Regulation of hormonal responses of sweet pepper as affected by salinity and elevated CO2 concentration

This study examines the extent to which the predicted CO₂‐protective effects on the inhibition of growth, impairment of photosynthesis and nutrient imbalance caused by saline stress are mediated by an effective adaptation of the endogenous plant hormonal balance. Therefore, sweet pepper plants (Capsicum annuum, cv. Ciclón) were grown at ambient or elevated [CO₂] (400 or 800 µmol mol^–1) with a nutrient solution containing 0 or 80 mM NaCl. The results show that, under saline conditions, elevated [CO₂] increased plant dry weight, leaf area, leaf relative water content and net photosynthesis compared with ambient [CO₂], whilst the maximum potential quantum efficiency of photosystem II was not modified. In salt‐stressed plants, elevated [CO₂] increased leaf NO₃^– concentration and reduced Cl^– concentration. Salinity stress induced ABA accumulation in the leaves but it was reduced in the roots at high [CO₂], being correlated with the stomatal response. Under non‐stressed conditions, IAA was dramatically reduced in the roots when high [CO₂] was applied, which resulted in greater root DW and root respiration. Additionally, the observed high CK concentration in the roots (especially tZR) could prevent downregulation of photosynthesis at high [CO₂], as the N level in the leaves was increased compared with the ambient [CO₂], under salt‐stress conditions. These results demonstrate that the hormonal balance was altered by the [CO₂], which resulted in significant changes at the growth, gas exchange and nutritional levels.     

Glutathione metabolism in soybean callus-cultures as affected by salinity

Induction and growth of soybean callus cultures were influenced by NaCl, especially at the highest concentration tested (150 mM). Protein content was raised as NaCl was increased in the Murashige and Skoog medium. Total sulfhydryl group (-SH) and glutathione (GSH) concentrations were also increased in NaCl treated cultures. The affinity (Km) of glutathione reductase (GR) for oxidized glutathione (GSSG) was gradually increased as NaCl level was raised in the medium. The GSH/GSSG ratio was raised significantly as the result of GR activity. The increase in GR activity may constitute an adaptive response of soybean callus to NaCl. This revised version was published online in July 2006 with corrections to the Cover Date.     

Polyploidy in tomato roots as affected by arbuscular mycorrhizal colonization

Nuclear changes in roots of tomato (Lycopersicon esculentum), a plant with a small genome, during the establishment of arbuscular mycorrhizal (AM) colonization were studied using light and electron microscopy, as well as flow and static cytometry. Nuclei of mycorrhizal root cortex cells were larger and had more decondensed chromatin than those of controls. Significant ploidy distribution differences were observed between nuclei of AM colonized and control roots, and a strong correlation between nuclear polyploidization and AM colonization was found. Polyploidization and decondensation are usually associated with high metabolic activity. The metabolic activity of mycorrhizal root cells, evaluated in this work as respiratory activity by using a cytochemical assay for succinate dehydrogenase combined with image analysis, increased in comparison to controls. The meaning of polyploidization is discussed in relation to the structural and metabolic modifications induced by mycorrhization.     

Leaf age and salinity influence water relations of pepper leaves

Plant growth is reduced under saline conditions even when turgor in mature leaves is maintained by osmotic adjustment. The objective of this study was to determine if young leaves from salt-affected plants were also osmotically adjusted. Pepper plants (Capsicum annuum L. cv. California Wonder) were grown in several levels of solution osmotic potential and various components of the plants' water relations were measured to determine if young, rapidly growing leaves could accumulate solutes rapidly enough to maintain turgor for normal cell enlargement. Psychrometric measurements indicated that osmotic adjustment is similar for both young and mature leaves although osmotic potential is slightly lower for young leaves. Total water potential is also lower for young leaves, particularly at dawn for the saline treatments. The result is reduced turgor under saline conditions at dawn for young but not mature leaves. This reduced turgor at dawn, and presumably low night value, is possibly a cause of reduced growth under saline conditions. No differences in leaf turgor occur at midday. Porometer measurements indicated that young leaves at a given salinity level have a higher stomatal conductance than mature leaves, regardless of the time of day. The result of stomatal closure is a linear reduction of transpiration.     

The growth and nodulation ofTrifolium alexandrinum as affected by salinity

The growth and nodulation ofTrifolium alexandrinum were compared at six levels (0 - 1.2 % NaCl) of salinity. Dry mass of shoots and roots, 14 and 20 weeks after the commencement of salinity treatment, increased at low levels of salinity (0.1 - 0.2 % NaCl) but decreased with higher NaCl concentrations (0.4 - 1.2 %). Nodulation occurred at NaCl concentrations up to 0.8 %. Nodule mass decreased with increasing salinity levels. The nodule size remained unaffected at NaCl concentrations up to 0.4 % but was reduced at higher concentrations.     

