Structural development of the shoot and root systems of two winter wheat cultivars, Triticum aestivum L.

The structural development of the stems and basal anchorageroots of Galahad and Hereward winter wheat cultivars (Triticumaestivum L.) were investigated and related to their mechanicalfunction. Stem and root morphology, anatomy and mechanical propertieswere examined from tillering (March) up to maturity (August),together with plant weight distribution. This allowed us tocalculate a ‘factor of safety’ against root andstem failure throughout development. As the plants grew taller the stem and the anchorage ‘coronalroots’ increased in bending strength countering the increasingmechanical demands. The bending strength, in turn, was correlatedwith the amount of lignified material around the stem and rootperimeter. Structural development ceased by ear emergence, whenthe plant was at its tallest, but because the ear weight continuedto rise the ‘self-weight’ moment pushing the plantover continued to increase. This meant that the ‘safetyfactors’ of both cultivars against both root and stemmechanical failure decreased throughout development. In bothcultivars the safety factors against root failure were lowerthan for stem failure, and Galahad had lower factors of safetythan Hereward. All these findings were consistent with resultsof field trials; failure tends to occur late in development,during grain filling, and is localized to the root system, whilstGalahad is more prone to lodging than Hereward. The pattern of mechanical development of winter wheat seemsto be one which would maximize its reproductive success, maintainingits structural integrity especially early in development whileinvesting in a minimum of structural material. Key words: Safety factor, anchorage, lodging, biomechan-ics, structural development     

Effect of Source of Nitrogen on the Growth of Fiskeby Soya Bean: the Carbon Economy of Whole Plants

Fiskeby V soya bean was grown from seed germination to seedmaturation with two contrasting patterns of nitrogen metabolism:either wholly dependent on dinitrogen fixation, or with an abundantsupply of nitrate nitrogen, but lacking root nodules. The carbonand nitrogen economies of the plants were assessed at frequentintervals by measurements of photosynthesis, shoot and rootrespiration, and organic and inorganic nitrogen contents. Plantsfixing atmospheric nitrogen assimilated only 25–30 percent as much nitrogen as equivalent plants given nitrate nitrogen:c. 40 per cent of the nitrogen of ‘nitrate’ plantswas assimilated after dinitrogen fixation had ceased in ‘nodulated’plants. The rates of photosynthesis and respiration of the shootsof soya bean were not markedly affected by source of nitrogen;in contrast, the roots of ‘nodulated’ plants respiredtwice as rapidly during intense dinitrogen fixation as thoseof ‘nitrate’ plants. The magnitude of this respiratoryburden was calculated to increase the daily whole-plant respiratory loss of assimilate by 10–15 per cent over thatof plants receiving abundant nitrate. It is concluded that ‘nodulated’plants grew more slowly than ‘nitrate’ plants inthese experiments for at least two reasons: firstly, the symbioticassociation fixed insufficient nitrogen for optimum growth and,secondly, the assimila tion of the nitrogen which was fixedin the root nodules was more energy-demanding in terms of assimilatethan that of plants which assimilated nitrogen by reducing nitratein their leaves.     

Nitrogen Utilization in N-limited Barley During Vegetative and Generative Growth: IV. TRANSLOCATION AND REMOBILIZATION OF NITROGEN

