Low Osmotic Potential Research Articles

Bulk elastic moduli and solute potentials in leaves of freshwater, coastal and marine hydrophytes. Are marine plants more rigid?

Bulk modulus of elasticity (ɛ), depicting the flexibility of plant tissues, is recognized as an important component in maintaining internal water balance. Elevated ɛ and comparatively low osmotic potential (Ψπ) may work in concert to effectively maintain vital cellular water content. This concept, termed the 'cell water conservation hypothesis', may foster tolerance for lower soil-water potentials in plants while minimizing cell dehydration and shrinkage. Therefore, the accumulation of solutes in marine plants, causing decreases in Ψπ, play an important role in plant-water relations and likely works with higher ɛ to achieve favourable cell volumes. While it is generally held that plants residing in marine systems have higher leaf tissue ɛ, to our knowledge no study has specifically addressed this notion in aquatic and wetland plants residing in marine and freshwater systems. Therefore, we compared ɛ and Ψπ in leaf tissues of 38 freshwater, coastal and marine plant species using data collected in our laboratory, with additional values from the literature. Overall, 8 of the 10 highest ɛ values were observed in marine plants, and 20 of the lowest 25 ɛ values were recorded in freshwater plants. As expected, marine plants often had lower Ψπ, wherein the majority of marine plants were below -1.0 MPa and the majority of freshwater plants were above -1.0 MPa. While there were no differences among habitat type and symplastic water content (θsym), we did observe higher θsym in shrubs when compared with graminoids, and believe that the comparatively low θsym observed in aquatic grasses may be attributed to their tendency to develop aerenchyma that hold apoplastic water. These results, with few exceptions, support the premise that leaf tissues of plants acclimated to marine environments tend to have higher ɛ and lower Ψπ, and agree with the general tenets of the cell water conservation hypothesis.

Linking transpiration reduction to rhizosphere salinity using a 3D coupled soil-plant model

Aims Soil salinitycan cause saltplant stressbyreducing plant transpiration and yield due to very low osmotic potentials in the soil. For predicting this reduction, we present a simulation study to (i) identify a suitable functional form of the transpiration reduction function and (ii) to explain the different shapes of empirically observed reduction functions. Methods We used high resolution simulations with a model that couples 3D water flow and salt transport in the soil towards individual roots with flow in the root system. Results The simulations demonstrated that the local total water potential at the soil-root interface, i.e. the sum of the matric and osmotic potentials, is for a given root system, uniquely and piecewise linearly related to the transpiration rate. Using bulk total water potentials, i.e. spatially and temporally averaged potentials in the soil around roots, sigmoid relations were obtained. Unlike for the local potentials, the sigmoid relations were non-unique functions of the total bulk potential but depended on the contribution of the bulk osmotic

Physiological Responses of Calystegia soldanella under Drought Stress

This study was conducted to determine the extent of drought resistance based on physiological responses of Calystegia soldanella under water deficit. In order to investigate the changes of plant growth, stomatal density, photosynthesis, chlorophyll fluorescence, the contents of chlorophyll and carotenoid, osmolality, total ion contents, the contents of carbohydrate and proline, C. soldanella was grown under well watered and drought stressed conditions for 12 days. In this study, water-deficit resulted in remarkable growth inhibition of C. soldanella. The effect of water-deficit on plant growth was associated with low osmotic potential of soil. On day 12 after drought treatment, dry weight, relative water contents, number and area of leaves and stem length were lower than those of control. The stomatal conductance and net photosynthetic rate were significantly reduced in water stressed plant to regulate inner water contents and <TEX>$CO_2$</TEX> exchange through the stomatal pore. Chlorophyll fluorescence and chlorophyll contents were not different in comparison with the control, indicating that the efficiency of photosystem II was not affected by drought stress. This results could be explained that water-deficit in C. soldanella limits the photosynthetic rate and reduces the plant's ability to convert energy to biomass. A significant increase in total ion contents and osmolality was observed on day 7 and day 12. Accumulation of proline in leaves is associated with the osmotic adjustment in C. soldanella to soil water-deficit. Consequently, this increase in osmolality in water stressed plant can be a result in the increase of ion contents and proline.

