Urease from leaves of Glycine max and Zea mays

Extracts of Glycine max and Zea mays leaves catalysed the release of ¹⁴CO₂ from [¹⁴C] urea with multiple pH maxima (5.5 and 9.0 for G. max; 5.5, 7.5 and 8.8 for Z. mays). Evidence was obtained that the principal activities, at pH 5.5 and 8.8–9.0 catalysed the same reaction stoichiometry as did urease purified from jackbean seeds (EC 3.5.1.5). The urease activities with these pH optima were not resolved by ammonium sulphate fractionation, DEAE-cellulose chromatography, or gel filtration chromatography. Many structural analogues of urea inhibited leaf urease, the most effective being amino acid hydroxamates, hydroxyurea and selenourea. Allantoic acid and ureidoglycolate are probably not alternative substrates because they showed at most only weak competitive inhibition with respect to radioactive urea.     

Ecophysiological adaptation of <Emphasis Type="Italic">Calligonum roborovskii</Emphasis> to decreasing soil water content along an altitudinal gradient in the Kunlun Mountains,central Asia

To understand the ecophysiological adaptation mechanisms of Calligonum roborovskii to altitude variation, this study analyzed chlorophyll a (Chl a), chlorophyll b (Chl b), Chl (a + b), carotenoid (Car), malondialdehyde (MDA), ascorbate (AsA), proline (Pro), membrane permeability (MP), reactive oxygen species (ROS), specific leaf area (SLA), leaf mass per area (LMA), leaf nitrogen content based on mass (N_mass), and the activities of peroxidase (POD), catalase (CAT), superoxide dismutase (SOD), and ascorbate peroxidase (APX) in leaves of plants inhabiting different altitudes (A1: 2100 m, A2: 2350 m, A3: 2600 m) on the northern slope of the Kunlun Mountains. The results showed that Chl a, Chl b, Chl (a + b), SLA, N_mass, and the activity of CAT increased with increasing altitude. LMA, MP, MDA, Car, Pro, AsA, O₂⁻, H₂O₂ and the activities of SOD, POD, and APX decreased with increasing altitude. The test results also showed that, changes in venvironmental factors along an altitudinal gradient are not obvious. Soil water content is the main ecological factor. With increasing altitude, soil water content increased significantly. More non-enzymatic and enzymatic antioxidants played an important role in eliminating intracellular ROS. They kept the cell membrane in a stable state and ensured the normal growth of C. roborovskii.     

Leaf Longevity,Construction, and Maintenance Costs of Three Mangrove Species Under Field Conditions

This study assessed the effect of leaf age on construction cost (CC) in the mangrove species Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle growing in their natural habitat. Leaf osmolality values were species-specific, the highest in A. germinans (1 693 mmol kg^–1) and the lowest in L. racemosa (1 270 mmol kg^–1). In the three species, contents of chlorophyll (a+b) (Chl_a+b) and nitrogen (N) per unit of leaf area were maximal in adult leaves and tended to decline with age. Leaf mass to leaf area ratio (LMA) and ash content increased during leaf ageing. Similarly, as leaves aged, a significant increase in leaf construction cost per leaf area (CC_a) was observed, while per leaf mass (CC_m) it remained almost constant, suggesting a sustained production of leaf compounds as leaves became older. CC was positively correlated with LMA and heat of combustion (Hc) per leaf area, suggesting differences among species in the quantity and composition of expensive compounds. Leaf half lifetime (t_0.5) showed contrasting values in the three mangrove species (60, 111, and 160 d in L. racemosa, R. mangle, and A. germinans, respectively). Overall, L. racemosa was the species with less expensive leaves to construct while leaves of A. germinans and R. mangle had the highest CC_m and CC_a, respectively. Leaf longevity was positively correlated with the ratio between CC and maximum photosynthetic rate (P _max), clearly showing the existence of a balance between leaf costs and benefits.     

