绿茶施用铵态氮和硝态氮后木质部导管中N运移模拟

An experiment was carried out to study the transport process of nitrogen （N） assimilation from tea roots by monitoring the dynamic composition of N compounds in xylem sap after 15^N-NO3 and 15^N-NH4 were fed to the root of tea plants （Camellia sinensis L.）. Results showed that the main amino acids were glutamine, theanine, axginine, asparic acid and glutamic acid, which accounted for 49%, 17%, 8%, 7%, and 4%, respectively, of the total amino acids in the xylem sap. After the tea plants were fed with 15^N-NO3 and 15^N-NH4 for 48 h, the amount of total amino acids in xylem sap significantly increased and those fed with 15^N-NH4 had higher increment than those with 15^N-NOa. Two hours after 15^N- NO3 and 15^N-NH4 were fed, 15N abundance in glutamine, asparagine, glutamic acid, alanine, and arginine were detected and increased quickly over time. This indicated that it took less than 2 h for NO3-N and NH4-N to be absorbed by tea roots, incorporated into the above amino acids and transported to the xylem sap. Rapid increase in 15^N-NO3 in the xylem sap of tea plants fed with 15^N-NO3 indicated that nitrate could be directly transported to the xylem sap. Glutamine, theanine, and alanine were the main amino acids transported in xylem sap of tea plants fed with both 15^N-NO3 and 15^N-NH4.     

Untersuchungen über den N-Stoffwechsel und den N-Transport dekapitierter Wurzeln

Investigations on the N-metabolism and N-transport in decapitated roots Experiments were carried out on roots and bleeding sap of decapitated pumpkin plants supplied with nitrate-N to investigate the nitrogen metabolism and transport of nitrogen in relation to the time after decapitation. 1. Up to two hours after decapitation absorbed nitrate was metabolized into organically bound nitrogen. The rate at which this occurred decreased with time and subsequently the turnover was strongly depressed. 2. 30 free amino acids were detected in the bleeding sap. These amounted to 25% of the organically bound nitrogen. Amino acids of the glutamate family dominated, especially glutamine and arginine. In the course of the first hour, the relative proportion of glutamine-N increased; at the same time the total concentration of amino acids was slightly reduced. More pronounced shifts in this fraction were observed after two hours, indicating the occurrence of catabolic processes. In the root neither the total concentration of free amino acids nor the composition of this fraction was altered within the two hour period. However, the composition differed substantially from that of the bleeding sap. 3. By aid of gel filtration, compounds in the bleeding sap were detected with molecular weights from 200 to 5000 and with free NH₂-groups. 4. Up to the end of the two hour period the activities of nitrate reductase, glutamate-dehydrogenase and glutamate-oxalacetate-transaminase remained almost fully functional. A slight and reversible decrease in the activity of nitrate reductase in the roots in the first half hour proved to be coincident with rising rates of nitrate accumulation and strongly decreasing rates of organically bound N accumulation. The results are discussed in relation to the interruption of carbohydrate supply to the root following decapitation. In addition, differences are pointed out between the contents of nitrogen compounds in the roots and in the bleeding sap.     

Mineral Nitrogen Associated Changes in Growth and Xylem-N Compounds in Amazonian Legume Tree

In nodulated young Inga edulis plants, nodule and plant growth, nitrogen (N) in xylem sap and tissues total contents of amino acid, ureide, and nitrate were determined in response to nutrition with nitrate, ammonium, or no mineral N. Additionally, the amount of soluble sugars in the different plant tissues was quantified. It was found that mineral N improved plant growth in height and diameter especially with ammonium. However, nitrate dramatically reduced nodule dry weight on a root dry weight basis and impaired N organic transport by xylem sap. Additionally, a higher amount of amino acids was observed in the roots and nodules of plants fed with mineral N but sugar levels remained constant. Although nitrate inhibited symbiosis, data support the idea that I. edulis is able to use both molecular and mineral nitrogen during the life cycle.     

