Berta Lasa

Pea responses to saline stress is affected by the source of nitrogen nutrition (ammonium or nitrate)

The effect of the source of nitrogen nutrition (ammonium or nitrate), onthe response of pea plants to a moderate saline stress (30 mMNaCl)was studied. Growth declined under saline stress but nitrate-fed plants wereless sensitive to salinity than ammonium-fed plants. This different sensitivitywas due mainly to a better maintenance of root growth in nitrate-fed plants.Organic nitrogen content decreased significantly in roots of ammonium-fedplants. Water relations changed slightly under saline stress leading to adecrease in stomatal conductance, which was correlated to a decline in carbonassimilation rates regardless of nitrogen source. Salinity affects the uptakeofseveral nutrients in a different way, depending on the nitrogen source. Thus,chloride was accumulated mainly in nitrate-fed plants, displacing nitrate,whereas sodium was accumulated mainly in ammonium-fed plants, especially inroots, displacing other cations such as ammonium and potassium. It is concludedthat the nitrogen source (ammonium or nitrate) is a major factor affecting pearesponses to saline stress, plants being more sensitive when ammonium is thesource used. The different sensitivity is discussed in terms of a competitionfor energy between nitrogen assimilation and sodium exclusion processes.

The sensitivity to ammonium nutrition is related to nitrogen accumulation

Abstract Ammonium and NO3− can be utilized as nitrogen sources by most plant species although the plant response to continuous ammonium nutrition is species dependent. The effect of the nitrogen source (nitrate and ammonium) on growth, photosynthetic parameters and nitrogen content in spinach (Spinacea oleracea L.), sunflower (Helianthus annuus L.) and pea (Pisum sativum L.) was studied. Results showed spinach to be highly sensitive, sunflower moderately sensitive and pea tolerant to ammonium nutrition. Ammonium accumulation in shoots was closely correlated to growth reduction. Moreover, ammonium accumulation was correlated to an increase of organic nitrogen content. The data suggest that the site of ammonium assimilation is a key factor controlling tolerance to ammonium nutrition in the different plant species, with plants being more tolerant when ammonium is assimilated in roots.

Short-term ammonium supply stimulates glutamate dehydrogenase activity and alternative pathway respiration in roots of pea plants

Summary The changes of C and N metabolism after switching Pisum sativum L. plants from nitrate to ammonium were studied. Pea plants were grown for three weeks in nutrient solution containing 0.5 mmol/L nitrate, and then randomly divided into five sets for five different nitrogen treatments: control (0.5 mmol/L nitrate) and four ammonium concentrations (0.5, 1, 2.5 and 5 mmol/L). After 72 hours, activities of enzymes related to C and N metabolism were measured. Ammonium content in roots increased with ammonium concentration showing saturation from a concentration of 2.5 mmol/L. Increasing external ammonium concentration also increased free amino acid content (mainly glutamine and asparagine), whereas starch content decreased and neither organic acid or soluble carbohydrates changed. Glutamine synthetase (GS; EC 6.3.1.2) activity decreased and root glutamate dehydrogenase (GDH; EC 1.4.1.2) activity increased with ammonium regardless of the concentration used. Root respiration rate increased with ammonium, due mainly to an increase of the alternative pathway. These results could be consistent with the assumption of a possible role for GDH in ammonium detoxification. Our results show a close relationship between GDH activity and respiration rate through alternate pathways in order to ensure the supply of C skeletons for ammonium assimilation, whereas surplus NADH is oxidised directly via the non-phosphorylating route.

Effects of low and high levels of magnesium on the response of sunflower plants grown with ammonium and nitrate

The effect of the nitrogen source (ammonium and nitrate) and its interaction with magnesium on various physiological processes was studied in sunflower plants (Helianthus annuusL.). Plants were grown in hydroponic culture with nitrate (5 mM) or ammonium (5 mM) and four concentrations of magnesium (0.1, 0.8, 5 and 10 mM). After 2 weeks, growth, gas exchange and fluorescence parameters, soluble carbohydrates, free amino acids, soluble protein and mineral elements were determined. Ammonium nutrition resulted in a reduction of dry matter accumulation, as well as in a decrease in the CO2 assimilation. Moreover, ammonium-fed plants showed a greater content of free amino acids, soluble protein, Rubisco and anions, and a lower cation content, mostly Mg2+. The presence of high levels of Mg2+ in the nutrient solution containing NH4+ resulted in a stimulation of growth and CO2 assimilation to the levels observed in nitrate-fed plants. The lower photosynthetic rate of ammonium-fed plants grown with low level of magnesium does not seem to be due to a lower photosynthetic pigment content, or a deficiency in Photosystem II activity, or to lower Rubisco content. Hence, Rubisco activity or other enzymes involved in CO2 fixation could have been affected in ammonium-fed plants.

Alternative pathway respiration is associated with ammonium ion sensitivity in spinach and pea plants

Spinach and pea plants were grown in hydroponic culture with nitrate orammonium salts as the nitrogen source. Dry matter accumulation andphotosynthetic rate declined in spinach plants fed with ammonium salts, whereasthey did not change in pea plants compared with nitrate-fed plants. Measurementof organic nitrogen and free amino acid content showed that ammonium ions wereassimilated in shoots in spinach plants and in roots in pea plants. Ammoniumionnutrition led to a decline in starch content in both species. Organic acidsincreased in roots of pea plants fed with ammonium ions whereas they declinedinspinach plants. In both species ammonium ions increased root respiration ratebut the contribution of both routes (cytochromic and alternative pathway) tothis increase was different depending on the species. In spinach plants,ammonium ions increased the cytochromic path and decreased the alternativepathway, whereas in pea plants both routes were stimulated mainly through thealternative pathway. The differences in the sensitivity to ammonium ionsbetweenboth species are discussed in terms of differences in the availability of Cskeletons and energy, which could be due in part to differences in the capacityto stimulate the alternative pathway.