Growth, photosynthesis and respiration in Plantago coronopus as affected by salinity

Plantago coronopus was grown in a non-saline culture solution and in a culture solution containing 50 m M NaCl. The rates of dry matter accumulation in both roots and shoots were not affected by 50 m M NaCl. Photosynthesis, expressed per shoot, was also the same in both environments. Neither the rate of shoot respiration nor that of root respiration was affected by salinity. In both environments the alternative respiratory pathway contributed to the same extent in root respiration. The activity of the alternative pathway decreased with increasing age. Since the respiratory activities were the same in plants grown under both saline and non-saline conditions and since the alternative respiratory pathway was also equally active in roots under both environmental conditions, it is concluded that respiratory costs involved in growth in 50 m M NaCl are negligible in terms of the plant's total energy costings.     

Nitrogen uptake and metabolism in Populus x canescens as affected by salinity

External salinization can affect different steps of nitrogen (N) metabolism (ion uptake, N assimilation, and amino acid and protein synthesis) depending on the inorganic N source. Here, we assessed the net uptake of N supplied as nitrate or ammonium and N assimilation (combining metabolite analyses with molecular biological approaches) in grey poplar (Populus x canescens) plants grown under saline (75 mM NaCl) and control conditions. The specific (micromol N g(-1) dry weight fine roots h(-1)) and total plant (micromol N per plant h(-1)) N net uptake rates, total plant N content, total plant biomass and total leaf protein concentration were reduced under saline conditions when plants were supplied with ammonium. In both nutritional groups, salt treatment caused pronounced accumulation of soluble N compounds in the leaves. The mRNAs of genes coding for enzymes catalyzing rate-limiting steps of both proline synthesis and degradation (delta-1-pyrroline-5-carboxylate synthase and proline dehydrogenase) as well as for NADH-dependent glutamate synthase were accumulated under saline conditions. Whereas under control conditions the plant N status seemed to be superior when ammonium was supplied, the N balance of ammonium-fed plants was more severely affected by salt stress than that of plants supplied with nitrate. Possible metabolic implications of stress-related accumulation of particular amino acids are discussed.     

Protein and free amino acids in wheat grain as affected by soil types and salinity levels in irrigation water

Summary The effects of four lysimeter soil series under three salinity levels were evaluated for grain yield, wt/1000 seeds, protein, and amino acids in Mexican dwarf wheat (Triticum aestivum L. var. Cajeme 71). The soil series consisted of: Holtville clay loam, Greenfield sandy loam, San Emigdio sandy loam, and Altamont clay loam. The irrigation water salinity levels were designated: low –2.2 mmho, medium –4.2 mmho, and high –7.1 mmho.No significant differences were found in the amount of grain harvested or wt/1000 seeds in the 1976 crop produced on the differential soil series. The yield of the 1977 crop was significantly affected by the soil types.Effects of soil type on the protein amino acids in the grain in both years were similar. Significantly higher protein amino acid levels of histidine, arginine, aspartic acid, threonine, serine, glutamic acid, glycine, alanine, cystine, valine, methionine, isoleucine, leucine, tyrosine, and phenylalanine were found in the grain grown on Altamont clay loam soil than the other types.The free amino acids in grain from the 1976 and 1977 crops were similarly affected by the soil types, except that the quantitative values of the free amino acids were substantially lower in 1977 than in 1976. The free amino acids significantly influenced by soil types were tryptophane, lysine, arginine, aspartic acid, threonine, serine, glycine, alanine, valine, isoleucine, tyrosine, and phenylalanine. In both years' crops, the sum of the free amino acid fractions was significantly higher in the grain produced on the Altamont soil than on the other soils.Salinity level in the irrigation water did not affect the 1976 crop yield or wt/1000 seeds. Although yields of the 1977 crop were significantly reduced by salinity, the wt/1000 seeds was not. The sum of protein amino acids was significantly higher in the 1976 and 1977 grain crops irrigated with high salinity water than in low salinity irrigated crops.An increased salinity irrigation water significantly reduced the sum of free amino acid fractions in the 1976 grain crop. Since some of the free amino acids in the 1977 grain crop increased while the others decreased due to the salinity level in the irrigation water, the sum of the free amino acid fractions was not significantly influenced.Significant interactions were found between soil types and salinity levels on free arginine, threonine, serine, glutamic acid, and alanine, and also on the sum of the free amino acids in the 1976 wheat grain. In the 1977 wheat grain, there were significant interactions between soil types and salinity levels on the free glutamic acid, valine, leucine, tyrosine, and phenylalanine, and on protein serine, glutamic acid, glycine, alanine, and the sum of the protein amino acids.The amounts of essential amino acids expressed as mg of amino acid/g of protein were not affected by the soil types or salinity levels. With the exception of lysine, and possibly threonine and methionine plus cystine, the essential amino acids were present in the grain at concentrations equal to or greater than recommended by WHO and FAO.     