Barley (Hordeum vulgare L., cvs Golf and Laevigatum) was grownunder nitrogen limitation in solution culture until near maturity.Three different nitrogen addition regimes were used: in the‘HN’ culture, the relative rate of nitrate-N additionwas 0·08 d^–1 until day 48 and then stepwise decreasedto, finally, 0·005 d^–1 during late grain-filling;the ‘LN’ culture received 45% of the nitrogen addedin HN; the ‘CN’ culture was maintained at RA 0·0375d^–1 throughout growth. At four different growth stages(vegetative,anthesis, and twice during grain-filling), ¹⁵N-nitrate was fedto the plants. In some cases (‘split root cultures’),label was fed only to one-half of the root system. These wereharvested directly after labelling, whereas ‘standardcultured’ plants were harvested at termination of theexperiment (day 148). Absorption of added nitrate was nearlycomplete in the HN and LN cultures, and translocation of nitrogenwithin the plants could thus be studied independently of differencesin nitrate absorption. Cycling of nitrogen absorbed by vegetativeplants accounted for up to 50% of the nitrogen recovered inthe roots. The sink strength of the roots for cycling nitrogen,however, declined during post-anthesis growth, and net lossof nitrogen from both roots and vegetative shoot tissue occurredconcomitantly with incorporation of labelled ¹⁵N-nitrogen. Thenitrogen of the vegetative shoot tissue was substantially lesslabelled than the nitrogen entering the ears, indicating thattranslocation of recently absorbed nitrogen to ears occurs withminor prior exchange with the bulk nitrogen of shoots. In caseswhere the sink strength of the ears was weak, as in LN-culturedLaevigatum (due to high frequency of sterile flowers) and inCN-cultured Golf, nitrogen translocated from roots appearedto be incorporated into the vegetative shoot tissue. There werealso indications that a fraction of the remobilized nitrogenwas actually lost from the plants in these cases. It is concludedthat the root remains efficient in translocation of nitrogento the aerial parts throughout ontogeny and that nitrogen takenup during grain–filling is preferentially directly translocatedto the developing grains. The further translocation of nitrogenreceived by vegetative shoot parts to ears appears mainly relatedto the potential of the ear to accumulate nitrogen. Nitrogenabsorbed/remobilized in excess of the sink strength of the earsis either invested in continued shoot growth, or is irreversiblylost from the plants. Key words: Barley, ¹⁵N-labelling, post-anthesis, remobilization, translocation     

Comparative Effects of Soil Moisture Stress and Restricted Root Zone Volume on Morphogenetic and Physiological Responses of Soybean [Glycine max (L.) Merr.]

Significant differences in response to soil moisture stress(SMS) and restricted root zone volume (RRZV) were found in twocultivars of soybean [Glycine max (L.) Merr.] (‘Forrest’and ‘Williams’) plants grown under controlled-environmentconditions. Leaf water potentials of SMS-treated plants were0·4-0·6 MPa lower than those of controls and stomata1conductances 23-56% lower. In the case of RRZV treatment, however,there were no differences in either parameter. Initiation ofnew leaves as reflected in the plastochron index was stronglyreduced by SMS but was unaffected by RRZV. Photosynthetic rates(CO₂ fixation dm² of leaf) of plants given SMS were reducedby 11-21% while those of RRZV-treated plants were unaffected.SMS caused a strong preferential allocation of dry matter tothe root at the expense of the shoot in both cultivars. RRZV,however, had no effect on assimilate distribution in ‘Forrest’and only slightly favoured root growth in ‘Williams.’Carbohydrate concentrations of both alcohol-soluble and insolublefractions were increased significantly by SMS, especially inthe leaves, but were little affected by RRZV. Nitrogen concentrationin the root fraction was reduced by 22-24% and that in the leafand stem fractions by 7-14% under SMS but was not affected appreciablyby RRZV. Phosphorus concentration in the leaf, stem, and rootfractions was reduced by 45-65% under SMS but was relativelyunaffected by RRZV. These findings suggest that SMS and RRZVare basically different in their mechanism of action and thatthe impairment of growth resulting from these two stresses mayinvolve different physiological processes. Our results alsoindicate that the suppressive effects of small containers onplant growth do not necessarily result from inadvertent SMS. Key words: Drought, Container effects, Glycine max (L.) Merr     

Dry Matter Partitioning in a Determinate and an Indeterminate Cultivar of Vicia faba L. Under Contrasting Plant Distributions and Densities

