Effect of Nitrate and Ammonium Nutrition of Nonnodulated Phaseolus vulgaris L. on Phosphoenolpyruvate Carboxylase and Pyruvate Kinase Activity

Young bean plants (Phaseolus vulgaris L. var Saxa) were fed with 3.5 or 10 millimolar N in either the form of NO₃⁻ or NH₄⁺, after being grown on N-free nutrient solution for 8 days. The pH of the nutrient solutions was either 6 or 4. The cell sap pH and the extractable activities of phosphoenolpyruvate carboxylase and of pyruvate kinase from roots and primary leaves were measured over several days.

The extractable activity of phosphoenolpyruvate carboxylase (based on soluble protein) from primary leaves increased with NO₃⁻ nutrition, whereas with NH₄⁺ nutrition and on N-free nutrient solution the activity remained at a low level. Phosphoenopyruvate carboxylase activity from the roots of NH₄⁺-fed plants at pH 4 was finally somewhat higher than from the roots of plants grown on NO₃⁻ at the same pH. There was no difference in activity from the root between the N treatments when pH in the nutrient solutions was 6. The extractable activity of pyruvate kinase from roots and primary leaves seemed not to be influenced by the N nutrition of the plants.

The results are discussed in relation to the physiological function of both enzymes with special regard to the postulated functions of phosphoenolpyruvate carboxylase in C3 plants as an anaplerotic enzyme and as part of a cellular pH stat.

     

Aspects of nitrogen metabolism in the rice seedling

The effects of nitrogen source NO₃⁻ or NH₄⁺ on nitrogen metabolism during the first 2 weeks of germination of the rice seedling (Oryza sativa L., var. IR22) grown in nutrient solution containing 40 μg/ml N were studied. Total, soluble protein, and free amino N levels were higher in the NH₄⁺-grown seedling, particularly during the 1st week of germination. Asparagine accounted for most of the difference in free amino acid level, in both the root and the shoot. Nitrate and nitrite reductase activities were present mainly in the shoot and were higher in the NO₃⁻-grown seedling, whereas the activity of glutamate dehydrogenase and glutamine synthetase in the root tended to be lower than that of the NH₄⁺-grown seedling during the 1st week of germination. Glycolate oxidase and catalase activities were present mainly in the shoot. Maximum activity of the above five enzymes occurred 7 to 10 days after germination. Differences in the zymograms of nitrate reductase, glutamate dehydrogenase, and catalase were mainly between shoot and root and not from N source. Nitrite reductase bands were observed only in plants grown in plants grown in NO₃⁻.     

Pyruvate Kinase Activity of Wheat Plants Grown under Potassium Deficient Conditions

The activity of pyruvate kinase was determined in the first leaves of wheat plants grown under K⁺-deficient conditions. An enhancement of the enzyme activity compared with the normal plants was found to start from eighth day of growth, and about 4-fold increase in the enzyme activity was observed in 14-day wheat leaves. The addition of K₂SO₄ to the nutrient solution given to the K⁺-deficient plants at tenth day resulted in the restoration of the enzyme activity to the normal level after 3 days. The levels of K⁺ as well as carbohydrates and chlorophyll were found to return normal over the same period. These findings are discussed in relation to the metabolic pattern of plants at the early stages of K⁺-deficiency.     

Uptake and transport of N,P and K in tomato supplied with nettle water and nutrient solution

