Sap-injury in Mango (Mangifera indica L.)
Sap-injury is an important post-harvest problem in mango . The mango fruit has a network of branching fruit ducts, which are present in both fruit and stalk and penetrate the transition zone between the fruit and its stalk. Several large fruit ducts continue into the stalk but end a short distance beneath the abscission zone. These fruit ducts contain a viscous, caustic liquid referred to as mango sap. Flow of sap between fruit and stalk is a part of the growth process. The amount of sap exuded varies with cultivar, maturity and production area. As the fruit matures and ripens, sap flow decreases and eventually stops. Formation of an abscission layer blocks further sap flow and the fruit falls from the tree. If this sap comes in contact with the peel of the fruit, it causes a darkening or browning of the peel in the region of contact. This discolouration of the peel is referred to as sap-injury or sap burn-injury.
Sap-injury in mango fruit is a major industry concern, as it results in poor fruit quality and lower prices, especially in the export markets. Different experiments have been conducted to devise strategy for sap-injury management in commercial mango cultivars of India
Factors influencing sap-injury
Though sap-injury is a superficial damage, it reduces consumer acceptance and decreases shelf life of the fruit. Sap-injured skin could be invaded by Aspergillus spp., especially in hot conditions. Thus, it is a very important post-harvest handling and marketing problem of the mango industry, particularly in the case of fresh fruit. Sap obtained from the fruit harvested in the afternoon (2.00 pm) caused more sap-injury than that collected from fruit harvested in the morning (7.00 am) and this was attributed to a dilution of injurious components of sap at higher fruit turgor. And also 'spurt sap' causes more injury than 'ooze sap'. organic layer of sap may be responsible for sap-injury. Thus the susceptibility of mango fruit to the injury as well as the severity of the injury seems to vary depending on the cultivar. Resorcinolic compound like 5-(12-heptadecenyl)- resorcinol, which is present in mango was also responsible for causing sap-injury.
The proteins and carbohydrates in the shoot ducts originated from the cell walls of integrating cells during the lysogenous process of duct formation. On the other hand, proteins and carbohydrates in the fruit sap of mango were found to be produced by special secretory epithelial cells lining the lumen of the ducts. The mode of mucilage production is summarized in the shoot ducts differ from fruit ducts in the absence of mucilage-secreting epithelial cells. This observation is in agreement with the findings the sap of mango fruit was found to contain more protein and carbohydrate than shoot sap. In general, mango sap has a wide variety of constituents including tannins, enzymes, resins and terpenes. And also presence of the enzymes polyphenol oxidase, 5-substituted resorcinol, terpinolene and laccase in the mango sap were responsible for causing mango dermatitis.
Alphonso and Batali varieties of mango contain the major component in each case constituted (3% of the total volatiles and had a similar green mango aroma note, though there was a significant difference in retention time (Rt) between them. The major component of the Alphonso variety was identified as as-ocimene and that of Batali variety was B-myrcene. These constituents responsible for the green aroma of raw mango of two different varieties were both acyclic hydrocarbons and identical subjectively in their aroma, they differed in molecular structure.
Methods used to control sap-injury
A) Physical methods
Different practices are followed in different parts of the world to control sap-injury in the harvested mangoes. In certain parts of India, the mangoes are de-sapped for 15-30 min on bamboo structures. In some parts of the world including India, one of the common practices was the inversion of fruits in soil for 15-30 min after harvest. This practice may, however, lead to infection of the fruit by soil-borne pathogens. Soil bleeding of mango caused a significant increase in post-harvest infection and also the earlier appearance of stem-end rot.
Standard harvesting practice in Australia is to clip the fruit from the tree with a long stem (5 to 10 cm). The long stems are then broken off by hand in the packing shed and the fruit are de-sapped by placing them stem end down for 20-30 min. However, during the shifting of fruits from the orchards to the packing sheds, care must be taken to keep the stalk intact. If the stalk is broken during the shifting, it contaminates the fruits with sap and causes sap-injury.
b) Chemical methods
In order to control sap-injury in mango fruits, several methods have been suggested.
- Plugging the stem-end of the fruit with chemicals after de-stalking, washing the fruit with 1% sodium bicarbonate or 1% aluminium potassium sulphate or applying surface coatings to the fruit prior to de-sapping, were a few of the methods that were suited.
- Washing the sap-contaminated fruits with 1% sodium bicarbonate and 1% aluminium potassium sulphate to control sap-injury.
- Paraffinic oils to pre-coat the fruits that were harvested with stalk, before de-sapping.
- Subjecting the freshly stained mangoes to hot water treatment (51-55°C, 10 min) minimized the sap-injury, whereas subjecting the fruits to this treatment after the sap had dried.
- Mango sap injury amelioration showed that dabbing in vegetable oil or dipping in DC Tron oil at 100 - 1000 ml/I was more efficacious.
In order to control sap-injury, therefore, in-depth knowledge of the components of mango sap in the different local varieties is essential. There are no reports so far of the exact composition of mango sap in general and in particular from Indian mango varieties. The role of mango sap in defense of the fruit against various pest infestations and infections has been long suspected.
Ph.D Scholar, Department of Plant Pathology, Center for Plant Protection Studies,
Tamil Nadu Agricultural University, Coimbatore -641003, Tamil Nadu, India.