Variability of leaf photosynthetic characteristics in rice and its relationship with resistance to water stress under different nitrogen nutrition regimes

The negative effects of water stress on rice can be alleviated by NH₄⁺ nutrition. However, the effects of mixed nitrogen (N) nutrition (NO₃^? + NH₄⁺) on resistance to water stress are still not well known. To investigate the response of rice growth to water stress and its relationship with photosynthetic characteristics, a hydroponic experiment supplying different N forms was conducted. Compared with NO₃^? nutrition, mixed‐N and NH₄⁺ nutrition greatly alleviated the reduction of leaf area, chlorophyll content, and photosynthesis under water stress, whilst subsequently maintaining higher biomass. In contrast, water stress inhibited the root‐shoot ratios in NH₄⁺‐ and mixed‐N‐supplied plants, indicating reduced root growth and higher photosynthate availability to shoots. The following key observations were made: (1) a similar stomatal limitation and low proportion of activated Rubisco were observed among the three different N nutrition regimes; (2) increased mesophyll conductance in NH₄⁺‐ and mixed‐N‐supplied plants simultaneously stimulated leaf photosynthesis and improved the water use efficiency and (3), the maximum carboxylation rate and actual photochemical efficiency of photosystem II in NH₄⁺‐ and mixed‐N‐supplied plants were significantly higher than that in NO₃^?‐supplied plants, thus resulting in higher photochemical efficiency under water stress. In conclusion, mixed‐N and NH₄⁺ nutrition may be used to develop strategies for improved water stress resistance and stimulated biomass production under conditions of osmotic stress and possibly drought.     

Role of sugars in nitrate utilization by roots of dwarf bean

Nitrate uptake and in vivo, nitrate reductase activity (NRA) in roots of Phaseolus vulgaris, L. cv. Witte Krombek were measured in nitrogen-depleted plants of varying sugar status, Variation in sugar status was achieved at the start of nitrate nutrition by excision, ringing, darkness or administration of sugars to the root medium. The shape of the apparent induction pattern of nitrate uptake was not influenced by the sugar status of the absorbing tissue. When measured after 6 h of nitrate nutrition (0.1 mol m^?3), steady state nitrate uptake and root NRA were in the order intact>dark>ringed>excised. Exogenous sucrose restored NRA in excised roots to the level of intact plants. The nitrate uptake rate of excised roots, however, was not fully restored by sucrose (0.03–300 mol m^?3). When plants were decapitated after an 18 h NO₃^? pretreatment, the net uptake rate declined gradually to become negative after three hours. This decline was slowed down by exogenous fructose, whilst glucose rapidly (sometimes within 5 min) stimulated NG^?₃ uptake. Presumably due to a difference in NO₃^? due to a difference in NO₃^? uptake, the NRA of excised roots was also higher in the presence of glucose than in the presence of fructose after 6 h of nitrate nutrition. The sugar-stimulation of, oxygen consumption as well as the release of ¹⁴CO₂ from freshly absorbed (U-¹⁴C) sugar was the same for glucose and fructose. Therefore, we propose a glucose-specific effect on NO₃^? uptake that is due to the presence of glucose rather than to its utilization in root respiration. A differential glucose-fructose effect on nitrate reductase activity independent of the effect on NO₃^? uptake was not indicated. A constant level of NRA occurred in roots of NO₃^? induced plants. Removal of nutrient nitrate from these plants caused an exponential NRA decay with an approximate half-life of 12 h in intact plants and 5.5 h in excised roots. The latter value was also found in roots that were excised in the presence of nitrate, indicating that the sugar status primarily determines the apparent rate of nitrate reductase decay in excised roots.     

