Plant anaerobic stress as a novel trend in ecological physiology, biochemistry, and molecular biology: 2. Further development of the problem

This review is a logical development of a previous publication, which summarized the main results of the early period of the systematic and active studying of hypoxic and anoxic stresses in plants. These studies laid a foundation for a new scientific discipline in biology, the investigation relevant to plant anaerobic stress. This review considers a further development of this trend when the investigations embraced a wider set of topics and the discipline acquired an international recognition. The results obtained during last decades by physiologists, biochemists, and molecular biologists engaged in the problem of plant anaerobic stress confirmed the correctness of a concept of the two principal strategies of plant adaptation to hypoxia and anoxia conditions. They are “true” tolerance manifesting at the molecular level under conditions of oxygen deficiency or its absence and “apparent” tolerance, which is realized by avoidance of anaerobiosis due to the long-distance oxygen transport. Therefore, experimental material available now is considered and discussed in this review mainly in the light of these principal notions. Especial attention is paid to the role of stress proteins, which synthesis is induced under hypoxia and anoxia. The results of these experiments confirmed earlier conclusions about the key role of energy (glycolysis and alcoholic fermentation) and carbohydrate (mobilization and utilization of reserved carbohydrates) metabolism in plant adaptation to oxygen deficiency or its absence from the environment. The phenomenon of hypoxic acclimation and its role in plant adaptation to anoxia are also considered. Along with these topics, a further development of pH-stat theory is discussed. A special attention is paid to plant strategy realized by the formation of the net of air-filled spaces (aerenchyma) and long-distance oxygen transport from aerated plant parts to those located in anaerobic environment (apparent tolerance). Among other important aspects, we consider (1) post-anaerobic plant injury by free oxygen radicals; (2) the physiological role of alternative pathways of plant adaptation (nitrate reduction and lipid synthesis); (3) the phenomenon of the adaptation syndrome in plants and possible molecular mechanisms of its realization; and (4) some biotechnological advances in the field of genetic and cell engineering used for the creation of plants more tolerant to anaerobic stress.     

Plant Anaerobic Stress as a Novel Trend in Ecological Physiology,Biochemistry, and Molecular Biology: 1. Establishment of a New Scientific Discipline

This review attempted to follow the establishment of a novel branch of biology arisen at the interfaces between plant physiology, biochemistry, and molecular biology—plant anaerobic stress. Most attention was given to the early period of these investigations, the activity of the members of International Society for Plant Anaerobiosis in particular, and the contribution of Russian scientists, who played a significant role at that time in the establishment and international recognition of this new trend. In this connection, the following points are considered: (1) Crawford's metabolic theory, which could not withstand experimental verification but induced an active discussion, thus stimulating further investigations in this field; (2) a concept of two main strategies of plant adaptation to anaerobic stress (true and apparent adaptation), which was put forward based on the following experimental data: (a) a discovery of a paradoxical phenomenon of hyper-sensitivity, but not hyper-resistance to anoxia, of the flood-tolerant plant roots (“apparent” tolerance); (b) the elucidation of the physiological role of oxygen transported from aerated organs of flood-tolerant plants to the roots inhabiting anaerobic environment; (c) demonstration of the key role of both energy metabolism, and (d) substrate providing for glycolysis and ethanolic fermentation in plants manifesting “true” tolerance to oxygen deprivation; (3) the discovery of plant stress proteins; and finally (4) pH-stat theory put forward by Davies.     

Anoxia tolerance in rice seedlings: exogenous glucose improves growth of an anoxia-'intolerant', but not of a 'tolerant' genotype