Effect Of Mineral Nutrition on Metabolic Change Induced in Crop Plant Roots (I)

Attempts have long been made to study the effect of mineral nutrition on the metabolic substances in excised roots from a numcer of plant species, but very little attention has teen given to an approach to the problem by using the bleeding sap from crop plants. Recently, however, an increasing number of reports about the occurrence of organic substances in bleeding sap from crop plants has completely revised an old view that xylem sap was essentially a rather dilute aqueous solution of inorganic salts. Evidence has already been obtained which suggests a significant role for the root system as a centre of metabolism and an upward transport of metabolites from the root via the xylem to the leaf. Although analyses of the nitrogenous compounds present in bleeding sap from herbaceous plants have shown glutamine and asparagine to be the most important constituents, in some species of plants nitrate nitrogen may be a predominant nitrogenous compound. Besides ami des and nitrate, the presence of amino acids in bleeding sap has ceen shown by Kulayeva, Silina, and Kursanov ¹⁾ for pumpkins, Wieringa and Bakhuis ²¹ for Lupins, and DIE³⁾ for cucumbers and tomatoes.     

Diurnal variations in uptake,transport and assimilation of NO3 ‐and efflux of OH‐ in maize plants

Maize plants, grown for 7 and 21 days on a nutrient solution with NO₃ ^‐ as the sole nitrogen source showed a clear diurnal pattern with respect to the in vivo NRA. Especially in roots dark/light fluctuations of the enzyme activity were high. Also in NO₃ ^‐ uptake, OH^‐ efflux and endogenous content of water soluble carbohydrates a diurnal variation was found. The plant age did not significantly affect the daily rhythm.

Because day/night changes of the in vivo root NRA and nitrate uptake were proportional, the relative content of reduced N in the xylem sap of the plants was constant during a day/night interval. At both day 7 and day 21 about 40–50% of the N was transported via the xylem as amino N. As a result of non‐synchronous variation of the specific root and shoot NRA, root reduction capacity showed a great within‐day variation. It varied between 20 and 40% of the whole plant reduction capacity. Since the ratio N‐organic to N‐total in the xylem sap was about 0.5, cycling of organic nitrogen was very likely in these maize plants.     

Effect of External Nitrogen and Potassium Supply on Xylem Sap Composition of Sugarcane

ABSTRACT

This study was conducted to evaluate the effect of nitrogen (N) and potassium (K) availability on root exudate composition of two sugarcane cultivars known to differ with regard to their resistance to drought and salinity stress. The plants were hydroponically grown in a greenhouse and subjected to three levels of N (0.1, 1.0, and 10 mM N) and three levels of K (0.02, 0.2, and 2 mM K). Nitrogen and K stress altered the xylem sap composition. Nitrogen stress significantly reduced nitrate (NO₃ ^?), ammonium (NH₄ ⁺), calcium (Ca), magnesium (Mg), and amino acid content and increased the pH, phosphorus (P), and K content. Whereas, K stress significantly decreased pH, K, NH₄ ⁺, and amino acid content but increased Ca, Mg, and P content. Nitrogen and K stress had opposing effects on xylem sap pH and osmolality. Results indicated that sugarcane plants recycle compounds between the phloem and xylem. The results also suggested that the NO₃ ^? and K concentration of xylem sap could be effectively used to estimate the N and K status of the soil solution.     

Responses of Root Growth and Nitrogen Transfer Metabolism to Uniconazole,a Growth Retardant,during the Seedling Stage of Soybean under Relay Strip Intercropping System

Relay strip intercropping of soybean has been widely developed in the southwest of China to secure China's soybean production. However, due to the shading from maize, soybean plants are thin and have a poor root system. Uniconazole is a plant-growth retardant that could enhance root vigor; increase root length, root volume, and root dry weight; and affect nitrogen (N) metabolism. To understand the effects of uniconazole on the root growth and N-transfer metabolism of soybean seedlings under relay strip intercropping, the changes in some morphological characteristics of root, dry-matter weight, root vigor, nitrate (NO₃ ^?)-N, ammonium (NH₄ ⁺)-N, and amino acid of xylem sap after seed treatment with uniconazole powder (0, 2, 4, and 8 mg kg^?1 seed) were investigated. Main root length, total lateral root lengths, first lateral root numbers, root nodule numbers, root vigor together with bleeding sap, bleeding sap–top ratio, root dry weight, and root/shoot ratio were increased, indicating uniconazole improved soybean root system in relay strip intercropping. Uniconazole powder treatment could increase NO₃ ^?-N, NH₄ ⁺-N, and total amino acid of xylem sap, to increase the potential of leaf and root N reduction and assimilation, and increase of leaf and root N contents. Thus, results suggested that uniconazole treatment can improve root growth and N transfer mechanism of soybean to support its further growth.     