Utilization of urea by leaves of bromeliad <Emphasis Type="Italic">Vriesea gigantea</Emphasis> under water deficit: much more than a nitrogen source

Vriesea gigantea Gaudichaud is an epiphytic bromeliad with a high capacity to take up urea. In plants, urea is hydrolyzed by urease into ammonium and CO₂, providing nitrogen to the plant. Most studies of urea nutrition have focused only on nitrogen metabolism, whereas scarce attention has been given to CO₂ assimilation. Therefore, this study attempted to investigate whether urea could play an important role as a carbon source, which could be of a significant importance under water deficit conditions because of the limitation in atmospheric CO₂ influx into the leaves due to stomatal closure. In this study, detached leaves of V. gigantea were exposed to water deficit and supplied with urea. The most photosynthetic parts of the leaf (mainly the apical leaf portion) showed higher urease activities and CO₂ buildup near chloroplasts, particularly during the nighttime under water deficit conditions when compared to urea application without the water deficit. Moreover, part of the CO₂ generated from urea hydrolysis was fixed into malate, probably via phosphoenolpyruvate carboxylase. Therefore, urea may contribute to the carbon balance of plants under water deficit conditions. Our data suggest that, besides being a source of nitrogen, urea might also be an important carbon source during CO₂-limited conditions in leaves of epiphytic bromeliads.     

Pleiotropic soybean mutants defective in both urease isozymes

Summary Two new soybean [Glycine max (L.) Merr. cv. Williams] loci, designated Eu2 and Eu3, were identified in which ethyl methanesulfonate (EMS)-induced mutation eliminated urease activity. These loci showed no linkage to each other or to the Sun-Eul locus described in the accompanying paper (Meyer-Bothling and Polacco 1987). Unlike sun (seed urease-null) mutations those at Eu2 and Eu3 affected both urease isozymes: the embryo-specific (seed) and the ubiquitous (leaf) urease. The eu2/eu2 mutant had no leaf activity and 0.6% normal seed activity. Two mutant Eu3 alleles were recovered, eu3-e1 and Eu3-e3. The eu3-e1/eu3-e1 genotype lacked both activities while Eu3-e3/Eu3-e3 had coordinately reduced leaf (0.1%) and seed (0.1%) activities. Only the Eu3-e3 mutation showed partial dominance, yielding about 5%–10% normal activity for each urease in the heterozygous state. Each homozygous mutant contained normal levels of embryo-specific urease mRNA and protein subunit, both of normal size. However, urease polymerization was aberrant in all three mutants. In all cases where urease could be measured, it was found to be temperature sensitive and, in addition, the embryospecific urease of Eu3-e3/Eu3-e3 had an altered pH dependence. These mutants may be defective in a urease maturation function common to both isozymes as suggested by the normal levels of urease gene product, coordinately (or nearly so) reduced urease isozyme activities, temperature sensitivity in both ureases (Eu3-e3) and the non-linkage of Eu2 and Eu3 to the locus encoding embryo-specific urease (Sun-Eul). Ubiquitous urease activity is reduced in mutant seed coat and callus culture as well as in leaf and cotyledon tissue. No mutant callus utilized urea (5 to 10 nM) as sole nitrogen source. However, all mutant cell lines tolerated normally toxic levels of urea (25 to 250 mM) added to medium containing KNO₃/NH₄NO₃ as nitrogen source. Urea thus may be used in cell culture as a selection agent for phenotypes either lacking or regaining an active ubiquitous urease.     

Ureide metabolism in leaves of nitrogen-fixing soybean plants

In leaf pieces from nodulated soybean (Glycine max [L] Merr cv Maple Arrow) plants, [¹⁴C]urea-dependent NH₃ and ¹⁴CO₂ production in the dark showed an approximately 2:1 stoichiometry and was decreased to less than 11% of the control (12-19 micromoles NH₃ per gram fresh weight per hour) in the presence of 50 millimolar acetohydroxamate, a urease inhibitor. NH₃ and CO₂ production from the utilization of [2-¹⁴C] allantoin also exhibited a 2:1 stoichiometry and was reduced to a similar extent by the presence of acetohydroxamate with a concomitant accumulation of urea which entirely accounted for the loss in NH₃ production. The almost complete sensitivity of NH₃ and CO₂ production from allantoin and urea metabolism to acetohydroxamate, together with the observed stoichiometry, indicated a path of ureide assimilation (2.0 micromoles per gram leaf fresh weight per hour) via allantoate, ureidoglycolate, and glyoxylate with the production of two urea molecules yielding, in turn, four molecules of NH₃ and two molecules of CO₂.     