Models for root: shoot, vegetative: generative and stem: leafpartitioning are presented to quantify dry matter partitioningof two contrasting genotypes of Vicia faba, an indeterminateand a ‘topless’. A third plant type, a determinateof which two to three inflorescences had been removed, was alsoincluded but behaved similarly to the intact plant. The root:shoot partitioning model predicts linear relationships betweenroot proportion and the product of air vapour pressure deficitand relative growth rate. Data from field experiments were consistentwith model prediction and coefficients estimated were similarfor both genotypes. Partitioning into pods was modelled as proportionalto the number of actively growing pods younger than 1000°Cd. Coefficients estimated were similar in both genotypes butdifferent between densities and years. In the indeterminategenotype, stem: leaf ratio was allometric throughout, whereasleaf growth ceased but stem growth continued in the determinateafter formation of the terminal inflorescence. Relatively moredry matter was allocated to stems than to leaves in high thanin low densities. In conclusion, the main differences in drymatter partitioning between genotypes concern leaf: stem partitioningduring early pod filling and pod partitioning due to pod numberdifferences. Partitioning, root, shoot, leaf, stem, Vicia faba L.     

Some Growth Regulators of Tobacco in Relation to the Symptoms of the Physiological Disease 'Frenching'

Extracrts of the shoot tips of normal and ‘frenched’tobacco plants were chemically separated into acidic, neutral,and basic ether–soluble fractions. On chromatograms ofthese, some plant growth regulators were assayed using the Avenacoleoptile section extension test. The acidic auxins and an acids and a neutral growth inhibitorwere found. One auxin, with the samew R_F value as indole-3-aceticacid, was four times more concentrated on normal as in ‘frenched’plants. No differences could be established between the twotypes of plants in regard to other growth regulators detected. It is argued that the symptoms of the physiological disease‘frenching’ could be explained in terms of a auxindeficiency.     

Mechanical Differences Between Free-standing and Supported Wheat Plants, Triticum aestivum L.

The effect of wind sway on the mechanical characteristics ofthe anchorage roots and the stem was investigated in maturewinter wheat (Triticum aestivumL., cv. Hereward). Wheat plantswere field-grown, either supported by a frame, which preventedwind sway, or unsupported (free-standing) and the morphologyand mechanical properties of the stems and the anchorage, ‘coronal’, roots were measured. Wind sway had little influence on either the stem height orear weight of the plants but did affect the mechanical propertiesof the stem. Stems of supported plants were weaker and moreflexible than the stems of free-standing plants. There werealso differences in the anchorage systems between the treatments:supported plants had just under half as many ‘coronal’ anchorage roots as the free-standing plants. This reducedthe anchorage strength of supported plants by a third. These differences in mechanical structure meant that the free-standingplants were more resistant to stem buckling and more resistantto anchorage failure. However, considering the difference inthe need for mechanical strength in plants from the two regimes,these differences were small. This suggests that wheat has inherentmechanical integrity and, as a monocotyledon with no secondarythickening, it differs little structurally between environments. Triticum aestivumL.; thigmomorphogenesis; anchorage; safety factor; mechanical stimulation     

Relative Sensitivity and Tolerance of some Gladiolus Cultivars to Sulphur Dioxide

Five Gladiolus cultivars, namely ‘Aldebaran’, ‘BrightEye’, ‘Illusion’, ‘Manisha’ and‘Manmohan’, were exposed to 1 and 2 µg l^–1sulphur dioxide to test their relative-sensitivity toleranceto the pollutant Plants were fumigated experimentally for 2h daily Foliar injury symptoms were observed first in ‘Manisha’followed by ‘Aldebaran’ and ‘Illusion’at the higher dose Photosynthetic pigments and leaf extractpH were significantly decreased, particularly in ‘Manisha’and ‘Illusion’ Overall disturbances in the plantmetabolism due to SO₂ treatment led to retarded growth of plants,as evident from decreased shoot length and phytomass valuesThe order of sensitivity of the five Gladiolus cultivars toSO₂ was as follows, with the greatest first Manisha, Illusion,Aldebaran, Bright Eye, Manmohan Cultivars, Gladiolus, sensitivity, sulphur dioxide, tolerance     