The nitrogen balance of Raphanus sativus x raphanistrum plants. II. Growth, nitrogen redistribution and photosynthesis under NO3− deprivation

Abstract. Wild radish plants deprived of, and continuously supplied with solution NO^?₃ for 7 d following 3 weeks growth at high NO^?₃ supply were compared in terms of changes in dry weight, leaf area, photosynthesis and the partitioning of carbon and nitrogen (NH₂-N and NO^?₃-N) among individual organs. Initial levels of NO^?₃-N accounted for 25% of total plant N. Following termination of NO^?₃ supply, whole plant dry weight growth was not significantly reduced for 3 d, during which time plant NH₂-N concentration declined by about 25% relative to NO^?₃-supplied plants, and endogenous NO^?₃-N content was reduced to nearly zero. Older leaves lost NO^?₃ and NH₂-N, and roots and young leaves gained NH₂-N in response to N stress. Relative growth rate declined due both to decreased net assimilation rate and a decrease in leaf area ratio. A rapid increase in specific leaf weight was indicative of a greater sensitivity to N stress of leaf expansion compared to carbon gain. In response to N stress, photosynthesis per unit leaf area was more severely inhibited in older leaves, whereas weight-based rates were equally inhibited among all leaf ages. Net photosynthesis was strongly correlated with leaf NH₂-N concentration, and the relationship was not significantly different for leaves of NO₃^?-supplied compared to NO^?₃-deprived plants. Simulations of the time course of NO^?₃ depletion for plants of various NH₂-N and NO^?₃ compositions and relative growth rates indicated that environmental conditions may influence the importance of NO^?₃ accumulation as a buffer against fluctuations in the N supply to demand ratio.     

Effects of elevated carbon dioxide concentration on photosynthesis and growth of small birch plants (Betula pendula Roth.) at optimal nutrition

Small birch plants (Betula pendula Roth.) were grown from seed for periods of up to 70d in a climate chamber at optimal nutrition and at present (350 μmol mol^?1) or elevated (700 μmol mol^?1) concentrations of atmospheric CO₂. Nutrients were sprayed over the roots in Ingestad-type units. Relative growth rate and net assimilation rate were slightly higher at elevated CO₂, whereas leaf area ratio was slightly lower. Smaller leaf area ratio was associated with lower values of specific leaf area. Leaves grown at elevated CO₂ had higher starch concentrations (dry weight basis) than leaves grown at present levels of CO₂. Biomass allocation showed no change with CO₂, and no large effects on stem height, number of side shoots and number of leaves were found. However, the specific root length of fine roots was higher at elevated CO₂. No large difference in the response of carbon assimilation to intercellular CO₂ concentration (A/C_i curves) were found between CO₂ treatments. When measured at the growth environments, the rates of photosynthesis were higher in plants grown at elevated CO₂ than in plants grown at present CO₂. Water use efficiency of single leaves was higher in the elevated treatment. This was mainly attributable to higher carbon assimilation rate at elevated CO₂. The difference in water use efficiency diminished with leaf age. The small treatment difference in relative growth rate was maintained throughout the experiment, which meant that the difference in plant size became progressively greater. Thus, where plant nutrition is sufficient to maintain maximum growth, small birch plants may potentially increase in size more rapidly at elevated CO₂.     

Induced leaf intercellular CO2, photosynthesis, potassium and nitrate retention and strawberry early fruit formation under macronutrient limitation

