Influence of rind water content on mandarin citrus fruit quality

Joubert, Jeanine (2016-03)

Thesis (MSc)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: ‘Nules Clementine’ and ‘Nadorcott’ mandarin are commercially important Citrus cultivars in South Africa. Both cultivars are prone to develop rind breakdown and pitting which are considered non-chilling related postharvest physiological rind disorders. The progressive and erratic nature of these rind disorders result in high financial losses. The incidence of a rind disorder is thought to be associated firstly with an increased susceptibility, as influenced by pre-harvest aspects; and secondly with a trigger in the postharvest environment. A study was conducted over two seasons a study to determine the effect of late nitrogen (stage II and after summer flush) application, pre-harvest water stress and postharvest handling were conducted. The rind quality of ‘Nules Clementine’ and ‘Nadorcott’ mandarin fruit harvested from Citrusdal and Riebeeck Kasteel was evaluated. Soil applications of nitrogen at 20 kg·ha-1 and 40 kg·ha-1 were done on 21 January and 26 March 2014/2015, respectively. This was in addition to the standard 300 kg·ha-1 nitrogen provided by the producer. During 2015 a 1% urea foliar application was sprayed on 26 March. During the post-harvest period all fruit were dehydrated at 25 °C and 60 to 80% RH (0.7 to 1.1 kPa vapour pressure deficit) for two days, followed by rehydration at 100% RH for one day. Subsequently fruit were stored at either -0.6 °C or 4 °C for a 30-day period. There were no significant differences in fruit colour or size between the different nitrogen treatments. No increase in rind disorders and no negative impacts on internal fruit quality were noted. To determine the impact of pre-harvest water stress, the soil below the tree was covered with plastic sheets three weeks prior to harvest to exclude rainfall or irrigation. The effect of postharvest stress was established by dehydrating and rehydrating fruit at 0.7-1.1 kPa vapour pressure deficit for different periods after harvest. Wax was applied on day 5 and thereafter fruit was stored at 4 °C for 30 days. The results indicated that pre-harvest water stress did not have a detrimental effect on fruit susceptibility to disorders. By early wax application, however, a decrease in moisture loss was recorded, coinciding with lower incidences of rind disorders. The final part of the study was aimed at determining if postharvest handling lending to water loss, as induced by high vapour pressure, could increase disorders. From the results of the trials it can be concluded that exposing fruit to dehydration increased rind disorder susceptibility. Dehydration prior to wax application on day 5 also increased pitting and rind breakdown, whereas an early wax application, 2 to 3 days after harvest, reduced incidence. This study serves as a step to resolve the impact of factors predisposing the citrus fruit rind to progressive postharvest disorders.

AFRIKAANSE OPSOMMING: 'Nules Clementine’ en ‘Nadorcott' mandaryne is kommersieël belangrike Citrus kultivars in Suid-Afrika. Beide kultivars is geneig tot skilafbraak en gepokte skil wat as na-oes fisiologiese skildefekte beskou word en is onafhanklik van koueskade tydens opberging. Die progressiewe- en wisselvallige aard van hierdie skildefekte het groot finansiële verliese tot gevolg. Vermoedelik hou die voorkoms van 'n skildefek eerstens verband met ‘n verhoogde vatbaarheid soos beïnvloed word deur voor-oes aspekte; en tweedens met 'n sneller vanuit die na-oes omgewing. Die invloed van ‘n laat stikstof toediening (fase II en na die somer groei), voor-oes waterstres en na-oes hantering op skildefekte is oor twee seisoene bepaal. 'Nules Clementine’ en ‘Nadorcott' mandaryn vrugskilgehalte, geoes vanaf Citrusdal en Riebeeck Kasteel, is geëvalueer. Toediening van korrelkunsmis (stikstof) teen 20 kg·ha-1 en 40 kg·ha-1 is gedoen op 21 Januarie en 26 Maart 2014/2015, respektiewelik. Dié toedienings was bykomend tot die produsent se standaardbehandeling van 300 kg·ha-1 stikstof. ʼn Blaartoediening van 1% ureum is addisioneel op 26 Maart 2015 aangewend. Alle vrugte was na-oes gedehidreer teen 25 °C en 60 tot 80% RH (0.7 tot 1.1 kPa dampdrukverskil) vir twee dae, gevolg deur ‘n een dag rehidrasie periode van 100% RH. Vrugte is vervolgens opgeberg by óf -0.6 °C óf 4 °C vir 30 dae. Geen beduidende verskil in kleur en grootte van die vrugte tussen die verskillende stikstof behandelings of toename in skildefekte of negatiewe effek op interne vrugkwaliteit was waargeneem nie. Die impak van voor-oes waterstres op na-oes skilgehalte is bepaal deur grond onder die eksperimentele bome drie weke voor oes met plastiekseile te bedek om sodoende die invloed van reënval en besproeiing uit te skakel. Die effek van na-oes stres was geëvalueer in vrugte wat gedehidreer en rehidreer was teen 0.7 tot 1.1 kPa dampdrukverskil. Die na-oes waks behandeling was op dag 5 toegedien, waarna vrugte opgeberg was by 4 °C vir 30 dae. Resultate dui daarop dat voor-oes waterstres geen negatiewe impak op die vrugte se vatbaarheid vir skildefekte gehad het nie. ‘n Afname in vogverlies is wel gemeet met die wakstoediening wat ooreenstem met 'n laer voorkoms van skildefekte. Die finale gedeelte van die studie was daarop gemik om te bepaal of na-oes waterverlies, soos geïnduseer deur ‘n hoë dampdruk verskil, die voorkoms van skildefekte kan induseer. Resultate dui aan dat die blootstelling van vrugte aan dehidrasie wel die vatbaarheid vir skildefekte verhoog het. Dehidrasie voor ‘n wakstoediening op dag 5 het ook verhoogde skilafbraak en gepokte skil tot gevolg gehad, terwyl 'n vroeë waks, 2 tot 3 dae na oes, die voorkoms verlaag het. Hierdie studie dien as ‘n skakel om die impak van faktore wat skil-vatbaarheid ten opsigte van fisiologiese defekte verhoog verder te ontrafel en dus sodoende aan te spreek.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98607
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