Symbiotic N2-fixation by the cover crop Pueraria phaseoloides as influenced by litter mineralization

The perennial legume Pueraria phaseoloides is widely used as a cover crop in rubber and oil palm plantations. However, very little knowledge exists on the effect of litter mineralization from P. phaseoloides on its symbiotic N₂-fixation. The contribution from symbiotic N₂-fixation (Ndfa) and litter N (Ndfl) to total plant N in P. phaseoloides was determined in a pot experiment using a ¹⁵N cross-labeling technique. For determination of N₂-fixation the non-fixing plant Axonopus compressus was used as a reference. The experiment was carried out in a growth chamber during 9 weeks with a sandy soil and 4 rates of ground litter (C/N=16,2.8% N). P. phaseoloides plants supplied with the highest amount of litter produced 26% more dry matter and fixed 23% more N than plants grown in soil with no litter application, but the percentage of Ndfa decreased slightly, but significantly, from 87 to 84%. The litter N uptake was directly proportional to the rate of application and constituted 10% of total plant N at the highest application rate. Additionally, a positive correlation was found between litter N uptake and the amount of fixed N₂. The total recovery of litter N in plants averaged 26% at harvest (shoot + root) and was not affected by the quantity added. A parallel incubation experiment also showed that, as an average of all litter levels, 26% of the litter N was present in the inorganic N pool. The amounts of fertilizer and soil N taken up by plants decreased with litter application, probably due to microbial immobilization and denitrification. It is concluded that, within the litter levels studied, litter mineralization will result in a higher amount of N₂-fixed by P. phaseoloides.     

Changes in growth,photosynthetic capacity and ionic relations in spring wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) due to pre-sowing seed treatment with polyamines

The influence of pre-sowing seed treatment with polyamines (2.5 mM putrescine, 5.0 mM spermidine and 2.5 mM spermine) on growth, photosynthetic capacity, and ion accumulation in two spring wheat (Triticum aestivum L.) cultivars MH-97 (intolerant) and Inqlab-91 (tolerant) was examined. The primed seeds of each treatment and non-primed seeds were sown in a field containing 15 dS m⁻¹ NaCl. Although all three polyamines were effective in improving shoot growth and grain yield in both cultivars under saline conditions, the effect of spermine was very pronounced particularly in improving grain yield. Different priming agents did not affect the net CO₂ assimilation rate and transpiration rate of either cultivar. However, pre-treatment with spermidine increased stomatal conductance (g_s) in the tolerant cultivar, whereas with spermine stomatal conductance decreased in the intolerant cultivar under salt stress. Priming agents had different effects on the accumulation of different ions in wheat plant tissues. When spermidine and distilled water were used as priming agents, they were effective in reducing shoot [Na⁺] in the tolerant and intolerant cultivars, respectively under saline conditions. Although all priming agents caused an increase in shoot [K⁺], distilled water was more effective in improving shoot [K⁺] in both cultivars under salt stress. Pre-treatment with spermidine was very effective in reducing shoot [Cl⁻] under saline conditions particularly in the tolerant cultivar. However, the pattern of accumulation of different ions in roots due to different seed priming treatments was not consistent in either cultivar except that root Na⁺ decreased due to priming with spermine and spermidine in the intolerant and tolerant cultivars under saline conditions. In conclusion, although all three priming agents, spermine, spermidine and putrescine, were effective in alleviating the adverse effect of salt stress on wheat plants, their effects on altering the concentration of different ions and growth were different in the two cultivars differing in salt tolerance.     

Intra-specific variation in growth and P accumulation of Leucaena leucocephala and Gliricidia sepium as influenced by soil phosphate status

