An easy method of estimating the stress-relaxation parameters T0 and b

Mechanical properties of the cell wall are represented by relaxationparameters. This paper describes an easy method of estimatingthe stress-relaxation parameters T₀ and b, in which they canbe calculated using stress-relaxation data directly obtainedwith a simple tensile tester. The value of A is calculated fromthe relaxation data at time = 1, 2, 3 and 5 sec, and the valueof T₀ is calculated by multiplying b by T, which is calculatedfrom the data at time = 0, 1 and 2 sec. (Received April 27, 1976; )     

Unchanged Molecular-Weight Distribution of Xyloglucans in Outer Tissue Cell Walls along Intact Growing Hypocotyls of Squash (Cucurbita maxima Duch.) Seedlings

The changes in the mechanical properties and compositions ofcell walls in outer and inner tissues were investigated alongthe hypocotyls of squash (Cucurbita maxima Duch.) seedlings.The endogenous growth capacity decreased and the minimum stress-relaxationtime (T_O) of cell walls in outer tissues increased from theapical to the basal region of hypocotyls. A high correlationwas observed between values of To in outer tissues and endogenousgrowth (r=–0.99). The values of T_O in inner tissues didnot change from the apical to the basal region of hypocotyls. In outer tissues, the levels of neutral sugars in pectin decreasedconsiderably from the apical to the basal region of hypocotyls.However, relative amounts of hemicellulose showed little differencealong the hypocotyls. Levels and molecular weights of hemicellulosicxyloglucans in outer tissues were about 2-3 times greater thanthose in inner tissues. The amount of xyloglucans in outer tissuesincreased in the middle region of hypocotyls, and xyloglucansin upper and basal regions had similar molecular weights. Bycontrast, in inner tissues, amounts of cell-wall material decreasedtoward the basal region. Amounts and molecular weights of hemicellulosicxyloglucans also decreased along the hypocotyls. These results clearly show that cell-wall metabolism duringaging of intact growing stem tissues differs markedly betweenouter and inner tissues, and the absence of a simple relationship between the molecular weights of xyloglucans and the mechanicalproperties of the cell walls in outer tissues indicates thatthe changes in the mechanical properties of the cell walls inintact growing tissues cannot be explained only by the molecularweights of xyloglucans. Thus, the regulation of the mechanicalproperties of cell walls in intact growing stems may be somewhatdifferent from that in auxin-treated stem sections, in whichauxin promotes the depolymerization of xyloglucan molecules. (Received November 28, 1991; Accepted November 16, 1992)     

Effect of Hydrogen Peroxide on Degradation of Cell Wall Associated Proteins in Growing Bean Hypocotyls

The possible involvement of active oxygen species and an apoplasticendopeptidase (EP) in the digestion of cell wall proteins wasstudied in extracellular fluid (EF) from hypocotyls of Phaseolusvulgaris at different stages of elongation. EF proteins underwentsignificant changes in polypeptide pattern during hypocotylgrowth, which were characterized by increases in 35, 39, 40and 50 kDa peptides and appearance of 61, 70 and 75 kDa peptidesat the exponential growth phase. EFs also contain endopeptidase[Gómez et al. (1994) Agriscientia 11:3]. Autolysis experimentswithout or with purified EP revealed that many cell wall polypeptidesare liable to degradation by the protease. Besides, EF polypeptidesincreased their susceptibility to EP during hypocotyl elongation.The 50 and 40 kDa polypeptydes were poorly degraded when extractedfrom hypocotyls in active growth, but greatly hydrolyzed whenextracted from fully elongated tissues, suggesting that in thecourse of growth proteins underwent modifications that renderedthem more prone to proteolytic attack. These modifications seemedto involve active oxygen species, as indicated by: (a) H₂O₂level rised when protein susceptibility to EP increased; and(b) EF proteins from growing hypocotyls (comparatively lesssusceptible to EP) treated with H₂O₂ were rapidly degraded bythe protease. (Received April 27, 1995; Accepted July 31, 1995)     

Monitoring Primary Response to Chilling Stress in Etiolated Vigna radiata and V. mungo Seedlings Using Thermal Hysteresis of Water Proton NMR Relaxation Times

Thermal hysteresis of longitudinal relaxation times (T₁) ofwater protons in hypocotyls of etiolated Vigna radiata and V.mungo seedlings was investigated by pulse nuclear magnetic resonance(NMR) spectroscopy. Various lengths of chilling exposures duringa cool-warm cycle between 20 and 0?C (below 10?C, about 4 h)for the T₁ hysteresis measurement did not cause any visibleinjury symptoms in hypocotyls. However, the profiles of T₁ hysteresisvaried as a result of different chilling exposures. The sumsof the T₁ ratio (for detail see Introduction) reflecting T₁prolongation or shortening upon the warming process were a goodquantitative index for the extent of T₁ hysteresis, and thewide dispersion of this value ranging on the "minus" side (T₁prolongation upon warming) suggested the occurrence of a primaryresponse of cells to chilling stress before obvious visiblesymptoms occur while the T₁ ratio sums on the "plus" side (T₁shortening upon warming) corresponded to a response of seriousvisible injury. Therefore, the sums of the T₁ ratio can be usedas a non-destructive diagnostic tool for monitoring the primaryevent of chilling injury when lacking any visible injury symptoms.The data indicate that the critical temperature for the occurrenceof primary response for chilling stress was around 7.5?C forV. radiata and 12.5?C for V. mungo. (Received February 1, 1988; Accepted June 1, 1988)     

Water Potential and Mechanical Properties of the Cell Wall of Hypocotyls of Dark-Grown Squash (Cucurbita maxima Duch.) under Water-Stress Conditions

