1986, 3: 21–26

Studies on the translocation system of guayule (Parthenium argentatum Gray)

SummaryObservations at ultrastructural level have been added to earlier descriptions of the vascular tissue inParthenium argentatum (guayule). Large transfer cells with pronounced cell wall projections were found in the phloem of the minor veins in the leaves. These cells have every appearance of being modified for the purpose of collecting photosynthates. In the large veins of the leaf the elements of the phloem, engaged in bulk transport of solutes, showed reduced transfer cell characteristics. Within these larger veins of the leaf nacreous thickenings of the sieve tubes was very common. Similar thickenings were found in the petiole and the stem and these persisted to maturity. Histochemical tests indicated a high level of pectic substances in these walls. In the stem, particularly in regions where rubber deposition was significant, the companion cells in the phloem had elaborate cell wall ingrowths. The adjacent parenchyma cells were adequately provided with pits. Occasionally transfer cells were found in the xylem.

Changes in the food reserves of viable and non-viable Protect compacta embryos during incubation

Summary Both viable and non-viable Protea compacta embryos show the same pattern of protein digestion during incubation. In the viable embryos however, the hydrolysis of proteins is accompanied by a much larger increase in free amino acids. Protein hydrolysis is followed by lipid digestion, probably by way of the glyoxylate pathway, and the accumulation of starch. Following this there is a rapid development of organelles. In non-viable embryos, digestion does not progress further than the hydrolysis of protein. It would appear as if pre-existing enzymes are associated with globoids, but that the majority of enzymes required for digestion, and in particular those required for lipid hydrolysis, are synthesized de novo .

Anatomical and ultrastructural changes associated with ethylene-induced defoliation of guayule explants

Explants of guayule,Parthenium argentatum Gray, were treated with concentrations of ethephon varying from 0.5 to 10gl-1 for six days. The cut ends of the shoots were immersed in the solutions and allowed to stand so that the ethylene-releasing agent entered the explant via the transpiration stream. With higher concentrations of ethephon, defoliation of the explants commenced after one day and was 100% effective after six days. Lower concentrations were less effective. Examination of the petiole bases of treated explants at the light microscope level revealed enhanced development of abscission layers and hydrolytic degradation of the tissue immediately distal to these layers. This led to separation of the leaf which had become senescent. Food reserves appeared to have been mobilised from the senescent leaves. Histochemical staining and ultrastructural observations indicated loss of insoluble polysaccharides and cellulose from the induced separation layer. Pectic substances were lost to a lesser extent.

The uptake, transport and metabolism of the bioregulator 2-(3,4-dichlorophenoxy) ethyldimethylamine in guayule

The bioregulator 2-(3,4-dichlorophenoxy) ethyldimethylamine was applied to five-month-old summer and winter guayule plants. Uptake of this molecule depended on the presence of viable trichomes and a well-developed cuticle, in the leaves. Winter plants absorbed the bioregulator more successfully than summer plants. The stem proved to be an active absorption site in young plants. Six days after bioregulator application, transport of the molecule was restricted to the lower stem in summer plants, and stem and leaves in winter plants. Transport was governed by the availability and development of conduits. The intact molecule was recovered two days after application but was not detectable after 4 and 6 days indicating that it is metabolized fairly rapidly. The significance of these findings is discussed in terms of the use of bioregulators to stimulate rubber production in guayule plants.

The Food Reserves and Ultrastructural Relationships of the Gametophyte and Sporophyte Tissues in Podocarpus henkelii Seeds

Summary Tissue of mature seeds of Podocarpus henkelii was fixed in glutaraldehyde and post-fixed in osmium tetroxide for electron microscopy. The major food reserve in both the gametophyte and sporophyte is starch, contained in amyloplasts. Well defined protein bodies without crystalline structures were found in the cotyledons. In the rest of the embryonic tissue, these were very sparse. The inner surfaces of the gametophyte cells in contact with the embryo are transversely ribbed. This greatly increases the area of the plasmolemma, which closely follows the wall contours. In view of their structure and function these cells are considered to be transfer cells, and confirms their existence in gymnosperms.

The Ultrastructural and Biochemical Changes Associated With Dormancy Breaking in Ricinodendron rautanenii Schinz. Seeds Following Ethylene Treatment

Summary Imbibition initiated digestion of both lipid and protein reserves in the manketti nut, although the embryo remained dormant. Dormancy was overcome by treatment with ethylene or ethrel whereupon the testa burst and the radicle emerged. Ultrastructural studies on the embryonic axis revealed that ethylene stimulation brought about changes in the nucleus and the pattern of protein digestion. The levels of proteins, lipids, sugars and starch were also monitored biochemically and it appeared that lipid was channelled into carbohydrates to support embryonic growth. Protein digestion took place at a slower rate than lipid digestion. The mobilization of reserves was more rapid in the embryonic axis than in the endosperm and cotyledons and all were accelerated by ethylene and ethrel treatments.