Twenty-three provenances of Gliricidia sepium and eleven isolines of Leucaena leucocephala were examined at a low and at high phosphate levels (20 and 80 mg P kg^-1 soil) for growth, phosphate (P) uptake and use efficiency. Large differences in growth at the low P level, and in growth response to the higher P rate occurred among L. leucocephala isolines and G. sepium provenances. Shoot dry weight at low P varied from 1.30 to 3.01 g plant^-1 for L. leucocephala and from 1.44 to 3.07 g plant^-1 for G. sepium.Leucaena isolines had only half the root weight of G. sepium provenances yet produced approximately 90% of the shoot weight of the corresponding G. sepium treatments, i.e. more than a 2-fold difference in root/shoot ratios. Total P in shoots of G. sepium was some 85% greater than of the respective L. leucocephala isolines in corresponding treatments. Physiological phosphate use efficiency (g shoot/mg P in shoots) (PPUE) was not a simple reciprocal relation, being markedly lower at higher shoot % P and content. However, for the same shoot P both species produced the same shoot weight. Nevertheless, there were apparent genotypic differences within species in the root development, shoot P and PPUE.In another study, the numbers of rhizobia in the rhizosphere of L. leucocephala, nodulation, N₂ fixation at five different levels of P were determined. The numbers of rhizobia in the rhizosphere of inoculated L. leucocephala during the first two weeks were lower when P was added but later became similar to those without added P. Nodules formed earlier than inoculated plants fertilized with P and in greater numbers (4- to 5-fold) and dry weights than in those without P. However, the percentage of N₂ derived from fixation did not change with increasing levels of P application. These results suggest that the observed P effect did not operate via stimulated growth of rhizobia in the rhizosphere, nor through increased N₂ fixation rate. The major effect appeared to be due to effects via plant growth.     

Assessment of genetic variability for N2 fixation between and within provenances of Leucaena leucocephala and Acacia albida estimated by 15N labelling techniques

Nitrogen fixed in 13 provenances of Acacia albida and 11 isolines of Leucaena leucocephala inoculated with effective Rhizobium strains was measured by ¹⁵N techniques and the total N difference method. In the test soil, on the average, L. leucocephala derived about 65% of its total N from atmospheric N₂ fixation compared to about 20% by A. albida. Significant differences in the percentage of N derived from atmospheric N₂ (% Ndfa) occurred, between provenances or isolines within species. The % Ndfa ranged from 37 to 74% within L. leucocephala and from 6 to 37 within A. albida; (equivalent to 20–50 mg N plant^–1 and 4–37 mg N plant^–1 for the two species over three months, respectively) and was correlated with the nodule mass (r=0.91). The time course of N₂ fixation of three selected provenances (low, intermediate and good fixers) was followed at 12 weekly intervals over a 36 week period. The % Ndfa of all provenances and isolines increased with time; and except for one of the L. leucocephala provenances, % Ndfa was similar within species at the 36 weeks harvest. There was a significant correlation between % Ndfa and the amount of N₂ fixed (r=0.96). Significant interactions occurred between provenances and N treatments and often growth of uninoculated but N fertilized plants was less variable than for inoculated unfertilized plants.     

Acclimation of potassium influx in rye (Secale cereale) to low root temperatures

The influx of K⁺(⁸⁶Rb⁺) into intact roots of rye (Secale cereale L. cv. Rheidal) exposed to a differential temperature (DT) between the root (8° C) and shoot (20° C) is initially reduced compared with warm-grown (WG) controls with both shoot and root maintained at 20° C. Over a period of 3 d, however, K⁺-influx rates into DT plants are restored to levels similar to or greater than those of the WG controls, the absolute rates of K⁺ influx being strongly dependent upon the shoot/root ratio. Acclimation in DT plants results in a reduction of K⁺ influx into the apical (0–2 cm) region of the seminal root which is associated with a compensatory increase in K⁺ influx into the more mature, basal regions of the root. Values of V _max and apparent K _m for K⁺ influx into DT plants were similar to those for WG plants at assay temperatures of 8° C and 20° C except for an increase in the apparent K _m at 8° C. The influx of K⁺ from solutions containing 0.6 mol·m^-3 K⁺ into both WG and DT plants was found to be linearly related to assay temperature over the range 2–27° C, and the temperature sensitivity of K⁺ influx to be dependent upon shoot/root ratio. At high shoot/root ratios, the ratio of K⁺ influx at 20° C:K⁺ influx at 8° C for WG plants approached a minimum value of 1.9 whereas that for DT plants approached unity indicating that K⁺ influx into DT plants has a large temperature-insensitive component. Additionally, when plants were grown in solutions of low potassium concentration, K⁺ influx into DT plants was consistently greater than that into WG plants, in spite of having a greater root potassium concentration ([K⁺]int). This result indicates some change in the regulation of K⁺ influx by [K⁺]int in plants exposed to low root temperatures. We suggest that K⁺ influx into rye seedlings exposed to low root temperatures is regulated by the increased demand placed on the root system by a proportionally larger shoot and that the acclimation of K⁺ influx to low temperatures may be the result of an increased hydraulic conductivity of the root system.Abbreviations DT differential temperature pretreatment - [K⁺]int root potassium concentration - [K⁺]ext potassium concentration of nutrient medium - WG warm-grown pretreatment     

