High Osmoregulation Research Articles

Combined effects of water stress and salinity on growth, physiological, and biochemical traits in two walnut genotypes.

Due to its great economic value, walnut (Juglans regia L.) has received increasing attention during recent years. However, water stress and salinity limit walnut growth, production, and quality. We employed two walnut genotypes, precocious walnut, and late-bearing walnut, to investigate their growth, photosynthetic capacity, non-structural carbohydrate contents, Cl- allocation, reactive oxygen species (ROS) accumulation, and osmotic regulation under water stress, salinity, and their combination. We found that late-bearing walnut showed higher total biomass and net photosynthetic rate, higher activities of antioxidant enzymes, higher osmoregulation, and lower ROS accumulation than precocious walnut under stressful conditions. In addition, late-bearing walnut restricted salt transport and allocated more Cl- into roots, whereas precocious walnut allocated more Cl- into leaves when exposed to salinity stress. These data collectively demonstrated that late-bearing walnut possesses better stress tolerance under water stress, salinity, and especially under their combination. Such knowledge of genotype-specific responses and tolerances to water stress and salinity is important for walnut plantation management under increasing drought and aggravated soil salinization occurring with climate change.

Critical Functions of Region 1-67 and Helix XIII in Retaining the Active Structure of NhaD Antiporter in Halomonas sp. Y2.

NhaD-type antiporters are mainly distributed in various Proteobacteria, especially in marine microorganisms and human pathogens. This distribution as well as the pathogenic properties of these strains suggest that these antiporters contribute to the regulation of high osmoregulation and are potential drug targets. Two NhaD homologs, NhaD1 and NhaD2, from the halotolerant and alkaliphilic Halomonas sp. Y2 exhibits similar, high in vitro activity, but remarkably different in vivo functions. To search for critical domains or residues involved in these differences of physiological functions, various chimeras composed of NhaD1 and NhaD2 segments were generated. Two regions at residues 1–67 and 464–492 were found to be responsible for the robust in vivo function of NhaD2, and region 464–492 is also crucial to the pH response of the antiporter. In particular, the completely abolished activity of KNabc/N463r, highly recovered activity while very weakly recovered ion resistance of the KNabc/N463r-C7 chimera, suggested that transmembrane helix (TM) XIII is crucial for the robust ion resistance of NhaD2. Using site-directed mutagenesis, seven hydrophobic residues in TM XIII were identified as key residues for the ion translocation of NhaD2. Compared with the fluorescence resonance energy transfer (FRET) profile in the wild-type NhaD2, the reduced FRET efficiency of N463r chimeras provided solid evidence for conformational changes in the N463r fusion protein and consequently verified the structural functions of TM XIII in the pH activation and physiological functions of NhaD2.

Adaptation of the white shrimp Litopenaeus vannamei to gradual changes to a low-pH environment

pH variation could cause a stress response in euryhaline penaeids, we evaluated the mortality, growth performance, osmoregulation gene expression, digestive enzyme activity, histology, and resistance against Vibrio parahemolyticus of white shrimp Litopenaeus vannamei reared under conditions of gradual changes to a low-pH environment (gradual-low pH, 6.65–8.20) or a high-pH environment (gradual-high pH, 8.20–9.81) versus a normal pH environment (8.14–8.31) during a 28-d experiment. Consequently, under gradual-high pH, the cumulative mortality rate (CMR) rose with time until 39.9% on days 28; the weight gain percentage (WGP) and length gain percentage (LGP) decreased continuously. However, under gradual-low pH, the CMR of shrimp stabilized at 6.67% during 7–28 d; the WGP and LGP decreased first and then returned to normal. These results indicated that L. vannamei displayed a moderate tolerance to gradual-low pH, compared with gradual-high pH. Under gradual-low pH, the Na+/K+-ATPase, cytoplasmic carbonic anydrase (CAc), and glycosyl-phosphatidylinositol-linked carbonic anhydrase (CAg) transcripts of shrimp increased continuously or then back to normal; the amylase, lipase, and trypsin activities decreased first and then returned to normal or increased; the hepatopancreases and midguts showed histopathological lesions first and then got remission. Thus, the major adaptation mechanism of shrimp to gradual-low pH might be its high osmoregulation ability, which made shrimp achieve a new, balanced steady-state, then promoted longer intestinal villi and recuperative hepatopancreases of shrimp with enhanced digestive enzyme activities to increase nutrient absorption after long-term exposure. Meanwhile, the enhanced resistance against V. parahemolyticus under gradual-low pH would probably inhibit disease outbreak in the shrimp farming.