Changes in leaf photosynthetic parameters with leaf position and nitrogen content within a rose plant canopy (Rosa hybrida)

This paper deals with changes in leaf photosynthetic capacity with depth in a rose (Rosa hybrida cv. Sonia) plant canopy. Measurements of leaf net CO₂ assimilation (A_l) and total nitrogen content (N_l) were performed in autumn under greenhouse conditions on mature leaves located at different layers within the plant canopy, including the flower stems and the main shoots. These leaves were subjected (i) to contrasting levels of CO₂ partial pressure (p_a) at saturating photosynthetic photon flux density (I about 1000 μ mol m ^{? 2} s ^{? 1}) and (ii) to saturating CO₂ partial pressure (p_a about 100 Pa) and varying I, while conditions of temperature were those prevailing in the greenhouse (20–38 °C). A biochemical model of leaf photosynthesis relating A_l to intercellular CO₂ partial pressure (p_i) was parameterized for each layer of leaves, supplying corresponding values of the photosynthetic Rubisco capacity (V_lm) and the maximum rate of electron transport (J_m). The results indicated that rose leaves growing at the top of the canopy had higher values of J_m and V_lm, which resulted from a higher allocation of nitrogen to the uppermost leaves. Mean values of total leaf nitrogen, N_l, decreased about 35% from the uppermost leaves of flower stem to leaves growing at the bottom of the plant. The derived values of non‐photosynthetic nitrogen, N_b, varied from 76 mmol_N m ^{? 2}_leaf (layer 1) to 60 mmol_N m ^{? 2}_leaf (layer 4), representing a large fraction of N_l (50 and 60% in layer 1 and 4, respectively). Comparison of leaf photosynthetic nitrogen (N_p = N_l–N_b) and I profiles supports the hypothesis that rose leaves acclimate to the time‐integrated absorbed I. The relationships between I and N_p, obtained during autumn, spring and summer, indicate that rose leaves seem also to acclimate their photosynthetic capacity seasonally, by allocating more photosynthetic nitrogen to leaves in autumn and spring than in summer.     

Changes in photosynthesis caused by adaptation of maize seedlings to short-term drought

Detached leaf is in the state of increasing water deficit; it is a good experimental model for looking into the hardening effect of adaptation of eight-day-old maize (Zea mays L.) seedlings to short-term drought (five days without watering). The light stage of photosynthesis and photosynthetic CO₂/H₂O exchange in detached leaves were studied. Specific surface density of leaf tissue (SSDL), the content of chlorophylls a and b, proline, MDA as well as photosynthetic parameters: quantum yield of photosystem II fluorescence, assimilation of CO₂, and transpiration at room temperature and light saturation (density of PAR quantum flux of 2000 μmol/(m² s)) at normal and half atmospheric CO₂ concentration were determined. The leaves of seedlings exposed to short-term drought differed from control material by a greater SSDL and higher content of proline. The hardening effect of the stress agent on the dark stage of photosynthesis was detected; it was expressed in the maintenance of the higher photosynthetic CO₂ assimilation against control material due to the elevation of stomatal conductance for CO₂ diffusing into the leaf. Judging from the lack of differences in the MDA content, short-term drought did not injure photosynthetic membranes. In detached leaves of experimental maize seedlings, photosynthesis was maintained on a higher level than in control material.     

Studies on phosphorylase isozymes in lower vertebrates: evidence for the presence of two isozymes in elasmobranchs.