Water extract of stinging nettle (Urtica dioica) has a growth stimulating effect on plants. This investigation elucidated effects of nettle water on uptake and transport of N, P and K. Tomato plants (Solanum lycopersicum L. cv. Dansk export) were grown in sand culture 6–8 weeks. Plants were supplied with nettle water and nutrient solution was used as a control medium. Uptake and transport of N, P and K⁺ were determined with isotopes (¹⁵N,³²P and⁸⁶Rb⁺ as a tracer for K⁺) and ion-selective electrodes and in exudation experiments. A 15% higher uptake of nitrogen (¹⁵N assay) was found after nettle water treatment compared with the nutrient solution control. The total amount of nitrogen was also higher in plants cultivated with nettle water. Transport of inorganic and organic nitrogen, measured in exudation experiments, was more than 50% higher for plants supplied with nettle water compared with plants supplied with nutrient solution. In contrast, nettle water had no effect on uptake, transport or total amount of phosphorus and potassium in the plants. Experiments in hydroculture showed that nettle water had a strong pH-elevating effect. Uptake of NH ₄ ⁺ was strongly stimulated by nettle water compared with nutrient solution. By holding pH at a constant level during the uptake period for 6 h, the uptake of NH ₄ ⁺ from nettle water was significantly lower when no adjustment of pH was made. Consequently a good deal of the NH ₄ ⁺ uptake enhancement by nettle water could be explained by pH-stimulation. Assays with the uncoupler/inhibitor 2,4-dinitrophenol (DNP) and dichlorophenyl-dimethyl-urea (DCMU) showed that uptake of nitrogen from nettle water was less metabolically-linked than uptake from a corresponding nutrient solution. All together, nettle water seems to stimulate the uptake of nitrogen, but not phosphorus or potassium.     

Effect of two nutrient solution temperatures on nitrate uptake,nitrate reductase activity,NH4+ concentration and chlorophyll a fluorescence in rose plants

The effect of two nutrient solution temperatures (cold (10 °C) and warm (22 °C)) during two flowering events of rose plants (Rosa × hybrida cv. Grand Gala) were examined by measuring chlorophyll (Chl) a fluorescence, ammonium (NH₄⁺) content and nitrate reductase (NR) activity in four different leaf types, that is, external and internal leaves of bent shoots and lower and upper leaves of flowering stems. Besides, nitrate (NO₃^?) uptake and water absorption, total nitrogen (N) concentration in the plant, dry biomass, and the ratios of shoot/root and thin-white roots/suberized-brown roots were determined. Generally, cold solution increased NO₃^? uptake and thin-white roots production but decreased water uptake, so plants grown at cold solution had to improve their NO₃^? uptake mechanisms to obtain a higher amount of nutrient with less water absorption than plants grown at warm solution. The higher NO₃^? uptake can be related to an increase in NR activity, NH₄⁺ content and total N concentration at cold solution. Nutrient solution temperature also had an effect on the photosynthetic apparatus. In general terms, the effective quantum yield (?_PSII) and the fraction of open PSII reaction centres (q_L) were higher in rose plants grown at cold solution. These effects can be associated to a higher NO₃^? uptake and total N concentration in the plants and were modulated by irradiance throughout all the experiment. Plants could adapt to cold solution by enhancing their metabolism without a decrease in total dry biomass. Nevertheless, the effect of nutrient solution temperature is not simple and also affected by climatic factors.     

NH(4)(+)-stimulated low-K(+) uptake is associated with the induction of H(+) extrusion by the plasma membrane H(+)-ATPase in sorghum roots under K(+) deficiency

The effect of external inorganic nitrogen and K⁺ content on K⁺ uptake from low-K⁺ solutions and plasma membrane (PM) H⁺-ATPase activity of sorghum roots was studied. Plants were grown for 15 days in full-nutrient solutions containing 0.2 or 1.4 mM K⁺ and inorganic nitrogen as NO₃^-, NO₃^-/NH₄⁺ or NH₄⁺ and then starved of K⁺ for 24, 48 and 72 h. NH₄⁺ in full nutrient solution significantly affected the uptake efficiency and accumulation of K⁺, and this effect was less pronounced at the high K⁺ concentration. In contrast, the translocation rate of K⁺ to the shoot was not altered. Depletion assays showed that plants grown with NH₄⁺ more efficiently depleted the external K⁺ and reached higher initial rates of low-K⁺ uptake than plants grown with NO₃^-. One possible influence of K⁺ content of shoot, but not of roots, on K⁺ uptake was evidenced. Enhanced K⁺-uptake capacity was correlated with the induction of H⁺ extrusion by PM H⁺-ATPase. In plants grown in high K⁺ solutions, the increase in the active H⁺ gradient was associated with an increase of the PM H⁺-ATPase protein concentration. In contrast, in plants grown in solutions containing 0.2 mM K⁺, only the initial rate of H⁺-pumping and ATP hydrolysis were affected. Under these conditions, two specific isoforms of PM H⁺-ATPase were detected, independent of the nitrogen source and deficiency period. No change in enzyme activity was observed in NO₃^--grown plants. The results suggest that K⁺ homeostasis in NH₄⁺-grown sorghum plants may be regulated by a high capacity for K⁺ uptake, which is dependent upon the H⁺-pumping activity of PM H⁺-ATPase.     