Relationships between induced high leaf intercellular CO₂ concentrations, leaf K⁺ and NO₃ ^? ion movement and early fruit formation under macronutrient limitation are not well understood. We examined the effects and interactions of reduced K/N input treatments on leaf intercellular CO₂, photosynthesis rate, carboxylation and water use efficiency, berry formation as well as leaf/fruit K⁺, NO₃ ^? and photosynthate retention of strawberry (Fragaria × ananassa Duch.) to enhance low-input agriculture. The field study was conducted in Nova Scotia, eastern Canada during 2009–2010. The experimental treatments consisted of five K₂O rates (0, 6, 12, 18, and 24 kg ha^?1) and five N rates (0, 5, 10, 15, and 20 kg ha^?1), representing respectively, 0, 25, 50, 75, and 100 % of regular macronutrient recommendations based on the soil testing. The treatments were arranged in a split-plot design with three blocks in the field. The cultivar was ‘Mira’, a June-bearing crop. The results showed that strawberry plants treated with 25 %-reduced inputs could induce significantly higher leaf intercellular CO₂ concentrations to improve plant photosynthesis, carboxylation and water use efficiency and translocation of leaf/fruit K⁺ and dissolved solids, which could advance berry formation by 6 days and produce significantly higher marketable yields (P < 0.05). Higher leaf intercellular CO₂ inhibited leaf/fruit NO₃ ^? ion retention, but this inhibition did not occur in leaf/fruit K⁺ retention. Linear interactions of the K/N treatments were significant on fruit marketable yields, intercellular CO₂, net photosynthesis, leaf transpiration rates, and leaf temperatures (P < 0.05). It was concluded that higher leaf CO₂ could enhance plant photosynthesis, promote plant carboxylation and water use efficiency, and advance berry formation, but it could inhibit leaf NO₃ ^? retention. This inhibition did not find in leaf K⁺ ion and dissolved solid retention. Overlay co-limitation of leaf intercellular CO₂ and translocation of leaf/fruit K⁺/NO₃ ^? and total dissolved solids could constrain more fruit formation attributes under full macronutrient supply than reduced inputs. It was suggested that low input would be an optimal and sustainable option for improving small fruit crop physiological development and dealing with macronutrient deficiency challenge.     

Ammonium nutrition increases water absorption in rice seedlings (Oryza sativa L.) under water stress

Water stress is a primary limitation on plant growth. In previous studies, it has been found that ammonium enhances the tolerance of rice plants to water stress, but how water is related to nitrogen form and water stress remains unknown. To study the effects of nitrogen form (NH ₄ ⁺ , NO ₃ ^? , and a mixture of NH ₄ ⁺ and NO ₃ ^? ) on the growth and water absorption of rice (Oryza sativa L.) seedlings, a hydroponic experiment with water stress, simulated by the addition of polyethylene glycol (PEG, 10% w/v, MW 6000), was conducted in a greenhouse. The results showed that, compared with non-water stress, under water stress, the fresh weight of rice seedlings increased by 14% with NH ₄ ⁺ nutrition, whereas it had decreased by about 20% with either NO ₃ ^? or mixed nitrogen nutrition. No significant difference was found in the transpiration rate of excised shoots or in xylem exudation of excised roots in NH ₄ ⁺ supply between the two water situations, whereas xylem flow decreased by 57% and 24% under water stress in NO ₃ ^? and mixed nutrition, and root hydraulic conductivity decreased by 29% and 54% in plants in NH ₄ ⁺ and NO ₃ ^? nutrition conditions, respectively. Although water absorption ability decreased in both NH ₄ ⁺ and NO ₃ ^? nutrition, aquaporin activity was higher in NH ₄ ⁺ than in NO ₃ ^? nutrition, regardless of water stress. We conclude that NH ₄ ⁺ nutrition can improve water handling in rice seedlings and subsequently enhance their resistance to drought.     

The nitrogen balance of Raphanus sativus X raphanistrum plants. I. Daily nitrogen use under high nitrate supply

Abstract Growth-chamber cultivated Raphanus plants accumulate nitrate during their vegetative growth. After 25 days of growth at a constant supply to the roots of 1 mol m^?3 (NO^?₃) in a balanced nutrient solution, the oldest leaves (eight-leaf stage) accumulated 2.5% NO^?₃-nitrogen (NO₃-N) in their lamina, and almost 5% NO₃-N in their petioles on a dry weight basis. This is equivalent to approximately 190 and 400 mol^?3 m^?3 concentration of NO^?₃ in the lamina and the petiole, respectively, as calculated on a total tissue water content basis. Measurements were made of root NO^?₃ uptake, NO^?₃ fluxes in the xylem, nitrate uptake by the mesophyll cells, and nitrate reduction as measured by an in vivo test. NO^?₃ uptake by roots and mesophyll cells was greater in the light than in the dark. The NO^?₃ concentration in the xylem fluid was constant with leaf age, but showed a distinct daily variation as a result of the independent fluxes of root uptake, transpiration and mesophyll uptake. NO^?₃ was reduced in the leaf at a higher rate in the light than in the dark. The reduction was inhibited at the high concentrations calculated to exist in the mesophyll vacuoles, but reduction continued at a low rate, even when there was no supply from the incubation medium. Sixty-four per cent of the NO^?₃ influx was turned into organic nitrogen, with the remaining NO^?₃ accumulating in both the light and the dark.     

