Starch Conversion in Leaves of Helianthus tuberosus and H. annuus: Preliminary Observations

Starch hydrolysis in leaves of Helianthus tuberosus, ,a short day variety, and in leaves of” H. annuus, flowering in long day, was investigated by a simple colorimetric starch estimation in situ (§ 2). Leaf halves, leaf discs, and leaves attached to considerable stem pieces, did not show conspicuous differences in starch hydrolysis. Moreover, the daily course of starch synthesis and breakdown was studied under natural conditions (figs 2, 3). The temperature curve of starch hydrolysis mostly shows a pronounced maximum at about 0-3° C, a minimum around 10° C, and a renewed increase at temperatures above 15° C (figs 4-9). An attempt has been made to explain the observed behaviour by assuming three processes, all possessing a temperature curve with a normal, positive rate increment with respect to temperature, viz., starch hydrolysis, (re) synthesis of starch, and respiration of sugars (fig. 12). Experiments on the effect of certain temperature sequences (3°, 13°; 31°, 13°) made in connection with the above tentative explanation of the temperature curve, failed to demonstrate a difference in behaviour at 13° C, depending on the preceding temperature. No resynthesis of starch at 13° C after hydrolysis at 3° C was found. The reason may be that during a preceding period of hydrolysis at a temperature which is not suitable for (re) synthesis, the sugar concentration is lowered too much by diffusion, thus preventing effective resynthesis (figs 10, 11). The temperature curve, in H. tuberosus, was found to be much the same in leaves of various physiological ages (fig. 9). In air with low CO2 content, light of various intensities was found not to influence starch hydrolysis in H. tuberosus, at various temperatures (figs 13, 14). In other experiments, 5 % CO2 in air decreased hydrolysis considerably in light, but also in darkness (at 25° C) (Table 4), and this time, also in air a difference between light and darkness was found. Lack of oxygen was found to decrease starch hydrolysis, especially at high temperature. (Table 5, 6). In large gas spaces this effect was less then in small ones, but in the large gas spaces used (dissiccators) anaerobiosis may not have been fully satisfactory (Table 7). The study of the influence of the composition of the gas phase still is incomplete. Attempts were made to study the effect of sugar solutions on starch hydrolysis. In many cases a decrease in hydrolysis was found, but it was observed that also water could bring about similar effects, bringing the phenomenon into contact with the effects of lack of oxygen. These experiments, therefore, still were felt to be unsatisfactory. It was observed occasionally that wilting promotes starch hydrolysis.