Increased Solute Leakage Research Articles

Primed Seeds of NERICA 4 Stored for Long Periods under High Temperature and Humidity Conditions Maintain Germination Rates

Agriculture depends on the ability of seeds to survive until the next planting season under ambient conditions that may be averse to seed quality even when the seed is in a quiescent state. Seed priming invigorates seeds, but the impact on the longevity of seeds has limited its adoption. This study investigated the effect of the storage of primed rice seed on seed viability, vigor, and longevity. Three seed priming methods were employed on the rice cultivar New Rice for Africa (NERICA 4) seeds. Subsequently, the seeds were stored for 120 days at 25 °C and 65% relative humidity, simulating the ambient seed storage conditions of the tropics and sub-tropics. The primed seed recorded increased vigor compared to the non-primed seed until 90 days of storage. However, seed storage for 120 days reduced seed vigor and viability for all the seeds. The results indicated a significant reduction in seed vigor, increased solute leakage, generation of hydrogen peroxide, and accumulation of malondialdehyde after storage. Priming enhances cell membrane integrity and maintains seed vigor in storage at near ambient conditions long enough before reversal of its performance by the storage conditions. This assures that primed seed can either be stored until the following planting season or remain viable in the soil during delayed germination.

Physiological roles of Casparian strips and suberin in the transport of water and solutes.

Summary The formation of Casparian strips (CS) and the deposition of suberin at the endodermis of plant roots are thought to limit the apoplastic transport of water and ions. We investigated the specific role of each of these apoplastic barriers in the control of hydro‐mineral transport by roots and the consequences on shoot growth.A collection of Arabidopsis thaliana mutants defective in suberin deposition and/or CS development was characterized under standard conditions using a hydroponic system and the Phenopsis platform.Mutants altered in suberin deposition had enhanced root hydraulic conductivity, indicating a restrictive role for this compound in water transport. In contrast, defective CS directly increased solute leakage and indirectly reduced root hydraulic conductivity. Defective CS also led to a reduction in rosette growth, which was partly dependent on the hydro‐mineral status of the plant. Ectopic suberin was shown to partially compensate for defective CS phenotypes.Altogether, our work shows that the functionality of the root apoplastic diffusion barriers greatly influences the plant physiology, and that their integrity is tightly surveyed.

Responses to chilling of recalcitrant seeds of Ekebergia capensis from different provenances

Recalcitrant seeds are shed at relatively high water contents and are metabolically active, initiating germination without exogenous provision of water. For whole seeds, specialised storage under hydrated conditions at the lowest possible temperatures not having adverse consequences, is required. This study aimed to establish the effect of chilling on the recalcitrant seeds of a single species, Ekebergia capensis, from three different populations occurring in different climates. These were collected from temperate Port Elizabeth (PE; Eastern Cape), sub-tropical St Lucia (KwaZulu-Natal [KZN]) and Tanzania (tropical southern Africa). Viability, axis ultrastructure, solute leakage and protein synthesis were assessed as indicators of response to chilling. These parameters were measured at intervals during storage under hydrated conditions. The seeds from PE retained 80% viability after 12 weeks in storage at 1° and 3 °C and 100% when stored at 6 °C. Those from St Lucia were dead after 38 d in storage at 3 °C, but had retained 100% viability at 6° and 16 °C for the same period. In contrast, all the seeds from Tanzania were dead after 9 d when stored at 3 °C and retained only 10 and 35% viability when stored for 9 d at 6° and 16 °C, respectively. A reduced rate of protein synthesis and increased solute leakage in axes during storage implied a breakdown in metabolic activity. Upon establishing that the PE seeds tolerated slow drying, it is suggested these seeds might be considered to be minimally recalcitrant. Studies of variation in nuclear ribosomal ITS1 sequences revealed the presence of three well-to-strongly-supported monophyletic clades corresponding to the geographical areas from which the seeds were sampled (PE, KZN and Tanzania). Observations indicated that axes from the seeds originating in St Lucia and Tanzania lost ultrastructural integrity when stored at 3 °C. Axes from the PE seeds did not show any deterioration in ultrastructure – for relatively prolonged storage periods. It is clear from the present study, that E. capensis seeds of temperate (PE) origin do tolerate chilling temperatures well, whereas those from sub-tropical St Lucia or the tropical Tanzanian provenance, do not. It is probable that what has been tacitly assumed to be a single species, may not be so.

