Effect of heat, ultraviolet-B and photosynthetic active radiation stress on apple peel photosystems

Hengari, Simeon Ngaitungue (2015-03)

Thesis (PhD)--Stellenbosch University, 2015.

Thesis

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.

AFRIKAANS OPSOMMING: Hierdie studie is onderneem om die respons van appelvrugskil fotosisteme van verskillende kultivars in reaksie op ultraviolet-B (UV-B) radiasie, fotosintetiese aktiewe radiasie (PAR) en hittestres onder laboratorium toestande te ondersoek. UB-B, PAR en hitte word gesien as die hoof stresfaktore wat sonbrand induseer. Die doelwit was om die biochemiese, fisiologiese en vrugskil anatomiese eienskappe wat beskerming teen die sonbrand induksie faktore verleen asook stres drumpelwaardes vir fotosisteemskade te identifiseer. Son blootgestelde appelvrugskil was strykdeur vrugontwikkeling weerstandig teen fotoskade onder ʼn hoë UV-B lading. Oorskadude vrugskil van volwasse vrugte het egter fotoskade ondergaan in reaksie op UV-B stres. Verder was fotosisteme van vrugte by alle ontwikkelingstadiums ewe sensitief tot hitteskade in kombinasie met matige PAR (500 µmol m-2 s-1). Hitte- en PAR stres induksie van fotoskade gedurende vrugontwikkeling was nie goed met vrugpigment, fenoolvlakke of met vrugskil anatomiese eienskappe gekorreleer nie. Daarmee saam het herhaaldelike hitte en PAR stres vir tot 9 ure nie enige vrug sonbrandsimptome geïnduseer nie. Die swak korrelasie en die onvermoë om sonbrandsimptome te induseer dui moontlik op die betrokkenheid van addisionele faktore in die manifestasie van vrug sonbrand. Die fotosisteme van die minder sonbrand sensitiewe ‘Golden Delicious’ en die meer sensitiewe ‘Granny Smith’ was klaarblyklik ewe sensitief vir hitte en PAR stres. Sonbrand sensitiwiteit hou daarom moontlik nie alleenlik verband met die hitte en PAR stres sensitiwiteit van vrugskil fotosisteme nie. Die moontlike betrokkenheid van die xantofielsiklus in vrugskil sonbrand sensitiwiteit behoort verder bestudeer te word, siende die variasie wat waargeneem is in die afhanklikheid van die verskillende kultivars op hierdie siklus vir fotobeskerming tydens hitte en PAR stres. Hitte stres opsigself veroorsaak klaarblyklik die grootste skade aan die vrug fotosisteme terwyl UV-B en PAR die effek van hitte versterk. Die resultate wat hier aangebied word, dui daarop dat direkte fotoskade in reaksie op hitte, UV-B en PAR stres nie, soos tans verstaan word, die alleen faktor in die induksie van sonbrand is nie. Die resultate dui verder ook daarop dat die sonbrand sensitiwiteit van verskillende kultivars, d.w.s hul geneigdheid om visuele sonbrandverbruining simptome te onwikkel, nie noodwendig saamhang met hul sensitiwiteit tot die verskillende faktore wat sonbrandverbruining induseer nie. Dit is moontlik omdat sonbrand simptomatologie in die geval van sonbrandverbruining dalk meer verband hou met die reaksie van die kultivar op die stres eerder as die sensitiwiteit daarvan tot die stres.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/96642
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