Effects of zinc deficiency on phosphate metabolism of the tomato plant.

A GROWING recognition of the importance of micronutrients has accelerated investigations in many fields and has materially extended our knowledge of the physiology, biochemistry, and cytology of plants. This paper attempts to elucidate certain phenomena exhibited by tomato plants grown in nutrient solutions of purified salts differing inter se in the amounts of zinc they received. The general technique of the preparation and care of the cultures has been described by Stout and Arnon (1939). Four cultures, each containing three plants in covered pyrex glass beakers, were employed for four concentrations of zinc, namely, O.O, 0.02, 0.1, and 0.5 parts per million. Where no zinc was added the plants were dwarfed from an early stage and eventually ceased growth; however, in the two higher concentrations they grew well and produced fruit. In the zinc deficient cultures the leaves were curved downwards,. the leaflets were chlorotic, inroluted, and often contained dead spots. The disrupted metabolism in the leaflet was correlated with the scarcity of plastids, production of melanotic material, and presence of calcium oxalate crystals (Reed, 1939). STEMs.-Distribution of phosphates.-Fresh sections of the penultimate internodes of zinc deficient stems when tested with molybdenic reagent in watch glasses produced more blue color in an hour than those of healthy stems; moreover, the color produced by the former was diffusible. When sections were rinsed, transferred to slides, and examined under high magnification there was no significant difference in the depth of color which they retained, indicating that the former were richer in diffusible phosphate compounds or that the protoplasm was more permeable. The walls of collenchyma and central pith remained deep blue in both lots of material. Other sections of similar stems from plants of the same age were tested for inorganic phosphates with the Gomori technique which precludes their diffusion. Sections of healthy stems (fig. 1) showed an over-all brown black color. Black material indicating phosphate was observed chiefly in collenchyma walls and inner and outer phloem, but it was scarce in cambium and primary xylem. The endodermal and collenchyma cells were optically empty, though walls of the latter were dark brown. Sections of affected stems (fig. 2) contained more black material in the extra-stelar region in comparison with the healthy, though there was none in the endodermal layer. Black material was abundant in the inner and outer phloem. The pictures therefore supplement the results with molybdenic reagent and agree with the results of unpublished work on other plants showing that phosphate may be and generally is 1 Received for publication June 15, 1946. more abundant in the phloem of deranged plants than in xylem or extra-stelar tissues. The retarded development of the phloem-xylem layers in the zinc deficient stem affords another contrast which should be recognized as a clinical symptoni. The parenchyma cells of the pith of the affected stem were generally larger than those of the healthy. (On the ratio of the sizes of affected cells of vegetative buds of the peach, see Reed, 1941.) Parenchyma cells contained more starch than similar cells in healthy stems, indicating an impairment of phosphorylase, hexokinase, or other enzymes in zinc deficient stems. They also contained more calcium carbonate.Convergent evidence on the higher level of inorganic phosphate in zinc deficient stems was obtained from the reaction of free hand sections immersed in weak silver nitrate solution for a short time, followed by washing and immersion in s-odium thiosulfate solution. After another washing, the sections were mounted in glycerine jelly. The contrast in appearance of the sections of healthy and of affected stems (fig. 3) after this treatment affirms the conclusions drawn from the preceding tests. The black material is interpreted as silver which had been precipitated by phosphate ions plus a certain amount of phenolic compounds, but the latter were comparatively scarce in these tissues. Inorganic phosphate was obviously more abundant in the cortex and pith of affected stems. The xylem in both stems contained phosphate which can be regarded as a solute in transit from the root to other centers of transformation. The heavier deposit in the cortex and pith of affected stems indicates that they had a higher level of phosphatase activity. The black precipitate in parenchyma cells of affected stems was discernible principally in the periphery of the cell when highly magnified (fig. 4). This is interpreted to mean that in the affected cells phosphate esters had been largely converted into inorganic phosphate, thereby liberating energy for maintenance. Further evidence of damage to this cell is afforded by the aggregates of desolvated phenolic compounds and by the clumping of starch grains about the nucleus. In a healthy cell the inorganic phosphate would enter esters through a process of phosphorylation coupled with oxidation, but in a hypoplastic cell, where the necessary mechanism fails to work, the inorganic phosphate accumulates. Enzymic action had apparently liberated phosphate anions from phosphate esters and possibly from partially disintegrated cytoplasm. To obtain further evidence on this point, I made tests for phosphatase by the method of Gomori (1939). Fresh sections of healthy and of zinc deficient stems were submerged in a solution which contained 2 per cent glycerophosphate and 2 per cent calcium nitrate where they were held at 300 to