Assessment of the preventive effect of vermicompost on salinity resistance in tomato (<Emphasis Type="Italic">Solanum lycopersicum</Emphasis> cv. Ailsa Craig)

To determine the effects of vermicompost leachate (VCL) on resistance to salt stress in plants, young tomato seedlings (Solanum lycopersicum, cv. Ailsa Craig) were exposed to salinity (150 mM NaCl addition to nutrient solution) for 7 days after or during 6 mL L^??1 VCL application. Salt stress significantly decreased leaf fresh and dry weights, reduced leaf water content, significantly increased root and leaf Na⁺ concentrations, and decreased K⁺ concentrations. Salt stress decreased stomatal conductance (g_s), net photosynthesis (A), instantaneous transpiration (E), maximal efficiency of PSII photochemistry in the dark-adapted state (F_v/F_m), photochemical quenching (qP), and actual PSII photochemical efficiency (ΦPSII). VCL applied during salt stress increased leaf fresh weight and g_s, but did not reduce leaf osmotic potential, despite increased proline content in salt-treated plants. VCL reduced Na⁺ concentrations in leaves (by 21.4%), but increased them in roots (by 16.9%). VCL pre-treatment followed by salt stress was more efficient than VCL concomitant to salt stress, since VCL pre-treatment provided the greatest osmotic adjustment recorded, with maintenance of net photosynthesis and K⁺/Na⁺ ratios following salt stress. VCL pre-treatment also led to the highest proline content in leaves (50 µmol g^??1 FW) and the highest sugar content in roots (9.2 µmol g^??1 FW). Fluorescence-related parameters confirmed that VCL pre-treatment of salt-stressed plants showed higher PSII stability and efficiency compared to plants under concomitant VCL and salt stress. Therefore, VCL represents an efficient protective agent for improvement of salt-stress resistance in tomato.     

Nitrogen source and water regime effects on durum wheat photosynthesis and stable carbon and nitrogen isotope composition

Water stress and nitrogen (N) availability are the main constraints limiting yield in durum wheat (Triticum turgidum L. var. durum). This work investigates the combined effects of N source (ammonium–NH₄⁺, nitrate–NO₃^– or a mixture of both–NH₄⁺:NO₃^–) and water availability (well‐watered vs. moderate water stress) on photosynthesis and water‐use efficiency in durum wheat (cv. Korifla) flag leaves grown under controlled conditions, using gas exchange, chlorophyll fluorescence and stable carbon isotope composition (δ¹³C). Under well‐watered conditions, NH₄⁺‐grown plants had lower net assimilation rates (A) than those grown with the other two N forms. This effect was mainly due to lower stomatal conductance (g_s). Under moderate water stress, differences among N forms were not significant, because water regime (WR) had a stronger effect on g_s and A than did N source. Consistent with lower g_s, δ¹³C and transpiration efficiency (TE) were the highest in NH₄⁺ leaves in both water treatments. These results indicate higher water‐use efficiency in plants fertilized with NH₄⁺ due to stomatal limitation on photosynthesis. Moreover, leaf δ¹³C is an adequate trait to assess differences in photosynthetic activity and water‐use efficiency caused by different N sources. Further, the effect of these growing conditions on the nitrogen isotope composition (δ¹⁵N) of flag leaves and roots was examined. Water stress increased leaf δ¹⁵N in all N forms. In addition, leaf δ¹⁵N increased as root N decreased and as leaf δ¹³C became less negative. Regardless of WR, the leaf δ¹⁵N of NO₃^–‐grown plants was lowest. Based on stepwise and canonical discriminant analyses, we conclude that plant δ¹⁵N together with δ¹³C and other variables may reflect the conditions of N nutrition and water availability where the plants were grown. Thus well‐watered plants grown with NH₄⁺:NO₃^– resembled those grown with NO3^–, whereas under water stress they were closer to plants grown with NH₄⁺.     