This study demonstrated that, in rice seedlings, genotypic differencein tolerance to anoxia only occurred when anoxia was imposedat imbibition, but not at 3 d after imbibition. When seeds wereimbibed and grown in anoxia, IR22 (anoxia-‘intolerant’)grew much slower and had lower soluble sugar concentrationsin coleoptiles and seeds than Amaroo (anoxia-‘tolerant’),while Calrose was intermediate. After 3 d in anoxia, the sugarconcentrations in embryos and endosperms of anoxic seedlingswere nearly 4-fold lower in IR22 than in Amaroo. Sugar deficitin the embryo of IR22 is presumably due to the limitation ofsugar mobilization rather than the capacity of transport asshown by similar sugar accumulation ratios of 1.8 between embryoand endosperm in IR22 and Amaroo at 3 d in anoxia. With 20 molm^–3 exogenous glucose, coleoptile extension and freshweight increments in anoxic seedlings of IR22 were much closerto those in the two other genotypes, nevertheless protein concentrationremained lowest on a fresh weight basis in the coleoptiles ofIR22; indicating that protein synthesis has a lower priorityfor energy apportionment during anoxia than processes crucialto coleoptile extension. In contrast to these responses to anoxiaimposed at imbibition, IR22 had nearly the same high toleranceto anoxia as Calrose and Amaroo, when anoxia was imposed onseedlings subsequent to 48 h aeration followed by 16 h hypoxicpretreatment. In fact, coleoptiles of anoxic IR22 had highersugar concentrations and grew faster than Calrose, and exogenousglucose had no effect on the coleoptile extension of IR22. Excisedcoleoptile tips of IR22 and Amaroo with exogenous glucose hadsimilar rates of ethanol production and were equally tolerantto anoxia. In conclusion, much of the anoxia ‘intolerance’of IR22 when germinated in anoxia could be attributed to limitedsubstrate availability to the embryo and coleoptile, presumablydue to slow starch hydrolysis in the endosperm. Key words: Anoxia, coleoptile, embryo, endosperm, ethanol production, germination, growth, Oryza sativa L., solute net uptake or loss, sugar availability.     

Black, M. and Pritchard, H.W. (eds). Desiccation and survival in plants. Drying without dying

This excellent volume provides a very readable and comprehensivecoverage of desiccation tolerance in plants. The opening chapter,from which the subtitle ‘Drying without dying’ isdrawn, gives a splendid overview of what desiccation toleranceis and how plants accommodate this potentially lethal stress.It includes a history of research into plant desiccation andthe development of ideas, its occurrence within the plant kingdomand its ecological significance. The subsequent chapters buildmasterfully upon this introduction, in     

A Developmental Window for Salt-Adaptation in Sorghum bicolor

Following exposure to specific NaCl treatments, certain Sorghumbicolor genotypes developed increased salinity tolerance, ‘adaptation’,which was accompanied by a morphological modification in thedevelopment of one or several leaves. Adaptation differed fromthe more common response of the plant to salinity, ‘pre–existingresistance’, which rested on the prevalent tolerance mechanismsexisting prior to the salinization treatment. Induction of adaptationwas possible only during a specific time period of plant development.This time-limited capacity for adjustment to a changing environmentalparameter, developmental window, resembled similar processesoccurring in animal development. Key words: Sorghum, adaptation, resistance, salinity, developmental window     

Patterns of Root Respiration Associated with the Induction of Aluminium Tolerance in Phaseolus vulgaris L.

Aluminium (Al) tolerance in an Al-tolerant cultivar of Phaseolusvulgaris L. (‘Dade’) was found to be an inducibletrait. Upon exposure to 10 µM Al, the rate of root elongationwas inhibited in comparison to controls. During the following72 h, the rate of elongation returned to levels comparable tocontrols. In contrast, root elongation of an Al-sensitive cultivar(‘Romano’) did not recover after exposure to Al.In Dade, the resumption of root elongation following exposureto Al was accompanied by increased rates of root respiration,whereas respiration rates slowly declined over the 72 h treatmentperiod in Romano. When partitioned into growth and maintenanceexpenditures, a larger proportion of root respiration of Dadeexposed to Al was allocated to maintenance processes, potentiallyreflecting diversion of energy to metabolic pathways that offsetthe adverse effects of Al toxicity. Romano did not show sucha pattern and respiration associated with both growth and maintenancewas reduced after exposure to Al. Root and shoot growth of bothcultivars were also measured to determine the effects of long-term(21 d) exposure to 10 µM Al. Dade plants exposed to Alexhibited enhanced growth in comparison to controls, whereasRomano plants were characterized by reduced shoot and root growth.Modelling the time-course of root respiration and measuringthe long-term growth responses to Al is a valuable method ofelucidating respiratory costs of stress tolerance. Key words: Aluminium, differential tolerance, maintenance respiration, Phaseolus vulgaris, root respiration     

The Nitrogen Content of Plants and the Self-thinning Rule of Plant Ecology: A Test of the Core-skin Hypothesis