Quantitative proteomics reveals the importance of nitrogen source to control glucosinolate metabolism in Arabidopsis thaliana and Brassica oleracea

Highlight A quantitative proteomic approach demonstrates how ammonium nutrition induces glucosinolate biosynthetic and catabolic pathways in Arabidopsis and broccoli.

Short term physiological implications of NBPT application on the N metabolism of Pisum sativum and Spinacea oleracea.

The application of urease inhibitors in conjunction with urea fertilizers as a means of reducing N loss due to ammonia volatilization requires an in-depth study of the physiological effects of these inhibitors on plants. The aim of this study was to determine how the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) affects N metabolism in pea and spinach. Plants were cultivated in pure hydroponic culture with urea as the sole N source. After 2 weeks of growth for pea, and 3 weeks for spinach, half of the plants received NBPT in their nutrient solution. Urease activity, urea and ammonium content, free amino acid composition and soluble protein were determined in leaves and roots at days 0, 1, 2, 4, 7 and 9, and the NBPT content in these tissues was determined 48h after inhibitor application. The results suggest that the effects of NBPT on spinach and pea urease activity differ, with pea being most affected by this treatment, and that the NBPT absorbed by the plant caused a clear inhibition of the urease activity in pea leaf and roots. The high urea concentration observed in leaves was associated with the development of necrotic leaf margins, and was further evidence of NBPT inhibition in these plants. A decrease in the ammonium content in roots, where N assimilation mainly takes place, was also observed. Consequently, total amino acid contents were drastically reduced upon NBPT treatment, indicating a strong alteration of the N metabolism. Furthermore, the amino acid profile showed that amidic amino acids were major components of the reduced pool of amino acids. In contrast, NBPT was absorbed to a much lesser degree by spinach plants than pea plants (35% less) and did not produce a clear inhibition of urease activity in this species.

Inhibition of endogenous urease activity by NBPT application reveals differential N metabolism responses to ammonium or nitrate nutrition in pea plants : a physiological study

Background and aimsUrea is the predominant form of N applied as fertilizer to crops, but it is also a significant N metabolite of plants themselves. As such, an understanding of urea metabolism in plants may contribute significantly to subsequent N fertilizer management. It currently appears that arginase is the only plant enzyme that can generate urea in vivo. The aim of this work was, therefore, to gain a more in-depth understanding of the significance of the inhibition of endogenous urease activity and its role in N metabolism depending on the N source supplied.MethodsPea (Pisum sativum cv. Snap-pea) plants were grown with either ammonium or nitrate as the sole N source in the presence or absence of the urease inhibitor NBPT.ResultsWhen supplied, NBPT is absorbed by plants and translocated from the roots to the leaves, where it reduces endogenous urease activity. Different N metabolic responses in terms of N-assimilatory enzymes and N-containing compounds indicate a different degree of arginine catabolism activation in ammonium- and nitrate-fed plants.ConclusionsThe arginine catabolism is more highly activated in ammonium-fed plants than in nitrate-fed plants, probably due to the higher turnover of substrates by enzymes playing a key role in N recycling and remobilization during catabolism and in early flowering and senescence processes, usually observed under ammonium nutrition.

The physiological implications of urease inhibitors on N metabolism during germination of Pisum sativum and Spinacea oleracea seeds

The development of new nitrogen fertilizers is necessary to optimize crop production whilst improving the environmental aspects arising from the use of nitrogenous fertilization as a cultural practice. The use of urease inhibitors aims to improve the efficiency of urea as a nitrogen fertilizer by preventing its loss from the soil as ammonia. However, although the action of urease inhibitors is aimed at the urease activity in soil, their availability for the plant may affect its urease activity. The aim of this work was therefore to evaluate the effect of two urease inhibitors, namely acetohydroxamic acid (AHA) and N-(n-butyl) thiophosphoric triamide (NBPT), on the germination of pea and spinach seeds. The results obtained show that urease inhibitors do not affect the germination process to any significant degree, with the only process affected being imbibition in spinach, thus also suggesting different urease activities for both plants. Our findings therefore suggest an activity other than the previously reported urolytic activity for urease in spinach. Furthermore, of the two inhibitors tested, NBPT was found to be the most effective at inhibiting urease activity, especially in pea seedlings.

Isotopic composition of maize as related to N-fertilization and irrigation in the Mediterranean region

Nitrate leaching as a result of excessive application of N-fertilizers and water use is a major problem of vulnerable regions. The farming of maize requires high N fertilization and water inputs in Spain. Isotopic techniques may provide information on the processes involved in the N and C cycles in farmed areas. The aim of this work was studying the impact of sprinkler and furrow irrigation and N input on maize (Zea mays L.) yields, and whether isotopic composition can be used as indicator of best farming practices. Trials were set up in Tudela (Spain) with three rates of N fertilization (0, 240 and 320 kg urea-N ha-1) and two irrigation systems (furrow and sprinkler). Yield, nitrogen content, irrigation parameters, N fate and C and N isotope composition were determined. The rate of N fertilization required to obtain the same yield is considerably higher under furrow irrigation, since the crop has less N at its disposal in furrow irrigation as a result of higher loss of nitrogen by NO3--N leaching and denitrification. A lower δ13C in plants under furrow irrigation was recorded.The δ15N value of plant increased with the application rate of N under furrow irrigation.