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The duckweed Spirodela oligorrhiza, growing in sterile defined nutrient media, was used to study some responses of plants to phosphorus deficiency. On a phosphate-deficient medium, growth of Spirodela soon slowed and eventually ceased. Older leaves became chlorotic, but newly formed leaves were dark green and contained much anthocyanin. The photosynthesis rate fell gradually, roots elongated, and chloroplasts became filled with starch.Nitrogen metabolism was not markedly affected: the total protein content changed only slightly, and. although levels of glutamine and asparagine increased, the concentrations of the other amino acids remained constant. The effects of phosphorus deficiency on Spirodela are discussed in relation to those found in other higher plants.The importance of phosphorus in plant nutrition has been long known, and the effects of phosphorus deficiency in plants have been frequently studied. Morphological and biochemical symptoms of phosphorus deficiency in crop plants have been reported (14): a number of workers have explained these symptoms in terms of changes in cell metabolism. Much of this past work on the biochemistry of phosphorus deficiency has suffered from the relatively insensitive techniques then available, and it is open to criticism on the grounds that plants were grown in nonsterile and usually undefined culture media.Recently, Spirodela oligorrhiza, a small free-floating monocotyledonous water plant, has been used for detailed examination of plant nutrient responses (1, 2, 4, 5). The present study compares the response of this plant to phosphorus deficiency with those reported for other higher plants.MATERIAILS AND METHODS Culture of Plants. S. oligorrhiza (Kurz) Hegelm. was obtained from Dr. K. V. Thimann in 1960 and has since been maintained in axenic culture at Fruit Research Division, D.S.I.R., Auckland (4). Normal plants (control plants) were grown on a complete mineral nutrient medium, and phosphorus-deficient plants were grown on medium from which phosphate had been omitted (phosphate-deficient medium) (1, 2). Nitrogen-deficient plants were grown on medium lacking (NH4)2S04 (4,5). The pH of media was kept at 7.2 by including about 50 mg of solid sterile CaCO3 in each flask. Plants were grown in 50-ml conical flasks on 20 ml of medium, at 24 C in continuous "daylight fluorescent" light, about 200 ft-c. To increase the growth rate and make it more uniform, glucose (1 %) was routinely added to mineral nutrient media. The doubling time was then 1.9 days instead of 2.4 to 3.2 days in the absence of glucose. Further details of inoculation and culture are given in References 1 and 4.Measurement of Growth. Three convenient criteria were used to measure growth. The frond number per flask was counted; all fronds extending beyond the margin of each parent frond were scored. The fresh weight per flask was measured: flasks were harvested, and plants were washed, blotted lightly, and weighed. The dry weight per flask was obtained, by drying each harvested sample for 24 hr at 110...

Onset of phosphorus deficiency in Spirodela oligorrhiza was accompanied by a 50-fold increase in phosphatase activity of cell extracts. The enzyme behaved like other plant acid phosphatase, and was both inhibited and repressed by inorganic phosphate. The phosphatase activity comprised at least three isozymes. Two, of low molecular weight, were present only in P-deficient Spirodela; one of high molecular weight was also present, though in smaller amounts, in normal Spirodela. Presence of 2-thiouracil during onset of P-deficiency partly inhibited the development of phosphatase activity. The nature and role of the increased phosphatase activity in P-deficient plants are discussed.

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