The ‘core-skin’ hypothesis postulates that secondarilythickened plants behave energetically as an inert ‘core’covered by an active ‘skin’, the ‘skin’being two-imensional, the ‘core’ three-dimensional.This would explain the ‘self-thinning ‘or‘–3/2’ rule of plant ecology, that is, the tendencyfor log (dry weight per plant) and log (number of plants perunit area) to progress along a straight line relationship, withslope = – 3/2’. The hypothesis was tested as follows. Plant nitrogen contentwas used as an estimate of the mass of ‘skin’ perplant, and dry weight as an estimate of the mass of the ‘core’.As plants mature the slope of the relationship between y = log(mass of nitrogen per plant) and x = log (mass of dry matterper plant) is expected to decline from an initial value of 1.0towards a final value of 0.66. The intercept of the relationshipis expected to reflect the intrinsic content of ‘skin’per unit of ‘core’. Genotypic variation in thisparameter should cause genotypic differences in the maximumattainable yield of biomass per unit area. The expectations were investigated by fitting the function y= p+qx+r exp – x to 30 sets of data on plant nitrogencontent, plant weight and time in 18 different vegetables. Simplelinear regressions of y on x were fitted to more limited setsof data on weights and nitrogen contents of mature trees. Theexpectations were, with some minor exceptions, confirmed. Nitrogen, yield, plant competition, self-thinning     

The Influence of the Plant Growth-regulator, 2-Methyl-4-Chlorophenoxyacetic Acid, on the Metabolism of Carbohydrate, Nitrogen and Minerals in Solanum lycopersicum (Tomato)

The effects of the foliar application of phytocidal concentrationsof 2-methyl-4-chlorophenoxyacetic acid (MCPA) on change in totaldry weight, and in ‘available carbohydrate’ (starch,‘total’ and ‘reducing’ sugars), totalnitrogen, phosphorus, potassium, calcium, and magnesium of ‘tops’and roots of tomato plants have been followed over a periodof 14 days following spraying. There were two main treatments—‘nutrient’(nutrient supply to roots continued after spraying) and ‘water’(distilled water only supplied to roots after spraying) and‘water’ (distilled water only supplied to rootsafter spraying)—the sub-treatments consisting of ‘MCPA’versus ‘no-MCPA’ for each of the main treatments.Twelve different times of sampling were used. In analysing the present data, the quantity ‘residualdry weight’ (total dry weight less ‘available carbohydrate’),which was originally introduced by Mason and Maskell as a basisof reference for analyses of plant organs in short-period experimentsnot involving appreciable growth, has been used as an estimateof the permanent structure of plant growth. This new use ofthe ‘residual dry weight’ basis has brought outimportant features which were obscured when the data were leftin their primary form (as percentages of total dry weight oramounts per plant). Growth, as measured by increase in ‘residual dry weight’,was greatly inhibited by 2-methyl-4-chlorophenoxyacetic acidshortly after spraying, in both the presence and the absenceof nutrient. In the presence of 2-methyl-4-chlorophenoxyacetic acid, netassimilation rate (estimated as rate of increase in total dryweight per gram ‘residual dry weight’ of the ‘tops’)was greatly diminished while uptake of total nitrogen and ofP₂O₅ (estimated as increase in total nitrogen or of P₂O₅ ofthe whole plant per day per 1 g. ‘residual dry weight’of the roots) appeared to undergo a similar but much smallerdiminution. It seemed probable, however, that in the presenceof MCPA a larger proportion of the carbohydrate actually formedwas utilized for synthesis of aminoacids and protein. In the plant as a whole there was no evidence of actual depletionof ‘available carbohydrate’ as a result of MCPAtreatment, this fraction showing a steady increase in all treatmentsthroughout the experiment. The rate of increase was, however,much reduced by MCPA treatment. The ‘tops’ presentedmuch the same picture as the whole plant, but for the rootsthe situation was quite different. While the roots of the ‘no-MCPA’plants and also of the ‘MCPA-water’ plants showeda steady increase in available carbohydrate, those of the ‘MCPA-nutrient’plants rose only very slightly (from the initial value of 8mg. per plant to about 10 mg.) during the first 2 days, andthen in the next 2 days declined to a value (about 6 mg.) belowthe initial and remained at this low level for the rest of theexperiment. It is suggested that the phytocidal effect of 2-methyl-4-chlorophenoxyaceticacid in the presence of nutrient may be due to depletion ofthe ‘available carbohydrate’ supplies in the roots,which is shown to be brought about, in part, by reduced transportfrom the tops, and partly by the relatively greater utilizationof the carbohydrate present. These results offer an explanationfor the facts that plants showing vigorous growth are more easilykilled by MCPA and that perennial plants, particularly thosewith storage tissues in their roots, are more resistant. Further,they suggest the useful practical application that MCPA treatmentshould be given when the carbohydrate reserves of the rootsare at a minimum. For perennial plants, conditions might beexpected to be optimal for the application of MCPA in late spring,at a time when the first ‘flush’ of growth is slowingdown and before any appreciable new reserves of carbohydratehave been accumulated. It was also shown that 2-methyl-4-chlorophenoxyacetic acid preventedthe net synthesis of starch, but still permitted an appreciablenet formation of sucrose. 2-methyl-4-chlorophenoxyacetic acid appeared to have no effecton the uptake of potassium, calcium, or of magnesium. The lackof effect on potassium is contrasted with the previous observationby Rhodes, Templeman, and Thruston (1950) that sub-lethal concentrationsof MCPA, applied over a relatively long period to the rootsof tomato plants, specifically depressed the uptake of potassium.     