Modulation of nutrient acquisition and polyamine pool in salt-stressed wheat (Triticum aestivum L.) plants inoculated with arbuscular mycorrhizal fungi

Two wheat (Triticum aestivum L.) cultivars, Sids 1 and Giza 168, were grown under non-saline or saline conditions (4.7 and 9.4 dS m^?1) with and without arbuscular mycorrhizal fungi (AMF) inoculation. Salt stress considerably decreased root colonization, plant productivity and N, P, K⁺, Fe, Zn and Cu concentrations, while it increased Na⁺ level, particularly in Giza 168. Mycorrhizal colonization significantly enhanced plant productivity and N, P, K⁺, Fe, Zn and Cu acquisition, while it diminished Na⁺ uptake, especially in Sids 1. Salinity increased putrescine level in Giza 168, however, values of spermidine and spermine increased in Sids 1 and decreased in Giza 168. Mycorrhization changed the polyamine balance under saline conditions, an increase in putrescine level associated with low contents of spermidine and spermine in Giza 168 was observed, while Sids 1 showed a decrease in putrescine and high increase in spermidine and spermine. Moreover, mycorrhizal inoculation significantly reduced the activities of diamine oxidase and polyamine oxidase in salt-stressed wheat plants. Modulation of nutrient acquisition and polyamine pool can be one of the mechanisms used by AMF to improve wheat adaptation to saline soils. This is the first report dealing with mycorrhization effect on diamine oxidase and polyamine oxidase activities under salt stress.     

The Changes with Age in the Distribution of Copper and Some Copper-containing Oxidases in Red Clover (Trifolium pratense L. cv. Dorset Marlgrass): Appendix--The Determination of Nanogram Amounts of Copper in Plant Materials Using the Copper-free Apoenzyme of Pea-seedling Diamine Oxidase

Changes in activity with age of four copper-containing enzymes-ascorbateoxidase, diamine oxidase, o-diphenol oxidase, and cytochromeoxidase-were studied in red clover plants (Trifolium pratenseL. cv. Dorset Marlgrass) grown in nutrient solutions with orwithout copper. In the first 2 weeks after germination the cytochromeoxidase activities of plants grown with or without copper weresimilar, but the activities of the other three enzymes wereless in plants grown without copper. At 6 weeks the activitiesof all four enzymes were less in plants grown without copper.The copper content of the cotyledons did not change during thefirst few weeks of growth suggesting that this copper is noteasily mobilized. A new method for determining 1–10 ng amounts of Cu²⁺ isdescribed based on the specific reactivation of the copper-freeapoenzyme of pea-seedling diamine oxidase.     

Stimulation of nodulation in field peas (Pisum sativum) by low concentrations of ammonium in hydroponic culture

Although the inhibitory effects of high concentrations of mineral N (> 1.0 mM) on nodule development and function have often been studied, the effects of low, static concentrations of NH₄⁺ (< 1.0 mM) on nodulation are unknown. In the present experiments we examine the effects of static concentrations of NH₄⁺ at 0, 0.1 and 0.5 mM in flowing, hydroponic culture on nodule establishment and nitrogenase activity in field peas [Pisum sativum L. cv. Express (Svalöf AB)] for the initial 28 days after planting (DAP). Peas grown in the presence of low concentrations of NH₄⁺ had significantly greater nodule numbers (up to 4-fold) than plants grown without NH₄⁺. Nodule dry weight per plant was significantly higher at 14, 21 and 28 DAP in plants grown in the presence of NH₄⁺, but individual nodule mass was lower than in plants grown without NH₄⁺. The nodulation pattern of the plants supplied with NH₄⁺ was similar to that often reported for supernodulating mutants, however the plants did not express other growth habits associated with supernodulation. Estimates of N₂ fixation indicate that the plus-NH₄⁺ peas fixed as much or more N₂ than the plants supplied with minus-NH₄⁺ nutrient solution. There were no significant differences in nodule numbers, nodule mass or NH₄⁺ uptake between the plants grown at the two concentrations of NH₄⁺. Nodulation appeared to autoregulate by 14 DAP in the minus-NH₄⁺ treatment. Plant growth and N accumulation in the minus-NH₄⁺ plants lagged behind those of the plus-NH₄⁺ treatments prior to N₂ fixation becoming well established in the final week of the experiment. The plus-NH₄⁺ treatments appeared not to elicit autoregulation and plants continued to initiate nodules throughout the experiment.     

