Physiology and ecology of geophytes in a winter rainfall receiving
semi-arid area of Southern Africa

The diversity and abundance of geophytes in the area of Nieuwoudtville in the Southwest of South Africa is unique on this globe. Especially the Bokkeveld Mountain Plateau is famous for its mass display of annuals and geophytes in late winter and spring. Strately elements of geophytes and annuals are similar as both groups have to complete their life cycle within a short period of time which is given by the availability of soil moisture. They differ, however, in the way they survive the dry season. Annuals survive in form of dry seeds, geophytes survive in a subsoil storage organ like bulb, tuber or rhizome. Although in terms of life strategies geophytes represent only one strategy they exhibit a high diversity of growth forms and life cycles. From a physiological point of view geophytes are characterized by allocation and translocation of reserves, nutrients and water from and to the storage organ. There is good information available for translocation and allocation processes in geophytes, but little attention has been paid to the production of these reserves in the leaves. Therefore it was one aim of the investigation to study the geophytes' photosynthesis and its interactions with the environment. Seedlings includes, the number of geophytes per m² ca be as high as 16 000. Adult subsoil storage organs, however, differ from 1321 on dolerite to 1955 per m² in the Renosterveld. Photosynthetic CO₂ uptake is characterized by rather low temperature optimum mostly below 20 °C. In comparison to annuals the CO₂uptake rate is low and between 2 and 24 µmol m^-2s^-1 when based on the projected leaf area. The decrease of CO₂ uptake with rising temperatures correlates with a substantial increase in photorespiration. Light saturation of photosynthesis occurred at 900 µmol photons m^-2s^-1 that of the electron transport through PSII at higher irradiance. Photorespiration and downregulation of PSII were found to be major factors preventing photoinhibition or photodamage at high temperatures and excess solar radiation. Leaf orientation determined interception of solar radiation and thus leaf temperatures which was highest for horizontal leaves. Leaf conductance and hence transpiration were found to be high. The accompanying study of carbon reserves and water in the subsoil storage organs of geophytes revealed a substantial loss of water during the dry season. Storage organs were able to refill their water reserves after an aseasonal rainfall. This reaction is regarded as main factor for the abundance of geophytes on the Bokkeveld Mountain Plateau.

Drittmittelgeber:

Schimper Stipendium

Beteiligte Wissenschaftler:

Prof. Dr. D.J. von Willert, B. Rossa, M. Steinberg

Veröffentlichungen:

Rossa, B., D.J. von Willert: Physiological characteristics of geophytes in semi-arid Namaqualand, South Africa. Plant Ecology 142, 121-132 (1999)

von Willert, D.J., B. Rossa, M. Steinberg: Ecological and physiological characteristics of geophytes of the Bokkeveld Montain Plateau, South Africa.1. Symposium der A.F.W. Schimper-Stiftung, 53-67 (2000)