Competition for phosphorus among plant parts in early and medium-duration cultivars of pigeon pea (Cajanus cajan L.)

Differences in absorption and distribution of phosphorus (P) between early and medium-duration cultivars of pigeon pea grown on nutrient solution at two P concentrations were investigated. Low-P treatment (0.03 ppm) significantly reduced shoot weight, root length and root surface area in both cultivars compared to the control (1.0 ppm), but the reduction was more pronounced in the early than the medium-duration cultivar. Dry-matter accumulation in shoots was slightly higher in the medium-duration than in the early cultivar.³²P-labelled P in the whole plant, P uptake rate and P-absorption ability were higher in the medium than in the early cultivar. At the low-P, however, no differences in these parameters were observed for control plants. In shoots,³²P-labelled P tended to accumulate in the stem of the early cultivar whereas more P was distributed to the leaves and petioles in the medium cultivar. The results suggest that when P supply is limited, medium-duration cultivars accumulate more dry matter through high efficiency of P absorption and distribution P to leaves and petioles.     

Influence of Phosphorus and Endomycorrhiza (Glomus Intraradices) on gas exchange and plant growth of Chile Ancho Pepper (Capsicum Annuum L. cv. San Luis)

Seedlings of chile ancho pepper were grown in pots containing a pasteurized mixture of sand and a low phosphorus (P) sandy loam soil, and either inoculated (VAM) or not inoculated (NVAM) with the endomycorrhizal fungus Glomus intraradices. Long Ashton nutrient solution (LANS) was modified to supply P to the seedlings at 0, 11, and 44 g(P) m^-3 (P₀, P11, P₄₄, respectively). Low P depressed net photosynthetic rate (P_N), stomatal conductance (gs), phosphorus use efficiency (P_N/P), and internal CO₂ concentration (C_i). The mycorrhiza alleviated low P effects by increasing P_N, g_s, P_N/P, and decreasing C_i. At P₀, C_i of NVAM plants was equal to or higher than that of VAM plants, suggesting nonstomatal inhibition of photosynthesis. Gas exchange of VAM plants at P₀ was similar to that of NVAM plants at P11. Endomycorrhiza increased leaf number, leaf area, shoot, root and fruit mass at P₀ and P₁₁ compared to NVAM plants. Reproductive growth was enhanced by 450 % in mycorrhizal plants at P₄₄. Root colonization (arbuscules, vesicles, internal and extraradical hyphae development) was higher at lower P concentrations, while sporulation was unaffected. The enhanced growth and gas exchange of mycorrhizal plants was in part due to greater uptake of P and greater extraradical hyphae development. This revised version was published online in June 2006 with corrections to the Cover Date.     

Root-zone temperature effects on photosynthesis, <Superscript>14</Superscript>C-photoassimilate partitioning and growth of temperate lettuce (<Emphasis Type="Italic">Lactuca sativa</Emphasis> cv. ‘Panama’) in the tropics