Fixation Drought Tolerance of the Slow‐Wilting Soybean PI 471938

ABSTRACTTo overcome the severe limitation of crop yield that may result from drought, specific plant traits are being explored. Soybean [Glycine max (L.) Merr.] genotypes that express a “slow‐wilting” phenotype have been evaluated as sources of useful drought traits. However, the unique physiological characteristic of PI 471938 resulting in slow‐wilting progeny and in high‐yielding progeny has not been resolved. This study was undertaken to explore the possibility that it may have especially tolerant N fixation activity as soil dries. In fact, greenhouse experiments confirmed this possibility with its N fixation tolerance being among the greatest reported for soybean genotypes. Like other genotypes with N fixation drought tolerance, the accumulation of ureides in its leaves is low compared to a fast‐wilting cultivar. Measurements of water status of PI 471938 as compared to the fast‐wilting cultivar Benning showed that PI 471938 is able to maintain a greater leaf turgor pressure during soil drying as a result of a lower osmotic potential.

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Water relation parameters of six Peltigera species correlate with their habitat preferences

Water relation parameters were measured in six congeneric lichen species with different requirements for water availability and with green algae (Peltigera aphthosa, Peltigera leucophlebia, Peltigera venosa) or cyanobacteria (Peltigera horizontalis, Peltigera praetextata, Peltigera rufescens) as main photobionts. Pressure–volume analysis was performed with a dewpoint hygrometer and integrated with anatomical analyses. The Peltigera species typical of arid environments were characterized by relatively lower osmotic potential (π0) and turgor loss point (ΨTLP), and higher values of bulk modulus of elasticity (ɛ). Both π0 and ΨTLP were correlated with the size of medullary cells, while ɛ was negatively correlated with cell dimensions. The adaptive value of low ΨTLP might reside in the capability to maintain cell turgor for longer time intervals under dry conditions. High ɛ might allow xerophilous lichens to regain cell turgor more promptly even for small amounts of water uptake, thus enlarging the cumulative period of positive carbon balance in environments with fluctuating water availability. The influence of the photobiont type is also discussed.

Differential freezing resistance and photoprotection in C3 and C4 eudicots and grasses

Globally, C4 plants dominate hot, open environments, but this general pattern is underpinned by important differences in the biogeography of C4 lineages. In particular, the species richness of C4 Poaceae (grasses) increases strongly with increasing temperature, whereas that of the major C4 eudicot group Chenopodiaceae correlates positively with aridity. Freezing tolerance is a crucial determinant of biogeographical relationships with temperature and is mediated by photodamage and cellular disruption by desiccation, but little is known about differences between C4 families. This study hypothesized that there is a greater risk of freezing damage via these mechanisms in C4 Poaceae than Chenopodiaceae, that freezing protection differs between the taxonomic groups, and that freezing tolerance of species is linked to arid habitat preference. Chlorophyll fluorescence, water relations, and freezing injury were compared in four C3 and six C4 species of Poaceae and Chenopodiaceae from the same Mongolian flora. Contrary to expectations, freezing-induced leaf mortality and photodamage were lower in Poaceae than Chenopodiaceae species, and unrelated to photosynthetic pathway. The freezing resistance of Poaceae species resulted from constitutive protection and cold acclimation and an ability to protect the photosynthetic apparatus from photodamage. Freezing protection was associated with low osmotic potential and low tissue elasticity, and freezing damage was accompanied by electrolyte leakage, consistent with cell-membrane disruption by ice. Both Chenopodiaceae and Poaceae had the potential to develop cold acclimation and withstand freezing during the growing season, which conflicted with the hypothesis. Instead, freezing tolerance was more closely associated with life history and ecological preference in these Mongolian species.