Dual N and O isotopes of nitrate in natural plants: first insights into individual variability and organ-specific patterns

Nitrate (NO₃ ^?) is an important form of nitrogen (N) available to plants. The measurements of NO₃ ^? concentration [NO₃ ^?] and isotopes (δ¹⁵N and δ¹⁸O) in plants provide unique insights into ecosystem NO₃ ^? availability and plant NO₃ ^? dynamics. This work investigated the variability of these parameters in individuals of a broadleaved (Aucuba japonica) plant and a coniferous (Platycladus orientalis) plant, and explored the applicability of tissue NO₃ ^? isotopes for deciphering plant NO₃ ^? utilization mechanisms. The NO₃ ^? in washed leaves showed concentration and isotopic ratios that were much lower than that in unwashed leaves, indicating a low contribution of atmospheric NO₃ ^? to NO₃ ^? in leaves. Current leaves showed higher [NO₃ ^?] and isotopic ratios than mature leaves. Moreover, higher leaf [NO₃ ^?] and isotopic enrichments (relative to soil NO₃ ^?) were found under higher soil NO₃ ^? availability for A. japonica. In contrast, leaves of P. orientalis showed low [NO₃ ^?] and negligible isotopic enrichments despite high soil NO₃ ^?. Higher [NO₃ ^?] was found in both fine and coarse roots of the P. orientalis plant, but significant isotopic enrichment was found only in coarse roots. These results reflect that the NO₃ ^? accumulation and isotopic effects decreased with leaf age, but increased with soil NO₃ ^? supply. Leaves are therefore identified as a location of NO₃ ^? reduction for A. japonica, while P. orientalis did not assimilate NO₃ ^? in leaves but in coarse roots. This work provided the first organ-specific information on NO₃ ^? isotopes in plant individuals, which will stimulate further studies of NO₃ ^? dynamics in a broader spectrum of plant ecosystems.     

Effects of 5-aminolevulinic acid treatment on photosynthesis of strawberry

Effects of root treatment with 5-aminolevulinic acid (ALA) on leaf photosynthesis in strawberry (Fragaria ananassa Duch.) plants were investigated by rapid chlorophyll fluorescence and modulated 820 nm reflection using 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) and methyl viologen (MV). Our results showed that ALA treatments increased the net photosynthetic rate and decreased the intercelluar CO₂ concentration in strawberry leaves. Under DCMU treatment, trapping energy for Q_A reduction per PSII reaction center increased greatly, indicating DCMU inhibited electron transfer from Q_A ^?. The maximum photochemical efficiency of PSII (F_v/F_m) decreased under the DCMU treatment, while a higher F_v/F_m remained in the ALA-pretreated plants. Not only the parameters related to a photochemical phase, but also that one related to a heat phase remained lower after the ALA pretreatment, compared to the sole DCMU treatment. The MV treatment decreased PSI photochemical capacity. The results of modulated 820 nm reflection analysis showed that DCMU and MV treatments had low re-reduction of P700 and plastocyanin (PSI). However, the strawberry leaf discs pretreated with ALA exhibited high re-reduction of PSI under DCMU and MV treatments. The results of this study suggest that the improvement of photosynthesis by ALA in strawberry was not only related to PSII, but also to PSI and electron transfer chain.     

Correlation of continuous ryegrass regrowth with cytokinin induced by root nitrate absorption

