On the Complex Formation of Nickel with Dimethylglyoxime.

Abstract

ABSTRACIT Nickel was established as an essential micronutrient for the growth of temperate cereal crops. Grain from barley (Hordeum vulgare L. cv 'Onda'; containing 40 to 80 nanograms of Ni per gram dry weight) grown in solution culture with negligible Ni concentrations (< 30 nanograms of Ni per liter) exhibited greatly reduced germination rates (i.e. 50% less than grain from Ni-adequate plants) and seedling vigor of the viable grain was greatly depressed. Grain containing less than 30 nanograms per gram dry weight was inviable. Under Ni-deficient conditions, barley plants fail to produce viable grain because of a disruption of the maternal plant's normal grain-filling and maturation processes that occur following formation of the grain embryo. The observations that (a) barley plants fail to complete their life cycle in the absence of Ni and (b) addition of Ni to the growth medium completely alleviates deficiency symptoms in the maternal plants satisfies the essentiality criteria; thus, Ni should be considered a micronutrient for cereals. Because Ni is required by legumes, and is now established as essential for cereals, we conclude that Ni should be added to the list of micronutrients essential for all higher plant growth. The discovery in 1975 that Ni is a component of the enzyme urease (6), which is present in a wide range of plant species (15), led to renewed scientific interest and research concerning the role ofNi in higher plants. Several researchers have since reported growth responses of plants to Ni fertilization under field conditions (for a review see Ref. 15) and in plants grown in nutrient solutions (13) or in tissue culture media furnished with urea as the sole N source (12). Eskew et al. (7) reported that Ni-deficient soybean (Glycine max L.) accumulate toxic levels of urea in their leaflet tips because of a depression in urease activity in their leaves. Walker et al. (14), working with cowpeas (Vigna ungui-culata L. Walp), suggested that Ni (and urease) participates in N metabolism of legumes during the reproductive phase ofgrowth. Checkai et al. (4) reported that Ni-deficient tomato plants (Ly-copersicon esculentum L.) developed chlorosis in the newest leaves and, ultimately, necrosis of the meristem. The earliest report of a growth response to Ni additions under controlled experimental conditions (2) indicated that Ni deficiency has a wide range of effects on plant growth and metabolism. These include effects on (a) plant growth, (b) plant senes-cence, (c) N metabolism, and (d) Fe uptake. Preliminary investigations also indicate that Ni may have a role in phytoalexin synthesis and plant disease resistance (9). Thus, low levels of Ni are known to be beneficial to plant growth. Previously, however, no study has satisfied all of the essentiality criteria for establishment of Ni as an essential element for all higher plants. For an element to be proven essential, one must demonstrate that a plant cannot complete its life cycle in the absence of the element, and that no other element can substitute for the test element (1). We report here that Ni satisfies these criteria and, therefore, should be classified as a micronutrient element essential for all higher plant growth. MATERIALS AND METHODS Establishing a trace element as essential requires techniques capable of reducing its levels in the nutrient media and environment to below those levels required by the plant. For investigations involving Ni, this involves lowering Ni in the seed or grain to a sufficiently low level by growing plants for several generations under low-Ni conditions and upon maintaining controlled, low levels of Ni in the growth medium (i.e. <30 ng Ni L'). Plant Material and Growth Conditions. Details of solution culture techniques and procedures used to minimize Ni contamination were described elsewhere (2). Briefly, to reduce Ni contamination , macronutrient salt stock solutions and deionized water supplied to growing plants were purified by column chromatography using an ion exchange column packed with 8-hydroxyquinoline controlled-pore glass beads (Pierce2) and by using only very high purity micronutrient chemicals and reagents (Ultrex, J. T. Baker; Puratronic and Specpure, Johnson Matthey; 7) Barley plants (Hordeum vulgare L. cv 'Onda') were grown for three generations in purified nutrient solutions supplemented with 0, 0.6, and 1.0 uM NiSO4 (40 plants per treatment). Grain from the third generation plants were used in the experiments reported here. Yield responses to the addition of Ni to the growth medium and Ni deficiency symptoms in third generation plants used to produce the grain for studies reported here, were described elsewhere (2). Germination. Germination tests were performed by imbibing grain in deionized water for 24 h and then placing the grain on wetted filter paper in sealed Petri dishes at 25°C for 36 h in the dark. In a second procedure, grain were first imbibed in a weak NiSO4 solution (1.0 FM Ni; pH 4.9), which is an effective method of supplying micronutrients to germinating grain (10). Nickel Determination. Concentrations of Ni in 5 to 10 grain (0.5-1.0 g total dry weight) were determined by an isotope dilution mass spectrometry technique as follows. Twenty ng of 2Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the United States Department of Agriculture and does not imply its approval to the exclusion of other products or vendors that may also be suitable. 801