Plant Physiology

Plant Physiology

Plant Physiology is the study of processes within plants. Research interests related to plant growth and development within the Research Centre are varied and cover two main themes: (1) Tissue culture and micropropagation; and (2) Plant growth regulators, including herbicides. Research is both pure and applied with an emphasis on biotechnology.

Areas of interest

Tissue culture and micropropagation

South Africa has a rich and diverse flora with over 30 000 species of flowering plants – almost 10% of the world’s higher plants. Traditionally, plants have been used by indigenous people for herbal medicines. Flowering bulbs and other fynbos species are prized in the cut-flower industry. Rapid urbanization and overexploitation of wild species by the horticultural trade and by traditional medical practitioners has led to a decline in many species.

Tissue culture is a means of preserving species that are rare and threatened and providing an alternative source of plants for commercial, horticultural and traditional medicinal trade. Tissue culture is the “aseptic culture of plant protoplasts, cells, tissues or organs on a culture medium which is as defined as possible; the cultures are maintained under controlled environmental condition”. Our the three main areas of expertise are: i) Tissue culture of ornamentals and medicinal plants; ii) Tissue culture in forestry; and iii) Tissue culture and secondary metabolite production.

Tissue culture of ornamentals and medicinal plants

The main difficulty in growing indigenous plants in large quantities is to obtain sufficient plant material. Seeds may germinate erratically and bulbs usually propagate by offsets. This makes production too slow to warrant their introduction as new commercial crops. In vitro methods are used to speed up propagation. The success of the system lies in the development of strict protocols for each species. This involves improving decontamination procedures and determining the effects of various cultural factors on plant growth, both in vitro and ex vitro to establish optimum growing conditions.

Tissue culture in forestry

Pinus patula is the most important commercially grown softwood species in South Africa. Clonal propagation of superior clones is gaining favour in the forestry industry. Conventional methods of clonal propagation involve the rooting of cuttings. However, the taking of cuttings from trees older than four years is problematic due to maturation effects. Somatic embryogenesis offers a means of mass producing superior clones. Somatic embryos can be encapsulated to produce large quantities of artificial seeds. Embryogenic tissue can be cryopreserved to maintain it in the juvenile state, thereby overcoming maturation constraints and can also be used in genetic engineering to improve disease resistance and tolerance to environmental stresses. Somatic embryos have been successfully induced in Pinus patula and these have been maintained in vitro within the Research Centre. Embryogenic tissue has also been cryopreserved and field trials of the somatic seedlings are being conducted.

Eucalyptus and wattle are also important in the wood-pulp industry in South Africa and ways of improving their production are being investigated. Areas of interest include improving vigor of cuttings and ways to store pollen to improve its viability.

Tissue culture and secondary metabolite production

Secondary metabolites are compounds produced in plants that are not necessary for the plants basic functions although some act as chemical defenses against microbes and animals. Secondary metabolites are used in the pharmaceutical industry as flavourants and dyes, and in perfumery. As conventional growing of medicinal plants is relatively expensive, production of medicinal and/or other compounds can be elicited in vitro. Hypoxoside produced by Hypoxis species, is used as an anticancer drug. This has been successfully produced in callus cultures within the Research Centre. Red pigmented anthocyanins, used as a food colourant, have been produced from the callus of Oxalis species in the Research Centre.

Plant growth hormones and growth regulation

Hormone Physiology

Hormones are vital in controlling the growth and development in plants. Plant hormones are classified into five major groups, namely cytokinins, auxins, gibberellins, ethylene and abscisic acid. Bioassays are an integral part of hormone research. A number of bioassays are routinely used in the Research Centre e.g. the soybean bioassay and the mung bean bioassay. Radiochemistry facilities are also available in the Research Centre, allowing for the elucidation of biochemical pathways within plants. Using chromatographic techniques such as paper chromatography, TLC, sephadex and HPLC, the occurrence and metabolism of hormones including cytokinins, auxins and abscisic acid have been investigated in a range of lower and higher plants. Cytokinins and auxins have also been identified in some commercial seaweed products used in agriculture.

Growth regulation and herbicides

Plant development is influenced by environmental signals which are transduced into physiological responses. These processes involve plant hormones and regulators. The nature of the plant’s reaction depends on endogenous levels and the interaction between the different classes of regulators. Understanding the effects of growth regulators allows for the improvement of plants, including commercial crops, by influencing various physiological processes such as flowering and fruit set.

Many weed biotypes are resistant to herbicides. Identification and characterization of herbicide resistant weed biotypes is necessary. The use of a newly developed plant growth regulator which alters plant growth, development and production is currently underway by members of the Research Centre. Addition of Plant Growth Regulators to herbicides to enhance efficiency of herbicides is also under investigation.

Seed biology

Smoke from burning vegetation contains a chemical messenger which triggers seeds of many species to germinate. This interesting mechanism ensures that seeds germinate at an optimal time when there is sufficient light and space immediately after a veld fire. Aqueous extracts of smoke can be used for commercial purposes. They are currently used in land restoration and have the potential to play an important role in the conservation of “difficult-to-germinate” plant species.

Poor seedling germination and vigor reduce the market value of many commercial seeds. Numerous reasons for poor seed quality are being investigated by members of the Research Centre.