Genotypic differences in partial resistance to crown rot caused by Fusarium pseudograminearum in relation to an osmoregulation gene in wheat

Both crown rot and osmoregulation are expressed when wheat plants undergo water stress. A possible genetic linkage between high osmoregulation and partial resistance to crown rot was investigated by using lines that had been bred for high osmoregulation (or gene) from parents with low osmoregulation and varying crown rot resistances. Analysis of the incidence and severity of the disease showed no association with the presence or absence of the or gene.

Osmoregulation in birdseed millet under conditions of water stress. II. Variation in F3 lines of Setaria italica and its relationship to plant morphology and yield

Variation in osmoregulation is reportedduring water stress for 40 F3 families ofSetaria italica L. (Foxtail millet).The lines were derived from a cross betweentwo genotypes known to differ in thischaracter. A clear separation was observedinto 23 low and 17 high osmoregulation F3families with coefficient variation 19.6%.Osmoregulation was not associated withplant height, length or width of flag leaf,length of panicle exertion, length or widthof inflorescence or dry matter, but wassignificantly associated with grain yield,grain number and harvest index. Mean yieldand harvest index for the highosmoregulation group were 1.58 and 1.57times greater, respectively, than yield andharvest index for the low osmoregulation.

Evaluation in barley lines of a simple pollen method for breeding for drought tolerance in wheat

A pollen test, which is used in wheat to detect an osmoregulation gene, was evaluated in 16 barley doubled-haploid lines. In the presence of water stress, created using a 40% polyethylene glycol 6000 solution on microscope slides, the application of a low concentration of KCl produced a size response in approximately 50% of lines. Visual selection was confirmed by quantification in 8 lines, with significant responses in 5. The lines were then evaluated in the field in low and high soil-water deficit sites. Water deficit was produced by exclusion of rain after initially wetting up the soil profile. The group selected for a pollen response showed significantly lower leaf zero turgor water potentials (ie. greater turgor maintenance). This was, in turn, due to differences in osmoregulation. At ear emergence, mean turgor pressures at low deficit and mean relative water contents at high deficit were higher in lines with high pollen osmoregulation. There was also a separate effect due to differences in maturity between the lines. Higher turgor maintenance and osmoregulation occurred in lines with faster maturity. Because lines were dissimilar, accurate dry weight and yield comparisons between osmoregulation groups were not feasible. A drought susceptibility index was used to accommodate variation in low stress growth and yield, and to allow comparisons with wheat experiments. In all experiments high osmoregulation groups had lower mean values than low groups. Indices calculated for osmoregulation groups in barley were not significantly different to those from similar wheat experiments conducted over 4 seasons with differing evaporative demand, suggesting operation of a similar process or gene. Further confirmatory work is needed using more closely related lines.

Changes in rheological properties and endosperm peroxidase activity associated with breeding for an osmoregulation gene in bread wheat

Backcross lines which had been bred for an osmoregulation gene to improve the drought tolerance of 3 commercial bread wheat cultivars were tested for standard grain, dough, and baking characteristics. Three field sites were used to provide a range of protein contents of 10–14%. It was found that backcross lines with high osmoregulation had alterations in dough strength which could only be understood in terms of genetic linkage. Evidence of a linkage effect was found by comparing lines with recurrent parents in a season of low water stress, i.e. where yields and hence protein contents of each group were the same. On average, lines which had been bred for high osmoregulation had significantly shorter development times and significantly lower maximum resistances to extension than recurrent parents. Other parameters were not significantly different. A probable explanation of the dough strength effect lay in a difference in peroxidase activity due to linkage between the endosperm peroxidase, Per-A4, locus, and the osmoregulation, or, locus. There was an expectation, from published work, that dough strength could be affected by peroxidase. The hypothesis was confirmed by measurements of peroxidase activity. On average, lines with high osmoregulation (lower dough strength) had lower peroxidase activities than recurrent parents (higher dough strength). This effect, however, depended on protein content and genotype. Significance for plant breeding is discussed.

Physiological attributes associated with early-season drought resistance in spring durum wheat cultivars

