Studies towards understanding sunburn in apples

Makeredza, Brian (2011-12)

Thesis (MScAgric)--Stellenbosch University, 2011.

Thesis

ENGLISH ABSTRACT: Research was carried out to increase the current knowledge on sunburn development and its control in apples. In addition to its chief causes, viz. high irradiance and high temperatures, water stress has long been thought to contribute to sunburn susceptibility. Certain fruit chemical and textural characteristics have also been suspected to increase sunburn, while it is not clear how factors such as crop load affect sunburn in relation to other fruit quality parameters. In red and blushed cultivars, sunburn could be underestimated due to masking by the red colour overlay. Half and total irrigation water were withheld for two weeks in ‘Cripps’ Pink’ apples while in a concomitant trial, mulching with vermicompost (plus a thin layer of woodchips), woodchips, plant compost and black geotextile were used to regulate plant water by reducing soil evaporative loss. Sunburn increased with an increase in moisture stress. Mulching had no effect on plant water status, but it generally enhanced plant photochemistry, reducing fruit surface temperature (FST) and sunburn. Chemical and textural characteristics of exposed, but non-burned ‘Cripps’ Pink’ apples were compared with that of fruit with induced and naturally occurring sunburn. Flesh firmness, total soluble solids (TSS), and titratable acidity (TA) of induced and naturally occurring sunburnt fruit did not differ from each other, but were significantly different from the non-burnt fruit. Based on this, we concluded that sunburn induces textural and compositional changes in sunburnt fruit rather than some fruit being predisposed to develop sunburn due to their texture and chemical composition. Differences in heat stress tolerance, flesh texture and chemical composition of sun exposed and shaded fruit sides seem to relate to their light exposure history. The effect of crop load on sunburn and fruit quality were assessed on a fruit cluster level in 2008-09 and on a whole tree basis in 2009-10 in ‘Cripps’ Pink’. In 2008-09, sunburn, red colour and the proportion of first grade fruit decreased with an increase in number of fruit per cluster. Crop load did not have an effect on sunburn and fruit colour in 2009-10 although the lowest thinning severity seemed to increase sunburn severity while the proportion of first grade fruit was highest for the highest thinning severity. Fruit number per cluster had no effect on fruit size, while at the whole tree level, fruit size decreased with increasing crop load. Flesh firmness, starch breakdown, TA and TSS were neither affected by the number of fruit per cluster nor the crop load of the tree as a whole. Green ‘Granny Smith’ and ‘Golden Delicious’, blushed ‘Royal Gala’, ‘Fuji’, ‘Braeburn’ and ‘Cripps’ Pink’, and full red ‘Topred’ were used to assess how red colour (anthocyanins) masks superficial sunburn browning and bleaching. The greater reduction in superficial sunburn, but not in sunburn necrosis in red and blushed compared to green cultivars with increasing red colour a month towards harvest seemed to confirm the effect of masking.

AFRIKAANSE OPSOMMING: Hierdie navorsing is verrig ter uitbreiding van die huidige kennis oor die ontwikkeling en beheer van sonbrand in appels. Addisioneel tot die hoof oorsake van sonbrand, naamlik hoë irradiasie en hoë temperatuur, word waterstres gesien as bydraend tot sonbrandsensitiwiteit. Daar word ook vermoed dat sekere chemiese en teksturele eienskappe van die vrug sonbrand kan verhoog terwyl daar nie duidelikheid is oor hoe faktore soos oeslading sonbrand relatief tot ander vrugkwaliteitparameters kan affekteer nie. Die omvang van sonbrand in rooi en bloskultivars kan dalk onderskat word vanweë maskering deur die oorliggende rooi pigmentasie. Besproeiing is vir twee weke gehalveer of gestop in ‘Cripps’ Pink’ appels. Sonbrand het toegeneem met ‘n toename in vogstres. ‘n Deklaag van vermikompos (plus ‘n dun laag houtstukkies), houtstukkies, plantkompos en ‘n swart geotekstiel is toegedien om die plant waterstatus te reguleer deur evaporasie vanuit die grond te beperk. Die deklae het geen effek op die plant waterstatus gehad nie, maar het wel die blaar fotochemie bevorder en terselftertyd die vrugoppervlaktemperatuur verlaag en sonbrand verminder. Die chemiese en tekturele eienskappe van sonligblootgestelde ‘Cripps’ Pink’ appels sonder sonbrand is vergelyk met vrugte met geïnduseerde sonbrand en vrugte met sonbrand wat natuurlik ontwikkel het. Vrugvleisfermheid, totale oplosbare vastestowwe (TOV) en titreerbare suur (TS) van vrugte met natuurlik en geïnduseerde sonbrand het nie onderling verskil nie, maar het wel betekesnisvol verskil van vrugte sonder sonbrand. Ons het gevolglik afgelei dat sonbrand teksturele en komposisionele veranderinge teweeg bring eerder as dat sekere vrugte gepredisponeer word om sonbrand te ontwikkel vanweë hul tekstuur en chemiese samestelling. Dit kom voor dat verskille in hittestres toleransie, tekstuur en chemiese samestelling tussen die sonligblootgestelde en skadu kante van appels verband hou met hul verskil in sonligblootstelling. Die effek van oeslading op sonbrand en vrugkwaliteit in ‘Cripps’ Pink’ is ge-evalueer op trosvlak in 2008-09 en op ‘n per boom basis in 2009-10. Sonbrand, rooi kleur en die proporsie eersteklas vrugte het afgeneem met ‘n toename in die aantal vrugte per tros. Op ‘n per boom basis is rooi kleur en sonbrand egter nie deur oeslading geaffekteer nie. Die laagste vlak van vruguitdunning het wel oënskynlik die graad van sonbrand verhoog terwyl die hoogste vlak van vruguitdunning tot ‘n toename in die proporsie eersteklas vrugte gelei het. Die aantal vrugte per tros het geen effek op vruggrootte gehad nie terwyl vruggrootte afgeneem het met oeslading op ‘n heelboombasis. Vrugvleisfermheid, styselafbraak, TOV en TS is nie deur aantal vrugte per tros of die oeslading per boom geaffekteer nie. Groen ‘Granny Smith’ en ‘Golden Delicious’, rooiblos ‘Royal Gala’, ‘Fuji’, ‘Braeburn’ en ‘Cripps’ Pink’, en volrooi ‘Topred’ is gebruik om die maskering van oppervlakkige sonbrandverbruining en –verbleiking deur rooi antosianien te bestudeer. Oppervlakkige sonbrand het minder toegeneem in die maand voor oes in die rooi- en bloskultivars as in die groen kultivars terwyl vrugkleur skynbaar nie die aanwesigheid van die meer sigbare sonbrandnekrose geaffekteer het nie. Maskering speel dus ‘n rol in die aanwesigheid van sigbare sonbrand.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/17908
This item appears in the following collections: