Bunch structure, rudimentary seed size and return fertility of Vitis vinifera L. ‘Sunred Seedless’ as affected by GA3 and GA4+7 thinning treatments

Claassen, Talana (2020-12)

Thesis (MScAgric)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: Market requirements for export grapes are consumer-driven and based on characteristics such as bunch size, bunch colour uniformity, berry size and distribution, seedlessness, flavour profile, texture and eating quality. In order to meet these requirements, the use of plant growth regulators (PGRs) has become an essential tool in producing grapes of high export quality, while contributing to reducing labour costs required for manual thinning or girdling to increase berry size. Increased costs associated with the production of table grapes, along with high expectations to meet increasing market demands, require attention to minimise input costs with the effective use of PGRs. The response of cultivars poses a challenge, as cultivars react differently towards a PGR application. Apart from cultivar response, the application timing and concentration used for the specific PGR also contribute towards the efficacy of the treatment applied. Limited research publications are available on the effect of GA4+7 used for thinning on table grapes, as well as the effect of GA3 and GA4+7 applications on rudimentary seed size and return fertility of specifically Sunred Seedless, but table grapes in general as well. The study aimed to determine whether an alternative gibberellic acid structure, GA4+7, could be used as a chemical thinning agent for cultivars that respond poorly to GA3 in order to improve bunch quality without negatively affecting the return fertility. The study was performed during the 2015/2016 and 2016/2017 growing seasons on 15-year-old Vitis vinifera L. cv. ‘Sunred Seedless’ vines, grafted onto Ramsey (Vitis champinii). The experimental site is situated in a commercial vineyard located on the premises of the ARC Infruitec-Nietvoorbij experimental farm at De Doorns, in the Hex River Valley, South Africa. A standard GA3 concentration of 5 parts per million (ppm) was evaluated against different concentrations of GA4+7, ranging from 7.5 ppm to 120 ppm, adjusted over the two seasons. These treatments were applied at different phenological stages in order to determine the most effective timing for a thinning application on Sunred Seedless. Eight treatments and an untreated control were evaluated during the 2015/2016 season. The treatments consisted of four early thinning applications applied 31 October 2015 and four late thinning applications applied 4 November 2015. Both the early and late treatments were applied at 5 ppm GA3, 7.5 ppm GA4+7,15 ppm GA4+7 and 30 ppm GA4+7. The two application dates refer to a difference in the predominant phenological stage of the vineyard, which a producer would have used to determine the timing of a thinning application. The early application timing represents a predominant phenological stage of 10% berry set (10%BS) and the late application represents berry set (BS). The treatment layout for the 2016/2017 season was adjusted to accommodate increased GA4+7 concentrations, as well as two sizing treatments. The nine treatments applied in this particular season consisted of an untreated control, six thinning (T) treatments (5 ppm GA3; 7.5 ppm GA4+7; 15 ppm GA4+7; 30 ppm GA4+7; 60 ppm GA4+7; 120 ppm GA4+7), a thinning and sizing (T+S) treatment (60 ppm GA4+7 + 60 ppm GA4+7) and a sizing (S) only treatment (60 ppm GA4+7). Each treatment had four replicates and each replicate consisted of four vines, referred to as an experimental unit. Within each experimental unit the two centre vines were used as the experimental data unit. Field sampling was performed in the experimental data unit. Additionally, within each experimental data unit, bunches were categorised and marked at four phenological stages to determine the optimal phenological stage for application. The stages for the 2015/2016 season included 80-100% flowering (80-100%F), 10% berry set (10%BS), berry set (BS) and berry set plus four days (BS+4D). The stages for the 2016/2017 season included 50% flowering (50%F), 80- 100%F, 10%BS and BS. Five bunches per experimental data unit were marked according to the phenological stages identified for