Biochemical studies on the development of corky tissue in sapota [Manikara achras (Mill.) Fosberg]

Abstract

Corky tissue (CT) of sapota is a physiological disorder characterized by hard lump in the pulp, slightly desiccated in nature and acidic to taste. This disorder shows no distinct external symptoms and becomes visible only when fruit is cut open. Under extremely severe conditions, corky skin eruptions are seen. Corky tissue incidence is estimated to vary from 20 to 50 per cent in Cricket Ball, while in other varieties like Kalipati it is up to 5 per cent. Biochemical analysis revealed that total and reducing sugars, soluble protein and activities of amylase and lipase were higher in the mesocarp of healthy fruits compared to CT affected fruits, while content of starch and free amino acids were lower. These indicated that degradation of starch into sugars was hampered in CT affected fruits. Starch content, soluble protein and free amino acid were higher in healthy seeds than seeds from CT affected fruits, while total and reducing sugars and amylase activity were lower. Under field conditions, regulation of sink strength by exogenous application of GA3 and PBZ showed that, CT occurred in the weaker sink thus confirming the role of itner-fruit competition in CT development. Seed viability seems to play an important role in CT development. Sink strength of fruit depends upon number of viable seeds it has. Moisture content in seed and mesocarp of healthy fruits was higher in comparison with that of CT affected fruits, showing thereby that seed from CT affected fruits had lost moisture leading to reduction in seed viability. Reduction in seed viability in corky tissue affected fruit was confirmed by data on germination, dehydrogenase activity, DNA content and analysis of endogenous levels of GA3, ABA and JA using HPLC. Conditions of increased temperature, evaporative demand and decreased relative humidity showed marked increase in corky tissue incidence.