Synthesis and Dissolution of Starch labelled with14Carbon in Tobacco Leaf Tissue

Abstract

Tobacco leaves depleted of starch, were detached and allowed to assimilate equal amounts of ¹⁴CO₂ and ¹²CO₂in succession, and vice versa. Distribution of radioactivity in starch, and sugars was determined after assimilation and after disks cut from the leaves had been kept in darkness for times up to 40 hours. The amount and activity of the CO₂ was also determined. ¹⁴C and ¹²C were incorporated in equal amounts into starch independently of the order in which they were supplied. In contrast sucrose had high activity ¹⁴C was given last, and hexose a low one. The reverse was true when ¹²C was given last. Activity of respiratory CO₂ was slightly higher when ¹⁴C was assimilated last as compared with ¹²C. In the dark only ¹⁴C or ¹²C was at first lost from starch, in accordance or removal of discrete layers. Analytical results show that starch is the main respiratory substrate and to account for the redistribution of radioactivity in passage to CO₂ it is concluded that sucrose occurs at two sites separated inter-or intra-cellularly, one of which is in equilibrium with the system intrconverting starch and CO₂ and at the other hexoses are produced by inversion. A starch-like polysaccharide is formed during assimilation which persists in the dark and there is a significant contribution to respiration of carbon from non-carbohydrate sources when leaf disks are kept on the dark.