N. Grobbelaar

The identification of the coralloid root endophytes of the southern African cycads and the ability of the isolates to fix dinitrogen

Forty-one isolates of the cyanobionts in the coralloid roots of 31 cycad species, which include all the species indigenous to the Republic of South Africa, were identified. One isolate (from Encephalartos hildebrandtii ) appears to be a species of Calothrix . All the other isolates were identified as species of Nostoc . Twenty-eight isolates were assigned to N. commune , five to N. punctiforme , two each to N. ellipsosporum , N. paludosum and N. sphaericum whilst one isolate was identified as N. muscorum . In the case of two cycad species, isolates were made from more than one individual. Of the 10 isolates from seven individuals of Encephalartos transvenosus eight were N. commune and two N. paludosum . One of the two isolates from Encephalartos manikensis was N. ellipsosporum and the other N. punctiforme . One isolate from each cycad species was shown to reduce acetylene to ethylene. One isolate from each of twelve cycad species was tested for its ability to fix 15 N. All yielded positive results. Although the cyanobacteria in all the isolates tested for acetylene reduction were contaminated with other bacteria (which were not identified), it could be demonstrated that they did not contribute to the nitrogenase activity of the cyanobiont cultures.

Photosynthetic Characteristics of Some South African Parasitic Flowering Plants

Summary The photosynthetic capacity of representatives of 24 species belonging to 16 genera of South African parasitic flowering plants had been investigated in respect of their chlorophyll concentration, photosynthetic activity, respiratory activities, CO 2 -compensation point, δ13 C-values and chloroplast ultrastructure. Some species were totally devoid of chlorophyll whilst other species had chlorophyll concentrations which varied from 2.0 to 9.3 mg chlorophyll (dm 2 plant area) -1 . Relatively high values (176.7 to 255.0 µl l -1 CO 2 ) for CO 2 -compensation points of stem hemiparasites were obtained in contrast to much lower values (51.7 to 148.3 µl l -1 CO 2 ) of the root hemiparasites. The hosts of the parasites could be classified as either C 3 - or C 4 -plants on the basis of their δ13 C-values, while those of the parasites all fell within the range characteristic of C 3 -plants. The chloroplasts ultrastucture of the chloroplast containing hemiparasites did not differ significantly from non-parasitic plants and were characterised by well-developed grana with an extensive thylakoid system.

Coning and sex ratio of Encephalartos transvenosus at the Modjadji Nature Reserve

From a random sample of 638 mature plants examined during 1987, the developing cones yielded a sex ratio of 2,3 males to 1 female. The sex expression of the taller plants (2,6xa0m – 9,5xa0m) during 1987 was 7 males to 1 female. This suggests a preferential dying off of the old females and/or a stronger decline in the production of cones by the ageing females compared with their male counterparts. Plants in the l,6-m – 3,5-m stem height range had the highest coning frequency during 1987. An increased water availability as well as fire in the preceding year appear to enhance coning whilst fire in the preceding year also appears to stimulate the females more than the males to cone during the ensueing season.

The occurrence of coralloid roots on the South African species of the Cycadales and their ability to fix nitrogen symbiotically

Thirty-three species of Encephalartos, which include all the known species indigenous to the Republic of South Africa, together with Stangeria eriopus , were found to form coralloid roots. In all cases, specimens were found in which the coralloid roots were infected with Cyanobacteria (blue-green algae) and these roots in all cases reduced acetylene to ethylene, and fixed nitrogen.

The occurrence of prostaglandin (PG) F2α in Pharbitis nil seedlings grown under short days or long days

The concentration of PGF 2 α present in Pharbitis nil kept under short days is about 20 times higher than in plants kept under long days.

The chlorophyll concentration and photosynthetic activity of some parasitic flowering plants.

Summary Fifteen species (7 stem and 8 root parasites) were investigated. Their chlorophyll concentration ranged from 0 (1 species) to 11.7 mg per dry leaf or shoot mass and their chlorophyll a : b ratio varied between 1.0 and 2.3. The relative rates of 14CO2 assimilation varied from 3 to 4 226 d.p.m. g-1 dry aerial material.

The vegetation of the Roodeplaat Dam Nature Reserve. IV. Phenology and climate

Phenological and environmental variables were analysed to determine their possible interrelationships. Growth activity in woody species such as the deciduous Berchemia zeyheri (Sond.) Grubov and the evergreen Olea europaea L. subsp. africana (Mill.) P.S. Green, commences when the air and soil temperatures exceed certain threshold values in conjunction with an increase in photoperiod. Growth and reproduction are influenced by rain only after certain threshold levels of temperature and the photoperiod have been exceeded.

Photosynthesis of certain South African parasitic flowering plants.

Summary The rate of apparent photosynthesis, the C0 2 compensation point and the chlorophyll concentration of a number of holo- and hemiparasitic plants were studied in order to establish the degree of their dependence on their hosts. The hemiparasitic stem-parasites exhibited an exceptionally high C0 2 compensation point and the low rate of apparent photosynthesis possibly indicating a high degree of dependence on their hosts for photosynthate.

The vegetation of the Roodeplaat Dam Nature Reserve. III. Phenological observations

The phenology of a number of phanerophytes, chamaephytes, hemi-cryptophytes, cryptophytes and therophytes was studied over a three-year period and it was found that certain phanerophytes exhibited activity as early as the second half of July. Most other species commence growing and/or flowering only in September, while the flowering phase of a number of species commences only in late summer or early winter.

Soil nitrogenase activity of the Nylsvley Nature Reserve

A dinitrogen fixation rate of 6,3 to 8,5xa0kg ha−xa01 a−xa01 was recorded for the Eragrostis pallens – Burkea africana savanna of the Nylsvley Nature Reserve by the acetylene reduction method. The bulk of the activity occurred in the summer and autumn after good rains and appears to be largely the result of legume – Rhizobium symbioses. No evidence of dinitrogen fixation by free-living Cyanobacteria or symbiotic systems involving either actinomycetes or grasses was found.