SummaryThe quantitative changes in starch content during the growth and senescence of tobacco leaves have been followed. The starch content was low while the leaves were expanding but rapidly increased after expansion stopped. The maximum was reached when more than half of the original chlorophyll content had gone. The starch content was then reduced rapidly and when the leaf was all yellow there was only a small quantity of starch and this remained when the leaf turned brown.The viscosities, iodine affinities, fl-amylolysis limits, and granule sizes of isolated starch granules and the chain lengths of amylopectins at different ages of leaves have been determined. The viscosities and iodine affinities increased with increasing age of the leaf and the fl-amylolysis limits of the whole starch and chain length of the amylopectins were constant throughout the growth of the leaf. The average granule size increased as the leaf matured until maximum starch content was reached and decreased as the starch content decreased. It is suggested that in tobacco leaves another starch accumulation pattern of longer duration than 'diurnal accumulation exists. The degradation of the starch resembles the breakdown of starch in sprouting potato tubers and not that in germinating barley where amylases are the active enzymes.The residue after ethanol extraction of leaves at different stages of growth was fractionated into water-soluble material, a perchloric acid extract precipitated by iodine, and a perchloric acid extract not precipitated by iodine but precipitated by ethanol. Glucan was present in significant concentration only in the perchloric acid extract precipitated by iodine and this had the properties of starch. Paper chromatography indicated that non-starchy extracts were composed of galactose, arabinose, xylose, rhamnose, and uronic acid.

The quantitative changes in starch in tobacco leaves have been determined over diurnal-nocturnal periods for rapidly expanding, fully expanded, and yellow leaves. The daily increase in starch lessens as the leaves mature and senesee, and the amount of stureh not undergoing diurnal variation in fully expanded leaves is more than in rapidly expanding leaves.

Tobacco leaf starch has been isolated as granules by mechanical extraction, and the viscosity, amylose content, f3-amylolysis limit, starch-iodine complex absorption spectrum, and apparent chain length compared with samples extracted by perchloric acid, sodium hydroxide, and chloral solution and purified as the starch-iodine complex. The efficiency of extraction of these methods was also compared. The same comparisons were made with potato granules and potato granules treated with these extracting reagents.Evidence is presented that the most efficient method of extraction of tobacco leaf starch is with perchloric acid but all solvent extraction methods give a degraded starch with low viscosity. The f3-amylolysis limits are similar, while the starchiodine absorption spectra and apparent amylose contents of the extracted starches and granules may differ slightly, but solvent·extracted starches show a quite different pattern of acid release on oxidation with periodate ion which makes estimation of apparent chain length difficult.EFFECT OF SOLVENTS ON STARCH 313 starches isolated by chemical methods has not been reported. Starch can be brought into solution, after gelatinization by boiling, by a number of agents that disperse the swollen grains. The most common extraction solutions used have been chloral at 80°C (Meyer and Bernfeld 1940), perchloric acid at or below room temperature (Pucher, Leavenworth, and Vickery 1948), and sodium hydroxide at room temperature (MacWilliam, Hall, and Harris 1956). It is well known that degradation of starch by alkali and hot water (Greenwood 1956; Machell and Richards 1958) and by acids (Greenwood 1956;Cowie and Greenwood 1957a) occurs. These solvents are not selective but remove other polysaccharides, e.g. cell wall polysaccharides, proteins, and inorganic salts, so that further purification is necessary. This is usually carried out by precipitation of the starch as the iodine complex in the presence of iodine-potassium iodide solution. Starch content can be estimated by hydrolysing this complex and estimating the glucose produced (Pucher, Leavenworth, and Vickery 1948). To make structural studies, e.g. apparent chain length of amylopectin fraction by estimation of formic acid produced on periodate oxidation of the whole starch (Anderson, Greenwood, and Hirst 1955) or isopotential absorption of iodine by potentiometric titration (Bates, French, and Rundle 1943) it is necessary to remove inorganic ions. This is usually carried out by decomposition of the starchiodine complex by alkali followed by dialysis, a process that involves the possibility of further degradation.