The Extraction, Solubility, and Characterization of Two Groups of Barley Storage Polypeptides

This paper reports further studies on the characteristics ofthe storage protein fraction (hordein) of barley. Hordein consistsof two groups of polypeptides (termed ‘B’ and ‘C’)coded by two separate but linked loci. Whereas the ‘C’polypeptides are readily soluble and extracted in 60% (v/v)ethanol at room temperature, the ‘B’ group is moresoluble in, and therefore more efficiently extracted by, 50%(v/v) propan-1-ol or 45% (v/v) propan-2-ol at elevated temperaturesand in the presence of 2-mercaptoethanol. However, the mostefficient conditions for hordein extraction (50% propan-1-ol+ 2% (v/v) 2-mercaptoethanol at 60 °C) also extract somecontaminating non-hordein polypeptides resulting in an apparentlyincreased lysine content of the hordein fraction. Amino acid analysis of the purified ‘B’ and ‘C’hordein groups shows that, whereas ‘C’ hordein containsmore glutamate + glutamine, proline, and phenylalanine than‘B’ hordein, it contains only traces of lysine andsulphur amino acids in contrast to ‘B’ hordein whichcontains 0·5% lysine 0·6% methionine, and 2·5%cysteine. Equilibrium sedimentation analyses carried out on the purified‘B’ and ‘C’ groups indicates that thepreparations were reasonably monodisperse with molecular weightsof approximately 32 000 and 52 000 respectively. These valuesare considerably lower than those previously determined by SDS-PAGE.     

Plant Morphology: The Historic Concepts of Wilhelm Troll, Walter Zimmermann and Agnes Arber

Recent molecular systematic and developmental genetic findingshave drawn attention to plant morphology as a discipline dealingwith the phenotypic appearance of plant forms. However, sincedifferent terms and conceptual frameworks have evolved overa period of more than 200 years, it is reasonable to surveythe history of plant morphology; this is the first of two paperswith this aim. The present paper deals with the historic conceptsof Troll, Zimmermann and Arber, which are based on Goethe'smorphology. Included are contrasting views of ‘unity anddiversity’, ‘position and process’, and ‘morphologyand phylogeny’, which, in part, are basic views of currentplant morphology, phylogenetic systematics and developmentalgenetics. Wilhelm Troll established the ‘type concept’and the ‘principle of variable proportions’. Hehas provided the most comprehensive overview of the positionalrelations of plant forms. Agnes Arber started from the universaldynamics of life and attempted to describe all structures asprocesses. She paid attention to ‘repetitive branching’,‘differential growth’, and ‘parallelism’.As a result she has recently been rediscovered by developmentalbotanists. Walter Zimmermann rejected any metaphysical influenceon plant form and instead called for objective procedures. Hewas mainly interested in phylogenetic ‘character transformation’and the ‘reconstruction of genealogical lines’.Guided by the example of flower-like inflorescences, a futurepaper will deal with functional and developmental constraintsinfluencing plant forms. Recent morphological concepts (‘trialectical’,‘continuum’/‘fuzzy’, ‘processmorphology’) will be discussed and related to currentmorphological and developmental genetic research. Copyright2001 Annals of Botany Company Plant form, plant morphology, natural philosophy, homology, phylogeny, Goethe, Troll, Arber, Zimmermann, typology, character transformation, differential growth, complementarity     