The Mechanics of Anchorage in Wheat Triticum aestivum L.: II. ANCHORAGE OF MATURE WHEAT AGAINST LODGING

The root system of mature wheat Triticum aestivum Marts Doveis dominated by the 7 to 15 adventitious roots which emergefrom the perimeter of the stem base, pointing radially outwardsand downwards. The basal, coronal region of these roots is thickand unbranched, attached to a rhizosheath of earth by a densecovering of root hairs and stiffened in bending by lignificationof outer layers of the cortex. Root lodging of plants involves bending of the coronal rootsat their base and axial movement of leeward and windward rootsthrough the soil; their resistance to these motions providemoments resisting lodging. A model of anchorage was producedby summing the resistance of each root to both forms of motionto give two anchorage components. The model was tested in aseries of mechanical experiments in which simulated lodgingwas followed by loading of individual roots; results supportedthe anchorage model and suggested that in the experimental conditionsthe two components of anchorage were approximately equal inmagnitude. The stem was about 30% stronger than the anchoragesystem. The coronal anchorage roots made up 4.4% of total dry mass;it is suggested that anchorage could be improved either by increasinginvestment in this region or by altering root orientation. Sequentialdevelopment of seminal and adventitious root systems is relatedto the changes in anchorage requirement with age.     

The Mechanics of Root Lodging in Winter Wheat, Triticum aestivum L.

The anchorage of winter wheat, Triticum aestivum L., is providedby a cone of rigid coronal roots which emerge from around thestem base. During root lodging this cone rotates at its windwardedge below the soil surface, the soil inside the cone movingas a block and compressing the soil beneath. A theoretical modelof anchorage suggested that lodging resistance should be dependenton the diameter of the root-soil cone, coronal root bendingstrength and soil shear strength. We tested the predictions of the anchorage model by carryingout two series of experiments. In the first, varieties of contrastinglodging resistances were artificially lodged. The moment requiredto rotate plants into the soil, the diameter of the root-soilcone, and the bending strength of the coronal roots were recorded.The lodging moment was correlated with the size of the soilcone, as predicted. Generally, differences in anchorage strengthbetween varieties were due to differences in root-soil conediameter, although coronal root strength was also important. A second series of tests was carried out using model plantsanchored by plastic discs. The behaviour of the models duringartificial lodging supported the anchorage model; the forceresisting lodging was similar to that of plants with root-soilcones of the same size and the resisting force was dependenton the soil strength. These results suggest that root lodging resistance might beimproved by increasing both the angle of spread and the bendingstrength of the coronal roots. Key words: Anchorage, root-soil cone, coronal roots, lodging, wheat     

A Comparison of Dry Matter Partitioning in Seminal and Adventitious Plants of Trifolium repens L

Trifolium repens has two types of root, one derived from theseed and the adventitious roots derived from the stolon nodes.It has been suggested that these two systems have differentpotentials for supporting growth. This paper presents a comparisonof plants grown on single seedling or adventitious roots anddemonstrates that although the shoot: root ratios for the twotypes differ this may be explained by differing shoot morphologies.Comparison of the lamina: root ratios for the two types of plantproduced no statistically significant differences and it isproposed that the two types of root system do not differ intheir relationship with leaf growth. A mechanism for large diameter‘tap’ root formation is suggested. white clover, Trifolium repens L., adventitious roots, seedling roots, shoot: root ratio     

