Is the sensitivity to ammonium nutrition related to nitrogen accumulation?

Nitrate and ammonium can be used as nitrogen sources by most plant species although plant response to continuous ammonium nutrition is species dependent. In the present study, the effect of the nitrogen source (nitrate and ammonium) on growth, photosynthetic parameters, nitrogen content and nitrogen assimilating-enzymes (nitrate reductase, glutamine synthetase and glutamate dehydrogenase) was investigated in wheat (Triticum aestivum L.), tomato (Solanum lycopersicum L.) and lucerne (Medicago truncatula L.). Obtained results showed that these plant species vary in their sensitivity to NH4+ nutrition, with wheat to be highly sensitive, tomato moderately sensitive and lucerne tolerant to ammonium nutrition. For the three plant species, the growth reduction was correlated closely to ammonium accumulation in leaves. Moreover, contrary to that was observed for wheat plants, glutamine synthetase and glutamate dehydrogenase activities were higher in roots than in leaves, for tomato and lucerne plants. Taken together, these 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

Does the Source of Nitrogen Affect the Response of Tomato Plants to Saline Stress?

The aim of the present study was to investigate the effects of the source of nitrogen (N) nutrition on the response of tomato (Solanum lycopersicum L. cv. Rio Grande) plants to saline stress (100 mM NaCl). To this end, plant growth, chlorophyll and carbohydrate levels, ion contents as well as N compounds and main N-metabolizing enzymes (nitrate reductase and glutamine synthetase) were analyzed in salt-treated and control plants grown in the presence of either NO3-, NH4+, or the mixture of NO3- and NH4+. Our results showed that plant growth declined under saline stress but NO3--fed plants were less sensitive to salinity than NH4+-fed plants. This different sensitivity was due mainly to a better maintenance of root growth and root nitrate reductase activity in NO3--fed plants. Concomitantly, leaf chlorophyll content was significantly decreased, regardless of the N source. Salinity affects the uptake of several nutrients in a different way, depending on the N source. Thus, sodium was accumulated mainly in NH4+-fed plants, especially in roots, displacing other cations such as NH4+and potassium. It is concluded that the N source is a major factor affecting tomato responses to saline stress, plants being more sensitive when NH4+ is the source used. The different sensitivity is discussed in terms of a competition for energy between N assimilation and sodium exclusion processes

Physiological Responses of Tomato Plants to the Combined Effect of Root Hypoxia And NaCl-Salinity

Flooding and salinity are important environmental factors restricting plant growth and productivity throughout the world because these two stresses frequently coexist. The objective in this work was to investigate the interactive effects of salinity and hypoxia on the physiological responses of tomato (Solanum lycopersicum L.) plants. To this end, growth, photosynthesis, stomatal conductance and organic solute accumulation was determined in hydroponically grown plants exposed for 4 weeks to hypoxia, salinity (100 mM) or to the combination of salinity and hypoxia.  Obtained results showed that plants exposed to salinity, either alone or in combination to hypoxia showed decreased root and shoot biomass production. However, root and shoot water contents were decreased only for plants exposed to the combination of the two stresses.  Concomitantly, leaf area, leaf mass per area, and K+ and sugar contents were significantly decreased in comparison with control (normoxia, 0 mM NaCl) plants. Na+ and proline significantly accumulated in roots and leaves of plants exposed to salinity, either alone or in combination to hypoxia. Taken together, these results suggest that tomato plants are strongly sensitive to the combination of hypoxia and salinity stresses. This is most probably due to a low K+-uptake selectivity, a strong Na+ absorption, and the disturbance of K+ translocation towards shoots and the loss of its use efficiency for biomass production. Key words: Tomato, hypoxia, salinity, growth, ions, solute accumulation Faouzi Horchani et al. Physiological Responses of Tomato Plants to the Combined Effect of Root Hypoxia And NaCl-Salinity. J Phytol 2/11 (2010) 36-4

Metabolic profile and development of tomato fruits as affected by flooding

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Prolonged root hypoxia effects on enzymes involved in nitrogen assimilation pathway in tomato plants

In order to investigate the effects of root hypoxia (1–2% oxygen) on the nitrogen (N) metabolism of tomato plants (Solanum lycopersicum L. cv. Micro-Tom), a range of N compounds and N-assimilating enzymes were performed on roots and leaves of plants submitted to root hypoxia at the second leaf stage for three weeks. Obtained results showed that root hypoxia led to a significant decrease in dry weight (DW) production and nitrate content in roots and leaves. Conversely, shoot to root DW ratio and nitrite content were significantly increased. Contrary to that in leaves, glutamine synthetase activity was significantly enhanced in roots. The activities of nitrate and nitrite reductase were enhanced in roots as well as leaves. The higher increase in the NH4+ content and in the protease activities in roots and leaves of hypoxically treated plants coincide with a greater decrease in soluble protein contents. Taken together, these results suggest that root hypoxia leaded to higher protein degradation. The hypoxia-induced increase in the aminating glutamate dehydrogenase activity may be considered as an alternative N assimilation pathway involved in detoxifying the NH4+, accumulated under hypoxic conditions. With respect to hypoxic stress, the distinct sensitivity of the enzymes involved in N assimilation is discussed

Prolonged root hypoxia effects on ethylene biosynthesis and perception in tomato fruit

The effects of root hypoxia on ethylene biosynthesis and perception have been documented in many vegetative organs, but not extensively in fruit. Therefore, in the present study, the effects of root hypoxia on ethylene biosynthesis and perception were investigated in tomato (Solanum lycopersicum L.) fruit at five stages of the maturation phase. Our results showed that root hypoxia does not affect ethylene biosynthesis in fruit, but stimulates its reception from other plant parts, as indicated by the increase in the expression of ethylene receptors ETR1 and 3

Both Plant and Bacterial Nitrate Reductases Contribute to Nitric Oxide Production in Medicago truncatula Nitrogen-Fixing Nodules1[W][OA]

Nitric oxide (NO) is a signaling and defense molecule of major importance in living organisms. In the model legume Medicago truncatula, NO production has been detected in the nitrogen fixation zone of the nodule, but the systems responsible for its synthesis are yet unknown and its role in symbiosis is far from being elucidated. In this work, using pharmacological and genetic approaches, we explored the enzymatic source of NO production in M. truncatula-Sinorhizobium meliloti nodules under normoxic and hypoxic conditions. When transferred from normoxia to hypoxia, nodule NO production was rapidly increased, indicating that NO production capacity is present in functioning nodules and may be promptly up-regulated in response to decreased oxygen availability. Contrary to roots and leaves, nodule NO production was stimulated by nitrate and nitrite and inhibited by tungstate, a nitrate reductase inhibitor. Nodules obtained with either plant nitrate reductase RNA interference double knockdown (MtNR1/2) or bacterial nitrate reductase-deficient (napA) and nitrite reductase-deficient (nirK) mutants, or both, exhibited reduced nitrate or nitrite reductase activities and NO production levels. Moreover, NO production in nodules was found to be inhibited by electron transfer chain inhibitors, and nodule energy state (ATP-ADP ratio) was significantly reduced when nodules were incubated in the presence of tungstate. Our data indicate that both plant and bacterial nitrate reductase and electron transfer chains are involved in NO synthesis. We propose the existence of a nitrate-NO respiration process in nodules that could play a role in the maintenance of the energy status required for nitrogen fixation under oxygen-limiting conditions