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The Effect Of Metalosate® Calcium On Browning And Metalosate® Potassium On Sugar Accumulation In 'Waltham Cross' (Table Grapes)

Introduction
 
Calcium plays an important role in maintaining fruit quality.It decreases browning, helps to retain fruit firmness, reduces respiration and ethylene evolution, increases vitamin C content, and decreases storage breakdown and decay. Calcium is essential for stabilizing and strengthening cell wall and membrane structure, including function of cell walls and membranes. Evidence for the positive role of calcium on fruit quality is three fold:

  1. Under calcium-deficient conditions, there is a profound deterioration of membranes.

  2. Calcium alters the actual architecture of membranes that changes the fluidity and water permeability thereof.

  3. Calcium can alter an array of physiological activities which are specifically associated with membrane function. Pre-harvest calcium sprays were shown to maintain fruit quality and prolong shelf life of apples. Fruit quality can be improved with potassium by raising the sugar level and increasing size and color.


Materials and Methods
  1. The trial was conducted on a 'Waltham Cross' vineyard situated in the Northern Paarl region of South Africa. The treatment design was a control plus three Metalosate® concentrations with six applications. Another three Metalosate Calcium concentrations with three applications, followed by three Metalosate Potassium concentrations were applied. Metalosate Calcium application commenced 14 days after 6.00 mm (0.24 in) berry size; whereafter cell division and seed production ceased and terminated 14 days before verasion. Metalosate Potassium was then applied from verasion onwards. Three 4.5 Kg (9.9 lbs) cartons were harvested from each treatment, packaged as for export, were used as an experimental unit for determining browning and other quality attributes after cold storage. The grapes were stored for five weeks at -0.5oC (31.1oF) and thereafter at 10oC (50oF) for one week.

  2. Normal viticultural practices for the cultivar and region, as required for export,were applied.

  3. 'Waltham Cross' was harvested when average berry total soluble solids (T.S.S.) reached export requirements (17oBrix)
  1. Evaluations at harvest included the following:
    • Calcium and potassium content of the berries
    • Color evaluation
    • Berry firmness
    • T.S.S., total titratable acids (T.T.A.), cell juice pH, and berry mass.
  1. Evaluations after cold storage included the following:
  • Browning
  • Scanning electron microscopy (SEM) on tissue samples taken from the center of berries. Cell walls and cell membranes were investigated.
  • T.S.S., T.T.A., cell juice pH, and berry mass.
  • Loose berries, decay, blemishes, berry crack, bruises, and SO2 burn.
  1. Nutrient status of soil samples from the trial plot was determined at the onset of the trial.
  2. Temperature and relative humidity in the vineyard were monitored.

Results and Discussion
  1. Calcium Content.  The calcium content of theMetalosate Calcium treated grapes at harvest tended to be higher than the control. Calcium in the combination treated grapes was, however, significantly higher than the control when Metalosate Calcium applications were followed by Metalosate Potassium applications. This indicates that the potassium applications on grapes and leaves led to increased calcium content in the grapes.

  2. Potassium Content.  The potassium content of the Metalosate Calcium treated grapes at harvest tended to be higher than the control. The potassium content of the combination treated grapes at harvest was significantly higher than the control when a Metalosate Calcium application was followed by a Metalosate Potassium application. This observation supports TSS results, namely that potassium improved fruit quality by raising the sugar level, notwithstanding delayed or stabilized ripening.

  3. Color.  The color of treated grapes at harvest tended to be greener that the control when a Metalosate Calcium application was followed by a Metalosate Potassium application. The same trend at harvest was not observed for the Metalosate Calcium-only applications.

  4. Berry Firmness.  The grapes receiving the highest concentration of Metalosate Calcium tended to be firmer at harvest. Thus, they were less wilted and flaccid than the control. The combination-treated grapes at harvest also tended to be firmer, thus less wilted and flaccid than the control.

  5. Total Soluble Solids.  The sugar level of the combination-treated grapes at harvest and after cold storage was significantly higher than the control.

  6. Total Titratable Acids.  The acid content of the combination-treated grapes differed only at harvest and was significantly higher than the control.

  7. Cell Juice pH.  The cell-juice pH of all treatments did not differ at harvest

  8. Evaluation of Browning.  The type of browning observed this season, although very low, was even browning of grape tissue. The skin of the berries was not affected. An average browning of 1.2% was observed in the control after cold storage. Despite this very low percentage of browning, the middle rate of Metalosate Calcium tended to reduce even browning to 0.8%. This effect is also supported by the SEM study as well as the calcium content in grapes.

  9. Scanning Electron Microscopy (SEM).  SEM micrographs (Figure 2) showed that the skin tissue cells of grapes that had been treated according to the spray program were not damaged, turgid, and regular. The fruit tissue cells of the control were squashed, non-turgid, irregular, creased, and damaged from the cuticle-epidermis into the mesodermis. The SEM study showed that the spray program stabilized and strengthened the cell walls and cell membranes. This also preserved the semi-permeability of cell membranes. The seemingly optimal condition of cell walls and cell membranes will result in the optimizing of skin metabolism (probably also through stomatal opening) and so keep bunches firmer, especially during daily temperatures exceeding 30oC (86oF). Control grapes during the previous season at harvest appeared to be flaccid at temperatures of 30oC (86oF) and higher, while treated grapes were firm. A certain concentration of calcium seems to be essential for optimal stomata functioning. This observation suggests that the cell membranes of spray treated grapes were not damaged, concomitant with repressed browning. Raschke (1979) and De Silva et al. (1985) reported that moderate concentrations of external calcium, also in combination with abscisic acid, were responsible for stomatal function. Calcium is responsible in controlling the gating of potassium and chloride channels, resulting in the opening and closing of stomata.

  10. Other Quality Aspects.  Loose berries in the combination treated grapes after storage were significantly lower than the treatment receiving only calcium sprays.

Conclusion

A calcium fertilization program should be complemented by a calcium and potassium spray program. This combination proved to be very effective at increasing many aspects related to fruit quality. For the complete text of this research project, or further information on the Metalosate products, please contact your local Albion Plant Nutrition representative.
Figure 2. The Cell Wall Structure of ‘Waltham Cross’ According to SEM Studies after Cold Storage

  1. Control

  2. 40 ml/100 L (0.5 fl. oz./10 gal.) Metalosate®Calcium as well as 40 ml/100 L (0.5 fl. oz./10 gal.) Metalosate® Calcium followed by 50 ml/100 L (0.6 fl. oz./10 gal.) Metalosate® Potassium

  3. 80 ml/100 L (1.0 fl. oz./10 gal.) Metalosate®Calcium as well as 80 ml/100 L (1.0 fl. oz./10 gal.) Metalosate® Calcium followed by 100 ml/100 L (1.3 fl. oz./10 gal.) Metalosate® Potassium

  4. 120 ml/100 L (1.5 fl. oz./10 gal.) Metalosate®Calcium as well as 120 ml/100 L (1.5 fl. oz./10 gal.) Metalosate® Calcium followed by 120 ml/100 L (1.5 fl. oz./10 gal.) Metalosate® Potassium.
References

DeSilva, D.L.R., Hetterington, A.M. & Mansfield, T.A. Synergism between calcium ions and abscisic acid in preventing stomatal opening. New Phytologist. 100, 473-482.

Raschke,K. 1979. Movements of stomata. In Haupt, W. & Feinleib, M.E. (eds). Encyclopedia of plant physiology, New Series, vol. 7. Berlin. Pp. 373-441.