each season. These marked bunches were used for bunch and berry evaluations at harvest and were therefore left in their natural state, with no bunch preparations applied or any berry sampling performed on them. Bunch structure assessments were performed in line with a protocol developed and applied by the Viticulture Division of ARC Infruitec-Nietvoorbij. Applications during flowering resulted in a better thinning effect of Sunred Seedless, based on the bunch and berry mass measurements. Bunch and berry mass measurements at harvest didn’t result in a specific trend concerning a specific GA concentration and application timing combination that could be recommended for effective thinning of Sunred Seedless. Based on the subjective visual assessment of bunch compactness, applying a GA thinning treatment at 50% flowering is too early for Sunred Seedless, as it resulted in straggly bunches. However, the longer a GA thinning treatment was delayed from flowering to berry set, the less effective the thinning results were, resulting in more compact bunches if applied around berry set. These findings correspond with the results obtained for the quantitative bunch compactness measurements. The mean total and normal berries per cm of lateral length were reduced significantly by GA treatments applied during flowering. The 5 ppm GA3 treatment applied at 80-100%F resulted in the most effective thinning, with a significantly reduced number of total berries per cm of lateral compared to the untreated control. There was a significant increase in the mean percentage of shot berries at the 50%F and 80-100%F stages compared to the 10%BS and BS stages, for GA treatments applied during the 2016/2017 season. These results indicate that Sunred Seedless has a higher sensitivity for the formation of shot berries when GA is applied during flowering. An increase in shot berry occurrence was observed with the use of higher GA4+7 concentrations and double applications at the 50%F stage. The sensitivity of Sunred Seedless towards GA applications applied during early flowering, along with poor response for GA applications applied after flowering observed in this study, confirms why GA thinning treatments for this particular cultivar do not give economically acceptable results. Reoccurring trends regarding the bunch phenological stage at the time of application were observed in this study, rather than trends regarding a specific GA treatment and treatment rates. These results confirm that the timing of a GA applications play a fundamental role in the treatment outcome for a specific cultivar. A trend was observed that applying GA treatments during flowering resulted in decreased average rudimentary seed mass per berry as well as an improved rudimentary seed size distribution with an increased percentage of small rudimentary seeds compared to GA applied during the early stages of berry development. No consistent trend regarding the effect of different GA3 or GA4+7 application timing and rates on rudimentary seed size could be concluded over two seasons. Commercially acceptable bud break percentages of ≥ 80% were obtained for all treatments, determined through forced budding in June 2016 and 2017 as well as through actual fertility assessments in October 2016. A reduction in the mean number of bunches per sprouted bud was reported from June 2016 to June 2017 for the potential fertility assessed through forced budding. Potential fertility assessed through bud dissections did not follow the same trend from June 2016 to June 2017 as mentioned above for forced budding. The use of GA3 reduced the actual fertility of Sunred Seedless in this study, after one season of GA treatment application compared to the untreated control. Similar results were not observed for GA4+7 treatments. There was a poor correlation between the potential fertility determined through bud dissection and forced budding were reported, compared to the actual fertility determined in the vineyard. Potential fertility assessments are therefore not advised for crop estimations, but rather to be used for verifying the pruning system used for a specific cultivar.

