Abstract: The effect of five levels of nitrogen fertilization on the growth and nutritional quality of Cos lettuce (Lactuca sativa L. cv. Parris Island) at harvest and after storage was studied during autumn and winter in South-West Greece. Plants were cultivated hydroponically in a greenhouse and the nitrate, chlorophyll and ascorbic acid (vitamin C) concentrations within the plant tissues were measured at harvest and following storage at 5 or 10 °C for 10 days. Nitrate accumulated in the leaves with increasing amounts of N within the nutrient solution and was higher in the winter than in the autumn. At the lowest N level (20 mg L-1), the inner leaves accumulated more nitrate than the outer leaves, whereas at higher N levels (140, 200 or 260 mg L-1) nitrate accumulation was higher in the outer leaves. Overall, the highest nitrate concentrations were detected in the petiole and the proximal end of the leaf, but at the lowest N application rate (20 mg L-1) nitrate accumulated in the distal region of the leaf too. Although the nitrate concentrations within the leaves did not change significantly during 10 days storage at 5 or 10 °C, the chlorophyll and vitamin C concentrations decreased. Chlorophyll loss was higher in lettuce that was grown under low N levels and was higher at 10 °C than at 5 °C, but was reduced by enclosure of the lettuce in polyethylene film. It is concluded that the optimum N application rate for Cos lettuce grown hydroponically under cover during autumn and winter in South-West Greece, and in other areas with a similar climate, is 200 mg N L-1 because at this N rate yield is satisfactory and leaf nitrate concentrations are below the maximum acceptable level for human consumption. Nutritional value (vitamin C concentration) and market quality (chlorophyll content) are highest at harvest and decrease during storage, but quality in terms of nitrate concentration does not change.
Abstract: Cd2+ binding on γ-Al2O3 was studied in the presence of the common electrolyte ions Mg2+, SO2-4, and NO-3 at high and low concentrations. Direct measurements were performed for Cd2+ as well as for electrolyte ion adsorption as a function of pH. The experimental data reveal that Cd2+ binding on γ-Al2O3 is modulated by the electrolyte ions in a complex manner. At high electrolyte concentration, Cd2+ uptake by γ-Al2O3 is inhibited. Theoretical analysis by a surface complexation model shows that this effect can be attributed partially to bulk, ionic strength, and effect of the electrolyte, but the most significant inhibition is due to direct competition between Mg2+ and Cd2+ ions for the {triple bond, long}SO- surface sites of γ-Al2O3. At low concentration of electrolyte ions, Cd2+ uptake by γ-Al2O3 can be enhanced due to synergistic co-adsorption of Cd2+ and electrolyte anions, particularly SO2-4 and to a lesser extent NO-3. The theoretical analysis shows that this co-adsorption is due to formation of ternary surface species ({triple bond, long}SOH2SO4Cd) and ({triple bond, long}SOH2NO3Cd) which enhance Cd-uptake at pH values well below the point of zero charge of the γ-Al2O3.
Abstract: In an experiment with roses grown hydroponically, a low (0.3 mM) and a high (2 mM) level of silicon were combined with a low (0.8 mM) and a high (40 mM) NaCl concentration in the nutrient solution supplied to the crop. The aim of the experiment was to detect possible beneficial effects of silicon on plant growth, yield and flower quality and to test whether the deleterious effects of NaCl-salinity on roses could be mitigated by increasing the Si concentration in the root zone. Silicon was added to the nutrient solution in form of a water-soluble potassium silicate compound. The electrical conductivity (EC) in the nutrient solutions with low and high NaCl concentrations was 1.8 and 6.1 dS m-1, respectively, while the corresponding values in the drainage water, which indicated the salinity status in the root zone, were 2.3 and 8.2 dS m-1, respectively. The increase of the NaCl concentration in the root zone restricted the above-ground vegetative weight of roses, the number of flowers per plant and the mean flower weight and stem length. The increased supply of Si significantly enhanced the vegetative growth of roses at both salinity levels, improved the overall appearance of the plants and resulted in a higher number of marketable flowers per plant at the low salinity level. However, silicon was unable to ameliorate the adverse effects of NaCl-salinity on flower production and quality. The increased Si concentration in the root environment restricted the translocation of Na and Cl to the young leaves of roses. However, net photosynthesis, stomatal conductance and transpiration rate were not affected either by Si or by NaCl-salinity at the concentration levels tested in this study. This finding indicates that the stimulation of the vegetative growth of roses by Si under conditions of high external salinity was not due to mitigation of toxic Na or Si effects on the photosynthetic apparatus.
