Effects of calcium and boron application on salak quality and nutrient composition

The most prevalent problem of salak fruit (Salacca zalacca) quality is the occurrence of localized flesh necrosis causing cell collapse and color change from orange to dark brown. The symptom usually occurs when fruits mature and reach harvesting stage during the rainy season. Calcium (Ca) and in some cases, boron (B) have been related to many physiological disorder of fruits. The objective of this study was to investigate whether flesh necrosis in salak is related to Ca and B deficiencies and if its incidence can be reduced by applying Ca and/or B via soil or foliar application. The experiment was carried out at a mature commercial salak orchard in Chanthaburi, eastern Thailand. The soil was sandy loam with pH 4.87, Ca content of 702 mg kg-1 and B content of 0.44 mg kg-1. The experiment consisted of four treatments: 1) control, 2) soil Ca application, 3) soil Ca and Ca foliar spray and 4) soil Ca and Ca plus B spray. Soil Ca was applied as gypsum at the rate of 3 kg/tree in September 2008. The Ca spray was applied six times using 2% CaCl2.2H2O while B spray was applied three times using 0.25% Solubor every two weeks. The fruits were harvested in May and September 2009. The numbers of normal and disordered fruits were evaluated. Concentrations of K, Ca, Mg and B in the normal and necrotic flesh were analyzed. The result showed that soil Ca application significantly reduced the number of fruit with necrosis from 63.2% to 24.9%. The additional Ca spray did not further reduce the number of necrotic fruit. The application of B spray not only had no effect in reducing the number of necrotic fruits, but also resulted in the appearance of a slightly deformed distal end of the fruits. Surprisingly, the Ca, B, and Mg concentrations in necrotic tissues were found to be higher than in the normal. Despite the discrepancy in tissue Ca association with necrotic tissue, the results confirmed that the incidence of necrosis in salak fruit can be reduced by the application of soil Ca.