Chemical composition of xylem sap of tomato grown on bicarbonate containing medium

Incorporation of bicarbonate (HCO₃ ^‐) by the roots of tomato seedlings resulted in an increase in biomass production and changed the chemical composition of xylem sap. In the xylem sap of seedlings grown on a medium enriched with HCO₃ ^‐ (5.68 mM dm^‐3, series II) compared with the control (series I) the element content increased by about 27% and 33% for cations and anions, respectively. Potassium was the major cation in the xylem sap and constituted 69% of the total concentration of all the inorganic cations determined. Calcium attained 19% of this amount. The anionic load in the xylem sap was chiefly nitrate, constituting about 90% of the content of all the inorganic anions. The exudate was analysed for seven organic acids. In general, malic (MA), maleic, and citric (CA) occurred in xylem exudate at greater concentrations, constituting about 95% of the total content of organic acids. Cultivation of seedlings on the medium enriched with HCOJ brought about an increased content of organic acids, exceeding the control by about 60%. The concentration of MA increased by about 104% and that of CA and maleic acid exceeded the control by about 40% and 14%, respectively. Twenty amino acids were identified in tomato xylem sap. Glutamic, aspartic and y‐arninobutyric acid, and particularly the amides aspargine and glutamine occurred in greater amounts. Their total concentration was about 60% and 70% of the total amino acid content in series I and II, respectively. The remaining amino acids occurred at concentrations ≥ 90 μM. The cultivation of plants on the medium containing HCO₃ ^‐ resulted in an increase in amino acid content in xylem sap by about 28% as compared with the control. The ratio of amino acid to organic acid content was 2: 2.5 and 2: 3 for series I and II, respectively. The similar value of the ratio in the two series suggests that the synthesis of both groups of compounds be equally favoured by the carbon source (endogenous in the control and from the medium enriched with HCO₃ ^‐ in series II). The increased level of elements and organic compounds in xylem sap in the case of plants supplied with HCO₃ ^‐ is discussed in the work in respect of metabolic processes of roots.     

Compositional changes of selected amino acids,organic acids,and soluble sugars in the xylem sap of N,P, or K‐deficient tomato plants

Xylem sap plays a major role in long‐distance transport of water, nutrients, and metabolites. However, there is little information on the behavior of metabolites in mineral‐deficient xylem sap. For this reason, the time‐dependent changes in selected metabolites (amino acids, organic acids, and soluble sugars) from tomato xylem sap in response to nitrogen (N), phosphorus (P), or potassium (K)‐deficient condition were investigated. Tomato plants (Solanum lycopersicum L.) were grown hydroponically in liquid culture under three different mineral regimes: N‐deficient [0.5 mM Ca(NO₃)₂ and 0.5 mM KNO₃], P‐deficient (0.05 mM KH₂PO₄), and K‐deficient (0.5 mM KNO₃), respectively. Xylem sap was collected at 10:00 am after 1, 5, 15, and 30 d, and the selected metabolites were analyzed with liquid chromatography. All N, P, or K deficiencies led to a substantial increase in metabolites in the xylem sap. The predominant amino acid in the xylem sap was glutamine and, interestingly, all mineral deficiencies resulted in a substantial amount of γ‐aminobutyric acid (GABA). Additionally, organic acids (citrate and malate) and soluble sugars were strongly increased in all mineral deficiencies, and, in particular, the level of shikimate was greatly affected by N deficiency. Based on these data, it is necessary to clearly elucidate an unknown event taking place in xylem loading in a variety of environmental impacts, and we are now studying to expand our knowledge on metabolic and proteomic responses using GC‐MS and LC‐MS.     

