Photo-thermal studies in Japanese plums

Makeredza, Brian (2019-12)

Thesis (PhDAgric)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Heat waves, occurring towards or during the harvesting window of Japanese plum cultivars, hamper production of premium quality plums in the Western Cape Province of South Africa by causing sunburn in the presence of high irradiance. In addition, high respiration rates initiated by high temperatures are thought to deplete internal fruit oxygen and trigger anaerobic respiration with subsequent accumulation of ethanol, resulting in internal damage. Damage that is not apparent at harvest can manifest during cold storage. No information is available on temperature thresholds for thermal damage of the peel and flesh of Japanese plums. In apples, maintaining high stem water potential (SWP) and applying shade netting were reported to alleviate sunburn. Summer pruning is a common practise in Japanese plums, but the timing may affect fruit quality and sunburn incidence. Research in this regard as it pertains to plums is lacking. The main objective of this study was to fill this information gap. ‘African Delight’ plums from exposed, upper canopy positions were larger, advanced in maturity but more susceptible to sunburn. Delaying summer pruning predisposed fruit to sunburn and did not enhance fruit quality. Early summer pruning decreased sunburn, increased fruit size, red colour and total soluble solids (TSS). Abstaining from pruning reduced sunburn but decreased overall fruit quality. Fruit that developed sunburn received >50% photosynthetic photon flux (PPF) of full sun on average while average fruit surface temperature (FST) exceeded 35 °C. Shade net during the hottest part of the season attenuated PPF, and subsequently, decreased FST and sunburn. Deficit irrigation late in the season elevated canopy temperature, FST and sunburn in ‘African Delight’ and ‘Laetitia’ while SWP, flesh firmness, TSS and gas exchange decreased. The increased heat load could be attributed to diminished evaporative cooling as a result of reduced transpiration. Excessive irrigation did not lower FST and sunburn compared to the control. There were no notable heat waves during the 2012/13 season so in subsequent seasons we assessed fruit respiration rate under simulated heat wave conditions at different fruit maturities in the laboratory. Increases in ethanol at harvest and internal damage after cold storage were higher in more mature fruit treated at 30 °C and 40 °C but tended to decline at 45 °C in ‘Laetitia’ due to curing. In ‘Fortune’, more mature fruit were consistently more susceptible to internal heat damage. No symptoms of internal heat damage were observed in ‘African Delight’ possibly due to this cultivar’s high peel permeability that prevented accumulation of threshold ethanol levels. In conclusion, plum producers should adopt early summer pruning practices and incorporate shade nets to reduce sunburn. However, the potential of shade nets and potential negative effects on reproductive development requires further evaluation over the entire growing season. Low SWP increases FST and sunburn possibly due to canopy heating and loss of convectional cooling, explaining why excessive irrigation did not reduce sunburn. High temperature treatments can potentially be used for curing against cold storage enhanced heat damaged if used with methods that circumvent external peel damage.

AFRIKAANSE OPSOMMING: Hittegolwe tesame met hoë vlakke van irradiasie kort voor of gedurende die plukvenster van Japanese pruimkultivars veroorsaak sonbrand en belemmer daardeur produksie van premium kwaliteit pruime in die Wes-Kaap Provinsie van Suid-Afrika. Verder word geglo dat hoë temperature interne suurstof in die vrug kan uitput deur respirasie te versnel. Lae interne suurstofvlakke kan anaerobiese respirasie aktiveer met gevolglike akkumulasie van etanol en gepaardgaande interne skade. Skade mag moontlik eers na koue opberging manifesteer. Die drempeltemperature vir skade aan die skil en vleis van Japanese pruime is onbekend. In appels is gerapporteer dat deurlopende hoë stamwaterpotensiale (SWP) en aanbring van skadunette sonbrand kan verminder. Somersnoei is ʼn algemene praktyk in Japanese pruime, maar die tydsberekening daarvan kan vrugkwaliteit en die voorkoms van sonbrand affekteer. Navorsing oor bogenoemde aspekte makeer vir pruime en die hoofdoelwit van hierdie studie was daarom om die kennisgebrek aan te spreek. ‘African Delight’ pruime van blootgestelde posisies aan die bokant van die blaredak was groter en meer ryp, maar meer onderhewig aan sonbrand. Die uitstel van somersnoei het vrugte meer vatbaar gemaak vir sonbrand sonder om vrugkwaliteit te verbeter. Vroeë somersnoei het sonbrand verminder asook vruggrootte, rooi kleur en totale oplosbare vaste stowwe (TOVS) verhoog. Geen somersnoei het sonbrand verminder, maar het algemene vrugkwaliteit verlaag. Vrugte wat sonbrand ontwikkel het, was blootgestel aan gemiddeld >50% fotosintetiese fotonvloei (PPF) van vol sonlig terwyl hul gemiddelde vrugoppervlaktemperatuur (FST) 35 ºC oorskry het. Die aanbring van skadunet gedurende die warmste deel van die seisoen het PPF verminder en gevolglik FST en sonbrand verminder. Tekort besproeiing laat in die seisoen het blaredak temperature, FST en sonbrand in ‘African Delight’ en ‘Laetitia’ verhoog terwyl SWP, vleisfermheid en gaswisseling verlaag is. Die verhoogde hittelading kon toegeskryf word aan verminderde evaporatiewe verkoeling as gevolg van die verlaagde transpirasie. Oormatige besproeiing het nie FST verlaag of sonbrand verminder nie. Daar was geen noemenswaardige hittegolwe gedurende die 2012/13 seisoen nie en daarom is vrugrespirasie by verskillende vrugryphede in daaropvolgende seisoene onder gesimuleerde hittegolf kondisies in die laboratorium ondersoek. Toenames in etanol by oestyd en interne skade na koue opberging was hoër in meer volwasse vrugte wat blootgestel was aan 30 ºC en 40 ºC maar het afgeneem by 45 ºC in ‘Laetitia’ vanweë kruisbeskerming teen koue deur die hitteblootstelling (curing). Meer volwasse ‘Fortune’ vrugte was deurlopend meer vatbaar vir interne hitteskade. Geen interne hitteskade simptome is in ‘African Delight’ waargeneem nie, moontlik vanweë die hoë permeabiliteit van hierdie kultivar se skil wat akkumulasie van drempelvlakke etanol voorkom. Ten slotte kan aanbeveel word dat pruimprodusente vroeë somersnoei toepas en van skadunette gebruik maak om sonbrand te verminder. Die potensiaal van skadunette en moontlike negatiewe effekte op reproduktiewe ontwikkeling benodig egter verdere evaluasie oor die hele groeiseisoen. Lae SWP verhoog FST en sonbrand moontlik deur opbou van hitte in die blaredak en verminderde konveksie verkoeling. Dit verklaar hoekom oormatige besproeiing nie sonbrand verminder het nie. Hittebehandeling kan moontlik gebruik word om vrugte te beskerm teen interne hitteskade wat tydens koue opberging te voorskyn kom indien eksterne skilskade voorkom kan word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107199
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