The effect of root growth temperature on maximal photosynthetic CO₂ assimilation (P _max), carbohydrate content, ¹⁴C-photoassimilate partitioning, growth, and root morphology of lettuce was studied after transfer of the root system from cool root-zone temperature (C-RZT) of 20 °C to hot ambient-RZT (A-RZT) and vice versa. Four days after RZT transfer, P _max and leaf total soluble sugar content were highest and lowest, respectively, in C-RZT and A-RZT plants. P _max and total leaf soluble sugar content were much lower in plants transferred from C-to A-RZT (C→A-RZT) than in C-RZT plants. However, these two parameters were much higher in plants transferred from A-to C-RZT (A→C-RZT) than in A-RZT plants. A-RZT and C→A-RZT plants had higher root total soluble sugar content than A→C-RZT and C-RZT plants. Leaf total insoluble sugar content was similar in leaves of all plants while it was the highest in the roots of C-RZT plants. Developing leaves of C-RZT plants had higher ¹⁴C-photoassimilate content than A-RZT plants. The A→C-RZT plants also had higher ¹⁴C-photoassimilate content in their developing leaves than A-RZT plants. However, more ¹⁴C-photoassimilates were translocated to the roots of A-RZT and C→A-RZT plants, but they were mainly used for root thickening than for its elongation. Increases in leaf area, shoot and root fresh mass were slower in C→A-RZT than in C-RZT plants. Conversely, A→C-RZT plants had higher increases in these parameters than A-RZT plants. Lower root/shoot ratio (R/S) in C-RZT than in A-RZT plants confirmed that more photoassimilates were channelled to the shoots than to the roots of C-RZT plants. Roots of C-RZT plants had greater total length with a greater number of tips and surface area, and smaller average diameter as compared to A-RZT plants. In C→A-RZT plants, there was root thickening but the increases in its length, tip number and surface area decreased. The reverse was observed for A→C-RZT plants. These results further supported the idea that newly fixed photoassimilates contributed more to root thickening than to root elongation in A-RZT and C→A-RZT plants.     

Genotypic variation in biological nitrogen fixation by common bean

Field experiments were performed in Austria, Brazil, Chile, Colombia, Guatemala, Mexico and Peru as part of an FAO/IAEA Co-ordinated Research Programme to investigate the nitrogen fixing potential of cultivars and breeding lines of common bean (Phaseolus vulgaris L.). Each experiment included approximately 20 bean genotypes which were compared using the ¹⁵N isotope dilution method. Great differences in nitrogen fixation were observed between and within experiments, with average values of 35% N derived from atmosphere (% Ndfa) and highest values of 70% Ndfa being observed. These values which were larger than had been reported previously for common bean, were observed only when environmental factors were favorable. Therefore, common bean lines are available, which can support high biological nitrogen fixation. These can be used either directly as cultivars for production or in breeding programmes to enhance nitrogen fixation in other cultivars.     

Low concentrations of ammonium inhibit specific nodulation (nodule number g-1 root DW) in soybean (Glycine max [L.] Merr.)

Experiments on peas (Gulden and Vessey, 1997) have indicated that NH ₄ ⁺ stimulates both whole plant (nodules plant^-1) and specific nodulation (nodules g^-1 root DW). The effect of low concentrations of NH ₄ ⁺ on the soybean/Bradyrhizobium symbiosis is unknown. The objectives of the current study were to determine the immediate and residual effects of NH ₄ ⁺ on nodulation and N₂ fixation in soybean (Glycine max [L.] Merr.) in sand culture. Soybean (cv. Maple Ridge) were exposed to 0.0, 0.5, 1.0 and 2.0 mM of ¹⁵N-labelled (NH₄)₂SO₄ for 28 days after inoculation (DAI). From 29 to 56 DAI the plants were grown on NH ₄ ⁺ -free nutrient solution. Plants were harvested at 7, 14, 21, 28 and 56 DAI for root, shoot and nodule dry weight (DW), total N content, nodule counts and ¹⁵N enrichment of plant composites. Nitrogenase activity was measured by gas exchange at 28 DAI. The plants in the control (0.0 mM NH ₄ ⁺ ) treatment had consistently lower relative growth rates than the plants in the NH ₄ ⁺ treatments during the first 28 DAI. Plant growth was also less at 2.0 mM NH ₄ ⁺ compared to growth at 0.5 and 1.0 mM NH ₄ ⁺ . At 28 DAI, plants exposed to 0.5 and 1.0 mM NH ₄ ⁺ had significantly more nodules per plant and larger individual nodules than either the NH ₄ ⁺ -free controls or the 2.0 mM NH ₄ ⁺ -supplied plants. However, specific nodulation (nodule number g^-1 root DW) and specific nitrogenase activity (nitrogenase activity g^-1 nodule DW) were on average approximately 286% and 60% higher in the control plants, respectively, than for plants in the NH ₄ ⁺ treatments at 28 DAI. Also at 28 DAI, specific nodule DW (nodule DW g^-1 root DW) were 17, 44 and 53% higher in control plants than plants that had been exposed to 0.5, 1.0 and 2.0 mM NH ₄ ⁺ . At 56 DAI, after an additional 4 weeks of NH ₄ ⁺ -free nutrition, the plants which had previously received 0.5 and 1.0 mM NH ₄ ⁺ still maintained the highest plant DW and N contents, however, specific nodule DW had become similar at 600 mg nodule DW g^-1 root DW among all treatments. It is concluded that NH ₄ ⁺ has a negative effect on the nodulation process in the soybean/Bradyrhizobium symbiosis (as best indicated by the negative effect of NH ₄ ⁺ on specific nodulation). Despite this negative effect on specific nodulation, 0.5 and 1.0 mM NH ₄ ⁺ resulted in higher whole plant nodulation and N₂ fixation due to a compensating, positive effect on overall plant growth (i.e. fewer nodules g^-1 root DW, but much larger roots). Once NH ₄ ⁺ was removed from all treatments, the soybean plants appeared to move toward a consistent level of nodule DW relative to root DW.     