This study investigated the separate and combined effects of nitrate (NO₃ ^?) and cytokinin additions on continuous ryegrass regrowth after defoliation and the underlying mechanisms. Our results showed that frequent defoliation reduced the biomass of newly grown leaves and roots, the root soluble carbohydrate contents, the root vitality (an indicator of root absorption capacity), and the leaf contents of NO₃ ^?, zeatin and zeatin riboside (Z + ZR), and isopentenyl adenine and isopentenyl adenosine (IP + IPA). NO₃ ^?addition to the roots or leaves increased the biomass of newly grown leaves as well as the leaf contents of NO₃ ^?, Z + ZR, and IP + IPA without increasing the root-to-shoot delivery of endogenous cytokinin. Interestingly, cytokinin directly added to the leaves also increased the biomass of newly grown leaves and their Z + ZR and IP + IPA contents, suggesting that nitrate-induced leaf cytokinin production mediates the growth-promoting effects of nitrate. We also found that cytokinin had a direct whereas NO₃ ^? had an indirect effect on the biomass of newly grown leaves. Taken together, our results indicate that leaf cytokinin production induced by NO₃ ^? absorbed through the roots plays a key role in continuous ryegrass regrowth after defoliation.     

Involvement of phosphoenolpyruvate carboxylase and antioxydants enzymes in nitrogen nutrition tolerance in <Emphasis Type="Italic">Sorghum bicolor</Emphasis> plants

Plants of Sorghum bicolor (C₄ species) were grown at different nitrate or ammonium concentrations (0.5, 5, 20 and 50 mM) in order to examine the effect of nitrogen nutrition on growth, phosphoenolpyruvate carboxylase (PEPC) and antioxidant enzymes activities in both roots and leaves of 30-day-old plants. At high NO₃^? levels (20 and 50 mM) the fresh weight was significantly higher. When the nitrogen source was in ammonium form, the leaf and root mass increased drastically at low concentration 5 mM and significantly at 20 mM, however similar fresh weight was found at high level of ammonium (50 mM). The leaves catalase (CAT), guaiacol peroxidase (POD), glutathione reductase (GR), and glutathione S-transferase (GST) activities and the roots glutathione reductase and glutathione S-transferase activities were significantly higher in the NH₄⁺-fed plants than those grown in the nitrate medium. Activity and proteins levels of phosphoenolpyruvate carboxylase in both leaves and roots of sorghum plants were increased progressively with increasing external nitrogen concentration. This increase was more pronounced at high level of ammonium (50 mM), being 2-fold at 50 mM of NO₃^? and 3-fold at 50 mM of NH₄⁺. Our results suggested that antioxidant enzymes activities and PEPC play a key role in ammonium detoxification and tolerance in sorghum plants.     

Increased photosynthetic capacity in response to nitrate is correlated with enhanced cytokinin levels in rice cultivar with high responsiveness to nitrogen nutrients

Background and aims

Ammonium (NH₄ ⁺) is the preferred nitrogen nutrient over nitrate (NO₃ ^–) in Oryza sativa L. (rice), but photosynthetic capacity is enhanced by partial NO₃ ^– nutrition (PNN). The role of cytokinin in the effects of PNN on photosynthetic capacity is unknown.

Methods

We investigated effects of PNN on six cytokinin fractions in roots, xylem sap, and leaves and on the expression of eight cytokinin synthesis genes in the roots of Nanguang and Elio rice cultivars. The effect of exogenous cytokinin (6-BA) on leaf growth and photosynthetic activity was examined.

Results

Cell expansion and CO₂ assimilation in the first fully expanded leaf were enhanced by PNN in Nanguang but not in Elio. The concentrations of cytokinins in roots, xylem sap, and leaves of Nanguang increased approximately 25–34 % with PNN compared with sole NH₄ ⁺, but no difference was observed in Elio. Exogenous 6-BA counteracted the effects of sole NH₄ ⁺ on leaf growth and photosynthetic activity in both cultivars. OsIPT3 was the key NO₃ ^–-responsive cytokinin synthesis gene in cv. Nanguang.

Conclusions

High NO₃ ^– responsiveness is associated with increased cytokinin synthesis and transport from the root to the leaf and is strongly related to a higher photosynthetic capacity in cv. Nanguang.     