Uptake,assimilation and transport of nitrogen compounds by plants

This paper reviews current knowledge and presents some new information on the metabolism of nitrogen in various species of higher plants.The role of the root system is considered. It is shown that the roots of many herbaceous and woody plants can manufacture organic compounds of nitrogen from the nitrate or other forms of inorganic nitrogen they absorb from the medium. The extent to which they do this varies greatly with the age and nutrition of the plant and with the environmental conditions under which it is growing. The relationship is examined between the synthetic activities of the root and its activity in upward transport of nitrogen to the shoot. The latter process takes place predominantly, if not exclusively, in the xylem, and in each species one or more nitrogen-rich compounds, e.g., amides, ureides and amino acids, carry the bulk of the nitrogen leaving the root. A second group of plants is described in which roots do not function to any extent in the reduction of nitrate.Consideration is given to the fate of recently absorbed nitrogen once it reaches the shoot system. An inorganic source such as nitrate, or molecules such as amides containing surplus amino groupings, are shown to serve as nitrogen sources for synthesis of amino acids required for protein synthesis. Some of these amino acids arise directly from the photosynthetic apparatus. Alternatively, surplus nitrogen arriving from the root may be stored in the shoot, from where it is drawn upon extensively if uptake by the root fails to keep pace with the shoot's demands for nitrogen.The transport system for nitrogen is examined for the whole plant. The classes of sources and sinks for nitrogen are described, and information presented on the types of nitrogenous solutes they receive from the xylem and phloem.     

Changes in growth and nitrogen assimilation in barley seedlings under cadmium stress

Barley (Hordeum vulgare L. cv. Martin) plants grown in solution culture, were exposed to increasing cadmium (Cd) concentration (0, 5, 10, 25, 50, and 100 μM) for a duration of 12 days. The sequence of important biochemical steps of nitrate (NO₃) assimilation were studied in roots and shoots as a function of external Cd concentration. Cadmium uptake in roots and shoots increased gradually with Cd concentration in the medium. This Cd accumulation lowered substantially root and shoot biomass. The nitrate reductase (NR, EC 1.6.6.1) and nitrite reductase (NiR, EC 1.6.6.4) activities declined under Cd stress. Concurrently, tissue NO₃ contents and xylem sap NO₃ concentration were also decreased in Cd‐treated plants. These results suggest that Cd could exert an inhibitory effect on the assimilatory NO₃ reducing system (NR and NiR) through a restriction of NO₃ availability in the tissues. We therefore examined, in short‐term experiments (12 h), the impact of Cd on NO₃ uptake and the two reductases in nitrogen (N)‐starved plants that were pretreated or not with Cd. It was found that Cd induced inhibition of both NO₃ uptake and activities of NR and NiR, during NO₃ induction period. The possible mechanisms of Cd action on NO₃ uptake are proposed. Further, in Cd‐grown plants, the glutamine synthetase (GS, EC 6.3.1.2) showed a decreasing activity both in shoots and roots. However, increasing external Cd concentration resulted in a marked enhancement of glutamate dehydrogenase (NADH‐GDH, EC 1.4.1.2) activity, coupled with elevated levels of ammonium (NH₄ in tissues. On the other hand, the total protein content in Cd‐treated plants declined with a progressive and substantial increase of protease activity in the tissues. These findings indicate that under Cd stress the usual pathway of NH₄ assimilation (glutamine synthetase/glutamate synthase) can switch to an alternative one (glutamate dehydrogenase). The changes in all parameters investigated were concentration‐dependent and more marked in roots than shoots. The regulation of N absorption and assimilation by Cd in relation to growth and adaptation to stress conditions are discussed.     