Methane emission from a wetland rice field as affected by salinity

The impact of salinity on CH₄ emission was studied by adding salt to a Philippine rice paddy, increasing pore water EC to approx. 4 dS.m^-1 Methane emission from the salt-amended plot and adjacent control plots was monitored with a closed chamber technique. The addition of salt to the rice field caused a reduction by 25% in CH₄ emission. Rates of methane emissions from intact soil cores were measured during aerobic and anaerobic incubations. The anaerobic CH₄ fluxes from the salt-amended soil cores were three to four times lower than from cores of the control plot, whereas the aerobic CH₄ fluxes were about equal. Measurements of the potential CH₄ production with depth showed that the CH₄ production in the salt-amended field was strongly reduced compared to the control field. Calculation of the percentage CH₄ oxidized of the anaerobic flux indicated that CH₄ oxidation in the salt-amended plot was even more inhibited than CH₄ production. The net result was about equal aerobic CH₄ fluxes from both salt-amended plots and non-amended plots. The data illustrate the importance of both CH₄ production and CH₄ oxidation when estimating CH₄ emission and show that the ratio between CH₄ production and CH₄ oxidation may depend on environmental conditions. The reduction in CH₄ emission from rice paddies upon amendment with salt low in sulfate is considerably smaller than the reduction in CH₄ emission observed in a similar study where fields were amended with high-sulfate containing salt (gypsum). The results indicate that CH₄ emissions from wetland rice fields on saline, low-sulfate soils are lower than CH₄ emissions from otherwise comparable non-saline rice tields. However, the reduction in CH₄ emission is not proportional to the reduction in CH₄ production     

Growth and ionic relations of six triticale cultivars as affected by salinity

Growth and ionic relations were studied in six triticale cultivars of different geographical origins grown in a greenhouse in nutrient solution with or without the addition of 100 mM NaCl. In 21 d old plants of all the six cultivars growth was little affected in the salt treatment, whereas in the subsequent three harvests during vegetative phase (after 31, 38 and 45 d), growth reduction effects of salinity were progressively pronounced. Generally, shoots of all the six cultivars accumulated relatively more K+ as compared to Na+ or Cl-. Differential accumulation of K+, Na+ and Cl- by various cultivars was coupled with variable rates of Na+ and Cl- transport from root to shoot which were — to some extent- related to cultivar differences in growth in saline root media. Chloride content of shoots of the six cultivars was negatively correlated with the relative growth reduction due to salinity at the four harvests.     

Growth and development ofRumex roots as affected by hypoxic and anoxic conditions

The growth characteristics of threeRumex species were determined under different solution oxygen concentrations in hydroculture. These species all occur in a river foreland ecosystem and they were found to differ in their flood tolerance. The flood-tolerantR. maritimus undR. crispus developed a large number of new, aerenchymatous roots within a short period under low solution oxygen concentrations. Biomass production was not affected. In the flood-intolerantR. thyrsiflorus, however, only few slow-growing new roots were developed and biomass production was significantly reduced at solution oxygen concentrations below 2% (v:v). These different responses could be partly explained by a differential aerenchyma formation in new roots of the flood-tolerant species. Aerenchyma can relieve the oxygen stress of the root systems via internal aeration.The fast development of new roots of the flood-tolerantR. maritimus andR. crispus after the onset of anaerobiosis coincided with the reduction or cessation of growth of the primary roots. Notwithstanding the cessation of growth, however, primary roots of both species were able to recover following restoration of aerobic conditions after a 13-day anaerobic period. However, the roots ofR. thyrsiflorus ceased growing very soon after the onset of anaerobiosis. All had died within 10 days.The balance between the growth rates of the primary and the newly formed root system are discussed and related to the differential tolerance of theRumex species to transient flooding.     

Structure and function of water channels

All aquaporins are efficient water transporters, while sustaining strict selectivity, even against protons, thereby maintaining the proton gradient across the cell membrane. Recently solved structures of these membrane channels have helped us to understand this remarkable property.     