AFRIKAANSE OPSOMMING: Die markvereistes vir uitvoerdruiwe word gedryf deur verbruikervoorkere. Dit word gebasseer op trosgrootte, eweredige troskleur, korrelgrootte en -verspreiding, pitloosheid, die geurprofiel, tekstuur en eetgehalte. Om aan hierdie vereistes te voldoen, het die gebruik van plantgroeireguleerders (PGRs) ‘n noodsaaklikke hulpmiddel geword om druiwe van ‘n hoë uitvoergehalte te produseer. Dit dra by tot verminderde arbeidskostes deur handuitdunning en korrelgrootte manipulasies grootliks te vervang. Meer aandag moet egter gegee word aan die effektiewe gebruik van PGRs, vanweë stygende produksiekostes, asook met die hoë vereistes wat deur verskillende markte gestel raak. Die PGR-konsentrasie wat toegedien word, gekombineer met die tydsberekening van daardie toediening, dra by tot die effektiwiteit van behandelings. Kultivar-spesifieke reaksies teenoor PGRtoedienings blyk egter steeds uitdagend te wees, aangesien kultivars verskillend reageer teenoor 'n PGR-behandeling. Beperkte literatuur is beskikbaar oor die effek van GA4+7 op uitdunning van tafeldruiwe asook die effek van GA3 en GA4+7 behandelings op pitresgrootte en opvolgvrugbaarheid van spesifiek Sunred Seedless, maar ook vir tafeldruiwe as geheel. Die doel van hierdie studie was om te bepaal of 'n alternatiewe struktuur van gibberelliensuur, GA4+7, gebruik kan word as 'n chemiese uitdunmiddel vir kultivars wat swak reageer op GA3, om sodoende trosgehalte te verbeter sonder om opvolgvrugbaarheid negatief te beïnvloed. Resultate van hierdie studie dra by tot beskikbare wetenskaplike gepubliseerde resultate wat handel oor die uitduneffek van GA4+7, sowel as die effek van GA3 en GA4 +7 op trosstruktuur, pitresgrootte en opvolgvrugbaarheid van tafeldruiwe. Die studie is uitgevoer gedurende 2015/2016 en 2016/2017 op 15-jarige Vitis vinifera L. cv. Sunred Seedless wingerd, wat op Ramsey (Vitis champinii) geënt is. Die proefperseel is geleë in 'n kommersiële wingerd op die perseel van die LNR Infruitec-Nietvoorbij proefplaas op De Doorns, in die Hexriviervallei, Suid-Afrika. ‘n Standaard GA3 konsentrasie van 5 dele per miljoen (dpm) is geëvalueer teenoor verskillende GA4+7 konsentrasies, wat gewissel het van 7.5 dpm tot 120 dpm oor twee seisoene. Die behandelings is op verskillende fenologiese stadiums toegedien, om die mees effektiewe tydsberekening vir 'n uitdunbehandeling op Sunred Seedless te bepaal. Agt behandelings en ‘n onbehandelde kontrole is tydens die 2015/2016 seisoen geëvalueer. Die behandelings het bestaan uit vier vroeë uitduntoedienings op 31 Oktober 2015 en vier laat uitduntoedienings op 4 November 2015. Beide die vroeë en die latere toedienings is teen 5 dpm GA3, 7.5 dpm GA4+7, 15 dpm GA4+7 en 30 dpm GA4+7 toegedien. Die twee toedieningsdatums verteenwoordig verskillende fenologiese stadiums van die wingerd, wat deur ‘n produsent gebruik sou word om die tydsberekening van 'n uitdunbehandeling te bepaal. Die vroeë toedieningstyd verteenwoordig 'n oorheersende fenologiese stadium van 10% set en die latere toediening verteenwoordig set. Die behandelings vir die 2016/2017 seisoen is aangepas om verhoogde GA4+7 konsentrasies, asook twee korrelvergrotingbehandlings in te sluit. Agt behandelings en ‘n onbehandelde kontrole is tydens die 2016/2017 seisoen geëvalueer. Die behandelings het bestaan uit ses uitdunbehandelings (5 dpm GA3; 7.5 dpm GA4+7; 15 dpm GA4+7; 30 dpm GA4+7; 60 dpm GA4+7; 120 dpm GA4+7), ‘n uitdun- en korrelvergrotingbehandlings (60 dpm GA4+7 + 60 dpm GA4+7) en ‘n korrelvergrotingbehandlings (60 dpm GA4+7). Elke behandeling is vier keer herhaal en elke herhaling bestaan uit vier stokke. Hierna word gesamentlik verwys as 'n eksperimentele eenheid. Die middelste twee stokke van elke eksperimentele eenheid is as die datastokke gebruik. Die optimale fenologiese stadium vir toediening is bepaal deur die blomtrosse binne elke data eksperimentele eenheid in vier fenologiese stadiums te kategoriseer. Die 2015/2016-seisoen se stadiums het bestaan uit: 80-100% blom, 10% set, set en set plus vier dae. Daarteenoor was die 2016/2017-seisoen se stadiums: 50% blom 50% blom, 80-100% blom, 10 set en set. Vyf trosse is per data eksperimentele eenheid gemerk volgens die bogenoemde fenologiese stadiums wat vir elke seisoen geïdentifiseer is. Hierdie gemerkte trosse is gebruik vir tros- en korrelevaluasies tydens oes. Trosvoorbereidingsaksies of korrelversameling is dus nie op hierdie trosse uitgevoer nie. Trosstruktuurevaluerings is gedoen volgens 'n protokol van die Wingerdkunde-afdeling van LNR Infruitec-Nietvoorbij. GA toedienings tydens blom het gelei tot 'n beter uitdun effek van Sunred Seedless, gebaseer op die evaluering van tros-en korrelmassa. Oes-evaluasies van tros- en korrelmassa het geen tendens gewys m.b.t. 