RETRACTED ARTICLE: Effect of root-zone salinity and form of N on photosynthate partitioning in wheat (Triticum aestivum L.)

Carbon-14 pulse labeling technique was used to study the effect of rooting medium salinity and form and availability of N on growth and rhizodeposition of wheat (Triticum aestivum L.). Thirty days old plants grown in continuously aerated Arnon and Hoagland nutrient solution were subjected to ¹⁴C pulse labeling for 24 h and transferred to aqueous rooting medium containing 0, 150, and 300 mM NaCl in all combinations with different forms (calcium nitrate, ammonium sulphate, and ammonium nitrate) and amounts (0.5, 1.0, 1.5, and 2.0 times the standard N concentration (150 ppm) of Arnon and Hoagland plant growth medium). Plant samples immediately after pulse labeling, following 7 days of growth under different rooting medium conditions, and the freeze-dried rooting medium were analyzed for total C and ¹⁴C. Length and fresh/dry weight of root and shoot portions and calculated values of unaccounted ¹⁴C were determined. Presence of NaCl in the rooting medium led to a decrease in root and shoot portions. However, NO₃ ⁻-fed plants showed better growth than NH₄ ⁺-fed plants at all the three salinity levels. Salinity in rooting medium led to higher rhizodeposition and lower loss of ¹⁴C. Relatively higher proportion of ¹⁴C was released as rhizodeposits and retained in root/shoot portions of plants fed with NH₄ ⁺ or NH₄ ⁺+NO₃ ⁻, than those with NO₃ ⁻, while less was respired. The specific activity of the rhizodeposits (kBq ¹⁴C g⁻¹ C) was also higher under saline conditions. The rhizodeposits in NH₄ ⁺-fed plants were more highly labeled as compared to NO₃ ⁻-plants.     

Molecular-marker-facilitated investigation on the ability to stimulate N2 fixation in the rhizosphere by irrigated rice plants

An F₂ population, consisting of 231 individuals derived from a cross between rice cultivars with a similar growing duration, Palawan and IR42, was utilized to investigate the genetic nature of rice varietal ability to stimulate N₂ fixation in the rice rhizosphere. To assess rhizospheric N₂ fixation, an isotope-enriched ¹⁵N dilution technique was employed, using ¹⁵N-stabilized soil in pots. IR42, an indica variety, had 23% higher N derived from fixation (Ndfa) than Palawan, a javanica genotype. Normal segregation of atom% ¹⁵N excess was obtained in the F₂ population, with an average of 0.218 with 8% of plants below IR42 (0.188) and 10% of plants above Palawan (0.248). One-hundred-and-four RFLP markers mapped on 12 chromosomes were tested for linkage to the putative QTLs. Significant (P<0.01) associations between markers and segregation of atom% ¹⁵N excess were observed for seven marker loci located on chromosomes 1, 3, 6 and 11. Four QTLs defined by the detected marker loci were identified by interval-mapping analysis. Additive gene action was found to be predominant, but for at least one locus, dominance and partial dominance effects were observed. Significant (P<0.01) epistatic effects were also identified. Individual marker loci detected between 8 and 16% of the total phenotypic variation. All four putative QTLs showed recessive gene action, and no phenotypic effects associated with heterozygosity of marker loci were observed. The results of this study suggest that rice genetic factors can be identified which affect levels of atom% ¹⁵N excess in the soil by interacting with diazotrophs in the rice rhizosphere.     

