On the physiological changes during the growth of head lettuce Lactuca sativa var. capitata

Abstract

Leaves of the head lettuce sown on March 25 started a radical growth on May 24 and the growth was most vigorous during the head formation. Their physiological situation was compared with that of the previous ones on June 3. The values of water content and specific electric conductivity of expressed cell sap decreased, but the specific gravity of tissue powder and the corrected concentration of expressed cell sap increased. The water content of the leaves of the upper parts during the early period of the growth was smaller than that of the leaves of the lower parts of the plant, while during the heading period the leaves of the upper parts kept more water than those of the lower parts. The relation of the water content to the leaf position was reversed on June 3. The values of the specific gravity of the tissue powder during the early period of growth were higher in the upper parts. During the heading period both the outer leaves and the inner parts of the head showed the similer tendency mentioned above, but the leaves on the outer parts of the head showed the converse tendency. The values of the specific electric conductivity of expressed cell sap and the water extract of the tissue powder of the leaves decreased in the leaves of the upper parts during the whole growing period. From the facts mentioned above, it is considered that the head lettuce remarkably showed the physiological changes both in growth and heading. The production, accumulation and translocation of the assimilated substance occurred in all leaves. Remarkable quantities of soluble matters were accumulated in the leaves on June 3, and at the same time both formation and growth of the leaves were active in the inner part of the rossett. Accordingly, this period is considered to be the begenning of the heading.