Proton efflux and transfer cell formation as responses to Fe deficiency of soybean in nutrient solution culture

Hydroponically grown Hawkeye soybeans with N supplied as NO ₃ ^– did not show any measurable pH decrease of the nutrient solution during the first week of Fe deficiency as has been observed for other Fe-efficient dicotyledonous species. Only after prolonged Fe stress with no renewal of the nutrient solution could an unspecific pH reduction be measured as a consequence of a decrease in the NO ₃ ^– content of the solution. On the other hand, Fe stress induced H⁺ efflux could be localized at the root tip region by day foru of-Fe treatment when intact plants were transferred from the nutrient solution to agar medium containing the pH indicator dye bromocresol purple. However, the activity of this H⁺ pump obviously was too weak to neutralize HCO₃-ions simultaneously excreted from older root parts and to acidify the bulk nutrient solution. Thus no remobilization of iron precipitated on older parts of the roots occurred and the plants remained chlorotic.Electron microscopy of the H⁺ extruding zone revealed hypodermal transfer cells with wall protuberances surrounded by cytoplasm especially rich in mitochondria. No transfer cells occurred in the rhizodermis as seen in other Fe-efficient dicots. Some cortical cells also showed transfer cell features with wall protuberances in the intercellular spaces. Often wall ingrowths were surrounded by a periplasmic space which reduced the potential surface amplification of the plasma membrane. It is concluded that the weak capacity of Hawkeye soybeans for Fe stress-induced H⁺ extrusion correlates with their less intense wall labyrinth formation as compared with other dicotyledonous species with higher Fe efficiency.     

Increased cucumber salt tolerance by grafting on pumpkin rootstock and after application of calcium

Self-grafted and pumpkin rootstock-grafted cucumber plants were subjected to the following four treatments: 1) aerated nutrient solution alone (control), 2) nutrient solution with 10 mM Ca(NO₃)₂ (Ca), 3) nutrient solution with 90 mM NaCl (NaCl), and 4) nutrient solution with 90 mM NaCl + 10 mM Ca(NO₃)₂ (NaCl+Ca). The NaCl treatment decreased the plant dry mass and content of Ca²⁺ and K⁺ but increased the Na⁺ content in roots and shoots. Smaller changes were observed in pumpkin rootstock-grafted plants. Supplementary Ca(NO₃)₂ ameliorated the negative effects of NaCl on plant dry mass, relative growth rate (RGR), as well as Ca²⁺, K⁺, and Na⁺ content especially for pumpkin rootstock-grafted plants. Supplementary Ca(NO₃)₂ distinctly stimulated the plasma membrane (PM) H⁺-ATPase activity which supplies the energy to remove excess Na⁺ from the cells. The expressions of gene encoding PM H⁺-ATPases (PMA) and gene encoding a PM Na⁺/H⁺ antiporter (SOS1) were up-regulated when Ca(NO₃)₂ was applied. The pumpkin rootstock-grafted plants had higher PM H⁺-ATPase activity as well as higher PMA and SOS1 expressions than the self-grafted plants under NaCl + Ca treatment. Therefore, the addition of Ca²⁺ in combination with pumpkin rootstock grafting is a powerful way to increase cucumber salt tolerance.     