Impact of total water potential and varying contribution of matric and osmotic potential on carbon mineralization in saline soils

In saline soils, microbial activity may be reduced by low matric (low water content) and low osmotic potential (presence of salts) but little is known about the impact of the relative contribution of matric and osmotic potential to water potential (sum of matric and osmotic potential) on microbial activity and biomass. A laboratory incubation experiment was conducted using a non-saline sandy loam; different osmotic potentials (−0.30 to −3.24 MPa) were achieved by adding different amounts of NaCl. After pre-incubation for 14 days, subsamples of these treatments were dried to achieve different contributions of matric potential (8–73%) and osmotic potential (27–92%) to water potential which ranged between −0.57 and −4.57 MPa. All treatments were amended with 20 g kg−1 pea residues to increase nutrient supply; carbon dioxide (CO2) emission was measured over 14 days. Microbial biomass C and K2SO4-extractable C were measured at the end of the experiment. Cumulative CO2–C (mg g−1 soil) was significantly (p < 0.05) lower at water potential −4 MPa than at water potential −1.5 MPa. Above water potential −4 MPa, cumulative CO2–C significantly decreased with increasing percentage contribution of osmotic potential to water potential, particularly if the contribution of osmotic potential was >50%. In contrast, K2SO4-extractable C and microbial biomass C were little affected by water potential above −4 MPa. Only at water potential −4 MPa, cumulative CO2–C and microbial biomass C were affected by matric potential and its contribution to water potential; that is when the soils are very dry. Our results show that cumulative CO2–C was more sensitive to decreasing water potential or the contributions of osmotic and matric potential than microbial biomass C. This suggests that not only water potential but also the contribution of osmotic and matric potential should be taken into account to understand microbial activity and growth in saline soils.

Tolerance of roadside and old field populations of common teasel (Dipsacus fullonum subsp. sylvestris) to salt and low osmotic potentials during germination

Plants inhabiting degraded habitats must contend with stressful environments. However, their ability to adapt may be constrained by available genetic variation and genetic correlations between traits. Here, we examine the correlation between salt and drought tolerance in germinating seeds from contrasting populations of common teasel (Dipsacus fullonum subsp. sylvestris) growing on roadsides that experience high salinity due to de-icing salts, or growing in an old field site, remote from roadsides and free of salinity stress. We examined the contribution of drought and salinity tolerance to the tolerance of roadside conditions in seedlings from five maternal families from three roadside and three old field populations. Germination and early growth were compared under high salinity, low water potential set at 20.5 MPa with solutions of polyethylene glycol 8000, sodium chloride or vermiculite wetted to 20.5 MPa with distilled water. Root length and the emergence of cotyledons (where appropriate) were used as a measure of performance. Maternal families from roadside populations displayed greater tolerance of both high salinity and drought than families from old field populations. However, no maternal family possessed tolerance to both drought and salinity. Salt and drought tolerance during germination were not correlated, indicating that they are separate traits in this species.

Ridge crest versus swale: contrasting plant–water relations and performance indexes in two understory plant species in a coastal maritime forest

Coastal-ridge plains are progradational landforms composed of elevated ridges and low-lying swales. The transitions between ridges and swales are steep, promoting dynamic shifts between xeric and mesic systems. Two understory plants that co-occur in ridge plains of North American mid-Atlantic maritime forests are Sabal minor and Ilex vomitoria. As coastal-ridge plains foster varying amounts of surface and sub-surface water driven largely by topography, the purpose of this study was to evaluate plant–water relations and chlorophyll a fluorescence in these two species. Ridge plants had lower leaf- and xylem-water potentials, lower osmotic potential (I. vomitoria), and lower symplastic water content (S. minor). Although there were no differences in potential- and effective-quantum yields, there were decreases in fluorescence performance index for ridge I. vomitoria and swale S. minor. While the data support potential water-stress conditions in ridge plants of both species, the data also suggest that I. vomitoria and S. minor use different physiological processes to tolerate hydrologically dynamic ridge–swale maritime forests.

