Thesis (MScAgric (Horticulture))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Peel colour is an important quality factor in the production of bi-coloured apple fruit. Most markets set minimum requirements for red colour coverage. Fruit that do not meet these requirements are downgraded and has a major impact on the profitability of apple production in South Africa. South African apple production areas are amongst the warmest in the world. Since anthocyanin accumulation requires induction at low temperature and synthesis require mild temperatures, experiments were conducted to investigate optimum day and night temperatures for red colour development throughout fruit development for red and bi-coloured apple cultivars grown in South Africa. We found that redder strains of bi-coloured apple cultivars did not appear to owe their enhanced pigmentation to higher temperature optima for anthocyanin synthesis. The optimum day temperatures for red colour development in the different cultivars seemed to differ between seasons, but not between production areas. In general, red colour in the cultivars evaluated developed maximally between 17 ºC and 25 ºC. The optimum day temperature for red colour development remained constant throughout fruit development for most cultivars, but increased roughly from 14 ºC to 22 ºC in ‘Cripps’ Pink’ between January and April. The extent of red colour development increased during fruit development in all the cultivars assessed. We were unable to determine optimum induction temperatures for red colour development. ‘Royal Gala’ from Ceres seemed to benefit from induction at 4 ºC while red colour in ‘Fuji’ decreased with decreasing temperature. To explain the presence of anthocyanins in immature apple fruit, we tested the hypothesis that anthocyanins protect the peel from photoinhibition and photooxidative damage during conditions of increased light stress. First we established that the rate of colour change in response to a passing cold front appears to be sufficient to provide photoprotection during a cold snap. Also in agreement with the hypothesis, ‘Cripps Pink’ peel incurred significantly more photoinhibition at low temperature (16 ºC) compared to mild (24 and 32 ºC) and high (40 ºC) temperature under high irradiance with visible light. Recovery rate was temperaturedependent, being the slowest at low temperature and increasing with temperature. The photoapparatus in ‘Cripps Pink’ peel appears to be particularly sensitive to light stress at low temperature throughout the season, with significant photoinhibition occurring even at moderate temperature (24 ºC). The sensitivity of the apple peel to photoinhibition increased throughout the season at lower irradiance levels, but remained the same at higher irradiance. In our final experiment, fruit were exposed to high irradiance at low and mild temperature before exposure to high temperature in combination with high irradiance. This was done to test the hypothesis that photoinhibition incurred during cold snaps predisposes peel to photothermal damage when temperature increases again after the cold snap. Unfortunately, due to the severity of the stress incurred in response to high temperature treatment, the results were inconclusive. / AFRIKAANSE OPSOMMING: Vrugkleur is ‘n belangrike kwaliteitsfaktor in die produksie van tweekleurappels. Die meeste markte stel minimum vereistes vir rooi kleurbedekking. Vrugte wat nie aan hierdie vereistes voldoen nie, word afgegradeer. Suid-Afrika se appel produksie areas word beskou as van die warmste ter wêreld. Antosianien akkumulasie benodig induksie by lae temperature gevolg deur sintese in lig by matige temperature. Gevolglik het swak rooi kleurontwikkeling onder plaaslike toestande ‘n groot impak op die winsgewendheid van appelproduksie in Suid-Afrika. Eksperimente is uitgevoer om die optimum dag- en nagtemperature vir rooi kleurontwikkeling tydens vrugontwikkeling vir die rooi en tweekleur appel kultivars wat in Suid-Afrika geproduseer word te bepaal. Ons het gevind dat die verhoogde pigmentasie van rooier seleksies van tweekleurappel kultivars nie aan ‘n hoër temperatuur optimum vir antosianiensintese toegeskryf kan word nie. Die optimum dag temperature vir rooi kleurontwikkeling vir die onderskeie kultivars verskil klaarblyklik tussen seisoene, maar nie tussen produksie areas nie. Oor die algemeen het kleurontwikkeling maksimaal plaasgevind tussen 17 ºC en 25 ºC. Die optimum dagtemperatuur vir rooi kleurontwikkeling het konstant gebly tydens vrugontwikkeling, buiten vir ‘Cripps’ Pink’ waar dit toegeneem het van ongeveer 14 ºC tot 22 ºC vanaf Januarie tot April. Die mate van rooi kleurontwikkeling het in al die kultivars toegeneem deur die loop van vrugontwikkeling . Ons kon nie daarin slaag om optimum induksie temperature vir rooi kleurontwikkeling vas te stel nie. Rooi kleurontwikkeling van ‘Royal Gala’ uit Ceres is moontlik bevorder deur induksie by 4 ºC, terwyl ‘Fuji’ se rooi kleur afgeneem het met ‘n verlaging in induksie temperatuur. Ten einde die teenwoordigheid van antosianien in onvolwasse appelvruggies te verduidelik, het ons die hipotese getoets dat antosianien die vrugskil beskerm teen fotoinhibisie en fotooksidatiewe beskadiging gedurende tydperke van verhoogde ligstres. Eerstens het ons bevestig dat die tempo van kleurontwikkeling in reaksie op ‘n koue front waarskynlik vinnig genoeg is om fotobeskerming te verleen. Vervolgens is gevind dat ‘Cripps’ Pink’ vrugskil aansienlik meer fotoinhibisie ervaar het by lae temperatuur (16 ºC) in vergelyking met matige (24 ºC en 32 ºC) en hoë (40 ºC) temperatuur onder hoë irradiasie met sigbare lig. Die hersteltempo was temperatuur-afhanklik; dit was die stadigste by lae temperatuur en het toegeneem met ‘n toename in temperatuur. Die foto-apparaat in ‘Cripps’ Pink’ vrugskil blyk besonder sensitief te wees vir ligstres by lae temperatuur regdeur die groeiseisoen met aansienlike fotoinhibisie by selfs matige temperatuur (24 ºC). Die sensitiwiteit van die vrugskil vir fotoinhibisie het toegeneem deur die groeiseisoen by laer ligvlakke, maar het dieselfde gebly by hoër vlakke van irradiasie. Laastens is vrugte blootgestel aan hoë irradiasie by lae en matige temperatuur voordat dit vervolgens blootgestel is aan hoë temperatuur in kombinasie met hoë irradiasie. Dit was om die hipotese te toets dat fotoinhibisie wat opgedoen word gedurende ‘n onverwagte koue periode, die skil meer vatbaar maak vir fototermiese skade sodra die temperatuur weer styg na die koue periode verby is. Ongelukkig het die hoë temperatuur stres al die behandelings tot so ‘n mate geaffekteer dat dit onmoontlik was om enige gevolgtrekkings vanuit ons resultate te maak.