'n spesifieke GA-konsentrasie in verhouding tot tyd van toediening van uitdunbehandelings nie. Resultate gebaseer op die subjektiewe visuele assessering van troskompaktheid, dui aan dat 'n GA uitdunningsbehandeling op 50% blom te vroeg is vir Sunred Seedless, aangesien dit yl trosse tot gevolg het. Hoe langer 'n GA toediening vanaf blom tot set vertraag is, hoe minder effektief is Sunred Seedless uitgedun. Dit kan toegeskryf word aan die toediening wat tot meer kompakte trosse lei indien dit rondom set toegedien word. Hierdie bevindings stem ooreen met resultate wat verkry is met die kwalitatiewe evaluasies van troskompaktheid. GA behandelings tydens blom het die gemiddelde totale- en normale korrels per sentimeter laterale lengte betekenisvol verminder. Die 5 dpm GA3 behandeling, wat toegedien is op 80-100% blom, was die mees effektiefste behandeling. Dit het gelei tot die effektiefste uitdunning en ‘n betekenisvolle vermindering in die totale korrels per sentimeter laterale lengte, teenoor die onbehandelde kontrole. 'n Betekenisvolle toename in die gemiddelde persentasie bokhaelkorrels is verkry met die 50% en 80-100% blomstadiums in vergelyking met die 10% set en set stadiums, vir GA behandelings toegedien gedurende die 2016/2017 seisoen. Sunred Seedless het dus ‘n verhoogde sensitiwiteit vir die vorming van bokhaelkorrels wanneer GA tydens blom toegedien word. 'n Toename in die voorkoms van bokhaelkorrels kan ook verwag word met die gebruik van hoër konsentrasies GA4 +7, asook met meer as een toediening tydens 50% blom. Sunred Seedless se sensitiwiteit teenoor GA toedienings tydens vroeë blom in hierdie studie, tesame met die swak reaksie teenoor hierdie toedienings wat na blom toegedien word, bevestig waarom GA uitdunbehandelings nie ekonomies aanvaarbare resultate vir hierdie kultivar lewer nie. Herhalende tendense met betrekking tot die fenologiese stadium van die tros tydens toediening is waargeneem in hierdie studie, eerder as tendense met betrekking tot 'n spesifieke GA behandeling en konsentrasie toegedien. Hierdie bevindings bevestig dat tydsberekening van 'n GA toediening 'n fundamentele rol speel in die resultate verkry met GA toedienings vir 'n spesifieke kultivar. GA behandelings wat tydens blom toegedien is, is vergelyk met toedienings tydens vroeë korrelontwikkeling. Eersgenoemde het tot ‘n afname in die gemiddelde pitresmassa per korrel gelei. Dit het ook ‘n verbeterde pitresgrootte verspreiding, met 'n verhoogde persentasie klein pitreste tot gevolg gehad. Geen konstante tendens is gevind t.o.v. van verskillende GA3 of GA4+7 toedieningstye en konsentrasies op pitresgrootte oor die twee seisoene nie. Kommersiëel aanvaarbare botpersentasies (≥ 80%) is verkry met uitbotproewe wat in Junie 2016 en 2017 gedoen is, asook met evaluerings wat in die wingerd uitgevoer is in Oktober 2016. 'n Afname in die gemiddelde aantal trosse per oogposisie is van Junie 2016 tot Junie 2017 verkry vir die potensiële vrugbaarheid bepaal deur uitbotproewe. Potensiële vrugbaarheid bepaal deur oogontledings het nie dieselfde tendens gevolg van Junie 2016 tot Junie 2017, soos gevind met die uitbotproewe nie. Die gebruik van GA3 het Sunred Seedless se werklike vrugbaarheid laat afneem na afloop van ‘n enkele seisoen se GA behandeling, teenoor die onbehandelde kontrole. Dieselfde resultate is nie vir GA4+7 behandelings verkry nie. Die potensiële vrugbaarheid wat deur uitbotproewe en oogontledings bepaal word het swak gekorreleer met die werklike vrugbaarheid wat in die wingerd bepaal is. Potensiële vrugbaarheidsassesserings word dus nie vir oesskattings aanbeveel nie, maar eerder om snoeistelsels wat gebruik word vir ‘n spesifike kultivar te verifieer. Die potensiële vrugbaarheid wat deur uitbotproewe en oogontledings bepaal word, het swak gekorreleer met die werklike vrugbaarheid wat in die wingerd bepaal is. Potensiële vrugbaarheidsevaluerings word dus nie vir oesskattings aanbeveel nie, maar eerder om snoeistelsels wat gebruik word vir ‘n spesifike kultivar te verifieer.

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