Gene expression of late embryogenesis abundant proteins, small heat shock proteins and peroxiredoxin and oxidation of lipid and protein during loss and re-establishment of desiccation tolerance in Pisum sativum seeds

The aim of the present work was to investigate the mechanism of loss and re-establishment of desiccation tolerance (DT) in germinated Pisum sativum seeds (seedlings). Dehydration of pea seedlings of 2 mm radicle length was accompanied by concomitant loss of DT from 100 to 60 and 20% when dehydrated to initial water content (IWC) and below IWC, respectively. Desiccation of seedlings was associated with accumulation of malondialdehyde (MDA) and carbonyls (oxidized product of lipid and protein, respectively) with increased solute leakage and decreased seedling viability. Also, the transcripts of all the protective proteins like late embryogenesis abundant proteins (LEA; PsDHN2, PsDHN3 and PsSBP65), small heat shock proteins (sHSP; PsHSP17.7, PsHSP18.1, PsHSP18.2 and PsHSP22.7) and peroxiredoxin (Prx; 2-Cys Prx) increased in seedlings desiccated to IWC but massive loss (5- to 9-folds) was discernible when desiccated below IWC. The PEG pre-treatment re-established the DT from 20 to 70% in seedlings dehydrated below IWC. The re-induction of DT in PEG-treated seedlings was accompanied with enhanced seedling viability and gene expression (5- to 10-folds) of all the protective proteins (LEA, sHSP and 2-Cys Prx) with substantial reduction in the levels of MDA, carbonyls and electrolyte leakage. The role of genes encoding the protective proteins, MDA, carbonyls, TTC reduction and solute leakage was discussed during loss and re-establishment of DT in pea seedlings.

Recalcitrant and orthodoxSpartinaseeds or isolated embryos exhibit similar leachate patterns immediately following desiccation

AbstractSolute leakage is used as an indicator of membrane damage during desiccation of recalcitrant seeds. We re-examined this phenomenon and its utility by comparing recalcitrantSpartina alternifloraand orthodoxSpartina pectinataseeds, as well as drying isolated embryos or embryos isolated after whole-seed drying. During drying, intact seed leakage (electrical conductivity and absorbance at 280 nm) was independent of seed moisture content, dormant/non-dormant state or desiccation tolerance. Embryos from bothSpartinaspecies, isolated after drying within the intact seed, exhibited increased leakage, especially below 40% (dry weight basis, DWB) seed moisture, the critical water content for viability loss inSpartina alterniflora. When isolated embryos were dried, the pattern and extent of increased solute leakage were similar in bothSpartinaspecies, even thoughS. pectinataembryos were >95% viable andS. alternifloraembryos were < 20% viable. We conclude that increased solute leakage is an artefact of embryo excision and not an accurate indicator of desiccation damage to recalcitrantSpartina alternifloraseeds.

Layered reactive barrier films

The lag time of a barrier film can be dramatically increased by the addition of scavengers which consume solute before it crosses the barrier. This increase in lag time theoretically depends on the location of the scavenger within the membrane. In this paper, we construct layered reactive barrier membranes to test this theory. The experiments show good agreement with the theory, which predicts that the lag time increase can be doubled over that of a homogenous reactive barrier membrane by confining the scavenger to the downstream interface. Computer modeling also supports this claim, but suggests that this increase may come at the expense of increased solute leakage prior to the lag time. Reasons for this leakage are discussed, with recommendations for balancing lag time and leakage increases.