Spermidine alleviates the growth of saline-stressed ginseng seedlings through antioxidative defense system

Protective effects of exogenous spermidine (Spd), activity of antioxygenic enzymes, and levels of free radicals in a well-known medicinal plant, Panax ginseng was examined. Seedlings grown in salinized nutrient solution (150 mM NaCl) for 7 d exhibited reduced relative water content, plant growth, increased free radicals, and showing elevated lipid peroxidation. Application of Spd (0.01, 0.1, and 1 mM) to the salinized nutrient solution showed increased plant growth by preventing chlorophyll degradation and increasing PA levels, as well as antioxidant enzymes such as CAT, APX, and GPX activity in the seedlings of ginseng. During salinity stress, Spd was effective for lowering the accumulation of putrescine (Put), with a significant increase in the spermidine (Spd) and spermine (Spm) levels in the ginseng seedlings. A decline in the Put level ran parallel to the higher accumulation of proline (Pro), and exogenous Spd also resulted in the alleviation of Pro content under salinity. Hydrogen peroxide (H₂O₂) and superoxide (O₂⁻) production rates were also reduced in stressed plants after Spd treatment. Furthermore, the combined effect of Spd and salt led to a significant increase in diamine oxidase (DAO), and subsequent decline in polyamine oxidase (PAO). These positive effects were observed in 0.1 and 1 mM Spd concentrations, but a lower concentration (0.01 mM) had a very limited effect. In summary, application of exogenous Spd could enhance salt tolerance of P. ginseng by enhancing the activities of enzyme scavenging system, which influence the intensity of oxidative stress.     

Effect of N concentration and N source on root colonization by Pseudomonas fluorescens 2-79RLI

Wheat (Triticum aestivum L.) was grown in nutrient solution with low or high N supply (NH₄NO₃ as N source). To further evaluate the influence of N form and its interaction with the nutrient solution pH, wheat plants were grown with NH ₄ ⁺ or NO ₃ ^- either in an conventional nutrient solution or in a nutrient solution in which the pH was maintained at pH 6.5 using a pH-stat system. The nutrient solution was inoculated with Pseudomonas fluorescens 2-79RLI, a genetically modified bacterium that contains lux genes activated by a ribosomal promoter. Cell numbers and physiological status of P. fluorescens 2-79RLI (length of the lag phase of bioluminescence) in the rhizosphere were determined at the root tip and in the lateral root zone. Nitrogen deficiency decreased both plant growth and root colonization by P. fluorescens 2-79RLI at the root tip while it had no effect on root colonization in the lateral root zone. The physiological status of P. fluorescens 2-79RLI was not affected by nitrogen deficiency. Ammonium nutrition increased root colonization by P. fluorescens 2-79RLI at the root tip and in the lateral root zone when the pH of the nutrient solution was allowed to change according to the N form provided. Under these conditions, the physiological status of P. fluorescens 2-79RLI was higher in the lateral root zone than at the root tip. In contrast, N source had no effect on root colonization or physiological status of P. fluorescens 2-79RLI in the nutrient solution maintained at pH 6.5. It is concluded that the stimulation of root colonization by NH ₄ ⁺ in the nutrient solution, not maintained at a constant pH, may be due to increased leakage of solutes into the rhizosphere as a result of impaired exudate retention by high H⁺ concentration in the rhizosphere or the apoplast. This revised version was published online in June 2006 with corrections to the Cover Date.     

Influence of inorganic nitrogen sources on K+/Na+ homeostasis and salt tolerance in sorghum plants

This work aimed to study the regulation of K⁺/Na⁺ homeostasis and the physiological responses of salt-treated sorghum plants [Sorghum bicolor (L.) Moench] grown with different inorganic nitrogen (N) sources. Four days after sowing (DAS), the plants were transferred to complete nutrient solutions containing 0.75 mM K⁺ and 5 mM N, supplied as either NO₃ ^? or NH₄ ⁺. Twelve DAS, the plants were subjected to salt stress with 75 mM NaCl, which was applied in two doses of 37.5 mM. The plants were harvested on the third and seventh days after the exposure to NaCl. Under the salt stress conditions, the reduction of K⁺ concentrations in the shoot and roots was higher in the culture with NO₃ ^? than with NH₄ ⁺. However, the more conspicuous effect of N was on the Na⁺ accumulation, which was severely limited in the presence of NH₄ ⁺. This ionic regulation had a positive influence on the K⁺/Na⁺ ratio and the selective absorption and transport of K⁺ in the plants grown with NH₄ ⁺. Under control and salt stress conditions, higher accumulation of free amino acids and soluble proteins was promoted in NH₄ ⁺ grown roots than NO₃ ^? grown roots at both harvesting time, whereas higher accumulation of soluble sugars was observed only at 7 days of salt stress exposure. Unlike the NH₄ ⁺ grown plants, the gas exchanges of the NO₃ ^? grown plants were reduced after 7 days of salt stress. These results suggest that external NH₄ ⁺ may limit Na⁺ accumulation in sorghum, which could contribute to improving its physiological and metabolic responses to salt stress.     