137Cs uptake in spring wheat (Triticum aestivum L.cv. Tonic) at varying K supply

Spring wheat (Triticum aestivum L. cv. Tonic) was grown for 16 days in a sandy loam soil which was contaminated with ¹³⁷Cs. The soil was fertilised with K at three rates (0,1 and 2 mmol K per 950 g dry soil) and with NO₃ ^--N at two rates (0 and 2 mmol per 950 g dry soil) in a factorial design. The ¹³⁷Cs Activity Concentration (AC) in the shoot tissue significantly reduced 8.2-fold (nil N treatment, p<0.001) and 9.3-fold (highest N dose, p<0.001) with increasing K supply. In contrast, the K application increased the ¹³⁷Cs AC in soil solution 1.7 fold (nil N treatment) or had no significant effect (highest N dose). At similar K application, the application of N increased the ¹³⁷Cs AC in the shoot compared to the control. This effect is most probably due to the increased NH₄ ⁺ concentration in soil solution which increased the ¹³⁷Cs AC in soil solution. The soil solution composition (¹³⁷Cs and K concentration) in the rhizosphere was estimated from the average soil solution composition at day 16 and solute transport calculations. The ¹³⁷Cs AC in the shoot tissue was predicted from the estimated soil solution composition in the rhizosphere and the relationship between K concentration and ¹³⁷Cs uptake derived from a nutrient solution experiment. The predictions of ¹³⁷Cs AC's in the shoot are qualitatively correct for the fertiliser effects but underestimate the observations between 1.4 and 9.9 fold.     

Ecophysiological response in leaves of Caragana microphylla to different soil phosphorus levels

Phosphorus (P) is one of the limiting mineral nutrient elements in the typical steppe of Inner Mongolia, China. In order to find out the adaptive strategy of Caragana microphylla to low soil P status, we grew plants in P-deficient soil in April 2009 and gave a gradient of P addition ranging from 0 to 60 mg(P) kg^?1(soil) from May 2010. Leaf traits were measured in September 2010. Both leaf growth and light-saturated photosynthetic rate (P _max) were similar among different groups. Leaf nitrogen (N):P ratio indicated that the growth of C. microphylla was not P-limited in most of the Inner Mongolia typical steppe, which had an average soil available P content equal to 3.61 mg kg^?1. The optimal P addition was 20 mg(P) kg^?1(soil) for two-year-old plants of C. microphylla. Leaf mass area (LMA) and leaf dry matter content (LDMC) were enhanced with low P, and significantly negatively correlated with photosynthetic N-use efficiency (PNUE). Photosynthetic P-use efficiency (PPUE) increased with decreasing soil P and increasing leaf inorganic P (Pi): organic P (Po) ratio, and showed no significant negative correlation with LMA or LDMC. P _max of C. microphylla did not decline so sharply as it was anticipated. The reason for this phenomenon might be due to the increased PPUE through regulating the leaf total P allocation. C. microphylla had high P-use efficiency via both high PPUE and long P-retention time at low-P supply. The adaptation of C. microphylla to low-P supply provided a new explanation for the increased distribution of the species in the degraded natural grassland in Inner Mongolia, China.     