Research on modeling germination response to salinity of barley seeds

Aims Salinization is a worldwide problem, and some researchers focus on the effect of salt on seed germination. The results are variable, and quantifying germination response to salt is significant. Furthermore, seed germina- tion is affected by salinity, as a result of both osmotic and ion effects. How the two effects act together is unclear. Thus our objectives are to propose a model to accurately describe the effect of salt on seed germination and to clarify the role of the osmotic and the ion effect act under different salinities. Methods Two varieties of barley (Hordeum vulgare) seeds ('Cask' and 'County') were cultured in five binate iso-osmotic polyethylene glycol (PEG) and NaCl solutions (-0.45, -0.88, -1.32, -1.76 and -2.20 MPa, distilled water as the control) at four constant temperatures of 5, 12, 20 and 27 °C. Germination time courses were re- corded and germination rates (the reciprocal of germination time) were calculated. The hydrotime model and the new salinity model were used to calculate the parameters and test which was the better fit. Important findings Results indicated that not only were seeds in saline conditions able to germinate at lower osmotic potentials than seeds germinating in an isotonic PEG-6000 solution, but that they were also able to do so faster. The hydrotime parameters of the NaCl treatments had great differences with the isotonic PEG treatments, which indicated the hydrotime model cannot describe salt effects on seed germination well. Barley seed germina- tion rates in salt solutions were negatively linear with salinity. We proposed a salinity model to quantify germina- tion response to salt. The germination time calculated from the salinity model approached the real data, compared to that calculated from the hydrotime model. Differences of germination rates in NaCl and the isotonic PEG treatments increased and then decreased with decreasing water potential. We suggest three situations of function mode by the osmotic and ion effects of salt. First, at low salinities the osmotic effect acts as the main negative role. Second, at medium salinities the two effects act together, with the positive ion effect stronger than the nega- tive osmotic effect. Third, at high salinities the ion effect begins to harm the germination process.

Differential response of potatoes (Solanum tuberosum L.) to salinity in an arid environment and field performance of the seed tubers grown with fresh water in the following season

Ten potato varieties, commonly grown in the Mediterranean region, were grown on sandy soil in an arid desert area of Israel. The potatoes were drip-irrigated with three water sources: fresh water (control – ENV1), saline water from a local well (intermediate salinity – ENV2) and the local well water enriched with NaCl (high salinity – ENV3). The average salinity levels expressed as the electrical conductivity (EC) measured in the soil extracts from the root zone were 2.35, 5.1 and 8.5dSm−1 for the ENV1, ENV2 and ENV3 treatments, respectively. Salinity stress decreased fresh tuber yields, increased tuber dry matter and soluble solids, increased tuber electrolyte content and lowered osmotic potential (OP) of the tuber sap in all varieties. ANOVA and Fisher's LSD pair-comparisons revealed highly significant differences among the varieties in all traits. Correlation analyses showed negative associations between tuber yield and tuber traits (dry matter, DM; total soluble solids, TSS, EC and OP). Positive associations were found between tuber DM and TSS, EC and OP. There was no positive association between early maturation or yielding capacity and tolerance to salinity. Vivaldi, a high-yielding variety and Almera, a low-yielding variety were found significantly less susceptible to salinity than Mondial, a late-maturing and high-yielding variety, and Charlotte, an early-maturing and low-yielding variety. The significant differences in the response to salinity stress indicated genotypic variation in the response to salinity and support the potential of breeding for tolerance to salinity in potatoes.When seed tubers of the three irrigation treatments were grown with fresh water irrigation during the following season, there were no significant differences in plant growth or tuber yields from the three seed sources in most varieties indicating the possibility of using saline water for the production of potato seed tubers.

