EFFECTS OF POTASSIUM DEFICIENCY ON THE FORMATION OF STARCH IN PISUM SATIVUM

Abstract

POTASSIUM IS essential to the normal growth of plants and it has long been assumed that one reason for this is the connection of this element with the carbohydrate supply, either in its formation or in its translocation. The investigations of Briggs (1922) and of Gregory and Richards (1929) indicate that limitation of potassium retards the rate of assimilation of carbon dioxide. Although sugar is rapidly produced by photosynthesis, so much is promptly stored as starch in the leaves that this substance is oftenused as a criterion of the process (Maskell, 1927; James, 1930). Factors that affect the storage of carbohydrates are fully discussed in relation to potassium deficiency by Hartt (1929, 1934a, 1934b), by Nightingale, Schermerhorn, and Robbins (1930), and by Phillips, Smitlh, and Dearborn (1934). It was the aim in the present work to test the plant as soon as the effects of potassium deficiency were definitely established but well before it had died from lack of this element. The Canada field pea was chosen becau.se previous Work (Day, 1935) had shown that this variety responds quickly to modifications in the nutrient solution, and it seemed probable that the leaf might be easily handled in qualitative starch tests similar to those of Maskell (1927). When the plants were harvested, fresh sections of stems and leaves were examined microscopically for differences in the internal structure, and material was fixed in formalinalcohol at that time, but only the starch analyses are reported here. CULTURAL METHODS AND APPEARANCE OF PLANTS.Canada field peas (Pisum sativum L., Idaho grown, lot 94785) were grown from seeds 2 in pure quartz sand in jars of glazed pottery, each with a lateral hole for drainage. This was covered with absorbent cotton to prevent loss of sand during the flushing; the rest of the time the hole was kept corked. The seeds were soaked in 0.25 per cent formalin, washed in distilled water, and then soaked in distilled water several hours before placing them in the sand previously moistened with the proper nutrient solution (table 1).The sand was kept sufficiently moist at all times. After two weeks from the sowing of the seeds, all jars were flushed twice with distilled water, drained for two hours, and then each jar was supplied with its respective solution. No difference was noted in the ability to germinate. The control plants, given the complete nutrient solution, displayed normal appearance of leaves, similar height of stems, and equal length of roots, while the usual characteristics of potassium starvation showed rapidly in the others. The stems were thinner and yellowish, with fewer and shorter internodes, and the