Browsing by Author "Claassen, Talana"

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    Bunch structure, rudimentary seed size and return fertility of Vitis vinifera L. ‘Sunred Seedless’ as affected by GA3 and GA4+7 thinning treatments
    (Stellenbosch : Stellenbosch University, 2020-12) Claassen, Talana; Eunice, Avenant; Avenant, J. H.; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology.
    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.