Identifying and using tuber characteristics to predict potato keeping quality

Thesis (MScAgric)--Stellenbosch University, 2018.ENGLISH ABSTRACT: Potato production in South Africa occurs in all nine provinces of South Africa and it is further divided into 16 production areas. The trial was undertaken in the Sandveld and Ceres production areas, which are characterised by dry and warm weather in summer, which presents obvious challenges to produce high-volume quality potatoes. A macronutrient that is very important for cell wall strength of potato tubers is calcium (Ca). In a previous study, no correlation between keeping quality and tuber Ca content was found (Bester, 1993). The reaction of Ca with organic acids might be part of the reason no correlation between keeping quality and tuber Ca could be found (Venter, 1989). Calcium oxalate forms when Ca reacts with organic acids and the cells it forms in are referred to as idioblasts. Calcium and magnesium (Mg) are strongly competitive and it seems that the plasma membrane binding site at the root has higher affinity for Ca2+ than for the highly hydrated Mg2+ (Marschner, 1995). Calcium has the ability to form a insoluble complex with pectin due to the free carboxyl groups on the pectin chain (Walter, 2012). Pectin methylesterase (PME) is an enzyme responsible for removing the methoxyl groups and enabling divalent cations Ca2+ and Mg2+ to react with pectin, creating rigid structures with an increase in firmness (Tajner-Czopek, 2003). Due to the high immobility of Ca in plants it is sometimes hard to increase the Ca content in tubers. On the other hand, Mg is mobile in the plant and increasing tuber concentrations is not as difficult as Ca, although Mg deficiencies caused by cation competition is a global phenomenon (Marschner, 2012). A study done in South Africa on several cultivars showed that of all the cultivars tested, Sifra had the lowest Mg content and that Fianna had the highest (Van Niekerk, 2015). Potato is highly susceptible to Mg deficiency, which has been shown to particularly affect the carbon assimilation and the transformation of energy (Hochmuth, 2007; Barker and Pilbeam, 2015). Considerable fewer studies have been done on Mg to the extent that it is often dubbed the “orphan nutrient” compared to Ca (Rosanoff, 2010). When producers export or sell their seed- or ware potatoes it is important for the buyer to know in advance the quality of the product. If the keeping quality can be predicted it will help both the producer and buyer to know the quality of the product and compensation can then be arranged more accurately, since good keeping quality potatoes should have higher value than poor keeping quality potatoes. To predict the keeping quality of seed- and ware potatoes, various quality characteristics must be identified and used. The main objective for this study was to develop a measuring tool that can be used to routinely predict the keeping quality of a tuber. Tubers were sampled throughout 2016 season and partially through the 2017 season. Inspection for IBS and hollow heart were assessed to see if any of the quality characteristics could correlate with these disorders. Tubers were stored at 25℃ and 5℃ respectively, while mass loss was determined as percentage (%) loss throughout the storage period. Different quality characteristics were measured to use in a prediction model.AFRIKAANSE OPSOMMING: Aartappelproduksie in Suid-Afrika vind plaas in al nege provinsies van Suid-Afrika en dit is verder opgedeel in 16 produksie areas. Die proef is in die Sandveld en Ceres produksie-areas uitgevoer, wat gekarakteriseer word deur warm weer in die somer wat uitdagings vir die produksie van hoë volume kwaliteit aartappels daarstel. Die makronutriënt kalsium (Ca) is baie belangrik vir selwandsterkte in aartappelknolle. In ‘n vorige studie is daar geen korrelasie gevind tussen houvermoë en knol Ca inhoud nie (Bester, 1993). Die reaksie tussen Ca en organiese sure mag deel wees waarom geen korrelasie tussen houvermoë en Ca gevind kon word nie (Venter, 1989). Kalsiumoksalaat vorm wanneer Ca reageer met organiese sure en die selle wat vorm, word idioblaste genoem. Kalsium en magnesium (Mg) is sterk kompeterend en dit lyk asof die plasmamembraanbindingsplekke by die wortel hoër affiniteit vir Ca2+ het as vir die hoogs gehidreerde Mg2+(Marschner, 1995). Kalsium het die vermoë om ʼn onoplosbare kompleks met pektien te vorm weens die vrye karboksielgroepe op die pektienketting (Walter, 2012). Pektienmetielesterase (PME) is ʼn ensiem verantwoordelik vir die verwydering van die metoksielgroepe groepe en dit laat toe dat divalente katione Ca2+ en Mg2+ reageer met pektien om sterk verbindings te vorm met hoër fermheid tot gevolg (Tajner-Czopek, 2003). As gevolg van die hoë immobiliteit van Ca in plante, is dit soms moeilik om die Ca-inhoud in die knolle te verhoog. Hierteenoor is Mg, ʼn mobiele element in die plant en om die konsentrasie Mg in die knol te verhoog is makliker as om Ca te verhoog, alhoewel Mg-tekorte as gevolg van katioonkompetisie ʼn globale verskynsel is (Marschner, 2012). Verskeie kultivars is in ʼn Suid-Afrikaanse studie gerbruik en van die getoets, het Sifra die laagste knol-Mg-inhoud en Fiana die hoogste gehad (Van Niekerk, 2015). Aartappels is hoogs vatbaar vir Mg tekorte, dit is al spesifiek getoon dat dit koolstofassimilasie en transfermasie van energie negatief affekteer (Hochmuth, 2007; Barker and Pilbeam, 2015). Aansienlik minder studies is al gedoen op Mg tot so ‘n mate dat dit gereeld beskryf word as die weeskind van nutriënte tenopsigte van Ca (Rosanoff, 2010). Wanneer produsente hul saad- of tafelaartappels uitvoer, is dit belangrik vir die koper om vooraf te weet wat die kwaliteit van die produk is. Indien die houvermoë voorspel kan word, sal dit beide die verkoper en koper bevoordeel want dan kan vergoeding vir die produk meer akkuraat wees, aangesien aartappels met goeie houvermoë beter pryse behoort te kry as die met swakker houvermoë. Om houvermoë te voorspel van saad- en tafelaartappels, moet verskeie kwaliteitseienskappe geïdentifiseer en gebruik word. Die hoofdoelwit van hierdie studie is om ʼn metingsinstrument te ontwikkel wat gebruik kan word om die kwaliteit van ʼn knol herhaaldelik te voorspel. Knolmonsters is reg deur die 2016-seisoen en deels deur die 2017-seisoen geneem. Inspeksie vir interne bruinvlek en holhart is gedoen om te kyk of enige van die kwaliteitseienskappe kon korreleer met die afwykings. Knolle is geberg by 25℃ en 5℃ en massaverlies was bepaal as persentasie (%) verlies deur die hele bergingsperiode. Verskeie kwaliteitseienskappe was gemeet om in die voorspellingsmodel te gebruik