Two distinct phosphorylase isozymes, skeletal muscle phosphorylase b and liver phosphorylase b, have been purified from skate (Raja pulchra) in a homogeneous form as judged by electrophoretic and immunological criteria. Both isozymes were dependent on AMP for activity and converted to a forms by rabbit muscle phosphorylase kinase. Their subunit molecular weight determined by sodium dodecyl sulfate-gel electrophoresis was 94,000. These isozymes were distinctly different in affinities for glycogen and AMP, while they were very similar in sensitivities to SO₄^2?. Rabbit antibodies against each of the muscle and liver isozymes inhibited completely the respective specific antigens. No cross-reaction was observed in double diffusion tests, but some immunological relatedness of these isozymes was demonstrated by inhibition tests with antibodies. Their similarity was also shown by amino acid analyses. No evidence has been obtained that the skate possesses such an isozyme as mammalian phosphorylase L, the b form of which is inactive even in the presence of AMP. Electrophoretic studies on phosphorylases of crucian carp, toad, and snake revealed that these animals possess three isozymes which strikingly resemble mammalian isozymes in the organ-specific distribution and electrophoretic behavior.     

Nitrogen deposition limits photosynthetic response to elevated CO2 differentially in a dioecious species

Sexual dimorphisms of dioecious plants are important in controlling and maintaining sex ratios under changing climate environments. Yet, little is known about sex-specific responses to elevated CO₂ with soil nitrogen (N) deposition. To investigate sex-related physiological and biochemical responses to elevated CO₂ with N deposition, Populus cathayana Rehd. was employed as a model species. The cuttings were subjected to two CO₂ regimes (350 and 700???mol?mol^?1) with two N levels (0 and 5?g?N?m^?2?year^?1). Our results showed that elevated CO₂ and N deposition separately increased the total number of leaves, leaf area (LA), leaf mass, net photosynthetic rate (P _n), light saturated photosynthetic rate (P _max), chlorophyll a (Chl a), and chlorophyll a to chlorophyll b ratio (Chl a/b) in both males and females of P. cathayana. However, the effects on LA, leaf mass, P _n, P _max, Chl a and Chl a/b were weakened under the combined treatment of elevated CO₂ and N deposition. Males had higher leaf mass, P _n, P _max, apparent quantum yield (??), carboxylation efficiency (CE), Chl a, Chl a/b, leaf N, and root carbon to N ratio (C/N) than did females under elevated CO₂ with N deposition. In contrast to males, females had significantly higher levels of soluble sugars in leaves and greater starch accumulation in roots and stems under the same condition. The results of the present work imply that P. cathayana females are more responsive and suffer from greater negative effects on growth and photosynthetic capacity than do males when grown under elevated CO₂ with soil N deposition.     

Chlorophyll fluorescence imaging of photosynthetic activity in sun and shade leaves of trees

The differences in pigment levels, photosynthetic activity and the chlorophyll fluorescence decrease ratio R _Fd (as indicator of photosynthetic rates) of green sun and shade leaves of three broadleaf trees (Platanus acerifolia Willd., Populus alba L., Tilia cordata Mill.) were compared. Sun leaves were characterized by higher levels of total chlorophylls a + b and total carotenoids x + c as well as higher values for the weight ratio chlorophyll (Chl) a/b (sun leaves 3.23–3.45; shade leaves: 2.74–2.81), and lower values for the ratio chlorophylls to carotenoids (a + b)/(x + c) (with 4.44–4.70 in sun leaves and 5.04–5.72 in shade leaves). Sun leaves exhibited higher photosynthetic rates P _N on a leaf area basis (mean of 9.1–10.1 μmol CO₂ m⁻² s⁻¹) and Chl basis, which correlated well with the higher values of stomatal conductance G _s (range 105–180 mmol m⁻² s⁻¹), as compared to shade leaves (G _s range 25–77 mmol m⁻² s⁻¹; P _N: 3.2–3.7 μmol CO₂ m⁻² s⁻¹). The higher photosynthetic rates could also be detected via imaging the Chl fluorescence decrease ratio R _Fd, which possessed higher values in sun leaves (2.8–3.0) as compared to shade leaves (1.4–1.8). In addition, via R _Fd images it was shown that the photosynthetic activity of the leaves of all trees exhibits a large heterogeneity across the leaf area, and in general to a higher extent in sun leaves than in shade leaves.     