Oxidative gating of water channels (aquaporins) in corn roots

An oxidative gating of water channels (aquaporins: AQPs) was observed in roots of corn seedlings as already found for the green alga Chara corallina. In the presence of 35 mM hydrogen peroxide (H2O2)--a precursor of hydroxyl radicals (*OH)--half times of water flow (as measured with the aid of pressure probes) increased at the level of both entire roots and individual cortical cells by factors of three and nine, respectively. This indicated decreases in the hydrostatic hydraulic conductivity of roots (Lp(hr)) and of cells (Lp(h)) by the same factors. Unlike other stresses, the plant hormone abscisic acid (ABA) had no ameliorative effect either on root LP(hr) or on cell Lp(h) when AQPs were inhibited by oxidative stress. Closure of AQPs reduced the permeability of acetone by factors of two in roots and 1.5 in cells. This indicated that AQPs were not ideally selective for water but allowed the passage of the organic solute acetone. In the presence of H2O2, channel closure caused anomalous (negative) osmosis at both the root and the cell level. This was interpreted by the fact that in the case of the rapidly permeating solute acetone, channel closure caused the solute to move faster than the water and the reflection coefficient (sigma s) reversed its sign. When H2O2 was removed from the medium, the effects were reversible, again at both the root and the cell level. The results provide evidence of oxidative gating of AQPs, which leads on to inhibition of water uptake by the roots. Possible mechanisms of the oxidative gating of AQPs induced by H2O2 (*OH) are discussed.     

Adenine nucleotide content of corn roots as affected by injury and subsequent washing

The adenine nucleotide content of the 2-centimeter segments excised from tray-grown corn (Zea mays L., WF9 × Mo17) roots declines for the first hour after excision. Concomitant with the loss of adenine nucleotides is a decline in respiration and a leakage of K⁺. With continued washing, these parameters partially or completely recover and increased phosphate influx develops. Increasing the wound effect by cutting 0.5-centimeter segments gives a more rapid and pronounced degradation of adenine nucleotides and slower recovery. Conversely, the mild injury caused by submerging intact roots induces less degradation and produces greater net adenine nucleotide synthesis during recovery; adding auxin to the washing medium produces a similar result. With all treatments, there is stabilization of energy charge at about 0.85.     

Seed reserve utilization and seedling growth of wheat as affected by drought and salinity

In germination stage, decreased wheat (Triticum aestivum L.) seedling growth (mg per seedling) as affected by drought and salinity stresses is a well-known phenomenon. The heterotrophic seedling growth can be defined as a product of two components: (1) the weight of mobilized seed reserve (WMSR; mg per seed), and (2) the conversion efficiency of utilized seed reserve to seedling tissue (mg seedling dry weight (SLDW) per mg utilized seed reserve). The first component can be further divided into (1) initial seed weight (mg per seed), and (2) the fraction of seed reserve, which is mobilized (mg mobilized seed reserve per mg initial seed weight). The objective of this study was the identification of the sensitive seedling growth component(s) in response to drought and salinity stresses. Two experiments were separately conducted using various osmotic pressures (OP) induced by polyethylene glycol (PEG; 0–1.8 MPa, with interval of 0.2) in experiment 1 and by NaCl (0, 0.4, 0.8, 1.2 and 1.6 MPa) in experiment 2. Two wheat cultivars were used in each experiment. In both experiments, seedling growth, fraction of seed reserve utilization and weight of mobilized seed reserve decreased with increasing drought and salt intensity. However, drought and salinity stresses had no effect on the conversion efficiency. It was concluded that the sensitive component of seedling growth is the weight of mobilized seed reserve. Thus, appropriate efforts such as plant breeding programs should be focused on improvement of seed reserve mobilization in order to obtain increased seedling growth under drought and salinity stresses.     

Growth and development of maxipak wheat as affected by soil salinity and moisture levels

The effect of soil salinity and soil moisture on the growth and yield of maxipak wheat (Triticum aestivum L.) was studied in a lath-house experiment in whih, chloride-sulphate salt mixtures were used to artificially salinize a sandy loam soil from Al-Jadyriah Baghdad. Five soil salinity levels of ECe's equal to 1.7 (Control) 4.2, 5.8, 8.1, 9.4 and 11.0dSm^–1 were prepared and used at 3 levels of available soil moisture depletion, namely, 25, 50, and 75% as determined by weight. Both growth (vegetative) and yield components were studied throughout the growing season.Results showed that increasing the soil salinity from 1.7 to 11.0 dSm^–1, and decreasing the available soil water from 75 to 25% resulted in independent and significant decreases in Mazipak wheat growth and yield components at different stages of plant development. Root growth showed more sensitivity to both available soil water and soil salinity level than other components. It has been concluded that at soil salinity levels of more than 8.0 dSm^–1, available soil water became a limiting factor on wheat growth and the maintenance of 75% of available soil water during the growth period is recommended to obtain satisfactory grain yield.