Relationship between the Accumulation of Putrescine and the Tolerance to Oxygen-Deficit Stress in Gramineae Seedlings

The accumulation of putrescine under anoxia was studied in seedlingsof 6 Gramineae species showing tolerance to the stress in thefollowing order: rice, barnyard grass>maize>rye, barleyand wheat. The accumulation of putrescine in shoots and rootsduring 6 h of oxygen deprivation was different among the speciesand correlated with their tolerance to anaerobic conditions.In both tissues, rice and barnyard grass accumulated more than0.6 µmol/g fresh weight. The lower increase in the putrescinetiter was observed in wheat tissues with less than 0.1 µmol/gfresh weight. Feeding experiments with [^l4C]arginine showedthat rice tissues have a greater capacity than wheat tissuesto synthesize putrescine under anoxia. The better tolerant speciesto anaerobic conditions showed high arginine decarboxylase activityin shoots and roots than the sensitive species. The presenceof exogenous Put partially increased the survival of wheat rootsafter 7.5 h of oxygen-deficit stress. The role of putrescinein determining differences in the tolerance to anoxia in Gramineaeseedlings is discussed. (Received December 20, 1989; Accepted March 12, 1990)     

Effects of Anoxia on Wheat Seedlings: I. INTERACTION BETWEEN ANOXIA AND OTHER ENVIRONMENTAL FACTORS

Anoxia was imposed on 4–6-d-old, intact wheat seedlings,after the roots had first been exposed for 1 d to O₂ concentrationsbetween 0·016 and 0·06 mol m^–3. Apices ofthe main axis of the seminal roots were considered to have toleratedanoxia if elongation occurred after return from anoxia to air,hereafter called ‘retention of elongation potential’.During anoxia, elongation potential was retained longer in rootsof intact seedlings than in 0–5 mm excised root tips suppliedwith 50 mol m^–3 glucose. In intact seedlings, elongation potential was retained longerat 15°C than at 25°C, and at pH 50 and 60 than at pH40. These differences between treatments were maintained inthe presence of exogenous glucose, and glucose supply prolongedthe retention of elongation potential in all anoxic treatments. Elongation potential was retained much longer at very low 02concentrations (0006 to 00l mol m^–3) than under anoxia;this was established at pH 40. Anoxia inhibited the transport of sugars from the shoots and/orendosperm to the root by 79-97%, as assessed from experimentswith roots of intact plants exposed to anoxia at pH 60 and 15°C. Overall, the results demonstrate: (i) that the occurrence ofadverse effects of anoxia during waterlogging in the field mayinteract with other environmental factors and (ii) that thereare pronounced difficulties integrating data on tolerance toanoxia obtained in different laboratories. Key words: Anoxia, wheat seedlings, pH, temperature     

Seasonal patterns of growth, dehydrins and water-soluble carbohydrates in genotypes of Dactylis glomerata varying in summer dormancy

• Background and Aims Summer dormancy in perennial grasseshas been studied inadequately, despite its potential to enhanceplant survival and persistence in Mediterranean areas. The aimof the present work was to characterize summer dormancy anddehydration tolerance in two cultivars of Dactylis glomerata(dormant ‘Kasbah’, non-dormant ‘Oasis’)and their hybrid using physiological indicators associated withthese traits. • Methods Dehydration tolerance was assessed in a glasshouseexperiment, while seasonal metabolic changes which produce putativeprotectants for drought, such as carbohydrates and dehydrinsthat might be associated with summer dormancy, were analysedin the field. • Key Results The genotypes differed in their ability tosurvive increasing soil water deficit: lethal soil water potential(_s) was –3·4 MPa for ‘Kasbah’ (althoughnon-dormant), –1·3 MPa for ‘Oasis’,and –1·6 MPa for their hybrid. In contrast, lethalwater content of apices was similar for all genotypes (approx.0·45 g H₂O g d. wt^–1), and hence the greater survivalof ‘Kasbah’ can be ascribed to better drought avoidancerather than dehydration tolerance. In autumn-sown plants, ‘Kasbah’had greatest dormancy, the hybrid was intermediate and ‘Oasis’had none. The more dormant the genotype, the lower the metabolicactivity during summer, and the earlier the activity declinedin spring. Decreased monosaccharide content was an early indicatorof dormancy induction. Accumulation of dehydrins did not correlatewith stress tolerance, but dehydrin content was a function ofthe water status of the tissues, irrespective of the soil moisture.A protein of approx. 55 kDa occurred in leaf bases of the mostdormant cultivar even in winter. • Conclusions Drought avoidance and summer dormancy arecorrelated but can be independently expressed. These traitsare heritable, allowing selection in breeding programmes.     