LIPID PEROXIDATION AND PLANT TISSUE DISORDERS

Increased solute leakage resulting from ozone injury, senescence, wounding and virus or bacteria-induced hypersensitive reaction [HR] have been related to membrane lipid peroxidation [LPX]. The genesis of LPX in various situations may derive from several different processes: a) the direct addition of O2 to unsaturated fatty acids forming hydroperoxide in an enzymatic reaction catalyzed by lipoxygenase, an enzyme widely distributed in plant seed, root, and leaf tissue or b) peroxidation by active oxygen species [AO], e.g. O2 and H2O2, that are generated during normal cellular metabolic activities. Increased production of AO could occur as the result of a metabolic change or a reduction in the amount or activities of antioxidants, i.e. ascorbic acid, vitamin E, and glutathione, and the detoxifying enzymes catalase and superoxide dismutase. The HR against plant pathogenic bacteria and viruses will be used as case histories to discuss the genesis of lipid peroxidation.

Techniques for Detecting Grass Tolerance to Sethoxydim and Fenoxaprop-Ethyl Herbicides

Measurements of net photosynthesis, chlorophyllafluorescence and solute leakage were evaluated as techniques for rapid detection of grass tolerance to fenoxaprop-ethyl and sethoxydim. Net photosynthesis measured as net CO2uptake of the youngest fully expanded leaf ofCalamagrostis arundinacea‘Karl Foerster’ (Feather Reed Grass) detected tolerance to fenoxaprop-ethyl and susceptibility to sethoxydim 4 days after treatment and prior to the observation of visual injury. Sethoxydim reduced photosynthesis compared to untreated controls but fenoxaprop-ethyl did not. Measurements of older leaf (second most recently fully expanded leaf) photosynthesis were less consistent than those of the younger leaf. Chlorophyllafluorescence ofCalamagrostisleaf segments following treatment with fenoxaprop-ethyl and sethoxydim was ineffective for detecting grass tolerance. Solute leakage from leaf discs treated with 50 to 500 μg ml−1fenoxaprop–ethyl and 10 to 100 μg ml−1sethoxydim indicated differential tolerance ofCalamagrostisand johnsongrass to fenoxaprop–ethyl and centipedegrass and johnsongrass to sethoxydim. Fenoxaprop–ethyl increased solute leakage from susceptible johnsongrass at 100 μg ml−1while the 500 μg ml−1concentration was required to cause substantial solute leakage from tolerantCalamagrostis. Sethoxydim caused greater leakage from johnsongrass than from centipedegrass at 50 and 100 μg ml−1. The best potential rapid screening technique was solute leakage measurements.

Changes in membrane lipid components and antioxidant levels during natural ageing of seeds of Acer platanoides

Changes in membrane lipid components and cellular antioxidant systems were investigated through 7 years in seeds of Acer platanoides L. after storage in natural conditions, i.e. ‐ 3°C and 10% water content. The loss of germination capacity in aged seeds was associated with increased solute leakage during imbibition, reduced content of phospholipids, especially phosphatidylcholine, and increased free fatty acid content. A marked decrease of unsaturated fatty acids in the phospholipid fraction was observed after one year of storage. Antioxidant potential in the lipid fraction and level of ‐SH groups decreased during storage. The results are consistent with the hypothesis that ageing in seeds is mediated by a free radical mechanism.

Effects of weathered fuel oil on solute leakage, respiration and plasmolysis in excised root sections of Spartina alterniflora Loisel. and Limonium carolinianum (Walt.) Britt.

This research examined the tolerance of two salt marsh plant species, Spartina alterniflora Loisel. and Limonium carolinianum (Walt.) Britt., to petroleum. Experiments were conducted on plasmalemma permeability, aerobic respiration and plasmolysis in excised root sections. Histochemical studies of fixed and fresh material were made using light microscopy. The results indicated that weathered No. 2 fuel oil increased solute leakage from root sections of Spartina and Limonium in a dose-dependent manner but greater leakage occurred from Spartina than from Limonium roots. Plasmolysis experiments indicated that the primary cause of solute leakage from Spartina root sections was not due to cell lysis up to 5 hr after treatment. Plasmolysis experiments with Limonium were inconclusive because the root tissues did not tolerate this assay procedure. Aerobic respiration was also inhibited by weathered fuel oil to a greater extent in Spartina than in Limonium root sections. Differences in secondary growth and suberization appeared to account for the differential responses of the two species.