Carbon mineralization in saline soils as affected by residue composition and water potential

In saline soils under semi-arid climate, low matric and osmotic potential are the main stressors for microbes. But little is known about the impact of water potential (sum of matric and osmotic potential) and substrate composition on microbial activity and biomass in field collected saline soils. Three sandy loam soils with electrical conductivity of the saturated soil extract (ECe) 3.8, 11 and 21 dS m−1 (hereafter referred to EC3.8, EC11 and EC21) were kept at optimal water content for 14 days. After this pre-incubation, the soils were either left at optimal water content or dried to achieve water potentials of −2.33, −2.82, −3.04 and −4.04 MPa. Then, the soils were amended with 20 g kg−1 pea or wheat residue to increase nutrient supply. Carbon dioxide emission was measured over 14 days; microbial biomass C was measured at the end of the experiment. Cumulative respiration decreased with decreasing water potential and was significantly (P < 0.05) lower in soils at water potential −4 MPa than in soils at optimal water content. The effect of residue type on the response of cumulative respiration was inconsistent; with residue type having no effect in the saline soils (EC11 and EC21) whereas in the non-saline soil (EC3.8), the decrease in respiration with decreasing water potential was less with wheat than with pea residue. At a given water potential, the absolute and relative (in percentage of optimal water content) cumulative respiration was lower in the saline soils than in the non-saline soil. This can be explained by the lower osmotic potential and the smaller microbial biomass in the saline soils. However, even at a similar osmotic potential, cumulative respiration was higher in the non-saline soil. It can be concluded that high salt concentrations in the soil solution strongly reduce microbial activity even if the water content is relatively high. The stronger relative decrease in microbial activity in the saline soils at a given osmotic potential compared to the non-saline soil suggests that the small biomass in saline soils is less able to tolerate low osmotic potential. Hence, drying of soil will have a stronger negative effect on microbial activity in saline than in non-saline soils.

Comparing genotypic variation in faba bean (Vicia faba L.) in response to salinity in hydroponic and field experiments

Faba bean (Vicia faba L.) is an important pulse crop in the Mediterranean region, west Asia, China and Australia where it is adapted to neutral-alkaline soils. In many cases faba bean is grown as a rainfed crop on saline–sodic soils where growth and yield are limited by salinity. Developing more salt tolerant varieties of faba bean would help improve productivity on these soils. This study assessed the genotypic variation of faba bean plants to salinity stress under controlled conditions and in the field and the suitability of various physiological traits to screen eleven faba bean genotypes for salt tolerance. Faba bean was grown in hydroponics at 0mM, 75mM or 150mM NaCl for 49 days and the shoot biomass, sodium (Na+) and chloride (Cl−) concentrations and osmotic potential of the last emerged leaves measured. These results were compared with similar measurements from field trials over two seasons. Significant genotypic variation for salinity tolerance in faba bean was measured among the varieties screened in hydroponics and in the field. Whole-plant Na+ and Cl− concentrations at 75mM NaCl were significantly correlated with biomass production under controlled conditions (r=−0.97 and −0.95) and ranked the genotypes with their grain yield production in the field. However, the importance of Na+ and Cl− exclusion as mechanisms of salt tolerance was diminished at 150mM NaCl and salt tolerance was more strongly related to the osmotic potential of the leaves: lower salt tolerance was associated with lower osmotic potential. The reduction in salt tolerance as the external salt stress increased varied among varieties and this was associated with the ability to maintain a high tissue water content. The present study also suggests that salt exclusion coupled with a synthesis of organic solutes is important components of salt tolerance in the tolerant genotypes. While Na+ and Cl− exclusion is an important primary mechanism of salt tolerance, the development of more salt-tolerant germplasm is likely to be accelerated if screening based on ion exclusion also takes into account genotypic differences in osmotic tolerance.

