Postharvest berry split and abscission in 'Thompson Seedless' and 'Waltham Cross' table grapes

[摘要] ENGLISH ABSTRACT:Postharvest berry split and abscission are prevailing physiological disorders that negativelyimpact on the quality of table grapes exported from South Africa. Inferior grape quality due tothese disorders results in a considerable decline in consumer confidence in the brandedproduct, which leads to a drop in demand, and consequently, lower prices. Since informationconcerning postharvest factors influencing postharvest berry split and abscission is limited, thesearch for reliable methods to adequately control these problems remains elusive. In anattempt to obtain the required information, the influence of harvest temperature, harvestmaturity, perforated liners, field heat removal prior to packing, delay periods before and afterpacking, storage duration and the elevation of storage temperature on the development of berrysplit and abscission in 'Thompson Seedless' (Vitis vinifera Linnaeus) table grapes wasinvestigated. Changes in abscission related factors during berry development, and the influenceof pre-and postharvest ethylene inhibitors on the development of berry abscission in 'WalthamCross' table grapes, was also studied.Berry split was aggravated by packing 'Thompson Seedless' grapes at high pulp temperaturesof approximately 30°C, especially if the grapes were packed in non-perforated bags. Theincidence of berry split could be reduced by between 80 and 90% by packing grapes inperforated instead of non-perforated liners. Perforated bags also reduced levels of S02damage. However, due to significantly more moisture loss from grapes in perforated bags,compared to non-perforated bags, the risk of higher fruit and stem desiccation and softer berriesexisted. Optimum size and density of perforations needs to be determined to reduce berry splitwithout excessive loss of moisture from the grapes, and S02gas from the air space surroundingthe product. The influence of harvest temperature and liner type on berry abscission was notconclusive. Advanced maturity increased grape resistance to berry split. However, grapesharvested too mature were prone to stem desiccation and the development of Botrytis decay.The occurrence of berry abscission also appeared to increase with advanced harvest maturity.Consequently, to ensure optimal post-storage quality, 'Thompson Seedless' grapes should beharvested as soon as horticultural maturity has been reached, which appears to be atapproximately 18°Brix.Field heat removal for 1.5 hours at 19°C prior to packing had no beneficial or adverse effect onberry split and abscission. Delay periods prior to packing aggravated berry abscission, but didnot influence berry split significantly. Grapes delayed for 12 hours showed a significant increasein berry abscission and Botrytis decay, compared to grapes delayed for only 3 or 8 hours.Considering that the absence of fungal decay is the most important quality prerequisite in tablegrapes, it is of vital importance to pack grapes with as short a delay period as possible. Grapespacked in non-perforated liners and delayed for different durations after packing, before theonset of forced-air cooling (FAC), showed significant differences regarding the incidence ofberry split. Grapes delayed for 18 hours had significantly higher levels of berry split directly afterthe delay period, compared to grapes delayed for 6 or 12 hours. No significant difference inberry abscission occurred between grapes delayed for different periods. To minimise theamount of berry split, FAC should be applied as rapidly as possible after the packing of grapesin non-perforated liners.Two storage related factors significantly influenced the incidence of berry split in 'ThompsonSeedless' grapes during cold storage significantly, viz. the duration of storage at -O.soC,and theincrease in temperature after low temperature storage. Berry split increased almost linearly withprolonged storage at -O.soC. An elevation of storage temperature from -O.soC to 10°C any timeduring the cold storage period, further aggravated the split problem. Consequently, thereduction of berry split in 'Thompson Seedless' table grapes during cold storage requires (a) theshortest possible cold storage period, and (b) good temperature management throughoutdistribution, from initiation of cooling until the final point of sale.The grape berry abscission potential, as quantitatively indexed by the measurement of the fruitremoval force (FRF), showed significant changes during berry development of 'Waltham Cross'table grapes, from 27 to 111 days after full bloom (OAFB). This showed that at certain stages offruit growth, 'Waltham Cross' grapes are more prone to berry abscission. At 27 OAFB, when theberries had an average diameter of 6.6mm, the grape bunches showed a significantly higherpotential for berry abscission, compared to grapes sampled at a later stage. 'Waltham Cross'has inherently straggly bunches with bare shoulders. Therefore, any abscission during berrydevelopment will aggravate the problem. Consequently, it is of vital importance that anyadverse factors such as moisture stress be avoided, especially during the period when'Waltham Cross' grapes appear to be very susceptible to berry abscission. Of all parametersmeasured, moisture loss showed the best correlation with abscission. Grapes harvested withtotal soluble solids (TSS) of 12.3°Brix, 83 OAFB, had a significantly higher abscission potentialthan grapes harvested more mature. Therefore, by harvesting 'Waltham Cross' grapes atoptimum maturity, at a TSS of approximately 16.4°Brix, berry abscission can be reduced to agreat extent. It was evident that at veraison, the metabolism of grape berries changesdrastically, and additional to the rapid increase in sugars and the rapid decrease in acidity, adecrease in FRF occurs.Preharvest sprays of ReTain™ (a derivative of aminoethoxyvinylglycine), which inhibits ethylenesynthesis, showed no promise as a means to reduce postharvest berry abscission. Apostharvest treatment with EthylBloc® (1-methylcyclopropene), which inhibits ethylene action,only reduced berry abscission during one season.

[发布机构] Stellenbosch University