The Role of Endogenous Abscisic Acid in the Response of Plants to Stress

When a continuous stream of warm air (38°C) was directedon to the leaves of dwarf bean seedlings they wilted and thengradually regained turgor. This process of adaptation was accompaniedby an increasing abscisic acid (ABA) level in the leaves andan increase in leaf resistance (R_L). It is suggested that theleaf-water deficit induced by the warm-air treatment causedthe increase in ABA level and that the latter was responsiblefor stimulating stomatal closure, enabling the plants to regainfull turgor. A similar type of adaptation, brought about byan increased level of ABA in the leaves, is believed to occurin tomato, dwarf bean, and wheat plants when they are flooded.Predictably, in rice, a species adapted to a flooded environment,seedlings showed no increase in ABA level as a result of flooding. It is proposed that adaptation may involve the formation ofan equilibrium between ABA and its conjugate form (i. e. theglucose ester). The ABA-conjugate was observed to disperse slowlyfrom leaves recovering from a water deficit and therefore itmay act as a metabolic ‘back-stop’, enabling the‘free’ ABA level to remain high for a period evenwhen the leaves have regained turgor. Abscisic acid appears to be responsible for alleviating theeffects of water stress in plants, making it possible for plantsto pass through periods of stress with little harm.     

Developmental control of xylem hydraulic resistances and vulnerability to embolism in Fraxinus excelsior L.: impacts on water relations

The freezing tolerance of many plants, such as pea (Pisum sativum),is increased by exposure to low temperature or abscisic acidtreatment, although the physiological basis of this phenomenonis poorly understood. The freezing tolerance of pea shoot tips,root tips, and epicotyl tissue was tested after cold acclimationat 2C, dehydration/rehydration, applications of 10^–4M abscisic acid (ABA), and deacclimation at 25C. Tests wereconducted using the cultivar ‘Alaska’, an ABA-deficientmutant ‘wil’, and its ‘wildtype’. Freezinginjury was determined graphically as the temperature that caused50% injury (T₅₀) from electrical conductivity. Endogenous ABAwas measured using an indirect enzyme-linked immunosorbant assay,and novel proteins were detected using 2-dimensional polyacrylamidegel electrophoresis. The maximum decrease in T₅₀ for root tissuewas 1C for all genotypes, regardless of treatment. For ‘Alaska’shoot tips and epicotyl tissue, exogenous ABA increased thefreezing tolerance by –1.5 to –4.0C, while coldtreatment increased the freezing tolerance by –7.5 to–14.8C. Cold treatment increased the freezing toleranceof shoot tips by –9 and –15C for ‘wil’and ‘wild-type’, respectively. Cold acclimationincreased endogenous ABA concentrations in ‘Alaska’shoot tips and epicotyls 3- to 4-fold. Immunogold labeling increasednoticeably in the nucleus and cytoplasm of the epicotyl after7 d at 2C and was greatest after 30 d at the time of maximumfreezing tolerance and soluble ABA concentration. Cold treatmentinduced the production of seven, three, and two proteins inshoot, epicotyl, and root tissue of ‘Alaska’, respectively.In ‘Alaska’ shoot tissue, five out of seven novelproteins accumulated in response to both ABA and cold treatment.However, only a 24 kDa protein was produced in ‘wil’and ‘wild-type’ shoot and epicotyl tissues aftercold treatment. Abscisic acid and cold treatment additivelyincreased the freezing tolerance of pea epicotyl and shoot tissuesthrough apparently independent mechanisms that both resultedin the production of a 24 kDa protein. Key words: Pisum sativum, cold acclimation, immuno-localization     

Freezing tolerance, protein composition, and abscisic acid localization and content of pea epicotyl, shoot, and root tissue 3

