D. William Rains

Differential effect of amino acids on nitrate uptake and reduction systems in barley roots.

This study was conducted to determine whether the inhibition of nitrate reductase activity (NRA; EC 1.6.6.1) in barley (Hordeum vulgare L. var. CM-72) roots by the amino acids (glutamic, aspartic, glutamine and asparagine) is a direct effect or indirect due to inhibition of the NO(3)(-) uptake system. Roots of 8-day-old intact seedlings were supplied with the amino acids (I mM) individually either with NO(3)(-) (0.1 or 10 mM) or roots were pretreated with the amino acids and then supplied with NO(3)(-) only. Nitrate uptake was determined by following NO(3)(-) depletion from the uptake solution containing 0.1 mM NO(3)(-). All the amino acids inhibited the increase in NO(3)(-) uptake similarly (50-60%) when the roots were supplied with 0.1 mM NO(3)(-). Pretreatment with glutamic and aspartic acids was more inhibitory (70-80%) than with glutamine and asparagine (30%). The amino acids partially inhibited (35%) the induction of NRA in roots supplied with 0.1 mM NO(3)(-); however, no inhibition occurred at 10 mM NO(3)(-). Likewise, pretreatment with glutamic or aspartic acid inhibited the induction of NRA at 0.1 mM NO(3)(-) but not at 10 mM NO(3)(-). In contrast, pretreatment with glutamine or asparagine had no effect on the subsequent induction of NRA, even at 0.1 mM NO(3)(-). The results suggest that, at low NO(3)(-) supply, the inhibition of induction of NRA by the amino acids is a result of the lack of substrate availability due to inhibition of the NO(3)(-) uptake system.

Effect of ammonium on the regulation of nitrate and nitrite transport systems in roots of intact barley (Hordeum vulgare L.) seedlings

The effect of NH4+on the regulation of NO3−and NO2−transport systems in roots of intact barley (Hordeum vulgareL.) seedlings grown in NO3−or NO2−was studied. Ammonium partially inhibited “induction” of both transport systems. The inhibition was less severe in NO2−-fed than in NO3−-fed seedlings, presumably due to lower uptake of NH4+in the presence of NO2−. In seedlings pretreated with NH4+subsequent “induction” was inhibited only when NH4+was also present during “induction”, even though pretreated roots accumulated high levels of NH4+. This indicates that inhibition may be regulated by NH4+concentration in the cytoplasm rather than its total accumulation in roots. L-Methionine sulfoximine did not relieve the inhibition by NH4+, suggesting that inhibition is caused by NH4+itself rather than by its assimilation product(s). Ammonium inhibited subsequent expression of NO3−transport activity similarly in roots grown in 0.1, 1.0, or 10 mM NO3−for 24 h (steady-state phase) or 4 d (decline phase), indicating that it has a direct, rather than general feedback effect. “Induction” of the NO3−transport system was about twice as sensitive to NH4+as compared to the NO2−transport system. This may relate to higher turnover rates of membraneassociated NO3−-transport proteins.

Enhancement of nitrate reductase activity and metabolic nitrate concentration by methionine sulfoximine in barley roots

Abstract The enhancement of nitrate reductase activity (NRA) and metabolic NO 3 − concentration by methionine sulfoximine (MSO) in roots of 8-day-old intact barley ( Hordeum vulgare L.) seedlings was studied. NRA was induced with 0.05–100 mM NO 3 − with or without 0.25 mM MSO and assayed in vitro using NADH as the electron donor. Metabolic NO 3 − concentration was estimated by the anaerobic in vivo NRA assay method. In the absence of MSO, induction of NRA reached a plateau at 0.1 mM external NO 3 − , even though root NO 3 − accumulation continued at higher external NO 3 − concentrations. MSO enhanced the induction of NRA in roots exposed to NO 3 − concentrations greater than 0.1 mM, although total NO 3 − accumulation in MSO-treated roots decreased. In the absence of MSO, induction of NRA reached a plateau after 6 h exposure to 100 mM NO 3 − , whereas in the presence of MSO the enzyme activity increased gradually up to 24 h. Metabolic NO 3 − concentrations were similar in roots induced with 0.1 and 100 mM NO 3 − . MSO had no effect on the metabolic NO 3 − concentration when roots were induced with 0.1 mM NO 3 − , but it increased the metabolic NO 3 − concentration in roots induced with 100 mM NO 3 − by 2-fold between 6 and 24 h. The results indicate that in the absence of MSO the metabolic NO 3 − pool is saturated at 0.1 mM exogenous NO 3 − and that enhancement of NRA by MSO at higher NO 3 − levels may be due to increased accumulation of NO 3 − in that pool.

Use of ion chromatography in agricultural research

Abstract Durint the last decade ion chromatography developed into a powerful analytical technique. Verstility, speed of operation, simultaneous multi-ion analysis, small sample size requirements and reasonable cost are some of the factors that have contributed to its popularity. One of the unique features of this technique has been the quantitative determination of various species of an ion ( e.g. , different oxidation states) that may exist in a sample. This paper describes the applications of ion chromatography technology in the agricultural research and some future directions.