Regeneration of a Natal coastal dune forest after fire

The composition of alcoastal dune forest in the MIalazi Nature Reserve, Natal was studied three years after a fire and compared with adjacent unburnt forest. The two communities had similar plant densities but there was a marked decrease in biomass and diversity in the burnt community. Measured as density, basal area or canopy volume, Trema orientalis contributed over 70% of the regenerating burnt forest. Saplings of some forest trees were-present but those of Mimusops caffra, the dominant tree of the unburnt community, were absent from the burnt area. S. A fr. J. Bot. 1985, 51 : 453 — 459Die samestelling van ‘n kusduinwoud te Mlalazi-natuurreservaat, Natal is drie jaar na ‘n brand bestudeer en met aangrensende ongebrande woud vergelyk. Die twee gemeenskappe se plantdigtheid was dieselfde, maar daar was ‘n merkbare afname in biomassa en verskeidenheid in die gebrande gemeenskap. Gemeet as digtheid, basale oppervlakte of blaredak-volume het Trema oriental is meer as 70% van die regenererende gebrande woud bygedra. Lote van sommige woudbome was aanwesig maar die van Mimusops caffra, die dominante boom van die ongebrande gemeenskap, was afwesig in die gebrande gebied. S.-Afr. Tyciskr. Plantk. 1985, 51: 453 – 45

Infection of the cones and seeds of Welwitschia mirabilis by Aspergillus niger var. phoenicis in the Namib-Naukluft Park

AbstractWelwitschia mirabilis Hook. fil. is a unique and rare dioecious desert gymnosperm endemic to the Namib Desert. The female plants bear 90–100 megasporophylls, of which 50–60% may be fertile, but up to 80% of those fertile seeds may be infected by Aspergillus niger var. phoenicis. This contamination results in seed and seedling death, potentially negatively affecting recruitment of plants into the population.The pattern of infection of the cones and seeds in the field was studied over a period of eight months at the Hope Mine in the Namib-Naukluft Park. Infection of the cones was found to peak coincident with the appearance of the pollination drops, and with high temperatures, winds and significant rainfall. W. mirabilis cones were microscopically examined and spores of A. niger var. phoenicis were found embedded in the dehydrating pollination drop at the tip of the micropyle, suggesting that this was one of the routes by which the seeds were infected. The heteropteran, Probergrothius sexpunctatis, was also implicated in the infection of the seeds, since it was found to be carrying spores of A. niger var. phoenicis and was observed feeding on mature seeds and immature cones. While most post-harvest treatments have proved ineffective in eradicating A. niger var. phoenicis from the seeds, promising results are presently reported using a fungicide containing tebuconazole as the active ingredient

The effect of drying rate on viability retention of recalcitrant propagules of Avicennia marina

Recalcitrant propagules of Avicennia marina were stored under different relative humidities to achieve both rapid and slow drying. Irrespective of conditions, short-term (4–8 days) storage was accompanied by increased rates of protein synthesis and respiratory activity, the initiation of vacuolation and cell division and also by enhanced rates of germination. These data indicate that the germination process is initiated upon shedding. Storage for longer periods resulted in reduced rates of germination and ultimately, in loss of viability. However, propagules dried rapidly retained viability to a lower moisture content than those dried slowly. It is suggested that as germination changes occur during storage, the propagules become increasingly sensitive to desiccation, which might coincide with the degree of vacuolation. Rapidly dried propagules have not proceeded as far along the germination pathway and, at a given moisture content, are not as desiccation sensitive as those dried slowly. Thus viability loss is dependent upon rates of drying rather than on absolute moisture content or storage time, considered independently. S. Air. J. Bot. 1985, 51: 432–438Weerspannige propagule van Avicennia marina is teen verskillende relatiewe vogtigheidswaardes geberg om vinnige sowel as stadige uitdroging teweeg te bring. Ongeag omstandighede, is korttermyn-berging (4–8 dae) gekenmerk deur ‘n verhoogde tempo van proteïensintese en respiratoriese aktiwiteit, aanvang van selholtevorming en seldeling, asook ‘n verhoogde ont-kiemingstempo. Hierdie gegewens dui daarop dat die proses van ontkieming ingelei word deur propaguulverlies. Berging vir langer tydperke het gelei tot verlaagde ontkiemingstempo, en uiteindelik tot verminderde kiemkrag. Propagule wat vinnig uitgedroog is, het egter hul kiemkrag tot op ‘n laer voggehalte behou as die wat stadig uitgedroog is. Daar word voorgestel dat die propagule toenemend sensitief word vir uitdroging, soos ontkiemingsveranderinge gedurende berging plaasvind, en dat dit moontlik saamval met die graad van selholtevorming. Vinnig-gedroogde propagule het nog nie so ver gevorder tot ontkieming nie, en by ‘n spesifieke voggehalte is hulle nie so droogte-sensitief soos die wat stadig uitgedroog het nie. Verlies aan kiemkragtigheid is gevolglik eerder afhanklik van uitdro-gingstempo as van absolute voggehalte of bergingstyd. S.-Afr. Tydskr. Plantk. 1985, 51: 432–43