Inter-conversion of free sugars and accumulation of galactomannan in response to supplied metabolic mediators during pod filling in guar.

ABSTRACT

Detached inflorescences of guar (Cyamopsis tetragonoloba), each bearing 4 uniformly-developing pods at 42 days post anthesis (DPA), were cultured for 6 days in complete liquid medium manipulated with a fixed concentration of mannose and varying concentration of myo-inositol. Such inflorescences, but with 2 pods, were also maintained in the solutions of (i) glucose(U-14C) containing myo-inositol or phytohormones, and (ii) mannose(U-14C) containing galactose for 36 hr. Effect of such exogenously supplied metabolic mediators on interconversion of free sugars in pod wall, endosperm and cotyledons and galactomannan accumulation in endosperm was studied. Myo-inositol decreased, over control, the relative proportion of invert sugars in pod wall, endosperm and cotyledons and at lower concentration (27.75 mM) it decreased the level of free sugars in pod wall and galactomannan in endosperm. In all pod tissues, 14C from both glucose and mannose got incorporated into myo-inositol as well as various sugars and maximum incorporation occurred in sucrose. High concentration of total free sugars and their 14C activity in pod wall indicated that this pod tissue was a potent accumulator of free sugars. With myoinositol, the relative proportion of 14C from glucose into raffinose sugars of pod wall and endosperm increased with a simultaneous decrease in this incorporation into galactomannan of the latter. Accompanying this, relative proportion of 14C into hexoses and myo-inositol decreased in pod tissues. Galactose increased 14C incorporation from mannose into total free sugars, sucrose and galactomannan with a concomitant decline in the labelling of hexoses. IAA and ABA enhanced 14C incorporation from glucose into total free sugars and this enhancement was much higher with IAA than ABA. The latter inhibited 14C incorporation into galactomannan. Based on these results, it was suggested that myo-inositol at lower concentration was inadequate to mediate the metabolism of sugars and, thereby, galactomannan synthesis. Galactose and mannose exhibited a mutual beneficial effect on their transportation to pods. Phytohormones stimulated the accumulation of sucrose in pod wall for its obligatory unloading into the seed.