Structural development and stability of rice Oryza sativa L. var. Nerica 1

The structural development of glasshouse-grown rice Oryza sativa L. var. Nerica 1 was studied in relation to its stability against lodging. The morphology and mechanical properties of both the stem and roots were examined from tillering, 4 weeks after transplantation up to maturity, together with plant weight distribution and anchorage strength. The "factors of safety" against root and stem failure were subsequently calculated throughout development. Rice plants showed similar morphology to wheat, although they possessed around twice as many tillers per plant and 10 times as many coronal roots. The mechanics of anchorage were also similar. The strength and rigidity of individual tillers increased throughout development as the plants grew taller and heavier and were around 15 times greater than in wheat. By contrast, individual root bending strength, the number of roots, and the anchorage strength levelled off earlier, and anchorage strength was only around twice that in wheat. Consequently, while the self-weight safety factor against stem failure was much higher than in wheat, increasing until late on in development from around 30 to 150, the self-weight safety factor against root anchorage failure was similar to wheat, decreasing from around 15 to 5. Consequently, plants subjected to anchorage tests always failed in their root system rather than their shoot system. The results suggest that, in the field, rice plants would be more likely to undergo root lodging than stem lodging, and that breeding efforts to reduce the incidence of lodging should act to strengthen the rather weak coronal roots.     

Uptake of 32P by Young Plants of Banksia hookeriana Meissner When Healthy and Infected with Phytophthora cinnamomi Rands

Young plants of Banksia hookeriana were grown in acid-washedsand with adequate phosphate and water supply, and a proportionwere inoculated with Phytophthora cinnamomi. There were no majordifferences in growth between uninoculated and infected plants,but there was a large increase in uptake of ³²P with increasingroot disease. In healthy plants ³²P uptake was greatest in youngleaf tissue, but in diseased plants labelled phosphate was directedmore towards older leaves where the activity was almost twicethat of young leaves. Enhanced uptake with disease was ascribed to possible blockageof the ‘message’ or ‘signal’ of phosphatetranslocation from shoot to root, such that the diseased rootincorrectly treated the shoot as P deficient and increased Puptake. Key words: Banksia hookeriana, Proteaceae, ³²P uptake, Phytophthora cinnamomi     

Patterns of Assimilate Distribution and Source--sink Relationships in the Young Reproductive Tomato Plant (Lycopersicon esculentum Mill.)

Patterns of distribution of ¹⁴C were determined in 47-day-oldtomato plants (Lycopersicon esculentum Mill.) 24 h after theapplication of [¹⁴C]sucrose to individual source leaves fromleaves 1–10 (leaf 1 being the first leaf produced abovethe cotyledons). The first inflorescence of these plants wasbetween the ‘buds visible’ and the ‘firstanthesis’ stages of development. The predominant sink organs in these plants were the root system,the stem, the developing first inflorescence and the shoot ‘apex’(all tissues above node 10). The contribution made by individualsource leaves to the assimilate reaching these organs dependedupon the vertical position of the leaf on the main-stem axisand upon its position with respect to the phyllotactic arrangementof the leaves about this axis. The root system received assimilateprincipally from leaf 5 and higher leaves, and the stem apexfrom the four lowest leaves. The developing first inflorescencereceived assimilates mainly from leaves in the two orthostichiesadjacent to the radial position of the inflorescence on thevertical axis of the plant; these included leaves which weremajor contributors of ¹⁴C to the root system (leaves 6 and 8)and to the shoot apex (leaves 1 and 3). This pattern of distributionof assimilate may explain why root-restriction treatments andremoval of young leaves at the shoot apex can reduce the extentof flower bud abortion in the first inflorescence under conditionsof reduced photoassimilate availability. Lycopersicon esculentum Mill, tomato, assimilate distribution, source-sink relationships     

Topophysis affects the Potential of Axillary Bud Growth, Fresh Biomass Accumulation and Specific Fresh Weight in Single-stem Roses (Rosa hybrida L.)