Biological nitrogen fixation in four Gliricidia sepium genotypes

A field experiment was conducted at the Coconut Research Institute in Sri Lanka to examine the biological nitrogen fixation potential of three Gliricidia sepium provenances (OFI 14/84, 17/84, 12/86) and a local landrace (designated LL), using the ¹⁵N isotope dilution method. There was marked variation in dry matter, total N, nodulation and ¹⁵N enrichment among the Gliricidia genotypes (=0.001), and the dry matter yield of Cassia siamea (syn. Senna siamea), the non-N₂ fixing reference plant was higher than for G. sepium. In all cases, highest biomass and total N were aboveground, with roots on average accounting for < 20 % of total dry matter or the total N in plants. Atom % ¹⁵N excess was highest in C. siamea, and lowest in OFI 14/84. Although atom % ¹⁵N excess was lower in Gliricidia leaves than in the other organs (all of which had similar ¹⁵N enrichments), values of % N derived from atmospheric N₂ fixation (% Ndfa) calculated for any individual organ or for the whole plant were similar. This was because the relative distribution of ¹⁵N in the different parts of the fixing plant followed the same trend as in the reference plant. There were significant differences (p=0.01) in N₂ fixation between the Gliricidia genotypes. The values ranged from 17.8 g N tree^-1 (equivalent to 86 kg N ha^-1 at 5000 trees ha^-1) in OFI 12/86 to 61.7g N tree^-1 (equivalent to 309 kg N ha^-1) in OFI 14/84. Although most of this variability was due to differences in both % Ndfa and total N in plant, amount of N fixed was more correlated with total N in plant (r=0.935) than with % Ndfa (r=0.707). On average, % Ndfa in all three G. sepium provenances and LL was about 55 % or 34.6 g N tree^-1 (equivalent to some 166 kg N ha^-1) in the 9 months within which N₂ fixation was measured. This represents a substantial contribution of N into the soil-plant system.     

Phosphorylation of the rat Ins(1,4,5)P3 receptor at T930 within the coupling domain decreases its affinity to Ins(1,4,5)P3

The Ins(1,4,5)P₃ receptor acts as a central hub for Ca²⁺ signaling by integrating multiple signaling modalities into Ca²⁺ release from intracellular stores downstream of G-protein and tyrosine kinase-coupled receptor stimulation. As such, the Ins(1,4,5)P₃ receptor plays fundamental roles in cellular physiology. The regulation of the Ins(1,4,5)P₃ receptor is complex and involves protein-protein interactions, post-translational modifications, allosteric modulation, and regulation of its sub-cellular distribution. Phosphorylation has been implicated in the sensitization of Ins(1,4,5)P₃-dependent Ca²⁺ release observed during oocyte maturation. Here we investigate the role of phosphorylation at T-930, a residue phosphorylated specifically during meiosis. We show that a phosphomimetic mutation at T-930 of the rat Ins(1,4,5)P₃ receptor results in decreased Ins(1,4,5)P₃-dependent Ca²⁺ release and lowers the Ins(1,4,5)P₃ binding affinity of the receptor. These data, coupled to the sensitization of Ins(1,4,5)P₃-dependent Ca²⁺ release during meiosis, argue that phosphorylation within the coupling domain of the Ins(1,4,5)P₃ receptor acts in a combinatorial fashion to regulate Ins(1,4,5)P₃ receptor function.     

Phosphorylation of the rat Ins(1,4,5)P3 receptor at T930 within the coupling domain decreases its affinity to Ins(1,4,5)P3

The Ins(1,4,5)P₃ receptor acts as a central hub for Ca²⁺ signaling by integrating multiple signaling modalities into Ca²⁺ release from intracellular stores downstream of G-protein and tyrosine kinase-coupled receptor stimulation. As such, the Ins(1,4,5)P₃ receptor plays fundamental roles in cellular physiology. The regulation of the Ins(1,4,5)P₃ receptor is complex and involves protein-protein interactions, post-translational modifications, allosteric modulation, and regulation of its sub-cellular distribution. Phosphorylation has been implicated in the sensitization of Ins(1,4,5)P₃-dependent Ca²⁺ release observed during oocyte maturation. Here we investigate the role of phosphorylation at T-930, a residue phosphorylated specifically during meiosis. We show that a phosphomimetic mutation at T-930 of the rat Ins(1,4,5)P₃ receptor results in decreased Ins(1,4,5)P₃-dependent Ca²⁺ release and lowers the Ins(1,4,5)P₃ binding affinity of the receptor. These data, coupled to the sensitization of Ins(1,4,5)P₃-dependent Ca²⁺ release during meiosis, argue that phosphorylation within the coupling domain of the Ins(1,4,5)P₃ receptor acts in a combinatorial fashion to regulate Ins(1,4,5)P₃ receptor function.     