Vergleich von Nitrat- und Amino-N-Schnelltest zur Charakterisierung des Stickstoff-Versorgungsgrades von Winterweizen

Comparison of the rapid tests for nitrate and amino-N for evaluating the N-status of winter wheat Nitrogen fertilizer trials with winter wheat were conducted in 1985 and 1986 to compare the efficacy of the rapid test for nitrate (in the stem base) and a newly developed rapid test for amino-N (in fully expanded green leaves) for evaluating the N-status of plants. In addition, the influence of weather conditions on the results given by both tests when using ammonium nitrate (NH₄NO₃ + CaCO₃) (AN) and urea-ammonium nitrate solution (UAN), was evaluated to determine wether the rapid test for amino-N gives more accurate information on the N-status of winter wheat than the rapid test for nitrate. The results show that the rapid test for nitrate is suitable to characterize the N-status of winter wheat when nitrogen is predominantly taken up by roots as nitrate. This is normaly the case when plants are fertilized with the salt form of nitrogen, als well as with liquid fertilizer, such as UAN applied through tubes in the soil, as for fertilization of winter wheat at later growing stages. However, during dry weather (1986 field trial) UAN application on leaves at shooting and ear emergence can result in high nitrogen uptake by leaves, causing an underestimation of plant N-status by the rapid test for nitrate, and thus, subsequent excess N-fertilizer application may be recommended (order of magnitude: 20 kg N/ha). Under these conditions the rapid test for amino-N in leaves (pressed sap) is a more accurate test for estimation of plant N-status because it determines glutamine and amino acids, the most important storage forms of reduced nitrogen in plants. When UAN fertilizer on leaves is washed off by rain (1985 field trial), crop N-fertilizer requirements predicted by both tests are comparable. If storage of nitrate in the stem base occurs, due, for example to low radiation intensity during spring (lower nitrate reduction), with the rapid test for amino-N an underestimation of plant N-status can be obtained.     

Ammonium assimilation in rice based on the occurrence of 15N and inhibition of glutamine synthetase activity

Assimilation of ammonium (NH₄) into free amino acids and total reduced nitrogen (N) was monitored in both roots and shoots of two‐week old rice seedlings supplied with 5 mM 99% (¹⁵NH₄)₂SO₄ in aerated hydroponic culture with or without a 2 h preincubation with 1 mM methionine sulfoximine (MSX), an inhibitor of glutamine synthetase (GS) activity. ¹⁵NH₄ was not assimilated into amino acids when the GS/GOGAT (glutamate synthase) cycle was inhibited by MSX. Inhibition of glutamine synthetase (GS) activity in roots with MSX increased both the amount of NH₄ and the abundance of ¹⁵N labeled NH₄. In contrast, the amount of Gln and Glu, and their proportions as ¹⁵N, decreased in roots when GS activity was inhibited. This research confirms the importance of GS/GOGAT in NH₄ assimilation in rice roots.

¹⁵N‐labeled studies indicate that NH₄ ions incorporated by roots of rice are transformed primarily into glutamine (Gln) and glutamic acid (Glu) before being converted to other amino acids through transamination (15). The formation of amino acids such as aspartic acid (Asp) and alanine (Ala) directly from free NH₄ in roots also has been reported (4,15). Translocation of free NH₄ to plant shoots, based on the concentration of free NH₄ in xylem exudate, has been reported in tomato (13), although NH₄ in shoots primarily originates from nitrate reduction in the shoot. Photorespiration also can contribute to the accumulation of NH₄ in leaves (7).

The GS/GOGAT cycle appears to be primarily responsible for the assimilation of exogenously supplied NH₄ and NH₄ derived from nitrate reduction in leaves, as well as NH₄ derived from photorespiration (2,3,6,8). Genetic evidence cited to support this conclusion includes the lethal effect of photorespiratory conditions on plant mutants deficient in chloroplast‐localized GS and GOGAT activities (2,3,9), and the rapid accumulation of free NH₄ in GS‐deficient mutants under photorespiratory conditions (2,3,5).

The present study was initiated to quantify the in vivo amino acid synthesis in rice roots and shoots by analysis of ¹⁵N labeling, and should provide a more complete understanding of this important system for NH₄ utilization.     

