Hydroponic Growth and the Nondestructive Assay for Dinitrogen Fixation

Abstract

Hydroponic growth medium must be well buffered if it is to support sustained plant growth. Athough 1.0 milar phosphate is commonly used as a buffer for hydroponic growth media, at that concentration It is generally toxic to a soybean plant that derives Its nitrogen solely from dinitrogen fixati On the other hand, we show that 1.0 to 2.0 milllmolar 2-(N-morpbolno)ethanesulfonic acid, pK. 6.1, has excelent buffering capacity, and It neither interferes with nor contributes nutritionally to soybean plant growth. Furthermore, it neither impedes nodulation nor the assay of dien fixatio Hence, soybean plants grown hydroponically on a medium pemented with 1.0 to 2.0 millm r 2-(N-morpho- Under conditions of nitrogen deprivation, however, 1.0 mm phosphate is often toxic to hydroponically grown soybeans (9, 13). This toxicity can be overcome by reducing the concentration of phosphate to 0.1 mM; this, however, essentially abolishes the buffering capacity of the growth medium. It has been suggested that excess solid CaCo3 may serve as a suitable buffer (22), but high concentrations of carbonate promote both phosphate uptake and chlorosis (13,16). Thus, Amberlite IRC resins were proposed as buffers (8, 13). Unfortunately, Amberlite IRC resins also can bind trace metal ions; hence, a simple solution to the buffering problem has not been reported.The most widely used procedure for the assay of dinitrogen fixation by leguminous plants relies upon the reduction of acetylene to ethylene (1,4,7,17,19). It should be recalled, however, that both acetylene and ethylene dissolve readily in water (3). Thus, the acetylene reduction assay procedure should be performed in the absence of aqueous medium or soil moisture.Combining the concept of hydroponic growth and the inexpensive, yet sensitive, acetylene reduction technique, we have devised a simple, rapid, reproducible assay procedure whereby the rate of dinitrogen fixation by individual plants can be measured throughout the lifetime of those plants. We also report that Mes (pKa of 6.1) satisfactorily buffers soybean hydroponic growth medium, permits excellent plant growth, and does not interfere with dinitrogen fixation.One of the major problems in attempting to increase dinitrogen fixation in legumes is the identification of those plants that are genetically endowed for enhanced dinitrogen fixation (1 1). Screen (2,8,13,(20)(21)(22). In most instances, the hydroponic growth medium contained nitrate and/or ammonia and was buffered with 1.0 mm phosphate. Ammonia and nitrate, however, may repress dinitrogen fixation in soybeans (8,21). Hence, to establish the maximum potential rate of dinitrogen fixation in a soybean plant, it is desirable to deprive the plant of ammonia and/or nitrate. '

Distribution and Mobilization of Sulfur during Soybean Reproduction

“…Plants in our system received 6.9 and 13.8 mg S (plant d)"' prior to and after R4.0, respectively. These values closely follow the 8.0 and 11 mg S (plant d)'^ suggested by Imsande and Ralston (1981).…”

Regulation of Soybean Nitrogen Fixation in Response to Rhizosphere Oxygen

“…In brief, soybean plants were grown in hydroponic chambers made from 102 mm diameter PVC pipe. Each chamber contained half-strength nitrogen-free nutrient solution (6) and was aerated by flowing 33 ml s-' of air through a glass bead bubbler in the bottom of the chamber. The chambers were submerged in a water bath to maintain a constant root temperature of 26°C and illuminated with 950 to 1400 ME m-2 s-I of photosynthetically active radiation depending on position in the canopy.…”

Differential Accumulation of Soybean Seed Storage Protein Subunits in Response to Sulfur and Nitrogen Nutritional Sources

“…Kenwood) was grown by hydroponic culture (Imsande and Ralston, 1981) in duplicate trials conducted in adjacent greenhouses between 22 Aug. 1995 and 15 Jan. 1996. Seeds were germinated on paper and, when the cotyledons opened, were transferred to opaque 3.8L pots that contained the hydroponic medium.…”