Plant-derived smoke: Old technology with possibilities for economic applications in agriculture and horticulture

Fire and smoke have been used in traditional agricultural systems for centuries. In recent years, biologically active compounds have been isolated from smoke with potential uses in agriculture and horticulture. This article highlights the possibilities of using smoke-water or smoke-derived butenolide (3-methyl- 2H-furo[2,3- c]pyran-2-one, termed karrikinolide, KAR 1) for the cultivation of agricultural and horticultural crops. Treatments with smoke-water show promising results for improving seed germination, seedling growth and crop productivity. In certain cases, even under adverse conditions, such as low or high temperatures and low osmotic potentials, smoke-water or a KAR 1 solution can promote seed germination and seedling growth. This phenomenon is of great significance when seeds are sown under drought conditions. Smoke-technology, therefore, has potential for use in arid and semi-arid regions. Possibilities may also exist for controlling some plant diseases and managing weeds with the use of smoke or KAR 1 solutions. In addition, smoke-technology can possibly economize the use of commercial chemical fertilizers, pesticides and herbicides, making it a feasible technology for organic farming and for resource-poor farmers in developing nations. The positive role of smoke-water in flowering and fruiting of crops cannot be overlooked as the karrikins found in smoke are now recognized as potential new plant growth regulators. Very low concentrations of smoke-water or a KAR 1 solution are effective in promoting germination and post-germination growth. Thus, early harvesting and increasing the productivity of crops using smoke-technology may be possible. Here we review some of the effects of smoke and KAR 1 on various crop species and discuss the potential uses of smoke technology in agriculture and horticulture.

Contrasting physiological responses of two co-occurring eucalypts to seasonal drought at restored bauxite mine sites

This study describes the physiological response of two co-occurring tree species (Eucalyptus marginata and Corymbia calophylla) to seasonal drought at low- and high-quality restored bauxite mine sites in south-western Australia. Seasonal changes in photosynthesis (A), stomatal conductance (g(s)), leaf water potential (ψ), leaf osmotic potential (ψ), leaf relative water content (RWC) and pressure-volume analysis were captured over an 18-month field study to (i) determine the nature and severity of physiological stress in relation to site quality and (ii) identify any physiological differences between the two species. Root system restriction at the low-quality site reduced maximum rates of gas exchange (g(s) and A) and increased water stress (midday ψ and daily RWC) in both species during drought. Both species showed high stomatal sensitivity during drought; however, E. marginata demonstrated a higher dehydration tolerance where ψ and RWC fell to -3.2 MPa and 73% compared with -2.4 MPa and 80% for C. calophylla. Corymbia calophylla showed lower g(s) and higher ψ and RWC during drought, indicating higher drought tolerance. Pressure-volume curves showed that cell-wall elasticity of E. marginata leaves increased in response to drought, while C. calophylla leaves showed lower osmotic potential at zero turgor in summer than in winter, indicating osmotic adjustment. Both species are clearly able to tolerate seasonal drought at hostile sites; however, by C. calophylla closing stomata earlier in the drought cycle, maintaining a higher water status during drought and having the additional mechanism of osmotic adjustment, it may have a greater capacity to survive extended periods of drought.

Water deficit and abscisic acid production of Salicornia bigelovii under salinity stress

We investigated the mechanism of growth reduction of dicotyledonous halophyte Salicornia bigelovii under salinity stress by growing it at 0.005 to 500 mol m−3 sodium chloride (NaCl). The optimal range for growth of S. bigelovii was between 50 and 200 mol m−3 NaCl. A significant correlation was found between growth and water content, which indicated that water deficit was an important factor in growth reduction at both suboptimal and supraoptimal salinities. Abscisic acid (ABA) concentration of the shoot was negatively related to growth and water content, which suggested that ABA induced by water deficit may inhibit growth at both the suboptimal and supraoptimal salinities. The cause of water deficit at supraoptimal salinity might be caused by nutritional imbalance and osmotic stress due to the low osmotic potential of the external solution. However, limited salt uptake may be one of the causes of water deficit under suboptimal salinity. We discuss a sodium ion (Na+) specific deficit rather than salt deficit as another possible cause of water deficit.