Ureide Catabolism of Soybeans : II. Pathway of Catabolism in Intact Leaf Tissue

Allantoin catabolism studies have been extended to intact leaf tissue of soybean (Glycine max L. Merr.). Phenyl phosphordiamidate, one of the most potent urease inhibitors known, does not inhibit ¹⁴CO₂ release from [2,7-¹⁴C]allantoin (urea labeled), but inhibits urea dependent CO₂ release ≥99.9% under similar conditions. Furthermore, ¹⁴CO₂ and [¹⁴C] allantoate are the only detectable products of [2,7-¹⁴C]allantoin catabolism. Neither urea nor any other product were detected by analysis on HPLC organic acid or organic base columns although urea and all commercially available metabolites that have been implicated in allantoin and glyoxylate metabolism can be resolved by a combination of these two columns. In contrast, when allantoin was labeled in the two central, nonureido carbons ([4,5-¹⁴C]allantoin), its catabolism to [¹⁴C]allantoate, ¹⁴CO₂, [¹⁴C]glyoxylate, [¹⁴C]glycine, and [¹⁴C]serine in leaf discs could be detected. These data are fully consistent with the metabolism of allantoate by two amidohydrolase reactions (neither of which is urease) that occur at similar rates to release glyoxylate, which in turn is metabolized via the photorespiratory pathway. This is the first evidence that allantoate is metabolized without urease action to NH₄⁺ and CO₂ and that carbons 4 and 5 enter the photorespiratory pathway.     

Study of the senescence process in primary leaves of sunflower (Helianthus annuus L.) plants under two different light intensities

Different parameters that vary during leaf development may be affected by light intensity. To study the influence of different light intensities on primary leaf senescence, sunflower (Helianthus annuus L.) plants were grown for 50 days under two photon flux density (PFD) conditions, namely high irradiance (HI) at 350 μmol(photon) m^?2 s^?1 and low irradiance (LI) at 125 μmol(photon) m^?2 s^?1. Plants grown under HI exhibited greater specific leaf mass referred to dry mass, leaf area and soluble protein at the beginning of the leaf development. This might have resulted from the increased CO₂ fixation rate observed in HI plants, during early development of primary leaves. Chlorophyll a and b contents in HI plants were lower than in LI plants in young leaves. By contrast, the carotenoid content was significantly higher in HI plants. Glucose concentration increased with the leaf age in both treatments (HI and LI), while the starch content decreased sharply in HI plants, but only slightly in LI plants. Glucose contents were higher in HI plants than in LI plants; the differences were statistically significant (p<0.05) mainly at the beginning of the leaf senescence. On the other hand, starch contents were higher in HI plants than in LI plants, throughout the whole leaf development period. Nitrate reductase (NR) activity decreased with leaf ageing in both treatments. However, the NR activation state was higher during early leaf development and decreased more markedly in senescent leaves in plants grown under HI. GS activity also decreased during sunflower leaf ageing under both PFD conditions, but HI plants showed higher GS activities than LI plants. Aminating and deaminating activities of glutamate dehydrogenase (GDH) peaked at 50 days (senescent leaves). GDH deaminating activity increased 5-fold during the leaf development in HI plants, but only 2-fold in LI plants. The plants grown under HI exhibited considerable oxidative stress in vivo during the leaf senescence, as revealed by the substantial H₂O₂ accumulation and the sharply decrease in the antioxidant enzymes, catalase and ascorbate peroxidase, in comparison with LI plants. Probably, systemic signals triggered by a high PFD caused early senescence and diminished oxidative protection in primary leaves of sunflower plants as a result.     

Nitrogen-molybdenum-manganese co-fertilization reduces nitrate accumulation and enhances spinach (Spinacia oleracea L.) yield and its quality