Growth and Major Nutrient Concentrations in Brassica campestris Supplied with Different NH4^＋/NO3 Ratios

In the present study, we investigated whether growth and main nutrient ion concentrations of cabbage （Brassica campestris L.） could be increased when plants were subjected to different NH4^＋/NO3- ratios. Cabbage seedlings were grown in a greenhouse in nutrient solutions with five NH4^＋/NO3- ratios （1：0; 0.75：0.25; 0.5：0.5; 0.25：0.75; and 0：1）. The results showed that cabbage growth was reduced by 87% when the proportion of NH4^＋-N in the nutrient solution was more than 75% compared with a ratio NH4^＋/NO3- of 0.5：0.5 35 d after transplanting, suggesting a possible toxicity due to the accumulation of a large amount of free ammonia in the leaves. When the NH4＋/NO3- ratio was 0.5：0.5, fresh seedling weight, root length, and H2PO4- （P）, K^＋, Ca^2＋, and Mg^2＋ concentrations were all higher than those in plants grown under other NH4^＋/NO3- ratios. The nitrate concentration in the leaves was the lowest in plants grown at 0.5： 0.5 NH4^＋/NO3-. The present results indicate that an appropriate NH4^＋/NO3- ratio improves the absorption of other nutrients and maintains a suitable proportion of N assimilation and storage that should benefit plant growth and the quality of cabbage as a vegetable.     

Adaptation of barley roots to low oxygen supply and its relation to potassium and sodium uptake

The uptake of Na⁺ and K⁺ by barley seedlings grown on aerated or non-aerated solutions was studied. Plants growing in culture solution took up K⁺ with high selectivity whether the solution was aerated or not. Roots of plants grown on aerated CaSO₄ and transferred to a solution of KCl and NaCl had a lower preference for K⁺ than roots of plants grown on non-aerated CaSO₄. Both kinds of low-salt roots were much less able to discriminate between K⁺ and Na⁺ than high-salt roots grown on a culture solution. The different levels of K⁺ selectivity are suggested to be related to H⁺ release from the tissue.     

Effect of salt stress on antioxidant system of plants as related to nitrogen nutrition

In plants of wheat (Triticum aestivum L.) grown in the media with nitrate (NO ₃ ^? plants), ammonium (NH ₄ ⁺ plants), and without nitrogen (N-deficient plants), the response to oxidative stress induced by the addition of 300 mM NaCl to the nutrient solution was investigated. Three-day-long salinization induced chlorophyll degradation and accumulation of malondialdehyde (MDA) in the leaves. These signs of oxidative stress were clearly expressed in NO ₃ ^? and N-deficient plants and weakly manifested in NH ₄ ⁺ plants. In none of the treatments, salinization induced the accumulation of MDA in the roots. Depending on the conditions of N nutrition, salt stress was accompanied by diverse changes in the activity of antioxidant enzymes in the leaves and roots. Resistance of leaves of NH ₄ ⁺ plants to oxidative stress correlated with a considerable increase in the activities of ascorbate peroxidase and glutathione reductase. Thus, wheat plants grown on the NH ₄ ⁺ -containing medium were more resistant to the development of oxidative stress in the leaves than those supplied with nitrate.     