Abstract: Sclerotinia minor accumulates β-carotene at levels dependent upon oxidative growth conditions and differentiation. β-carotene accumulation is 2.5-fold higher in differentiated mycelia at high than at low oxidative stress, and approx. 3-fold higher in differentiated than in undifferentiated mycelia. It is proposed that β-carotene may be produced by the fungus to counteract oxidative stress that develops during growth. This is shown by the finding that exogenous β-carotene at growth non-inhibiting concentrations causes a concentration-dependent reduction of oxidative stress (lipid and protein peroxidation) and sclerotial differentiation in this fungus. The data of this study support our hypothesis that sclerotial differentiation in phytopathogenic fungi may be induced by oxidative stress.
Abstract: The portable chlorophyll meter (SPAD-502) has been successfully used for a rapid and direct estimation of total chlorophyll content (TCHL) in the leaves of some crops. In this work, SPAD-502 meter readings and TCHL concentration were compared for the leaves of Amaranthus vlitus L., a common weed. SPAD readings were linearly and positively correlated to TCHL concentration in the leaves. A linear correlation was also shown between SPAD-502 readings and some physiological parameters of the leaves, such as photosynthesis, transpiration, and stomatal conductance.
Abstract:
Phosphoenolpyruvate carboxylase (PEPC) and pyruvate orthophosphate dikinase (PPDK) cold inactivation was studied in leaf extracts from Atriplex halimus L. Both enzyme activities gradually reduced as the temperature and the total soluble protein decreased. Mg2+ at a concentration of 10 mM stabilized PEPC and PPDK activities against cold inactivation. At low Mg 2+ concentration (4 mM), PEPC was strongly protected by phosphoenolpyruvate, glucose-6-phosphate, and, partially, by L-malate, while PPDK was protected by PEP, but not by its substrate, pyruvate. High concentrations of compatible solutes (glycerol, betaine, proline, sorbitol and trehalose) proved to be good protectants for both enzyme activities against cold inactivation. When illuminated leaves were exposed to low temperature, PPDK was partially inactivated, while the activity of PEPC was not altered.
Abstract: When Tris-SO4 was used as an extraction buffer for phosphoenolpyruvate carboxylase (PEPC) from leaves of the C4 plant Cynodon dactylon (L.) Pers., a higher extractable activity was obtained as compared to Tris-HCl, especially at low phosphoenolpyruvate concentrations and an assay pH of 7.2. The Tris-SO4-extracted PEPC activity was stable under dilution and remained unchanged for at least 24 h at 22°C. This enzyme was less sensitive to both activation by glucose-6-phosphate and inhibition by L-malate. The effects of Tris-SO4 could be attributed to its preferential exclusion from the enzymic protein domain and, therefore, to a shifting of this oligomeric enzyme to a more aggregable form that is more stable and active.
Abstract: Among various C4 plants we found a wide range in the level of inactivation of phosphoenolpyruvate carboxylase (PEPC) at low temperature (0 °C). The activity of the 2-fold diluted enzyme in crude leaf extracts after 60 min incubation (compared to zero time incubation) at pH 7.5, remained above 87 % at low temperatures for the species Setaria verticillata, Portulaca oleracea, and Saccharum officinarum, and between 11 and 17 % in the species Cynodon dactylon and Atriplex halimus. The enzyme exhibited intermediate levels of inactivation (42 to 58 %) for the species Amaranthus sp., Zea mays, Salsola kali, and Digitaria sanguinalis. The enzyme activity for S. verticillata was unaffected between pH 5.7 and 8.4 during incubation at room and low temperatures. Under similar conditions, the activity of the enzyme from C. dactylon was stable between pH 5.7 and 7.0 and decreased at pH above 7.0, but for Z. mays it was enhanced between pH 5.7 and 6.8 and decreased at pH above 7.0.
