Browsing by Author "Hengari, Simeon Ngaitungue"

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    Effect of heat, ultraviolet-B and photosynthetic active radiation stress on apple peel photosystems
    (Stellenbosch : Stellenbosch University, 2015-03) Hengari, Simeon Ngaitungue; Steyn, Willem J.; Theron, K. I.; Midgley, S. J. E.; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.
    ENGLISH SUMMARY: The study was undertaken to analyse the response of apple fruit peel photosystems of different cultivars to ultraviolet-B (UV-B) radiation, photosynthetic active radiation (PAR) and heat stresses under laboratory conditions. UV-B, PAR and heat are claimed to be the main fruit sunburn inducing stress factors. The aim was to identify biochemical, physiological and fruit peel anatomical characteristics that provide photoprotection against sunburn inducing factors and to determine stress threshold levels for photodamage. Previously sun-exposed peels of apple fruits were resistant to photodamage under high UV-B dosage throughout fruit development. However, the shaded peels of mature fruits incurred photodamage under UV B stress. Furthermore, fruit photosystems at all development stages were equally sensitive to heat stress combined with moderate PAR (500 µmol m-2 s 1). Photodamage induced by heat and PAR stress during fruit development was not well correlated to fruit pigments, phenolic levels or fruit peel anatomical characteristics. In addition, repeated heat and PAR stress up to 9 hours did not induce any fruit sunburn symptoms. The photosystems of the less sunburn susceptible ‘Golden Delicious’ and more susceptible ‘Granny Smith’ appeared to be equaly sensitive to heat and PAR stress. The possible involvement of the xanthophyll cycle in fruit sunburn susceptibility needs further investigation as a variation in the dependancy of different cultivars on this cycle for photoprotection under heat and PAR stress was observed. Heat stress alone appears to cause the highest damage to fruit photosystems, while the presence of UV-B and PAR enhances this effect. The results presented in this document suggest that sensitivity to sunburn browning may not only be related to the heat, PAR and UV-B stress sensitivity of fruit peel photosystems. General non-photoprotective biochemical responses to the experienced stress may also play a role in sunburn symptom development.
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    The growth response of Eucalyptus grandis x e. camaldulensis to salt stress, ectomycorrhizae and endomycorrhizae double colonisation
    (Stellenbosch : University of Stellenbosch, 2007-03) Hengari, Simeon Ngaitungue; Valentine, A. J.; Theron, J. M.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Forest and Wood Science.
    The study was undertaken to determine the potential physiological benefits to plants provided by the double colonisation of host plant roots by endomycorrhizal (AM) and ectomycorrhizal (ECM) fungi, when growing under normal and under salt stress conditions. Plants of the Eucalyptus grandis x E. camaldulensis clone were grown in a sterile soil with 0 and 75 mM NaCl and with or without infection with the fungi Glomus etunicatum (an AM fungus) and Pisolithus tinctorius (an ECM fungus). The Eucalyptus clone formed both ECM and AM in single and double inoculation. The mycorrhizal symbiosis did not provide any nutritional benefits to the hosts. The double colonisation had no effect on plant growth under normal growth conditions while single colonisations of AM and ECM reduced growth. Double colonisation reduced host plant specific leaf mass by 12% and increased total leaf area by 43% compared with the control under these growth conditions. This colonisation also reduced photosynthesis per leaf area by 29% compared with the control. The reduced photosynthesis of the double colonisation did not result in reduced plant growth because these plants may have had a high total plant photosynthesis because of their large total leaf area. The double symbiosis however did not reduce salt stress when host plants were exposed to 75 mM NaCl, while the AM fungus increased plant dry weight by 13% compared to the control. AM and ECM colonisation in the double colonised roots under salt stress was decreased by 18 and 43% compared to that in plants under normal growth. The reduced colonisation may have reduced the fungi’s abilities to be beneficial to the host plant. The double symbiosis is recommended based on the documented positive effects of this symbiosis to plant growth and the considered possible long-term benefits to host plants growing in saline soils.