Water relations and sunburn in apple fruit

Mupambi, Giverson (2017-12)

Thesis (PhDAgric)--Stellenbosch University, 2017.

Thesis

ENGLISH SUMMARY: Sunburn in apple is a major problem in South Africa with losses of up to 50% of total yield. The primary objective of this study was to investigate the relationship between tree water relations and sunburn development under South African conditions. Cultural practices that may affect sunburn development through modification of plant water status and the effect of microclimate on fruit peel anatomy was also studied. Induced water stress increased sunburn necrosis and incidence in ‘Granny Smith’ and sunburn severity in ‘Cripps’ Pink’ apple. Although not very pronounced, chlorophyll fluorescence pointed to photochemical changes within the peel under water stress. Water stress influenced ascorbic acid levels in peel, indicating the occurrence of oxidative stress. Leaf net carbon assimilation was reduced due to lower stomatal conductance and possibly also photoinhibition. Pulsing irrigation reduced the incidence of sunburn in ‘Golden Delicious’ and ‘Granny Smith’ compared to an under irrigated control. The reduction in sunburn might be attributed to either the improved plant water status, which reduced tree stress compared to the under irrigated control, or the comparatively more vegetative growth resulting in shading of fruit. Canopy ambient temperature was lower under pulsing irrigation and plant water status was improved. Foliar S˗ABA (active form of abscisic acid) application had no effect on sunburn incidence and severity under South African conditions over three growing seasons. The main apple growing regions in South Africa (33-34º S latitude) experience high radiation, while temperatures often exceed 40 ºC during the growing season. S-ABA application resulted in lower stomatal conductance with a consequent decrease in gas exchange and reduction in fruit size. Tree water status in terms of stem water potential was improved due to the reduction in transpiration. Fruit transpiration and water loss was higher in young compared to mature fruit, which seemingly corresponds with increasing susceptibility of fruit to sunburn during fruit growth. Stomata are present in young fruit and are under regulatory control, which makes them more effective for transpiration. In mature fruit, the predominant lenticel type was partially open and not effective for transpiration. Lenticel density and morphology varied considerably within and between cultivars, between different canopy positions, and between production areas. Although fruit transpiration and water loss decreased over the growing season, it did not always follow the same pattern as fruit surface morphology. In addition, the contribution of transpiration and water loss to the fruit heat balance is negligible. Hence, differences in fruit surface morphology may not explain differences in cultivar and fruit sunburn sensitivity. In conclusion, this study provides insight into ecophysiological and biochemical changes that occur under water stress and that might have a bearing on sunburn development in apple fruit. The study provides further proof of the link between plant water status and sunburn development and shows that good management of plant water status through irrigation is instrumental to controlling sunburn in a warm production region.

AFRIKAANSE OPSOMMING: Sonbrand is ʼn ernstige probleem in Suid-Afrika met verliese van tot 50% van totale produksie wat ondervind word. Die primêre doelwit van hierdie studie was om die verband tussen boom waterverwantskappe en sonbrand onder Suid-Afrikaanse kondisies te ondersoek. Verbouingspraktyke wat sonbrand ontwikkeling mag affekteer deur die modifikasie van die plantwaterstatus asook die effek van makro- en mikroklimaat op die vrugskil anatomie is ook ondersoek. Geïnduseerde waterstres het die graad van sonbrand in ‘Granny Smith’ en ‘Cripps’ Pink’ appels vererger. Ten spyte van ʼn relatiewe klein effek, het chlorofiel fluoressensie gedui op fotochemiese veranderinge in die skil vanweë waterstres. Waterstres het askorbiensuurvlakke in die skil geaffekteer, wat dui op die voorkoms van oksidatiewe stres. Netto koolstofassimilasie is verminder vanweë laer huidmondjiegeleiding en moontlik ook fotoinhibisie. Pulsbesproeiing het sonbrand in ‘Golden Delicious’ en ‘Granny Smith’ verminder in vergelyking met ‘n onderbesproeide kontrole. Die vermindering in sonbrand kan moontlik toegeskryf word aan enige van of beide verbeterde plant waterstatus wat stres verlaag het vegeleke met die onderbesproeide kontrole, of the relatief verhoogde vegetatiewe groei en gevolglike oorskaduwing van vrugte. Pulsbesproeiing het die temperatuur van die blaredak verlaag asook die plantwaterstatus verbeter. Blaartoediening van S-ABA (aktiewe vorm van absissiensuur) oor drie groeiseisoene het geen effek op die voorkoms en graad van sonbrand onder Suid-Afrikaanse toestande gehad nie. Die hoof produksie-areas in Suid-Afrika (33-34ºS breedtegraad) ondervind hoë straling terwyl temperature dikwels 40 ºC oorskry. S-ABA toediening het huidmondjiegeleiding verlaag met ʼn gevolglike verlaging in gaswisseling en vruggrootte. Boomwaterstatus, in terme van stamwaterpotensiaal, is verbeter vanweë die verlaging in transpirasie. Vrugtranspirasie en waterverlies was hoër in onvolwasse vergeleke met volwasse vrugte. Dit stem oënskynlik ooreen met die toenemende vatbaarheid van vrugte vir sonbrand soos wat hul groei. Huidmondjies is aanwesig in onvolwasse vrugte en is onder regulatoriese beheer. Dit maak hul meer effektief vir evaporatiewe verkoeling. Gedeeltelik oop lentiselle was die mees algemene lentisel tipe in volwasse vrugte. Hierdie lentiselle is oneffektief vir transpirasie. Die digtheid van lentiselle asook hul morfologie het heelwat tussen en binne kultivars, tussen vrugposisies in die blaredak en tussen produksie-area verskil. Alhoewel vrugtranspirasie en waterverlies gedurende die groeiseisoen afgeneem het, het dit nie altyd dieselfde patroon as vrugskilmorfologie gevolg nie. Bowendien is die bydrae van transpirasie en waterverlies tot die vrug hittebalans weglaatbaar klein. Gevolglik mag verskille in oppervlakmorfologie waarskynlik nie verskille in kultivar en vrug sonbrand sensitiwiteit verklaar nie. In gevolgtrekking; hierdie studie verskaf insig in ekofisiologiese en biochemiese veranderinge wat tydens waterstres ervaar word en wat moontlik ʼn strekking mag hê op die ontwikkeling van sonbrand in appels. Die studie verskaf verder ook bewyse van die skakel tussen plantwaterstatus en die ontwikkeling van sonbrand en dui daarop dat goeie bestuur van die plantwaterstatus deur besproeiing instrumentaal is tot die beheer van sonbrand in warm produksiestreke.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/102660
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