Topophysis, the effect on growth and differentiation of positionof axillary buds along the shoot, was studied by propagatingfive-leaflet-leaf single-node cuttings which were excised fromseven stem positions and grown as single stemmed plants. InRosahybrida ‘Korokis’ Kiss®, ‘Tanettahn’Manhattan Blue®, and ‘Sweet Promise’ Sonia®,following release of the buds from apical dominance by excision,morphogenetic development was studied until anthesis. The timefrom excision/planting until onset of bud growth, visible flowerbud appearance, and anthesis was generally shorter in plantsoriginating from apical bud positions than from basipetal positions.Topophysis mainly affected the onset of axillary bud growth;the earliest growth and development was found in cuttings fromthe second uppermost node position. This node tended to havethe lowest plastochron value, which indicated the existenceof a transition between sylleptic and proleptic buds. Stem lengthat visible flower bud and at anthesis generally increased asthe cutting position changed basipetally until the second lowestposition, and the number of five-leaflet-leaves at anthesisand the total number of nodes generally increased basipetally.For internode length, growth rate, and fresh biomass efficiencythe cuttings taken from the uppermost and lowermost positionsgenerally had significantly lower values than cuttings fromall medial positions. At anthesis, plants originating from cuttingsexcised from lower medial positions generally had a higher freshweight, greater flower stem diameter, and a significantly higherspecific fresh weight than those plants originating from apicalor basal positions. Among the cultivars, Sonia was the mostefficient in increasing fresh biomass and had the highest growthrate, whereas Manhattan Blue possessed the highest specificfresh weight, indicating a higher plant quality. It is suggestedthat topophysis inRosa is an independent phenomenon intrinsicto the axillary bud. apical dominance; axillary bud growth; fresh biomass accumulation; cut rose; flowering; Rosaceae; Rosa hybrida L.; rose; shoot growth; single-stem roses; specific fresh weight; topophysis; quality     

Growth and Dry-weight Distribution in Callistephus chinensis as Influenced by Lighting Treatment

Plants of two cultivars of Callistephus chinensis (Queen ofthe Market and Johannistag) were grown in 8 h of daylight perday with one of the following treatments given during the 16h dark period: (a) darkness—‘uninterrupted night’,(b) I h of light in the middle of the dark period—a ‘nightbreak’, (c) I min of light in every hour of the dark period—‘cycliclighting’, (d) light throughout—‘continuouslight’. The plants receiving uninterrupted dark periods remained compactand rosetted in habit with small leaves, while leaf expansion,stem extension, and flower initiation were promoted in all threeillumination treatments (b, c, d). Although these three treatmentsproduced similar increases in leaf area, continuous light wasthe most effective for the promotion of both stem growth andflower initiation while cyclic lighting was generally more effectivethan a I-h night break. Continuous light also caused more dry matter to be divertedto stems at any given vegetative dry weight and it was shownthat the stem weight ratio of both varieties was correlatedwith stem length.     

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     

Anchorage Mechanics of the Tap Root System of Winter-sown Oilseed Rape (Brassica napus L.)

The anchorage mechanics of mature winter-sown oilseed rape (‘Envol’)were investigated by combining a morphological and mechanicalstudy of the root system with anchorage tests on real and modelplants. Oilseed rape plants were anchored by a rigid tap root;the few laterals all emerged below the centre of rotation ofthe root system (approx. 30 mm below the soil surface). Whenplants were pulled over, the tap root bent and the top 30 mmmoved in the soil towards the direction of pull, creating acrevice on the opposite side. The maximum anchorage moment was2.9 ± 0.36 N m. Two main components of anchorage wereidentified: the bending resistance of the tap root and the resistanceof the soil on the near side to compression. The relative importanceof these components was determined by measuring both the bendingresistance of the tap root, and the resistance of metal tubesof varying diameter, inserted to various depths in the soil,to being pulled over. These tests showed that the tap root bendingmoment at failure could account for around 40% of anchoragemoment, while soil resistance could account for around 60%.The model tests on the tubes also help to shed light on theway in which the dimensions of tap roots will influence theiranchorage capability. Copyright 2001 Annals of Botany Company Anchorage, lodging, root bending resistance, mechanical properties, oilseed rape, Brassica napus L     

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     

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