Nitrogen fixation in soybean as influenced by cultivar and Rhizobium strain

Summary The¹⁵N-substratum labeling technique and other indirect methods were used to compare nitrogen (N₂) fixation in soybean varieties grown in the field in Greece and Romania. Significant variation in the amount (Ndfa) and proportion of N derived from fixation (% Ndfa) was found in different varieties. With 20 kg N/ha applied to soil, N₂ fixed ranged from 22 to 236 kg N/ha in Greece and from 17 to 132 kg N/ha in Romania. In general, varieties or treatments with higher dry matter yield supported greater fixation. Also, varieties with high Ndfa had high % Ndfa andvice versa. Breeding N₂-fixing legumes for high yields at low soil N levels therefore appears to be a reasonable strategy for enhancing N₂ fixation. Heavy applications of inorganic N fertilizer severely depressed N₂ fixation in two out of the three varieties used in Romania. One variety, F 74–412, however, derived slightly higher amounts of N₂ from fixation at 100 kg N/ha rate than when fertilized with 20 kg N/ha. In Greece, Chippewa, Williams and Amsoy-71 inoculated with a Nitragin inoculant fixed similar amounts of N₂ at both 20 and 100 kg N/ha fertilizer rates. However, when Chippewa and Williams were inoculated with amother, locally-isolated Rhizobium strain, N₂ fixation was substantially depressed at the higher N rate.     

Varietal differences in potassium uptake by barley

Potassium influx isotherms were obtained for 10 cultivars of barley using plants which had been grown with or without potassium (high K⁺ and low K⁺ plants, respectively), and the cultivars ranked with respect to K_m or V_max values for influx with a view to using these rankings as a predictive measure of long term performance under conditions of potassium-limited growth. Analyses of these rankings revealed significant differences between cultivars. Net uptake rates for low K⁺ plants, determined over a 24-hour period, confirmed the differences between upper (Herta) and lower (Conquest) ranked cultivars, and established similar differences in the rates of translocation to the shoot. Efflux analyses showed no differences in potassium efflux from the cytoplasm or from the vacuole for these cultivars. Growth rate studies under different conditions of potassium limitation indicated, with some exceptions, strong positive correlations between ranks accorded cultivars on the basis of influx kinetics and those based upon growth rates.     

Allocation of carbon to shoots,roots, soil and rhizosphere respiration by barrel medic (Medicago truncatula) before and after defoliation

The allocation of carbon to shoots, roots, soil and rhizosphere respiration in barrel medic (Medicago truncatulaGaertn.) before and after defoliation was determined by growing plants in pots in a labelled atmosphere in a growth cabinet. Plants were grown in a ¹⁴CO₂-labelled atmosphere for 30 days, defoliated and then grown in a ¹³CO₂-labelled atmosphere for 19 days. Allocation of ¹⁴C-labelled C to shoots, roots, soil and rhizosphere respiration was determined before defoliation and the allocation of ¹⁴C and ¹³C was determined for the period after defoliation. Before defoliation, 38.4% of assimilated C was allocated below ground, whereas after defoliation it was 19.9%. Over the entire length of the experiment, the proportion of net assimilated carbon allocated below ground was 30.3%. Of this, 46% was found in the roots, 22% in the soil and 32% was recovered as rhizosphere respiration. There was no net translocation of assimilate from roots to new shoot tissue after defoliation, indicating that all new shoot growth arose from above-ground stores and newly assimilated carbon. The rate of rhizosphere respiration decreased immediately after defoliation, but after 8 days, was at comparable levels to those before defoliation. It was not until 14 days after defoliation that the amount of respiration from newly assimilated C (¹³C) exceeded that of C assimilated before defoliation (¹⁴C). This revised version was published online in June 2006 with corrections to the Cover Date.