Nitrate nutrition enhances zinc hyperaccumulation in Noccaea caerulescens (Prayon)

Nitrate fertilization has been shown to increase Zn hyperaccumulation by Noccaea caerulescens (Prayon) (formerly Thlaspi caerulescens). However, it is unknown whether this increased hyperaccumulation is a direct result of NO₃ ^? nutrition or due to changes in rhizosphere pH as a result of NO₃ ^? uptake. This paper investigated the mechanism of NO₃ ^?-enhanced Zn hyperaccumulation in N. caerulescens by assessing the response of Zn uptake to N form and solution pH. Plants were grown in nutrient solution with 300 μM Zn and supplied with either (NH₄)₂SO₄, NH₄NO₃ or Ca(NO₃)₂. The solutions were buffered at either pH 4.5 or 6.5. The Zn concentration and content were much higher in shoots of NO₃ ^?-fed plants than in NH₄ ⁺-fed plants at pH 4.5 and 6.5. The Zn concentration in the shoots was mainly enhanced by NO₃ ^?, whereas the Zn concentration in the roots was mainly enhanced by pH 6.5. Nitrate increased Zn uptake in the roots at pH 6.5 and increased apoplastic Zn at pH 4.5. Zinc and Ca co-increased and was found co-localized in leaf cells of NO₃ ^?-fed plants. We conclude that NO₃ ^? directly enhanced Zn uptake and translocation from roots to shoots in N. caerulescens.     

Distribution of NO3− reduction between roots and shoots of peachtree seedlings as affected by NO3− uptake rate

The distribution of NO₃^? reduction between roots and shoots was studied in hydro-ponically-grown peach-tree seedlings (Prunus persica L.) during recovery from N starvation. Uptake, translocation and reduction of NO₃^?, together with transport through xylem and phloem of the newly reduced N were estimated, using ¹⁵N labellings, in intact plants supplied for 90 h with 0.5 mM NH₄⁺ and 0.5, 1.5 or 10 mM NO₃^?. Xylem transport of NO₃^? was further investigated by xylem sap analysis in a similar experiment. The roots were the main site of NO₃^? reduction at all 3 levels of NO₃^? nutrition. However, the contribution of the shoots to the whole plant NO₃^? reduction increased with increasing external NO₃^? availability. This contribution was estimated to be 20, 23 and 42% of the total assimilation at 0.5, 1.5 and 10 mM NO₃^?, respectively. Both ¹⁵N results and xylem sap analysis confirmed that this trend was due to an enhancement of NO₃^? translocation from roots to shoots. It is proposed that the lack of NO₃^? export to the shoots at low NO₃^? uptake rate resulted from a competition between NO₃^? reduction in the root epidermis/cortex and NO₃^? diffusion to the stele. On the other hand, net xylem transport of newly reduced N was very efficient since ca 70% of the amino acids synthesized in the roots were translocated to the shoots, regardless of the level of NO₃^? nutrition. This net xylem transport by far exceeded the net downward phloem transport of the reduced N assimilated in shoots. As a consequence, the reduced N resulting from NO₃^? assimilation, principally occurring in the roots, was mainly incorporated in the shoots.     

Photosynthesis and nitrogen utilization in exponentially growing nitrogen-limited cultures of Lemna gibba

The photosynthetic performance and nitrogen utilization of Lemna gibba L. G3 adapted to limited nitrogen supply was studied. The plants were adapted to two levels of nitrogen limitation where the nitrogen addition rates were calculated to sustain relative growth rates (RGR) of 0.15 day^?1 and 0.25 day^?1, respectively. The photosynthetic performance of these cultures was compared to nitrogen-sufficient cultures with an average RGR of 0.32 day^?1. Plants transferred from nitrogen-sufficient conditions attained RGR values corresponding to the nitrogen addition rates after 6 to 10 days. Light-saturated net photosynthesis declined during adaptation according to the drop in growth rate, and a concomitant decrease in the respiration rate was recorded. The efficiency of net photosynthesis on a dry weight basis increased with increased nitrogen supply, whereas it was the same in all cultures when expressed on a chlorophyll basis. The light compensation point was unaffected by the nitrogen regime. Limited nitrogen supply resulted in an increased proportion of dry matter in the roots, which led to decreased leaf area ratios. The net assimilation rates also decreased, but not to the same extent as the leaf area ratios. Growth-limiting amounts of nitrogen were added to the cultures once daily, and the net influx of N was higher than the requirement for N, also in adapted cultures with a steady growth rate. This resulted in transient, periodic fluctuations in the NO₃^?, NH₄⁺ and amino acid pools. Also the rates of NO₃^? reduction and NH₄⁺ assimilation fluctuated as did the amino acid assimilation which paralleled NH₄⁺ assimilation. The role of flux rates over the plasmalemma and tonoplast for control of nitrogen assimilation rates are discussed.     