Potential Nitrate Leaching Under Common Landscaping Plants

Two Elsholtzia haichowensis S. populations, copper-tolerant (TLS) and non-tolerant (HA) ones were studied in hydroponic experiment for the nitrogen assimilation and plant growth under excess Cu conditions. The results demonstrated that there were surely the differences in nitrogen assimilation and plant growth between the two populations. Excess Cu caused evident decreases in the shoot and root biomass and root/shoot biomass ratio in HA population while no significant changes happened in TLS population. In addition, in HA population, excess Cu also induced apparent declines in activities of nitrate reductase (NR, EC 1.6.6.1) and glutamine synthetase (GS, EC 6.3.1.2) in the leaves and roots as well as the contents of nitrate, ammonium and amino acids in the roots. In TLS population, excess Cu did not significantly affect the NR activities in the leaves and roots and the nitrate content in the roots, and apparently elevated the root ammonium and amino acids contents, although it also clearly reduced the GS activities in the leaves and roots. Besides, with the addition of Cu in the culture solution, the Cu contents in the leaves and roots of the two populations markedly increased. But this increase was significantly lower in TLS population than that in HA population; the fact might be partly responsible for the relative stabilization of nitrogen assimilation in TLS population compared to that in HA population.     

Nitrogen assimilation in sunflower leaves and upward and downward transport of nitrogen

The assimilation of ammonium and nitrate nitrogen into amino acids of mature sunflower leaves and their transport to the other plant parts were investigated using nitrogen-15 as a tracer. In the leaf, to which ¹⁵N-labelled ammonium was vacuum-infiltrated, the ¹⁵N content of glutamine was always the highest of the amino acids tested and that of alanine was higher than that of glutamic acid and aspartic acid at 15 min after the infiltration. On the other hand in the leaf to which ¹⁵N-labelled nitrate was vacuum-infiltrated, the ¹⁵N content of glutamic acid and aspartic acid was superior to that of glutamine. The incorporation of ¹⁵N into serine was not active in the case of either ¹⁵N-labelled ammonium or nitrate. In the internodes above and below the treated leaf, through which photosynthates were transported into other parts, the ¹⁵N content of γ-aminobutyric acid and glutamine was markedly high when both nitrogen sources were supplied. There were no differences in the labelling patterns of amino acids between the upper and lower internodes. From these results it may be concluded that glutamine, glutamic acid, and aspartic acid play an important role in the assimilation of ammonium and nitrate nitrogen in leaves and that nitrogen is transported mainly in the forms of γ-aminobutyric acid and glutamine from the leaves to the other plant parts,     

Cycling of nitrogen and potassium between shoot and roots in maize as affected by shoot and root growth

In nitrate-fed plants cycling of nitrogen (N) and potassium (K) may serve several functions including supply of the roots with nutrients needed for growth, signalling of the growth-related shoot demand for nutrients to the roots, and removal of excess K from the shoot. In the present study, cycling and recycling of N and K were estimated in plants showing different rates of shoot and root growth. To induce these variations in growth, the plants were cultured with the same optimal nutrient supply but with the root zone temperature (RZT) at 12°C or 24°C. Additionally at both RZT, the plants were grown with their shoot base including apical shoot meristem at high or low temperature (SBT). Decreasing the RZT to 12°C drastically diminished root growth and accumulation of N and K in the roots. Cycling of N and K were less reduced by low RZT. At both RZT, N and K cycling were markedly reduced at low in comparison to high SBT although root growth was not affected by the SBT. Obviously, N and K cycling from shoot to roots were more affected by shoot growth than by the growth related demand of the roots for nutrients. At both RZT, N and K cycling exceeded accumulation in the roots. It was estimated that at least 20—33% of the N, and 24—51% of the K translocated from the roots to the shoot in the xylem is not directly derived from root uptake but from cycling. Plant culture at low shoot base temperature (SBT) drastically diminished shoot growth, and the accumulation of N and K in the shoot to less than 50% of the values measured in plants grown at high SBT. The low SBT-induced decrease of N accumulation in the shoot, at both RZT was associated with a reduction of K circulation and recirculation rates to less than 50% of those found in plants grown at high SBT. These findings are in accordance with the suggested role of K⁺ for charge balance facilitating the transport of NO₃^— in the xylem and disposal of the negatively charged products of NO₃^— assimilation from shoot to roots in the phloem. In plants cultured at low SBT, net uptake and translocation rates of N and K were diminished to less than 50% of those measured in plants grown at high SBT. This repression was associated with reduced rates of N and K cycling from the shoot to the roots. Obviously, low rates of N and K cycling from the shoot to the roots are not necessarily signals to increase uptake in the roots. It is suggested that for plants adequately supplied with N, high rates of N cycling and recycling might be the consequence of an apparent lack in control of phloem loading of amino acids in the leaves.     

