Cellular factors that affect table grape berry firmness
Date
2008-03
Authors
Du Plessis, Beatrix W.
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
The South African table grape industry is under great pressure to produce table grapes
of the best quality for the export markets. Quality defects such as poor colour,
inadequate berry firmness, browning and soft tissue breakdown cause great losses in
export. The firmness of table grapes is one of the major factors determining the eating
quality of grapes. Consumers prefer grapes with a firmer flesh above those with soft
flesh. Firmer berries are commonly accepted to have better eating quality and longer
cold storage capacity. Factors that promote and maintain berry firmness are only
speculated about; therefore producers cannot effectively control the development of
firmer berries by managerial practises or by applying specific sprays.
The study was done on Redglobe and two Waltham Cross clones (the firmer Clone
8 and softer Clone 13). The aim of this study was two-fold. Firstly the cellular and
ultracellular differences between the tissues of firm and soft berries were determined.
The effect of gibberellic acid (GA3), synthetic cytokinin (CPPU) and bunch applied
calcium sprays on the cellular and ultracellular structure of berry tissues were also
under investigation. Secondly, the effects of GA3, CPPU and bunch directed calcium
sprays on berry firmness, eating quality and storage capacity were determined.
To determine the cellular and ultracelular structure of berry tissues, light microscope
(LM) and transmission electron (TEM) studies were done. In order to investigate the
effect of different sprays on berry firmness, 20 mg/L GA3 (GA3 treatment) was applied at
10mm average berry size; 20 mg/L GA3 plus 3 mg/L CPPU (CPPU treatment) was
applied at 10 mm average berry size; and a mixture of 8 L/ha Stopit® and 5 L/ha
Caltrac® (calcium treatment) was applied directly to the bunches every two weeks from
berry set till veraison for the calcium treatments. The control received no plant
bioregulators (PBR’s). The treatments were the same for both cultivars.
Grapes were stored three weeks at 0 °C and one week at 10 °C after which it was
evaluated for loose berries, botrytis infections, rachis browning and berry split.
Afterwards it was tasted by an independent tasting panel.
Firm berries were found to have an opaque coloured flesh while soft berries had a
gel-like translucent flesh. For berries with normal firmness, the opaque flesh is limited to
the outer mesocarp of the berry. Extremely firm berries’ whole mesocarp consisted of
the opaque coloured flesh while soft berries’ mesocarp consisted of mostly the gel-like
translucent flesh with, in some cases, a very thin layer of opaque flesh just under the
skin.
Berry firmness was not related to cell size as the cell size of the tissues in the firm
and soft berries were identical. Cell shape seems to play an important role in berry
firmness. The cells in the opaque coloured flesh of the outer mesocarp are more turgid
and oval than those in the gel-like flesh of the inner mesocarp. Berry firmness is
therefore determined by the thickness of the outer mesocarp with the opaque coloured
flesh that contains turgid cells. The thickness of cell walls between the different tissues did not differ. There was however a difference between the cell contents and the
plasmalemmas of the inner and outer mesocarp. The plasmalemma and tonoplast of
the outer mesocarp cells was more intact than those of the inner mesocarp. The
membranes in the inner mesocarp are more subtracted form the cell wall than in the
outer mesocarp.
Both the PBR’s and calcium treatments cause a delay in sugar accumulation in the
case of Redglobe and Waltham Cross. The CPPU treatment results in significantly
bigger and firmer berries for both cultivars. In the case of Redglobe, this treatment
cause bigger cells in the outer mesocarp suggesting a correlation between berry
firmness and cell size. In the case of Waltham Cross, however, cell size did not play a
role in berry size and firmness; instead the rate of cell division earlier in berry
development. The CPPU treatment was the only treatment that maintains berry
firmness during cold storage for Redglobe while GA3 and CPPU did so in the case of
Waltham Cross.
PBR’s seems to have no effect on cell wall thickness. In the case of Redglobe, the
calcium treatments resulted in significantly thinner cell walls, but this can not be
explained.
Calcium and GA3 treatments had a negative effect on grape quality after cold
storage of both Redglobe and Waltham Cross. The Waltham Cross CPPU treatment
results in better taste and colour as observed by the tasting panel, while in the case of
Redglobe, the tasting panel preferred the control.
It is found that the use of CPPU in combination with GA3 had the best effect on the
eating quality, storage capacity, berry size and firmness. When a producer decides to
use the CPPU treatment in order to improve berry firmness, he must realize that it can
cause delayed ripening which can affect the export of the fruit.
Description
Thesis (MscAgric (Viticulture and Oenology))--Stellenbosch University, 2008.
Keywords
Berry firmness, Cell structure, Plant bioregulators, Calcium, Dissertations -- Agriculture, Theses -- Agriculture, Dissertations -- Viticulture and oenology, Theses -- Viticulture and oenology, Table grapes -- Quality, Table grapes -- Cytology