Spinach (Spinacia oleracea L.) is considered a nitrogen (N) intensive plant with high nitrate (NO₃^?) accumulation in its leaves. The current study via a two-year field trial introduced an approach by combining N fertilization from different sources (e.g., ammonium nitrate; 33.5 % N, and urea; 48 % N) at different rates (180, and 360 kg N ha^?1) with the foliar spraying of molybdenum (Mo) as sodium molybdate, and/or manganese (Mn) as manganese sulphate at rates of 50 and 100 mgL^?1 of each or with a mixture of Mo and Mn at rates of 50 and 50 mg L^?1, respectively on growth, chemical constituents, and NO₃^? accumulation in spinach leaves. Our findings revealed that the highest rate of N fertilization (360 kg N ha^?1) significantly increased most of the measured parameters e.g., plant length, fresh and dry weight plant^?1, number of leaves plant^?1, leaf area plant^?1, leaf pigments (chlorophyll a, b and carotenoids), nutrients (N, P, K, Fe, Mn, Zn), total soluble carbohydrates, protein content, net assimilation rate, and NO₃^? accumulation, but decreased leaf area ratio and relative growth rate. Moreover, plants received urea-N fertilizer gave the highest values of all previous attributes when compared with ammonium nitrate –N fertilizers, and the lowest values of NO₃^? accumulation. The co-fertilization of N-Mo-Mn gave the highest values in all studied attributes and the lowest NO₃^? accumulation. The best treatment was recorded under the treatment of 360 kg N-urea ha^?1 in parallel with the combined foliar application of Mo and Mn (50 + 50 mg L^?1). Our findings proposed that the co-fertilization of N-Mo-Mn could enhance spinach yield and its quality, while reducing NO₃^? accumulation in leaves, resulting agronomical, environmental and economic benefits.     

Development of urea biosensor using non-covalent complexes of urease with aldehyde derivative of PEG and analysis on serum samples

Abstract

Non-covalent complexes of urease/polyethylene glycol (PEG)-aldehyde were synthesized using regular molar ratios of urease and PEG-aldehyde at room temperature. The physical properties of the non-covalent complexes were analyzed in order to investigate the impact of coupling ratio, temperature, pH, storage stability, and thermal stability. Urease activity was analyzed by UV–Vis spectrophotometer at 630?nm. The results showed that the strongest thermal resistance was obtained using n_U/n_PEG:1/1 (mg/mL) complex within all molar ratios tested. The enzymatic activity of n_U/n_PEG:1/1 complex doubled the activity of the free enzyme. Therefore, this complex was chosen to be used in the analyses. When coupled with PEG-aldehyde, urease exhibited improved activity between pH 4.0–9.0 and the optimum pH was found to be 7.0. The thermal inactivation results of the complex demonstrated that higher activity remained (40%) when compared with the free enzyme (10%) at 60?°C. The storage stability of the non-covalent complex was 4 weeks which was greater than the storage stability of the free enzyme. A kinetic model was suggested in order to reveal the mechanism of enzymatic conversion. Potentiometric urea biosensor was prepared using two different membranes: carboxylated poly vinyl chloride (PVC) and palmitic acid containing PVC. The potentiometric responses of both sensors were tested against pH and temperature and the best results were obtained at pH 7.0 and 20–30?°C. Also, selectivity of the suggested biosensors toward Na⁺, Li⁺ Ca²⁺, and K⁺ ions was evaluated and the reproducibility responses of the urea biosensors were measured with acceptable results.     

Nitrogen metabolism in leaves of a tank epiphytic bromeliad: characterization of a spatial and functional division

The leaf is considered the most important vegetative organ of tank epiphytic bromeliads due to its ability to absorb and assimilate nutrients. However, little is known about the physiological characteristics of nutrient uptake and assimilation. In order to better understand the mechanisms utilized by some tank epiphytic bromeliads to optimize the nitrogen acquisition and assimilation, a study was proposed to verify the existence of a differential capacity to assimilate nitrogen in different leaf portions. The experiments were conducted using young plants of Vriesea gigantea. A nutrient solution containing NO₃⁻/NH₄⁺ or urea as the sole nitrogen source was supplied to the tank of these plants and the activities of urease, nitrate reductase (NR), glutamine synthetase (GS) and glutamate dehydrogenase (NADH-GDH) were quantified in apical and basal leaf portions after 1, 3, 6, 9, 12, 24 and 48 h. The endogenous ammonium and urea contents were also analyzed. Independent of the nitrogen sources utilized, NR and urease activities were higher in the basal portions of leaves in all the period analyzed. On the contrary, GS and GDH activities were higher in apical part. It was also observed that the endogenous ammonium and urea had the highest contents detected in the basal region. These results suggest that the basal portion was preferentially involved in nitrate reduction and urea hydrolysis, while the apical region could be the main area responsible for ammonium assimilation through the action of GS and GDH activities. Moreover, it was possible to infer that ammonium may be transported from the base, to the apex of the leaves. In conclusion, it was suggested that a spatial and functional division in nitrogen absorption and NH₄⁺ assimilation between basal and apical leaf areas exists, ensuring that the majority of nitrogen available inside the tank is quickly used by bromeliad's leaves.     