Water potential affects Coniothyrium minitans growth, germination and parasitism of Sclerotinia sclerotiorum sclerotia

Water availability is an important environmental factor which has major effects on fungal activity. The effects of osmotic (KCl amended agar) and matric Polyethylene glycol ((PEG) 8000 amended agar) potentials over the range −0.1 to −5.0 MPa on mycelial growth and conidial germination of eight isolates of the sclerotial parasite Coniothyrium minitans was assessed. The influence of soil water potential on the ability of three selected isolates (LU112, LU545, and T5R42i) to parasitise sclerotia of the plant pathogen Sclerotinia sclerotiorum was determined. For all eight C. minitans isolates, decreasing osmotic and matric potentials caused a reduction in mycelial growth and conidial germination. Isolates were more sensitive to decreasing matric potential than osmotic potential. Across the isolates, growth at an osmotic potential of −5.0 MPa was 30–70 % of the growth seen in the control, whereas less than 20 % of the control growth was seen at the corresponding matric potential. Across all isolates no conidial germination was seen at matric potential of −5.0 MPa. The C. minitans isolates varied in their sensitivity to decreasing water potentials. Mycelial growth and conidial germination of three isolates (LU112, Conio, and CH1) were more tolerant of low osmotic potential and matric potential with respect to mycelial growth. Isolates T5R42i and LU430 were least tolerant. In contrast, conidial germination of isolates Conio, LU545, and T5R42i were less sensitive to decreasing matric potential. Soil water potential was seen to affect infection and viability of sclerotia by the three C. minitans isolates. Isolate LU545 reduced sclerotial viability over a wider water potential range (−0.01 to −1.5 MPa) compared with LU112 (−0.01 to −1.0 MPa), with isolate T5R42i being intermediate. Indigenous soil fungi ( Trichoderma spp. and Clonostachys rosea) were recovered from sclerotia but did not result in reduction in sclerotial viability. The relevance of these results in relation to biocontrol activity of C. minitans in soil is discussed.

Salinity effects on carbon mineralization in soils of varying texture

In salt-affected soils, soil organic carbon (SOC) levels are usually low as a result of poor plant growth; additionally, decomposition of soil organic matter (SOM) may be negatively affected. Soil organic carbon models, such as the Rothamsted Carbon Model (RothC), that are used to estimate carbon dioxide (CO 2) emission and SOC stocks at various spatial scales, do not consider the effect of salinity on CO 2 emissions and may therefore over-estimate CO 2 release from saline soils. Two laboratory incubation experiments were conducted to assess the effect of soil texture on the response of CO 2 release to salinity, and to calculate a rate modifier for salinity to be introduced into the RothC model. The soils used were a sandy loam (18.7% clay) and a sandy clay loam (22.5% clay) in one experiment and a loamy sand (6.3% clay) and a clay (42% clay) in another experiment. The water content was adjusted to 75%, 55%, 50% and 45% water holding capacity (WHC) for the loamy sand, sandy loam, sandy clay loam and the clay, respectively to ensure optimal soil moisture for decomposition. Sodium chloride (NaCl) was used to develop a range of salinities: electrical conductivity of the 1:5 soil: water extract (EC 1:5) 1, 2, 3, 4 and 5 dS m −1. The soils were amended with 2% (w/w) wheat residues and CO 2 emission was measured over 4 months. Carbon dioxide release was also measured from five salt-affected soils from the field for model evaluation. In all soils, cumulative CO 2–C g −1 soil significantly decreased with increasing EC 1:5 developed by addition of NaCl, but the relative decrease differed among the soils. In the salt-amended soils, the reduction in normalised cumulative respiration (in percentage for the control) at EC 1:5 > 1.0 dS m −1 was most pronounced in the loamy sand. This is due to the differential water content of the soils, at the same EC 1:5; the salt concentration in the soil solution is higher in the coarser textured soils than in fine textured soils because in the former soils, the water content for optimal decomposition is lower. When salinity was expressed as osmotic potential, the decrease in normalised cumulative respiration with increasing salinity was less than with EC 1:5. The osmotic potential of the soil solution is a more appropriate parameter for estimating the salinity effect on microbial activity than the electrical conductivity (EC) because osmotic potential, unlike EC, takes account into salt concentration in the soil solution as a function of the water content. The decrease in particulate organic carbon (POC) was smaller in soils with low osmotic potential whereas total organic carbon, humus-C and charcoal-C did not change over time, and were not significantly affected by salinity. The modelling of cumulative respiration data using a two compartment model showed that the decomposition of labile carbon (C) pool is more sensitive to salinity than that of the slow C pool. The evaluation of RothC, modified to include the decomposition rate modifier for salinity developed from the salt-amended soils, against saline soils from the field, suggested that salinity had a greater effect on cumulative respiration in the salt-amended soils. The results of this study show (i) salinity needs to be taken into account when modelling CO 2 release and SOC turnover in salt-affected soils, and (ii) a decomposition rate modifier developed from salt-amended soils may overestimate the effect of salinity on CO 2 release.

