Differential effect of calcium and strontium on flesh firmness and properties of cell walls in apples

Influence of pre-harvest sprays of calcium nitrate on storability and quality attributes of plum fruits

The effects of pre-harvest applications of calcium nitrate at different concentrations were investigated on storability and quality attributes of plum fruits under low temperature storage. Uniform and healthy trees of plum cv. Satluj Purple were sprayed with calcium nitrate (1.0%, 1.5% and 2.0%) in 2nd and 3rd week of April and control trees were water sprayed only. Fruits from treated and control trees were harvested at colour break stage, and then stored at low temperature conditions (0-1 °C and 90-95% RH) for 35 days. Evaluation of physico-chemical parameters and enzymatic activities were made on the day of harvest and after 7, 14, 21, 28 and 35 days of storage. The colour of the fruits with respect to a* and b* values (green-red and blue-yellow spectrum, respectively) improved progressively up to 35 days of storage, whereas, fruit firmness decreased. Total sugars and pectin methylesterase (PME) and cellulase activities increased up to 28 days of storage before showing a declining trend. Two superimposed sprays of calcium nitrate (2.0%) enhanced the storage life of plum fruits up to 28 days by reducing physiological loss in weight (PLW), PME and cellulase activities, delaying development of fruit colour & anthocyanin and retaining sensory quality, firmness and total sugars in plum fruits.

Inhibition of cell-wall autolysis and pectin degradation by cations

Modification of cell wall components such as cellulose, hemicellulose and pectin plays an important role in cell expansion. Cell expansion is known to be diminished by cations but it is unknown if this results from cations reacting with pectin or other cell wall components. Autolysis of cell wall material purified from bean root (Phaseolus vulgaris L.) occurred optimally at pH 5.0 and released mainly neutral sugars but very little uronic acid. Autolytic release of neutral sugars and uronic acid was decreased when cell wall material was loaded with Ca, Cu, Sr, Zn, Al or La cations. Results were also extended to a metal-pectate model system, which behaved similarly to cell walls and these cations also inhibited the enzymatic degradation by added polygalacturonase (EC 3.2.1.15). The extent of sugar release from cation-loaded cell wall material and pectate gels was related to the degree of cation saturation of the substrate, but not to the type of cation. The binding strength of the cations was assessed by their influence on the buffer capacity of the cell wall and pectate. The strongly bound cations (Cu, Al or La) resulted in higher cation saturation of the substrate and decreased enzymatic degradability than the weakly held cations (Ca, Sr and Zn). The results indicate that the junction zones between pectin molecules can peel open with weakly held cations, allowing polygalacturonase to cleave the hairy region of pectin, while strongly bound cations or high concentrations of cations force the junction zone closed, minimising enzymatic attack on the pectin backbone.