Abstract: The extraction of phosphoenolpyruvate carboxylase, PEPC (EC 4.1.1.31) from leaves of Cynodon dactylon (L.) Pers. with phosphate buffer (pH 7.4, 105 mM) was advantageous in comparison to the usual extraction with Tris-HCl buffer (pH 7.4, 100 mM); a higher activity was obtained, which was most evident at low substrate (phosphoenolpyruvate) concentrations. The PEPC activity was stable under dilution or in storage for at least 48 h at room temperature. The effects of phosphate buffer were not due to inhibition of phosphatase(s) action during the extraction, since they were also observed when the phosphates were added after the extraction with Tris-HCl. The phosphate-extracted enzyme was less responsive to both L-malate inhibition and activation by glucose-6-phosphate. The effects of phosphates might be due to preferential exclusion from the enzymic protein domain and, therefore, to a confinement of the enzyme to a fraction of the total volume.
Abstract: The effect of phosphate, sulfate and other inorganic ions on the activity of phosphoenolpyruvate carboxylase (PEPC) from the C4 plant Cynodon dactylon were investigated for the first time, as well as their interaction with Glc-6-P, AMP and malate. Activation of PEPC by phosphate and sulfate ions was demonstrated and it was not dependent on the accompanying cations, something that was not clarified for PEPCs from other plant sources. No activation of this enzyme was observed by nitrate. PEPC activation was found to be competitive with glucose-6-phosphate (Glc-6-P) and AMP stimulation and less sensitive to malate inhibition. This work showed that PEPC from C4 plants could exhibit similar activation properties with the enzyme from CAM plants and different activation properties in plants of the same type, rendering the study of this enzyme from different plant sources necessary.
Abstract: High concentration (1.25 M) of trehalose fully protects the activity of phosphoenolpyruvate carboxylase (PEPC, E C 4.1.1.31) from the C4-plant Cynodon dactylon against cold inactivation (at 0°) and heat inactivation (at 37°). The concentration of glycerol needed for the same degree of protection was much higher (2.7 M). Phosphates at 100 mM only partially protect the enzyme against cold inactivation and at 60 mM fully protect it against heat inactivation. At temperatures higher than 45°the enzyme was better protected by trehalose than glycerol or phosphates.
Abstract: Glycerol stabilizes the activity of pyruvate, orthophosphate dikinase extracted from darkened or illuminated maize leaves. It serves as a better protectant of activity than dithiothreitol for the active day-form and the glycerol concentration needed for full protection is inversely related to the level of protein. The night-form of the enzyme is also protected by glycerol not only against inactivation, but also against partial reactivation in storage. Glycerol does not prevent the Pi-dependent activation nor the ADP-dependent inactivation of pyruvate, orthophosphate dikinase, but the rates of both processes are substantially decreased. The ability of the inactive night-form for Pi-dependent activation is also sustained by glycerol for at least 2 h at 20°C, apparently through stabilization of the labile regulatory protein.
Abstract: Phospho enolpyruvate carboxylase (EC 4.1.1.31), used as a coupling enzyme in the assay of the pyruvate, orthophosphate dikinase (EC 2.7.9.1) forward reaction, is a serious limiting factor for the overall rate when added at a level of 0.2-0.3 unit/ml of assay medium. Nonlimiting assay conditions are obtained by either increasing the level of the coupling enzyme to 3 units/ml or adding 6mM glucose-6-phosphate as an activator/stabilizer of phospho enolpyruvate carboxylase.
