Sunburn control in apple fruit

Thesis (MSc)--Stellenbosch University, 2005.

ENGLISH ABSTRACT: Sunburn on apple and pear fruit results in large losses of export volumes in South African orchards. Producers estimate that losses due to sunburn are often about 10- 20%. Knowledge of temperature thresholds for sunburn as well as technologies available to manage sunburn is important to increase producer earnings. The effect of evaporative cooling on fruit surface temperature and sunburn was evaluated on 'Cripps' Pink', 'Royal Gala' and 'Fuji' apples as well as 'Forelle' pears. Evaporative cooling was effective in reducing fruit surface temperature by 0.6-3.7°C. Apple fruit under evaporative cooling treatments appeared to have acclimatised and had significantly higher surface temperatures when dry (0.9-4.7°C) than non-cooled fruit. This acclimitasing effect could be due to a modification of the cuticular wax layer. Futher trials are needed to investigate the cause of this acclimatising effect. The acclimatising effect was not found on 'Forelle' pears. Sunburn was reduced significantly only on 'Royal Gala' apples. Spraying 'Braeburn', 'Granny Smith', and 'Fuji' apples with reflective kaolin-based particle film (Surround WP) significantly reduced sunburn on 'Braeburn', 'Granny Smith' and 'Fuji' apple cultivars. Surround WP applications reduced leaf stomatal conductance and photosynthetic rate. Four applications of Surround WP with a commercial blower sprayer increased total soluble solids in 'Braeburn' . Four applications with a blower and five applications with a tower sprayer resulted in increased fruit size, higher flesh firmness, and lower percentage starch breakdown in 'Granny Smith'. Surround WP treatments slightly improved fruit colour in 'Granny Smith' apples. Futher trials are needed to establish whether Surround WP can,be reliably and economically removed from apple fruit and to evaluate the storage quality of fruit cleansed in this way. The effect of fruit surface temperatures (24°C, 30°C, 33-35°C, 40-43°C and 48-49°C) for increasing periods of time (two, four, six or eight hours) on maximum quantum yield of fluorescence (Fv/Fm) was measured on exposed and shaded sides of detached 'Cripps' Pink' and 'Royal Gala' apples. Measurements were taken directly after removal from the chamber as well as 12 hours later to assess recovery. Permanent damage to Photo system II (PSII) occurred between 48-49°C irrespective of duration, with permanent damage possible at 42-43°C when exposure exceeded six hours. There were no meaningful differences in Fv/Fm between shaded and exposed sides of apple fruit after exposure to high temperature. Duration of exposure was important in lowering PSU efficiency in 'Cripps' Pink' but less so in 'Royal Gala'. 'Cripps' Pink' apples harvested later in the season appeared to be able to recover from heat stress better than apples harvested earlier in the season. However, we only studied the effects of high temperature on apple tissues. More research is needed to establish the reaction of apple tissues to high temperatures in the presence of sunlight.

AFRIKAANSE OPSOMMING: Sonbrand op appel en peervrugte kom jaarliks voor in Suid-Afrikaanse boorde en lei tot groot afnames in vrugte wat geskik is vir die uitvoermark. Produsente beraam dat verliese a.g.v. sonbrand jaarliks 10-20% beloop. Kennis van drempelwaarde temperature vir die ontwikkeling van sonbrand sowel as die tegnologieë wat beskikbaar is om sonbrand te bekamp, is van groot belang om uitpakke te verbeter. Die effek van oorhoofse besproeïng op vrugtemperature en sonbrand van 'Cripps' Pink', 'Royal Gala' en 'Fuji' appels sowel as 'Forelle' pere is ondersoek. Oorhoofse besproeïng het vrugtemperature betekenisvol verlaag in al die proewe. Dit wil voorkom of appels onder oorhoofse besproeïng geakklimatiseer het en dié appels het hoër skil temperature geopenbaar wanneer hulle droog was as die kontrole vrugte. Moontlik is die effek te wyte aan 'n verandering in die kutikulêre waslaag. Verdere proewe om hierdie verskynsel te probeer verklaar is nodig. Die effek is nie waargeneem op die 'Forelle' pere nie. Sonbrand is betekenisvol verminder op 'Royal Gala' appels. Toediening van weerkaatsende kaolin-gebaseerde partikels (Surround'" WP) het sonbrand betenisvol verlaag op 'Braeburn' , 'Granny Smith', en 'Fuji' appels. Surround® WP toedienings het blaarhuidmondjie-geleiding en die tempo van fotosintese verlaag. Vier toedienings van Surround® WP met 'n kommersiële waaierblaser het totale oplosbare vastestowwe (TOVS) beduidend verhoog in 'Braeburn' appels. Vier toedienings met 'n waaierblaser en vyf toedienings met 'n toring spuitkar het gelei tot 'n verhoging in vrugmassa en vrugferrnheid, en 'n verlaging in persentasie styselafbraak in 'Granny Smith' appels. Surround® WP toedienings het gelei tot effens beter kleur op 'Granny Smith' appels. Verdere proewe is nodig om te bepaal of Surround® WP op 'n betroubare en ekonomiese wyse van vrugte verwyder kan word in die pakstoor, en wat die hou-vermoë is van appels wat só behandel is. Die effek van vrugoppervlaktemperature (24°C, 30°C, 33-35°C, 40-43°C en 48-49°C) vir toenemende tydsdure (twee, vier, ses en agt ure) op maksimum kwantum-opbrengs van fluoressensie is gemeet op sonkante en skadukante van geplukte 'Cripps' Pink' en 'Royal Gala' appels. Metings is geneem direk na verwydering uit die oonde sowel as 12 ure later om moontlike herstel waar te neem. Permanente skade aan Fotosisteem II (PSII) het plaasgevind tussen 48-49°C ongeag tydsduur, met permanente skade ook moontlik by 42-43°C wanneer blootstelling aan genoemde temperature ses ure oorskry het. Daar was geen betekenisvolle verskille in Fv/Fm tussen skadu- en sonkante van appels na blootstelling aan hoë temperature nie. Die tydsduur van blootstelling was belangrik om PSII effektiwiteit te verlaag in 'Cripps' Pink', maar minder so in 'Royal Gala'. Dit wil voorkom of 'Cripps' Pink' appels later in die seisoen beter daartoe in staat is om te herstel van hitte stres as vroeër in die seisoen. Ons het egter slegs die effek van hoë temperature op appel weefsels ondersoek. Verdere proewe is nodig om die reaksie van appelweefsels op hoë temperature in die teenwoordigheid van sonlig te ondersoek.