Studies towards understanding sunburn in apples

[摘要] ENGLISH ABSTRACT: Research was carried out to increase the current knowledge on sunburn development and itscontrol in apples. In addition to its chief causes, viz. high irradiance and high temperatures, waterstress has long been thought to contribute to sunburn susceptibility. Certain fruit chemical andtextural characteristics have also been suspected to increase sunburn, while it is not clear howfactors such as crop load affect sunburn in relation to other fruit quality parameters. In red andblushed 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 aconcomitant trial, mulching with vermicompost (plus a thin layer of woodchips), woodchips,plant compost and black geotextile were used to regulate plant water by reducing soilevaporative loss. Sunburn increased with an increase in moisture stress. Mulching had no effecton plant water status, but it generally enhanced plant photochemistry, reducing fruit surfacetemperature (FST) and sunburn.Chemical and textural characteristics of exposed, but non-burned 'Cripps' Pink' apples werecompared with that of fruit with induced and naturally occurring sunburn. Flesh firmness, totalsoluble solids (TSS), and titratable acidity (TA) of induced and naturally occurring sunburnt fruitdid not differ from each other, but were significantly different from the non-burnt fruit. Based onthis, we concluded that sunburn induces textural and compositional changes in sunburnt fruitrather than some fruit being predisposed to develop sunburn due to their texture and chemicalcomposition. Differences in heat stress tolerance, flesh texture and chemical composition of sunexposed 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 in2008-09 and on a whole tree basis in 2009-10 in 'Cripps' Pink'. In 2008-09, sunburn, red colourand 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 lowestthinning severity seemed to increase sunburn severity while the proportion of first grade fruitwas 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 thecrop 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) maskssuperficial sunburn browning and bleaching. The greater reduction in superficial sunburn, but notin sunburn necrosis in red and blushed compared to green cultivars with increasing red colour amonth towards harvest seemed to confirm the effect of masking.

[发布机构] Stellenbosch University