Browsing by Author "Bowie, Minnelise Rouchelle-Ann"

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    Ecophysiological studies on four selected succulent Karoo species
    (Stellenbosch : Stellenbosch University, 1999-03) Bowie, Minnelise Rouchelle-Ann; Esler, Karen J.; Wand, S.; Stellenbosch University. Faculty of Science. Department of Botany and Zoology.
    ENGLISH ABSTRACT: A study was undertaken in three greenhouses of differing temperature regimes (low, intermediate and high), to determine the ecophysiological plasticity of four selected Succulent Karoo shrub species. The temperature acclimation potential was determined for species grown under different ambient temperature treatments and seasonal temperature regimes. The four study species were Zygophyllum prismatocarpum and Stoeberia utilis (from the Namaqualand-Namib Domain) as well as Tripteris sinuata and Ruschia spinosa (from the Little Karoo ). All the study species are leaf succulent shrubs, except for Tripteris sinuata, which is a drought-deciduous shrub. The acclimation potential of the species was investigated with regard to the following: photosynthetic and transpiration rates, stomatal conductance, photosynthetic temperature optima, photosynthetic mode (C3 or CAM), instantaneous water-use efficiency, vegetative growth and flowering phenology. Gas exchange measurements performed on the evergreen species, Zygophyllum prismatocarpum, and the drought-deciduous species, Tripteris sinuata, indicated that both species are capable of photosynthetic acclimation when grown under different growth and seasonal temperature regimes. Z. prismatocarpum generally obtained the highest photosynthetic rates in the high temperature treatment and in September 1997 (spring). T. sinuata obtained highest photosynthetic rates in the intermediate and high temperature treatments and in June 1997 (winter). T. sinuata had much higher photosynthetic rates compared to Z. prismatocarpum. Photosynthetic temperature optima of Z. prismatocarpum and T. sinuata plants increased with an increase in growth and seasonal temperature regimes and differed between species. Temperature optima of Z. prismatocarpum plants varied from 21-24°C in spring and from 28-32°C in summer. T. sinuata plants had temperature optima ranging from 24-28°C in spring and from 28-33°C in summer. Leaf stable carbon isotope composition (813C) varied between temperature treatments, seasons and species. All four species studied had 813C values indicating the C3photosynthetic mode in both winter and summer, ranging from -24 to -31 %0. Less negative 813C values (higher water-use efficiency) were obtained for plants grown in the high temperature treatment, regardless of season, and during summer, regardless of greenhouse temperature regime. S. utilis and R. spinosa had less negative 813C values compared to Z prismatocarpum and T sinuata plants. Vegetative growth measurements (leaf area and plant volume) indicated continuous growth for Z prismatocarpum and R. spinosa plants, regardless of treatment temperature. By contrast, vegetative growth was greatest for T sinuata and S. utilis plants in autumn and winter, and in spring and summer respectively. The study species showed great plasticity in their flowering phenology, especially for T sinuata plants. T sinuata plants showed mass flowering in the high temperature treatment, especially during summer. S. utilis plants grown in the low temperature treatment flowered in autumn and early winter and plants in the high temperature treatment flowered later in winter. R. spinosa plants only flowered in autumn in the low temperature treatment and in spring in the intermediate temperature treatment. Z prismatocarpum plants did not flower at all. Plants of Z prismatocarpum and S. utilis, grown under the same temperature regime, were used in a water stress experiment. The plants were grown under three water treatments: short-term water stress, long-term water stress and non-stressed (control). Predawn acidity measurements of S. utilis leaves grown under different water stress treatments indicated increasing levels of titratable acidity in long-term stressed compared to non-stressed plants. This indicates the possibility that S. utilis plants from the long-term water stress treatment, are facultative CAM/C3. Z prismatocarpum and S. utilis plants from the short-term water stress treatment responded very rapidly to re-watering. The predicted increase in global mean temperature by approximately 2-4°C during the next century requires an understanding of plant responses to temperatures above current ambient temperatures. As an important ancillary objective, this study attempted to improve our understanding of the responsiveness of succulent Karoo shrubs to changing temperatures, and to provide a framework to facilitate theprediction of the responses of Karoo plants to future climatic scenarios. The study indicated that Z. prismatocarpum and S. utilis plants are capable of photosynthetic acclimation when grown under different temperature regimes. It is thus suggested that these plants are capable of photosynthetic acclimation to a predicted future global temperature increase. Water-use efficiency studies of the four study species also suggest that, these species as well as possible other succulent Karoo species, will increase their water-use efficiency in response to a predicted climate change. With higher global temperatures, the study also suggests that succulent plants would rely more on CAM as a means of conserving water to tolerate drought stress.