CHILLING INJURY. A REVIEW OF QUALITY ASPECTS

ABSTRACTThe various quality aspects of chilling injury (CI) serve as the focus of this review in which symptoms, occurrence and its alleviation are discussed. CI is a term used to describe the physiological damage that occurs in many plants and plant products, particularly those of tropical and subtropical origin, as a result of their exposure to low but nonfreezing temperatures. The substantial economic consequences of CI have provided the impetus for studying/developing effective means of alleviating symptoms which manifest this disorder. A diversity in plant responses to low temperature stress exists, including alterations in ethylene biosynthesis, increased respiration rates, cessation of protoplasmic streaming, increased solute leakage, and uncoupling of oxidative phosphorylation. These various responses ultimately give rise to an array of visual symptoms (e.g., surface pitting, water rot, poor color development, general loss of structural integrity) which can render severe losses in product quality both pre‐ and postharvest. A number of different methods are available by which to alleviate symptom development, including manipulation of storage conditions (e.g., temperature cycling, hypobaric and modified atmosphere storage), exogenous chemical treatments (e.g., application of phospholipids, antioxidants, calcium) and genetic modification of chill sensitive species. These are discussed with respect to their effectiveness and possible control mechanisms.

Uniconazole-Induced Chilling Tolerance in Tomato and its Relationship to Antioxidant Content

Summary Five-week old tomato (Lycopersicon esculentum L. cv. UC-82) seedlings were chilled at 4°C for 4 days at a 16 h light and 8 dark cycle. The leaves of chilled plants showed a loss of turgor, chlorosis, increased solute leakage followed by leaf death. All these symptoms of chilling injury were prevented by a soil application of the triazole uniconazole (8 mg·l−1) to the plants 4 days prior to chilling treatment. Leaf chlorophyll fluorescence measurements indicated that chilling induced an inhibition of the energy transducing system which did not recover after transfer to room temperature. While the triazole treatment did not alter the degree of this inhibition during chilling, it prevented the permanent inhibitory effect of chilling observed in the control plants after their return to 25°C. Chilling injury to the membranes was observed as a loss of phospholipid and an accumulation of free fatty acids, which was prevented in the triazole treated plants. Changes in fatty acid saturation after chilling were relatively minor. This is consistent with injury to membranes which is promoted by oxygen free radicals. Application of the triazole resulted in an increase in total lipid soluble antioxidants, alpha-tocopherol and ascorbic acid levels in leaves. This may have protected the membranes from oxidative stress and contributed to the chilling tolerance.

Age‐induced changes in cellular membranes of imbibed soybean seed axes

The physical and chemical properties of microsomal membranes and cellular antioxidant systems were investigated in imbibed soybean (Glycine max L. Merr. cv. Maple Arrow) seeds following aging for 5 years at room temperature. The loss of germination capacity in aged seeds was associated with increased solute leakage during imbibition and with a loss of membrane phospholipid. Higher levels of free fatty acids were observed in the microsomal membranes from aged seeds. However, there was no change in fatty acid saturation. Wide angle X‐ray diffraction studies indicated the presence of gel phase in addition to liquid‐crystalline phase lipid domains in the membranes of aged seeds. Those from fresh seeds were exclusively liquid‐crystalline. Fluorescence depolarization, using diphenylhexatriene, suggested that the microviscosity of the membrane bilayer was increased by aging. Aged seeds had a lower antioxidant potential in the lipid fraction, lower tocopherol content, and reduced ascorbate:dehydroxyascorbate ratio indicating that the aging process was associated with exposure to an oxidative stress.

