Ecophysiological studies on four selected succulent Karoo species

Bowie, Minnelise Rouchelle-Ann (1999)

Thesis (M.Sc.) -- University of Stellenbosch, 1999.

Thesis

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.

AFRIKAANSE OPSOMMING: 'n Studie was ondemeem in drie glashuise met verskillende temperatuurstoestande (laag, gemiddeld en hoog), om die ekofisiologiese aanpasbaarheid van vier spesies afkomstig van die Sukkulente Karoo te bepaal. Die temperatuur akklimasie potensiaal van die spesies was bepaal onder verskillende temperatuur en seisonale temperatuurstoestande. The vier studie spesies was Zygophyllum prismatocarpum en Stoeberia utilis (afkomstig vanaf die Namaqualand-Namib area) en Tripteris sinuata en Ruschia spinosa (vanaf die Klein Karoo ). Die spesies is almal blaar sukkulente plante, behalwe Tripteris sinuata wat 'n somer bladwisselende plant is. Die akklimasie potensiaal van die spesies was bepaal in terme van die volgende: fotosintetiese en transpirasie tempo, huidmondjie begeleiding, fotosintetiese temperatuur optima, fotosintetiese weg (C3 of CAM), effektiewe waterhouvermoe, vegetatiewe en reproduktiewe groei. Gaswisseling studies wat ondemeem was op die immergroen spesies, Zygophyllum prismatocarpum en die bladwisselende spesies, Tripteris sinuata het getoon dat beide spesies in staat is tot fotosintetiese akklimasie under verskillende groei en seisonale temperatuurstoestande. Z. prismatocarpum het die hoogste fotosintetiese tempo behaal in die behandeling met die hoogste temperatuur en in September 1997 (lente ). T sinuata het die hoogste fotosintetiese tempo behaal in die behandeling met gemiddelde en hoe temperature en in Junie 1997 (winter). Die fotosintetiese tempo van T sinuata was baie hoer as die van Z. prismatocarpum. Die fotosintetiese temperatuur optima van Z. prismatocarpum en T sinuata het gestyg met 'n styging in groei en seisonale temperatuurstoestande en het verskil tussen die twee spesies. Die temperatuur optima van Z. prismatocarpum het gewissel vanaf 21-24 °C in die lente en vanaf28-32 °C in die somer. Die temperatuur optima van T sinuata het gewissel vanaf24-28 °C in die lente en vanaf 28-33 °C in die somer. Die koolstof isotoop samestelling (813C) van die blare het gewissel tussen temperatuur behandelings, seisoene en spesies. Die 813C waardes van al vier studie spesies het dieC3 fotosintetiese weg getoon in beide winter en somer, wisselende vanaf -24 tot -31 %0. Minder negatiewe 813C waardes (hoer effektiewe waterhouvermoe) was verkry vir die plante in die hoe temperatuur behandeling, en in die somer. Die 813C waardes van S. utilis en R. spinosa was minder negatief in vergelyking met die van Z. prismatocarpum en T sinuata. Metinge van vegetatiewe groei (blaararea en plantvolurne) het aaneenlopende groei getoon vir Z. prismatocarpum en R. spinosa, teenspytende die temperatuur behandelinge. In teenstelling, vegetatiewe groei was die · meeste vir T sinuata en S. utilis plante in die herfs en somer, en in lente en somer onderskeidelik. Die studie spesies het goeie aanpasbaarheid in hul reproduktiewe groei getoon, veral T sinuata. T sinuata het grootskaals geblom in die hoe temperatuur behandeling, veral tydens somer. Die plante van S. utilis in die lae temperatuur behandeling het geblom in die herfs en vroe winter en die plante in die hoe temperatuur behandeling het laat in winter geblom. Die plante van R. spinosa het slegs geblom in herfs in die lae temperatuur behandeling en in die lente in die gemiddelde temperatuur behandeling. Die plante van Z. prismatocarpum het glad nie geblom nie. Plante van Z. prismatocarpum en S. utilis wat gegroei het in dieselfde temperatuur behandeling was gebruik in 'n droogte eksperiment. Die plante was gegroei in drie water behandelings: kort termyn droogte, lang termyn droogte en geen droogte (kontrole). Vroe suur bepalinge van die blare van S. utilis wat gegroei het onder verskillende droogte behandelings het 'n styging getoon in die suurvlakke in die plante van die lang termyn droogte in teenstelling met die plante van die kontrole behandeling. Dit toon die moontlikheid dat die lang termyn droogte plante van S. utilis CAM/C3 fakultatief is. Die plante van Z. prismatocarpum en S. utilis van die kort termyn droogte behandeling het baie vinnig gereageer toe dit weer water ontvang het. Die voorspelde styging in gemiddelde wereldwye temperature van gemiddeld 2-4 °C gedurende die volgende eeu, verg begrip van hoe die plante gaan reageer op 'n moontlike temperatuurstyging. 'n Belangrike bykomende objektiefvan die projek was 'n poging om die begrip te verbeter van die responsiwiteit van sukkulente Karooplante met temperatuurveranderinge en om 'n basis te verskaf om te voorspel hoe die plante gaan reageer onder toekomstigde klimaatstoestande. Die studie het getoon dat Z. prismatocarpum en S. utilis in staat is tot fotosintetiese akklimasie onder verskillende temperatuurstoestande. Dit word dus voorgestel dat die plante in staat is tot fotosintetiese akklimasie met 'n voorspelde toekomstige temperatuurstyging. Effektiewe waterhouvermoe studies van die vier studie spesies stel ook voor dat die spesies sowel as moontlike ander sukkulente Karoo spesies hul effektiewe waterhouvermoe sal verbeter met 'n moontlike temperatuurstyging. Die studie voorspel ook dat met hoer temperature, meer sukkulente plante die CAM fotosintetiese weg sal aanneem as 'n strategie om water te bespaar en om droogte toestande te weerstaan.

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