CARBOHYDRATES OF WHEAT LEAVES

Previous work on respiration and carbohydrate metabolism of wheat leaves on prolonged starvation (3) revealed the presence in these leaves of two different respiratory substrates, which were provisionally called the primary and the secondary substrates. The primary substrate has been found to be responsible for the production of C02 by leaves early in starvation, while its contributions in later stages were little or nothing. On the other hand the secondary substrate, which might or might not be used on the first day, was becoming with the progress of starvation of greater and greater importance, until in later stages either (as in some cases) all the C02 emitted by leaves was derived from it, or (as in others) the major portion. It was shown in the same paper that the primary substrate was represented by sugars extracted from leaves by ethyl alcohol. While nothing definite was said on the nature of the secondary substrate it was suggested that this probably consisted of a variety of substances, some of which are of importance in protoplasmic organization. Deleano (1) and Yemm (5) reported that proteins serve as the respiratory substrate in later stages of starvation in the leaves of grape and of barley. But before attention is definitely focused on proteins as the source of secondary substrate in wheat leaves, it ought to be determined whether or not some other carbohydrates, which might serve in this capacity, are present in leaves in sufficiently large amounts. Newton and Brown (4), for example, reported that wheat leaves in later stages of maturity contain considerable amounts of pentosans. Such carbohydrates, if present, must remain in the leaf residue after alcoholic extraction, and they might exist there in the two following forms: (1) as hemicelluloses, (hydrolyzable by 1 per cent. H2S04) ; (2) as part of a carbohydrate-protein complex, from which carbohydrates could be liberated on digestion of its protein fraction by trypsin. To test these two possibilities, the experiments reported below were performed.