Effects of Phosphorus Deficiency on Growth and Metabolism of Sunflower

1. The effects of phosphorus deficiency on sunflower plants were studied in sand culture. An early symptom was the deeper green color of the leaves. Later the lower leaves became chlorotic, and the chlorosis was followed by necrosis of the tissue. In certain cases the tissue of green areas of the leaves collapsed, turned black or dark brown, and became dry and brittle. The leaves were small, and the stems were short, thin, and hard. Growth of tops was affected more than that of roots, resulting in a lower toproot ratio of minus-phosphorus than of plus-phosphorus plants. Roots of the former plants were much darker brown than those of the latter. 2. Phosphorus-deficient stems were low in moisture but were high in total sugars, reducing sugars, sucrose, and starch, especially in the younger plants. At later stages of development there was less difference in the two sets of plants, and at certain levels of the third harvest all the carbohydrate fractions except sucrose were higher in the plus-phosphorus stems. There was little starch or sucrose in the stems of either group of plants. Reducing sugars made up a large percentage of the total sugars. 3. Phosphorus deficiency does not seem to interfere with the nitrogen metabolism of sunflower stems. When figured on the dry-weight basis, total nitrogen, soluble nitrogen, and insoluble nitrogen were higher at all levels in the plus-phosphorus stems of each harvest than in the deficient ones. On the fresh-weight basis the differences were smaller and in certain cases slightly in favor of the minus-phosphorus stems. None of the soluble-nitrogen fractions-nitrate nitrogen, ammonia nitrogen, amino nitrogen, and amide nitrogen-accumulated in the phosphorus-deficient stems, at least at the time of the second harvest. These nitrogen fractions have been reported to accumulate in other plants suffering from phosphorus starvation. 4. Total phosphorus, soluble phosphorus, and insoluble phosphorus were much higher in plus-phosphorus stems than in minus-phosphorus ones. The soluble phosphorus of the latter stems made up a much larger percentage of the total phosphorus than in the case of the former. Phosphorus seems to be readily translocated from the lower parts of phosphorus-deficient plants to the upper regions. The upper leaves of plants which had been grown about 9 weeks with a minus-phosphorus solution and which had developed flower buds appeared normal. Other plants which had been grown 7 weeks without phosphorus grew rapidly when phosphorus was supplied.