The freezing tolerance of many plants, such as pea (Pisum sativum),is increased by exposure to low temperature or abscisic acidtreatment, although the physiological basis of this phenomenonis poorly understood. The freezing tolerance of pea shoot tips,root tips, and epicotyl tissue was tested after cold acclimationat 2C, dehydration/rehydration, applications of 10^–4M abscisic acid (ABA), and deacclimation at 25C. Tests wereconducted using the cultivar ‘Alaska’, an ABA-deficientmutant ‘wil’, and its ‘wildtype’. Freezinginjury was determined graphically as the temperature that caused50% injury (T₅₀) from electrical conductivity. Endogenous ABAwas measured using an indirect enzyme-linked immunosorbant assay,and novel proteins were detected using 2-dimensional polyacrylamidegel electrophoresis. The maximum decrease in T₅₀ for root tissuewas 1C for all genotypes, regardless of treatment. For ‘Alaska’shoot tips and epicotyl tissue, exogenous ABA increased thefreezing tolerance by –1.5 to –4.0C, while coldtreatment increased the freezing tolerance by –7.5 to–14.8C. Cold treatment increased the freezing toleranceof shoot tips by –9 and –15C for ‘wil’and ‘wild-type’, respectively. Cold acclimationincreased endogenous ABA concentrations in ‘Alaska’shoot tips and epicotyls 3- to 4-fold. Immunogold labeling increasednoticeably in the nucleus and cytoplasm of the epicotyl after7 d at 2C and was greatest after 30 d at the time of maximumfreezing tolerance and soluble ABA concentration. Cold treatmentinduced the production of seven, three, and two proteins inshoot, epicotyl, and root tissue of ‘Alaska’, respectively.In ‘Alaska’ shoot tissue, five out of seven novelproteins accumulated in response to both ABA and cold treatment.However, only a 24 kDa protein was produced in ‘wil’and ‘wild-type’ shoot and epicotyl tissues aftercold treatment. Abscisic acid and cold treatment additivelyincreased the freezing tolerance of pea epicotyl and shoot tissuesthrough apparently independent mechanisms that both resultedin the production of a 24 kDa protein. Key words: Pisum sativum, cold acclimation, immuno-localization     

The Importance of Light Intensity for Pollen Tube Growth and Embryo Survival in Wheat x Maize Crosses

The success of Triticum aestivumxZea mays crosses, used to producewheat doubled haploids, is influenced by light intensity. Toexamine the basis for this response, pollen tube growth, embryosurvival and indicators of photosynthetic rate were measuredin two wheat cultivars (‘Karamu’ and ‘Kotuku’)crossed with maize at two irradiance levels (250 or 750 µmolm^-2s^-1, PAR). Pollen tube growth was significantly affectedby light intensity in ‘Karamu’ plants but not in‘Kotuku’ plants, despite both cultivars being pollinatedby the same maize source. The percentage of pollen tubes reachingthe cavity between the ovarian wall and integuments, or in themicropyle of ‘Karamu’ plants at high light intensity(65%) was nearly three-times greater than that at low lightintensity (22%). Thus, either low light intensity can affectthe maternal wheat plant in a way that inhibits pollen tubegrowth and/or high light intensity may promote pollen tube growthin ‘Karamu’ plants. Significant differences in ratesof electron transport in plants grown at the two light intensitiesindicated that the rate of photosynthesis may also have an effecton pollen tube growth. These results have importance for improvingthe efficiency of wheat x maize crosses and other wide cerealcrosses. Copyright 2001 Annals of Botany Company Intergeneric hybridization, light intensity, pollen tube growth, embryo survival, Triticum aestivum, wheat,Zea mays , maize     

The Potassium Relations of Lemna minor L.: I. POTASSIUM UPTAKE AND PLANT GROWTH

A simple expression has been derived to predict the rate ofnet K uptake into exponentially growing plants of Lemna minor.Net uptake predictions are in good quantitative agreement withmeasurements of ‘steady-state’ K influx, indicatingthat, in the ‘steady state’, K movements in theplant are essentially undirectional and that efflux is small.This close matching of inward K movement to the demands of theexpanding tissue is temporarily disturbed if plants are transferredto media of different K status. Uptake rates in the ‘step-up’are initially enhanced and then fall gradually towards a new‘steady-state’ rate. In contrast, the ‘step-down’causes an initial depression of uptake and then rates increasegradually towards the new ‘steady-state’ rate. Itis argued that these changes in uptake rates are associatedwith alterations in the cytoplasmic K content.