Calcium-mediated conversion of sucrose to starch in relation to the activities of amylases and sucrose-metabolizing enzymes in sorghum grains raised through liquid culture

Abstract

Detached ears of sorghum (<i>Sorghum vulgare</i>)<i> </i>were cultured in complete liquid medium containing Ca²⁺(0, 3,10 and 30 <i>mM</i>)<i> </i>and effect of this ion on the conversion of· sucrose to starch with respect to the activities of amylases, sucrose synthase, sucrose phosphate synthase and soluble invertases were studied in developing grains. Presence of 3 m<i>M </i>Ca²⁺ in culture medium enhanced both accumulation of starch and activity of α-amylase in grain but without having any influence on the activity of β-amylase. However, with 10 and 30 m<i>M </i>Ca²⁺, the accumulation of starch and activities of both amylases decreased and with advancement in culturing period, starch accumulation was further decreased. Irrespective of its concentration, Ca²⁺ enhanced the activities of sucrose synthase (synthesis), sucrose-phosphate synthase, soluble acid invertase and soluble-neutral invertase. Increase in the concentration of Ca²⁺ in culture medium was concomitant with an elevation in relative proportion of sucrose in the grain reflecting a net balance in per cent increase with Ca²⁺ in the activities of sucrose-synthesizing enzymes over sucrose-hydrolysing ones. Based on the results. it is suggested that assimilation of Ca²⁺ by grain is essential for maintaining high activity of α-amylase to generate starch primers required for the conversion or sucrose to starch during grain filling in sorghum.