Doctoral Degrees (Horticulture)

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Browsing Doctoral Degrees (Horticulture) 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.