Short-term effects of urea amended with the urease inhibitor N-(n-butyl) thiophosphoric triamide on perennial ryegrass

The current study investigated the short-term physiological implications of plant nitrogen uptake of urea amended with the urease inhibitor N-(n-butyl) thiophosphoric triamide (nBTPT) under both greenhouse and field conditions. ¹⁵N labelled urea amended with 0.0, 0.01, 0.1 and 0.5% nBTPT (w/w) was surface applied at a rate equivalent to 100 kg N ha^–1 to perennial ryegrass in a greenhouse pot experiment. Root, shoot and soil fractions were destructively harvested 0.75, 1.75, 4, 7 and 10 days after fertilizer application. Urease activity was determined in each fraction together with ¹⁵N recovery and a range of chemical analyses. The effect of nBTPT amended urea on leaf tip scorch was evaluated together with the effect of the inhibitor applied on its own on plant urease activity.nBTPT-amended urea dramatically reduced shoot urease activity for the first few days after application compared to unamended urea. The higher the nBTPT concentration the longer the time required for shoot activity to return to that in the unamended treatment. At the highest inhibitor concentration of 0.5% shoot urease activity had returned to that of unamended urea by 10 days. Root urease activity was unaffected by nBTPT in the presence of urea but was affected by nBTPT in the absence of urea.Transient leaf tip scorch was observed approximately 7–15 days after nBTPT + urea application and was greatest with high concentrations of nBTPT and high urea-N application rates. New developing leaves showed no visual sign of tip necrosis.Urea hydrolysis of unamended urea was rapid with only 1.3% urea-N remaining in the soil after 1.75 days. N uptake and metabolism by ryegrass was rapid with ¹⁵N recovery from unamended urea, in the plant (shoot + root) being 33% after 1.75 days. Most of the ¹⁵N in the soil following the urea+0.5% nBTPT application was still as urea after 1.75 days, yet ¹⁵N plant recovery at this time was 25% (root+shoot). This together with other evidence, suggests that if urea hydrolysis in soil is delayed by nBTPT then urea can be taken up by ryegrass as the intact molecule, albeit at a significantly slower initial rate of uptake than NH₄ ⁺-N. Protein and water soluble carbohydrate content of the plant were not significantly affected by amending urea with nBTPT however, there was a significant effect on the composition of amino acids in the roots and shoots, suggesting a difference in metabolism.Although nBTPT-amended urea affected plant urease activity and caused some leaf-tip scorch the effects were transient and short-lived. The previously reported benefit of nBTPT in reducing NH₃ volatilization of urea would appear to far outweigh any of the observed short-term effects, as dry-matter production of ryegrass is increased.     

Chlorophyllase activities and chlorophyll degradation during leaf senescence in non-yellowing mutant and wild type of Phaseolus vulgaris L

The activities of chlorophyllase, contents of pigments including chlorophyll a and b, chlorophyllide a and b, and phaeophorbide a during leaf senescence under low oxygen (0.5% O2) and control (air) were investigated in a non-yellowing mutant and wild-type leaves of snap beans (Phaseolus vulgaris L.). Chlorophyllase from leaf tissues had maximum activity when incubated at 40C in a mixture containing 50% acetone. In both mutant and wild type, chlorophyllase activity was the highest in freshly harvested non-senescent leaves and decreased sharply in the course of senescence, indicating that the loss of chlorophylls in senescing leaves is not directly related to the activity of chlorophyllase and that chlorophyllase activity is not altered in the mutant. The wild type had higher ratios of chlorophyll a to chlorophyll b than the mutant and chlorophyll a : b ratios increased during senescence in both types. In the senescent mutant leaves, accumulations of chlorophyllide a and chlorophyllide b were detected, but no phaeophorbide a was found. Chlorophyllide b had a greater accumulation than chlorophyllide a in the early stage of senescence. Low oxygen treatment not only delayed chlorophyll degradation but also enhanced the accumulations of chlorophyllide a and b and lowered the ratios of chlorophyll a to chlorophyll b.     