One of the common features of the Mediterranean climate in North Africa is the uncertainty of rainfall immediately after wheat (Triticum durum Desf) emerges. Relatively little work has been done to compare the drought resistance of spring durum wheat cultivars under early-season drought stress. There is a limited insight into the physiological basis of spring durum wheat drought resistance in rainfed Mediterranean regions. Field experiments were conducted in 1995 and 1996 growing seasons, and a greenhouse experiment was conducted in 1996 to examine differences in some physiological characters among six spring durum wheat cultivars in response to different durations of early-season drought, and rewatering; and to determine the relationships of these characters to drought resistance. Six spring durum wheat cultivars were evaluated under four water regimes. Water regime treatments were: a well-irrigated treatment; and three water deficit treatments imposed during the period from emergence through either the onset of tillering, mid-tillering or the end of tillering. Cultivars differed widely in their response to early season water stress. Under drought stress conditions, grain yield, aboveground dry matter yield, water use efficiency for the grain (WUEg) and for the total dry matter (WUEdm) were strongly positively associated with net CO2 uptake:transpiration ratio (A/E), and osmoregulation capacity. It is concluded that drought-induced changes in A, A/E, stomatal resistance, and osmotic adjustment are possible key control points in determining the drought-resistance of a cultivar. Furthermore, there is a substantial degree of intraspecific variation for the above mentioned physiological attributes to explore as a selection tool. Selection for high osmoregulation capacity and high A/E ratio would seem to be a justifiable means of improving total dry matter and grain yield under conditions of early-season water stress. Key words: Wheat, Triticum durum Desf., physiological attributes, early-season drought

Growth and yield of wheat lines with differing osmoregulative capacity at high soil water deficit in seasons of varying evaporative demand

Wheat lines with differing capacities for osmoregulation were grown on a full profile of soil water at sowing in seasons of contrasting evaporative demand. Watered experiments were also included. Across seasons the relative increase in dry weight and yield associated with high osmoregulation was positively correlated with cumulative pan evaporation during the period of most rapid growth before anthesis. In experiments showing the most extreme responses to osmoregulative capacity there was no association with the overall soil water deficit at anthesis or harvest suggesting a direct effect of evaporative demand. This was further confirmed by correlations between vapour pressure deficit and leaf water potential and differences in turgor pressure. Significant differences in turgor were, in turn, associated with significant differences in dry weight. It was therefore concluded that osmoregulative effects on growth were mediated through leaf or shoot responses to leaf water stress rather than through root responses to soil water deficit, and that it was necessary to screen lines specifically for osmoregulation rather than for growth responses to soil water deficit.

Osmotic Components and Properties Associated With Genotypic Differences in Osmoregulation in Wheat

Solutes contributing to genotypic differences in osmoregulation in water-stressed wheat plants were investigated using samples of sap from expanded flag leaves of breeding lines with high and low osmoregulation. The sap samples were collected from leaves using a hand press after freezing and thawing. The leaves were sampled during a period of increasing stress created by withholding water from breeding lines growing in potting mix in the glasshouse. Measurements of potassium, free amino acids, proline, glucose, sucrose and fructose, as well as conductivity and refractive index, were made on the sap. The osmotic potential of the same sap samples was determined with a thermocouple psychrometer both before and after these measurements were made. Separate determinations of osmotic potential were also made on sections of frozen and thawed tissue from the same leaves used for sap samples, but without expressing the sap. The level of osmoregulation was expressed as the solute accumulation between full and zero turgor. The lines showed either very low (0.18 MPa) or very high (1.35 MPa) levels of solute accumulation. These levels were similar to a single gene effect observed in other lines. High osmoregulation was largely (78%) due to potassium accumulation, with amino acids the only other important contributor (22%). No sugar accumulation was found. The osmotic potential due to the solutes potassium, amino acids and sugars accounted for, on average, 90% of the sap osmotic potential measured with a thermocouple psychrometer. The major components were potassium (42-53%) and amino acids (35-43%). In high lines, increases in the proportion of potassium with increased stress were associated with changes in the relationship between sap osmotic potential and either tissue osmotic potential or that derived from the component solutes. The various methods of measuring osmotic potential and effects on the calculation of solute accumulation are discussed.

A Gene Controlling Differences in Osmoregulation in Wheat

Evidence is presented for a single gene controlling differences in osmoregulation in wheat in response to water stress, confirming earlier results. Analyses of osmoregulation were made on the flag leaves of wheat plants which were grown in pots in the glasshouse and stressed in a controlled environment chamber by withholding water after the flag leaf had fully emerged. Osmoregulation was derived from responses of osmotic potential to relative water content or from responses of relative water content and osmotic potential to water potential. Usable estimates of osmoregulation were obtained for 67 F2 lines derived from contrasting parents, to test for gene number, and for one substitution series with contrasting parents, to determine chromosomal location. The F2 frequency response, which consisted of two overlapping distributions, was compatible with a single recessive gene, the estimated ratio being 2.79 : 1 (low: high osmoregulation). This confirmed previous measurements made on F1s and F4s Results for the substitution series were also compatible with these results in indicating a single chromosome, 7A, which had an identical response to the low osmoregulation parent, Red Egyptian. The effects of the gene were confined to solute accumulations at water potentials above, but not below, zero turgor.