High supply of NO3 ? mitigates salinity effects through an enhancement in the efficiency of photosystem II and CO2 assimilation in Jatropha curcas plants

This study was performed to determine if a high supply of N-NO₃ ^? is capable of mitigating negative salinity effects on photosynthesis and growth through the stimulation of nitrate assimilation, which could act as an sink from photosynthetic electron transport chain and restrict the over reduction in thylakoid membrane in Jatropha curcas leaves. The experiment was arranged in a factorial design with two nitrate concentrations (1 and 10?mM) and two NaCl levels (0 and 100?mM). Salt-stressed plants supplied with high NO₃ ^? demonstrated a higher nitrate uptake rate, nitrate reductase activity and soluble-protein content when compared with plants that presented low nitrate uptake. High nitrate assimilation was associated with higher leaf growth, CO₂ assimilation and lower membrane damage in salt-stressed plants. The superior performance of salt-stressed plants grown with high NO₃ ^? was indicated by a higher effective quantum yield of PSII and electron transport rate and lower energy excess at the PSII level and non-photochemical quenching. Interestingly, a high NO₃ ^? level in the absence of NaCl did not alter the leaf growth, photochemical activity and gas exchange parameters when compared with plants supplied with low nitrate. The proline and glycinebetaine contents were similarly increased in both low- and high-NO₃ ^? salt-stressed plants. Our data suggest that the favorable effects induced by high nitrate supply were possibly associated with stimulation in the nitrate assimilatory pathway. This process might have acted as a sink of electrons from the thylakoid membranes minimizing photo-damage and stimulating CO₂ assimilation under salinity in J. curcas.     

Ammonium under solution culture alleviates aluminum toxicity in rice and reduces aluminum accumulation in roots compared with nitrate

Al stress and ammonium–nitrogen nutrition often coexist in acidic soils due to their low pH and weak nitrification ability. Rice is the most Al-resistant species among small grain cereal crops and prefers NH₄ ⁺ as its major inorganic nitrogen source. This study investigates the effects of NH₄ ⁺ and NO₃ ^? on Al toxicity and Al accumulation in rice, and thereby associates rice Al resistance with its NH₄ ⁺ preference. Two rice subspecies, indica cv. Yangdao6 and japonica cv. Wuyunjing7, were used in this study. After treatment with or without Al under conditions of varying NH₄ ⁺ and NO₃ ^? supply, rice seedlings were harvested for the determination of root elongation, callose content, biomass, Al concentration and medium pH. The results indicated that Wuyunjing7 was more Al-resistant and NH₄ ⁺-preferring than Yangdao6. NH₄ ⁺ alleviated Al toxicity in two cultivars compared with NO₃ ^?. Both NH₄ ⁺-Al supply and pretreatment with NH₄ ⁺ reduced Al accumulation in roots and root tips compared with NO₃ ^?. NH₄ ⁺ decreased but NO₃ ^? increased the medium pH, and root tips accumulated more Al with a pH increase from 3.5 to 5.5. Increasing the NO₃ ^? concentration enhanced Al accumulation in root tips but increasing the NH₄ ⁺ concentration had the opposite effect. These results show NH₄ ⁺ alleviates Al toxicity for rice and reduces Al accumulation in roots compared with NO₃ ^?, possibly through medium pH changes and ionic competitive effects. Making use of the protective effect of NH₄ ⁺, in which the Al resistance increases, is advised for acidic soils, and the hypothesis that rice Al resistance is associated with the preferred utilization of NH₄ ⁺ is suggested.