Uptake of nitrate by root cells followed by reduction and assimilation in plant tissues is the main route by which mineral N is converted into organic N by living organisms. Like photosynthesis, these are life-dependent processes that members of the animal kingdom are unable to perform for themselves. Nitrate and other mineral nutrients required for optimal plant growth and development frequently exist at relatively low concentrations in soil. To thrive on these dilute nutrients, plants have developed high-performance uptake systems in their root cells. To cope with wide variations in mineral concentrations in soil, plants have evolved mechanisms to regulate the activity of uptake systems so that net intake of a nutrient depends on the plant's need for this element rather than its concentration in the rooting medium. Indeed, uptake rates of most ions are seemingly controlled by specific demand-driven regulatory mechanisms. Such processes set the uptake rate of a given element to match the plant's current growth rate and developmental stage. Nitrate uptake is of special interest because nitrate is absorbed at a relatively high rate and because compounds that function as uptake sensors may have been identified. This paper focuses on whole-plant signaling processes involved in the regulation of nitrate uptake by N demand. NITRATE UPTAKE LlMlTS NITRATE ASSlMlLATlONNitrate-fed plants seldom accumulate an excess of either nitrite or ammonia, indicating that reduction of nitrate to nitrite usually is the rate-limiting step in nitrate assimilation. The rate of nitrate reduction in situ, however, is controlled primarily by the rate of nitrate uptake rather than by alterations in NR activity (Wilkinson and Crawford, 1993) or limitations in reducing power (Warner and Huffaker, 1989). Thus, nitrate uptake appears to control N assimilation in nitrate-fed plants. WHAT LlMlTS NITRATE UPTAKE?Experiments in which both nitrate availability and plant growth rate are manipulated independently show that nitrate uptake rates are determined mainly by regulatory processes that coordinate nitrate uptake and biomass production. This generalization is seemingly independent of plant species. Legumes, however, are a particularly interesting family because they can obtain N via nitrate uptake, symbiotic NZ fixation, or both. Hence, even when N concentrations of legumes are rather constant, their rates of nitrate uptake vary appreciably, depending on nitrate availability, the extent of nodulation, and plant developmental stage.In higher plants, two types of variation in nitrate uptake rate have been identified: (a) temporal responses to modifications of environmental factors such as light intensity, temperature, or stress conditions; and (b) variations that occur during ontogeny. In the first case, when a discrepancy exists between interna1 N supply and growth rate, nitrate uptake varies so that the amount of N in the different interna1 pools remains relatively constant. During ontogeny, the rate of nitrate uptake varies dramatically. Nitrat...

Phytophthora root rot, caused by Phytophthora megasperma Drechs. f. sp. glycinea T. Kuan & D.C. Erwin, is one of the most serious diseases of soybean [Glycine max (L.) Merr.]. Six loci with alleles giving race‐specific resistance of soybean to phytophthora have been reported. The first objective of this study was to map the phytophthora resistance (Rps) loci using restriction fragment length polymorphism (RFLP) markers. The second objective was to map the Rj2 locus for ineffective nodulation with RFLP markers, because of the linkage of Rj2 to Rps2. The mapping was conducted using a series of ‘Williams’ near‐isogenic lines (NILs) with each having one or two phytophthora resistance alleles. The NILs were screened with 141 mapped RFLP markers. At least one polymorphism was found between each NIL and the recurrent parent. Linkage tests among the polymorphic RFLP markers, Rps loci, and the RJ2 locus were conducted using F2 populations. Linkage was found between RFLP markers and Rpsl, Kps2, Rps3, Rps4, Rps5, and RJ2. Linkage was also found between Rps2 and Rj2.

Green plants deficient in nitrogen, sulfur, or iron develop a similar yellow coloration. In each case, the yellow coloration is accompanied by a lowered chlorophyll concentration. This review attempts to collate some of the biochemical information concerning these three seemingly diverse nutritive deficiencies and bares a need for a more integrative approach to plant physiology. The biochemical and biological roles of nitrogen, sulfur and iron in living systems are examined, with emphasis on sulfur and iron. Mechanistically, iron and/or sulfur are highly reactive components of many enzymes. Indeed, iron and sulfur sometimes form Fe2S2, Fe3S4, or Fe4S4 clusters which are very active electron transfer agents. Recently, iron‐sulfur clusters have been reported to serve as sensors of oxidative stress, to couple photosynthesis with several metabolic pathways, to participate in the reduction of sulfite and nitrite, and to participate in regulation of gene expression. Thus, there are several mechanisms by which a deficiency of nitrogen, sulfur, or iron could produce the same low‐chlorophyll, yellow phenotype in plants. Unless the interactions and coordination of the various pathways connected to chlorophyll synthesis are elucidated, it is unlikely that we will select the quickest and most direct path to plant improvement.

There is interest in increasing the protein concentration of soybean [Glycine max (L.) Merr.] seed. Potentially, this could change its protein composition and perhaps its value as a livestock feed. Our objective was to examine the change in protein composition of soybean seed with change in protein concentration as influenced by nitrogen source supplied. ‘Harper’ soybean was grown in hydroponic culture and supplied with different nitrogen sources (KNO3, NH4NO3N2, urea) during pod filling. Protein quality was monitored as a function of soybean seed storage subunit composition. With N2 fixation as sole N source, the concentration of total seed protein decreased about 6%, storage protein 5%. Each seed storage protein subunit decreased, but the concentration of [β‐conglycinin, especially the sulfur‐poor [β‐subunit, decreased more than glycinin. Seed storage protein concentration was increased up to 4% by substituting reduced‐N for NO−3. The increased protein concentration resulted from a disproportionate increase of the β‐subunit of β‐conglycinin. The relative abundance of α‐ and α′‐subunits was not changed by N form. Thus, N form seems to control, at least in part, the relative portion of the β‐subunit of [β‐conglycinin, and consequently affects the 11S/7S ratio of seed storage protein. This change in composition with increased concentration diminishes the protein quality because the [β‐submit lacks sulfur‐amino acids. It is noteworthy that the concentration of glycinin remained constant while the total seed protein concentration increased. We think these variations in seed storage protein composition result from the relative abundance of S‐ and N‐metabolites available to developing soybean seed.

Ilarslan, H.; Palmer, R. G.; Imsande, J.; and Horner, Harry T., "Quantitative determination of calcium oxalate and oxalate in developing seeds of soybean (Leguminosae)" (1997). Botany Publication and Papers. 41.