Chlorophyll fluorescence kinetics, photosynthetic activity, and pigment composition of blue-shade and half-shade leaves as compared to sun and shade leaves of different trees

The chlorophyll (Chl) fluorescence induction kinetics, net photosynthetic CO₂ fixation rates P _N, and composition of photosynthetic pigments of differently light exposed leaves of several trees were comparatively measured to determine the differences in photosynthetic activity and pigment adaptation of leaves. The functional measurements were carried out with sun, half-shade and shade leaves of seven different trees species. These were: Acer platanoides L., Ginkgo biloba L., Fagus sylvatica L., Platanus x acerifolia Willd., Populus nigra L., Quercus robur L., Tilia cordata Mill. In three cases (beech, ginkgo, and oak), we compared the Chl fluorescence kinetics and photosynthetic rates of blue-shade leaves of the north tree crown receiving only blue sky light but no direct sunlight with that of sun leaves. In these cases, we also determined in detail the pigment composition of all four leaf types. In addition, we determined the quantum irradiance and spectral irradiance of direct sunlight, blue skylight as well as the irradiance in half shade and full shade. The results indicate that sun leaves possess significantly higher mean values for the net CO₂ fixation rates P _N (7.8–10.7 μmol CO₂ m^?2 s^?1 leaf area) and the Chl fluorescence ratio R _Fd (3.85–4.46) as compared to shade leaves (mean P _N of 2.6–3.8 μmol CO₂ m^?2 s^?1 leaf area.; mean R _Fd of 1.94–2.56). Sun leaves also exhibit higher mean values for the pigment ratio Chl a/b (3.14–3.31) and considerably lower values for the weight ratio total chlorophylls to total carotenoids, (a + b)/(x + c), (4.07–4.25) as compared to shade leaves (Chl a/b 2.62–2.72) and (a + b)/(x + c) of 5.18–5.54. Blue-shade and half-shade leaves have an intermediate position between sun and shade leaves in all investigated parameters including the ratio F _v/F _o (maximum quantum yield of PS2 photochemistry) and are significantly different from sun and shade leaves but could not be differentiated from each other. The mean values of the Chl fluorescence decrease ratio R _Fd of blue-shade and half-shade leaves fit well into the strong linear correlation with the net photosynthetic rates P _N of sun and shade leaves, thus unequivocally indicating that the determination of the Chl fluorescence decrease ratio R _Fd is a fast and indirect measurement of the photosynthetic activity of leaves. The investigations clearly demonstrate that the photosynthetic capacity and pigment composition of leaves and chloroplasts strongly depend on the amounts and quality of light received by the leaves.     

Tolerance of Arabidopsis thaliana to the Allelochemical Protocatechualdehyde

We investigated the effects of the secondary metabolite protocatechualdehyde (PCA, 3,4-dihydroxybenzaldehyde) on stress markers, including fluorescence parameters and the concentrations of pigments, free radicals, protein, and lipid peroxides, in adult plants of Arabidopsis thaliana. The content of proline, carotenoids, and chlorophylls a and b peaked 9?h after administration of 3?mM PCA (the highest concentration used in this study), although malonyldialdehyde and dry mass contents peaked 24?h following PCA treatment. Leaf staining revealed peak production of O₂ ^? between 30 and 90?min post-treatment and peak production of H₂O₂ between 1 and 9?h post-treatment. Several effects, including the observed furling of leaf margins (leaf rolling), the increases in proline content and dry to fresh weight ratio, and the oxidative burst, are reminiscent of the plant response to drought. Early dehydration in PCA-treated plants was confirmed by decreases in leaf water potential, relative water content, and stomatal opening in the first hours of treatment. Thus, PCA seems to be either inducing water deficiency stress (probably through action in the roots) or directly triggering antidrought defenses. In either case, plants showed tolerance to the concentrations employed in this study, with most of the parameters observed having recovered control values within a week of treatment.