Nitrogen assimilation studies using 15N in soybean plants treated with imazethapyr, an inhibitor of branched-chain amino acid biosynthesis

The pattern of nitrogen assimilation in soybean plants treated with a herbicide that inhibits branched-chain amino acid biosynthesis was evaluated by (15)N isotopic analysis. The herbicide imazethapyr caused a strong decrease in nitrate uptake by roots, partly due to a reduced stomatal conductance. The inhibition of (15)N uptake was accompanied by a decrease in the (15)N content in the plant and, concomitantly, an inhibition of translocation to the shoot. Imazethapyr inhibited nitrate reductase activity in leaves and roots. Among all parameters studied, "de novo" synthesis of proteins was the first parameter of the N assimilation metabolism affected by the herbicide. These results show that this class of herbicides totally damages N metabolism and indicates a regulatory effect on N uptake and translocation that would be mediated by the increase in free amino acid pool provoked by the inhibition of branched-chain amino acid biosynthesis.     

Effects of low root temperature on sap flow rate,soluble carbohydrates,nitrate contents and on cytokinin and gibberellin levels in root xylem exudate of sand‐grown tomato

Tomato (Lycopersicon esculuntum Mill.) grown in open fields in dry land areas or in non‐controlled greenhouses are subjected to substantial daily changes in root temperature. In the field, root‐zone temperatures fluctuate both diurnally and during the growing season. The purpose of this study was to monitor root‐zone temperature effects on tomato initial growth, transpiration, sap flow rate, leaf and air temperatures differences, nitrate accumulation, total nitrogen, and soluble carbohydrates in the shoot and roots as well as levels of endogenous cytokinins and gibberellins in xylem exudate. Tomato seedlings were grown in three growth cabinets with variable control of root temperatures. Three day/night root temperature regimes (12/12, 16/8 and 20/20°C) were employed. Low day root temperatures of 12 and 16°C reduced shoot dry weight by 47 and 26%, root dry weight by 36 and 14%, shoot nitrate by 79 and 50%, root nitrate by 49 and 16%, levels of cytokinins in root xylem exudate by 27 and 13% and gibberellins by 65 and 23%, in relation to the respective values of 20°C day root temperature. Soluble carbohydrates in the shoot and roots were increased significantly (18 and 111%) by 12°C root temperature. The main effects of low root temperatures on shoot growth stem from slow upward transport of plant hormones and nitrate rather than reduction in their rate of biosynthesis or entry to the root, respectively.     

Influence of potassium:rubidium ratios on the xylematic transport of solutes in cucumber plants grown with nitrate plus ammonium

The influence of three potassium:rubidium (K:Rb) ratios (6:0, 5:1, and 4:2) on the xylematic transport of solutes in cucumber plants cv. Medusa supplied with both nitrate (NO₃ ^‐) (60%) and ammonium (NH₄ ⁺) (40%) was studied in greenhouse conditions. In the xylem sap of plants grown with a K:Rb ratio of 4:2, there was an increase in the transport of NO₃ ^‐, phosphate (H₂PO₄ ^‐), calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), manganese (Mn) and boron (B) while that of organic‐N, organic‐P, K⁺, zinc (Zn), organic acids, and carbohydrates decreased, if compared with the sap of the plants supplied with K alone. The translocation of NO₃ ^‐, H₂PO₄ ^‐, Ca²⁺, Mg²⁺, and Mn was enhanced and that of K⁺ and organic acids decreased when the plants were supplied with a K:Rb ratio of 5:1. The K:Rb ratio detected in the xylem sap was the same K:Rb ratio as in the solutions. However, in the cucumber plant substituting 33% of total K by Rb resulted in an alteration in the transport of solutes, probably due to a competition between Rb and K rather than between the latter two and NH₄ ⁺.