Deficienoy on Levels of Phosphorus Compounds in Spirodela

When Spirodela plants are transferred to a phosphate-deficient medium, growth slows down immediately, and ceases after 14 days. During this time, inorganic phosphate content falls from 30 to 0.7 ,umoles/g fresh weight of tissue, phosphate ester content from 3.5 to 0.6 gmoles/g, phospholipid content from 3.5 to 1.2 ,umoles/g, and residual phosphate (mainly RNA) content from 7.5 to 2.0 mmoles/g. Relative proportions of the various phosphate esters, and relative proportions of the various phospholipids, are not markedly affected by phosphate deficiency. Turnover rates of phosphate esters are somewhat higher in phosphatedeficient tissue. In control tissue, inorganic phosphate is present in 2 pools; a metabolic (12 No%) and a non-metabolic pool (88 % ). In phosphate-deficient tissues, most of the inorganic phosphate (>90 %) is in the metabolic pool. Non-metabolic phosphate is presumably stored in the vacuole, and is not readily accessible to the tissue, so that growth normally occurs at the expense of external phosphate. During deficiency, growth is limited by the rate at which phosphate can be transported through the tonoplast and tissue to the growing point. Growth ceases when the supply of non-metabolic phosphate is exhausted. Metabolic phosphate is presumably located in the cytoplasm: it can not be used for growth. Nor can the plant respond to deficiency by making some phosphorus compounds at the expense of others. In this respect, phosphorus deficiency and nitrogen deficiency are dissimilar. Phosphorus is a major plant nutrient, and phosphate deficiency is a major factor limiting plant productivity. Even under conditions where phosphate supply is apparently sufficient, addition of further phosphate can often produce desirable biological effects, as in increasing the storage quality of fruits (11). At least 2 factors stand in the way of understanding the basis of phosphorus deficiency: the complex nature of soil as a nutrient medium, and the multitude of roles that various phosphorus compounds can play in the life of the plant. The previous paper (3) describes the use of a duckweed, Spirodela, growing axenically in a completely defined nutrient solution, as a model system to study the way in which phosphorus distributes between the different organic compounds in an actively growing, nutrient-sufficient plant. In the following paper, the same system has been used to study the wav in which this distribution is affected after phosphate is removed from the medium: that is, during the onset of phosphorus deficiency. As before, plants have been grown for several generations in 32P1, in order to label uniformly all phosphorus-containing compounds, and thereby provide a simple method of measuring the amount of each compound through its radioactivity (3). Such plants have then been transferred to a phosphate-deficient (minus-P) medium, and the subsequent effects studied. Materials and Methods Culture of Tissue and Measurement of Growth. Spirodela oligorrhiza (Kurz) Hegelm. was grown in axenic culture (3). Minus-P medium was made up: it contained 4 mm (NH4)2S04, 2 mm CaSO4, 2 mm MgSO4, 1 mM KoS04, minor and trace elements, and 1 % glucose. Control medium (3) was obtained by adding KH2PO4 at 1 mm to the deficient medium; and labeled medium, by adding KH232PO0 of the appropriate specific activity. Aliquots of 20 ml within 50 ml conical flasks were autoclaved, and sterile CaCO3, about 50 mg, was added to each flask to stabilize the pH at 7.4. When required, filter-sterilized kinetin (at 1 part per million) or zeatin (0.1 part per million) was added to each flask. Fronds for inoculation were washed first in sterile distilled water, then inoculated as before (3). Growth could be measured either by counting the number of fronds present in 1 flask at successive times, or by preparing many flasks in a treatment then harvesting some at successive times. In the latter case, plants from each flask were washed in 3 changes of distilled water during 2 minutes, then blotted and weighed. Distributtion of Phosphorus in Fractions During Deficiency. Firstly, tissue was inoculated into 1309 www.plantphysiol.org on September 22, 2017 Published by Downloaded from Copyright © 1968 American Society of Plant Biologists. All rights reserved.