Humic acid differentially improves nitrate kinetics under low‐ and high‐affinity systems and alters the expression of plasma membrane H+‐ATPases and nitrate transporters in rice

Humic acids (HAs) have a major effect on nutrient uptake, metabolism, growth and development in plants. Here, we evaluated the effect of HA pretreatment applied with a nutrient solution on the uptake kinetics of nitrate nitrogen (N‐NO₃^?) and the metabolism of nitrogen (N) in rice under conditions of high and low NO₃^? supply. In addition, the kinetic parameters of NO₃^? uptake, N metabolites, and nitrate transporters (NRTs) and the plasma membrane (PM) H⁺‐ATPase gene expression were examined. The plants were grown in a growth chamber with modified Hoagland and Arnon solution until 21 days after germination (DAG), and they were then transferred to a solution without N for 48 h and then to another solution without N and with and without the addition of HAs for another 48 h. After this period of N deprivation, the plants received new nutrient solutions containing 0.2 and 2.0 mM N‐NO₃^?. Treatment of rice plants with HA promoted the induction of the genes OsNRT2.1‐2.2/OsNAR2.1 and some isoforms PM H⁺‐ATPase in roots. The application of HAs differentially modified the parameters of the uptake kinetics of NO₃^? under both concentrations. When grown with 0.2 mM NO₃^?, the plants pretreated with HA had lower K_m and C_min values as well as a higher V_max/K_m ratio. When grown with 2 mM NO₃^?, the plants pretreated with HA had a higher V_max value, a greater root and shoot mass, and a lower root/shoot ratio. The N fractions were also altered by pretreatment with HA, and a greater accumulation of NO₃^? and N‐amino was observed in the roots and shoots, respectively, of plants pretreated with HA. The results suggest that pretreatment with HA modifies root morphology and gene expression of PM H⁺‐ATPases and NO₃^? transporters, resulting in a greater efficiency of NO₃^? acquisition by high‐ and low‐affinity systems.     

Stimulation of CAM Photosynthesis in Kalanchoë blossfeldiana by Transferring to Nitrogen-Deficient Conditions

Kalanchoë blossfeldiana Poelln. cv Hikan plants were grown hydroponically with nutrient solution containing 5 millimolar NO₃⁻ (or NH₄⁺) for 1 to 2 months and then transferred to nutrient solution containing no nitrogen. CO₂ uptake at night, nocturnal increase in titratable acidity, and activity of phosphoenolpyruvate carboxylase increased after the transfer. Thus, transfer to nitrogen-deficient conditions stimulates Crassulacean acid metabolism (CAM photosynthesis) in K. blossfeldiana. The importance of the plant nitrogen status (nitrogen-withdrawal status) for induction and stimulation of CAM photosynthesis is discussed.     

Decrease in nitrate uptake and increase in proton release in zinc deficient cotton,sunflower and buckwheat plants

The effect of varied Zn supply on the pH of the nutrient solution and uptake of cations and anions was studied in cotton (Gossypium hirsutum L.), sunflower (Helianthus annuus L.) and buckwheat (Fagopyrum esculentum Moench) plants grown under controlled environmental conditions in nutrient solutions with nitrate as source of nitrogen. With the appearance of visual Zn deficiency symtoms, the pH of the nutrient solutions decreased from 6 to about 5 whereas the pH increased to about 7 when the plants were adequately supplied with Zn. In Zn deficient plants the pH decrease was associated with a shift in the cation-anion uptake ratio in favour of cation uptake. Of the major ions, uptake of Ca²⁺ and K⁺ was either not affected or only slightly lowered whereas NO₃ ^- uptake was drastically decreased in Zn deficient plants. Although the Zn nutritional status of plants hardly affected the NO₃ ^- concentrations in the plants, the leakage of NO₃ ^- from roots of Zn deficient plants into a diluted CaCl₂ solution was nearly 10 times higher than that of plants adequately supplied with Zn. In contrast to Zn deficiency, Mn deficiency in cotton plants neither affected NO₃ ^- uptake nor the pH of the nutrient solution.The results indicate that, probably as a consequence of the role of Zn in plasma membrane integrity and nitrogen metabolism, when Zn is deficient in dicotyledonous species net uptake of NO₃ ^- is particularly depressed which in turn results in an increase in cation-anion uptake ratio and a corresponding decrease in external pH. The ecological relevance of this rhizosphere acidification is discussed.