The effects of sodium fluoride on cytoplasmic leakage and the lipid and fatty acid composition of jack pine (Pinus banksiana) seedlings

The effects of sodium fluoride on the lipid and fatty acid composition of jack pine (Pinus banksiana Lamb.) seedlings were studied using chromatography techniques. Sodium fluoride markedly reduced levels of phospholipids and increased those of nonpolar lipids. Fatty acid composition was little affected. In several fluoride treatments, elevated levels of palmitic and stearic acid were found, while those of behenic acid declined. The changes in lipid and fatty acid composition were accompanied by increased solute leakage from cells. Cellular membranes are implicated as possible sites of fluoride injury.

Root growth and solute accumulation in pea in response to water stress and powdery mildew

Peas growing in soil columns that allowed unrestricted downward growth and development of the root system and inoculated with Erysiphe pisi were subjected to 3, 4-day drying cycles. Water stress did not affect growth of the tap root but it did stimulate the growth of shallower lateral roots. Infection did not affect tap root growth in either well-watered or water-stressed plants. Infection severely reduced the growth of all lateral roots of well-watered plants and of those lateral roots of water-stressed plants which had reached the lower wetter regions of the soil. Infection of water-stressed plants only reduced the growth of lateral roots in the upper, drier regions of the soil after the third drying cycle. This loss reduced the root to shoot dry weight ratio of infected-stressed plants which had previously been equal to that of healthy-stressed plants and higher than that of unstressed plants whether the latter were healthy or infected. The decline in root weight was associated with the rapid death of older infected leaves on water-stressed plants and so the ratio of root dry weight to leaf area remained unaltered by infection. Solute and water potentials of the shoot apex of healthy and infected plants were lower in stressed than in unstressed plants but in infected plants they increased with time after inoculation. However, no change in turgor resulted in infected plants since solute and water potentials changed at similar rates. When plants were grown in culture solution and were water-stressed by addition of polyethylene glycol to the medium, growth of intact root systems was reduced by both mildew and stress, the effects being additive when they occurred together. The solute potential of tips of first lateral roots was lowered by water stress, but was raised by infection. Infection did not cause increased solute leakage from roots but reduced the ability of root tips to accumulate solutes in response to stress. Thus, when tips were excised from such roots and immersed in mannitol, extension was reduced in solutions of low osmotic potential. It is concluded that mildew leads to slower growth, or even death, of lateral roots in dry regions of the soil because it interferes with the supply of photosynthates needed for normal osmoregulation. Rapid growth of the tap root and changes in the balance between the absorbing and transpiring regions of the mildewed plant protect it from increased water stress in dry soil.

Effects of Chemical Protectants against Chilling Injury of Young Cucumber Seedlings

Abstract Chilling of 8-day-old cucumber (Cucumis sativus L.) seedlings for 48 hours at 2°C caused reversible damage; chilled seedlings collapsed but regained turgor within 6 hours at 25°. Hypocotyl segments from chilled seedlings exhibited increased solute leakage, ethylene production, and O2 uptake after transfer to 25°. Pretreatment of seedlings with 10−4m abscisic acid generally reduced symptoms of damage but did not prevent chilling injury. Pretreatment with 10−3m CaCl2 was generally ineffective, and pretreatment with 10−3m ascorbate generally intensified symptoms of injury. Oxygen uptake increased during the first 6 hours at 25° but returned to the level of unchilled seedlings within 24 hours. Chilled tissue was sensitive to 2,4-dinitrophenol during this period, although sensitivity was less than in controls during the first 12 hours after chilling. Unchilled tissue was insensitive to SHAM, but O2 uptake was inhibited by SHAM during the respiratory burst following chilling. Tissue was no longer SHAM-sensitive after 24 hours at 25°, by which time the respiratory burst had subsided. Thus, the alternative respiratory pathway was apparently operating only during the time of the respiratory burst, when the seedlings were recovering from chilling injury.