APPLICATIONS OF XEROPHYTOPHYSIOLOGY IN PLANT PRODUCTION - TOMATO FRUIT YIELD AND QUALITY IMPROVED BY RESTRICTED IRRIGATIONS IN SOIL-BASED GREENHOUSES

Plant can perceive environment changes and adjust itself internally to get ready to resist the coming adverse conditions. In the present research, drip infiltration, a kind of deficit irrigation, was imposed to tomato plants. This deficit irrigation was perceived by the tomato plants as a false stimulus of drought and made the plants stronger by internal physiological adaptation, such as osmotic adjustment and symplastic water compartmenting. Tomato plants under infiltration irrigation showed a higher osmotic concentration or lower osmotic potential in symplasm at fully turgid status and a consequent higher leaf turgor potential. Leaf symplastic water fraction was larger or apoplastic water fraction was smaller in tomato leaves under infiltration irrigation. Both osmotic potential and relative water content at the point of incipient plasmolysis were lower in tomato leaves under infiltration irrigation, suggesting a higher resistance to water stress. The higher leaf turgor potential and the larger symplastic water fraction might have supported the higher leaf photosynthesis in the treated plants. Soil microbial activities were also improved by infiltration irrigation. In conclusion, horizontal infiltration irrigation can improve plant physiology and consequently increase tomato fruit yield. The higher physiological activities might be resulted from xerophytophysiological responses under unwet or modest dry conditions. INTRODUCTION Irrigation has developed from basin and furrow, through sprinkler, to drip and trickle irrigation (Masse, 1981). Drip or trickle irrigation has come into favor because it conserves water and plastic tubing makes the installation convenient (Adamsen, 1992). Efficient irrigation means applying enough water to prevent plant water deficits without surplus that drains below the root zone, in addition to improvements in crop yield and product quality. Sometimes irrigation must be controlled at a minimum sufficient level even if the water resource is plenty. For example, greenhouse production requires a dry air condition to prevent infection from fungus diseases and a modest drought in soil to obtain a good fruit quality (Xu at al., 2007). This kind of controlled irrigation is also called regulated deficit irrigation (RDI). As reported by Goodwin and Jerie (1992), RDI for fruit trees has increased water use efficiency by approximately 60% with no loss in yield. It is also useful for reducing excessive vegetative vigor and for minimizing nutrient loss through leaching. Chalmers et al. (1986) have reported higher yield and better quality a huilian@janis.or.jp b jialing_300@163.com Proc. IS on High Technology for Greenhouse Systems GreenSys2009