Hypocotyl growth of seedlings of dark-grown squash (Cucurbitamaxima Duch.) was greatly reduced by the addition of 60mM polyethyleneglycol (PEG) to hydroponic solution (water stress). Apoplastic solution (A) and cell sap (C) were separately collectedfrom the hypocotyl segments by a centrifugation method. Theosmotic potentials of A (_A) and C (_c), and (=_c–_A) ofstressed hypocotyls were always lower than those of unstressedhypocotyls. Suction force was measured by immersing the segments into solutionsof different concentrations of mannitol. Suction force was significantlycorrelated with _C (r= –0.99). The mechanical properties of the cell wall of hypocotyl segmentswere measured by stressrelaxation technique. Minimum stressrelaxation time (T_o), relaxation rate (R) and residual stressof unstressed hypocotyls were low during the growth period andincreased when the growth ceased. T_o and R of stressed hypocotylsdecreased one day after the stress treatment, but the residualstress was not decreased by the water stress throughout theexperiment. These results suggest that the suppressed growth of dark-grownsquash hypocotyls under water stress was due neither to thereduction of the osmotic potential difference between innerand outer space of the cell, nor to the decrease in suctionforce, but was partly due to the unchanged mechanical propertiesof the cell wall, as represented by one stress-relaxation parameter,residual stress. (Received February 5, 1988; Accepted September 8, 1988)     

Stress relaxation properties of the cell wall of tissue segments under different growth conditions

Stress-relaxation parameters were compared under different experimentalconditions using 5th internode segments of light-grown pea seedlingsand coleoptile segments of dark-grown Avena seedlings. The followingresults were obtained. 1. In a short incubation period at 25?C, IAA caused a decreasein the minimum relaxation time, To, of the epidermal cell wallof pea internodes when it induced elongation; the optimum concentrationof IAA for decreasing To was 10 mg/liter. 2. At all concentrations of IAA used, 0.1–1000 mg/liter,the relationship between the To value of the epidermal cellwall peeled from segments incubated for 2 hr and the subsequentelongation rate in 2–3 hr incubation was linear, indicatingthat the To value of the cell wall at a certain time regulatesthe rate of the following elongation. 3. When segments of pea epicotyls or Avena coleoptiles wereincubated in mannitol solution of various concentrations inthe presence and absence of IAA and then allowed to grow inthe absence of both mannitol and IAA, the segments extendeddifferently depending upon the mannitol concentration, whichwas less than 0.3 M, given during preincubation. 4. The T_o and b (relaxation rate, S/log t) values were smallerin the cell wall of segments which extended more, than in thosewhich extended less. In this case, 0.2 M mannitol solution wasmost effective, since it inhibited IAA-induced elongation duringpre-incubation and the segments thus incubated extended themost afterward. 5. Extensibility, mm/gr, seemed to parallel the elongation whichhad occurred during pre-incubation, indicating that this value,contrary to To, represented at least partly the result of elongation. From these results we concluded that the growth rate to followis regulated by the minimum stress relaxation time, To, andpossibly by the relaxation rate, b, of the cell wall beforeextension, and these parameters may represent certain biochemicalmodifications of the cell wall components needed for cell extension. (Received August 12, 1974; )     

Factors Affecting the Prolongation of NMR Relaxation Times of Water Protons in Leaves of Woody Plants Affected by Formation of Insect Galls

Changes in NMR relaxation times (T₁) of water protons and watercontents of leaves of woody plants affected by formation ofinsect galls were studied in Machilus, Zelkova and Cinnamomumparasitized with a gall-midge, an aphid and two different Triozinepsyllids, respectively. The presence of galls in Machilus leavesincreased both T₁ and water contents in the galled leaf tissues,while such tissues in Zelkova showed only increases in T₁. Similartrends for both parameters were also observed in gall-bearingleaf tissues of Machilus and Cinnamomum, with galls caused bytwo different psyllids. It seems that it is the particular characteristicsof leaf tissues of the host plant that determine whether thesystemic effect of the presence of galls is reflected both inT₁ and in water content, or only in T₁. Histologic features,including the presence of tannins in and leakage of electrolytesfrom these materials, were compared with those of normal (ungalled)leaves to determine possible causative factors involved in theprolongation of T₁ relaxation times that were associated withthe presence of insect galls. The eco-physiological implicationof tannins with respect to the host-parasite relationship isalso discussed. (Received October 27, 1989; Accepted April 24, 1990)     

Time, Temperature and Germination of Pearl Millet (Pennisetum typhoides S. & H.): I. CONSTANT TEMPERATURE

The germination of pearl millet (Pennisetum typhoides S. &H.) seeds was investigated at constant temperatures between12 ?C and 47 ?C on a thermal gradient plate. The rate of germination increased linearly with temperaturefrom a base T_b to a sharply defined optimum T_o beyond whichthe rate decreased linearly with temperature, reaching zeroat T_m. The linearity of the response both above and below T_oallowed time and temperature to be combined in a thermal timeat which a specified fraction of the seeds germinated. Withinthe population T_b and T_m were constant.     

Cessation of cell elongation in rye coleoptiles is accompanied by a loss of cell-wall plasticity

The mechanism by which endogenous cessation of coleoptile elongationafter emergence of the primary leaf is brought about was investigatedin rye seedlings (Secale cereale L.) that were either grownin darkness or irradiated with continuous white light. In 3-d-oldetiolated (growing) coleoptiles a turgor pressure of 0.59 MPawas measured. In 6-d-old coleoptiles, which had ceased to elongate,cell turgor was 0.51 MPa and thus only 13% lower than in therapidly growing organ. Hence, the driving force for growth (turgor)is largely maintained. Cell-wall plasticity (E_pl) and elasticity(E_Ql were determined with a constant load extensiometer bothin vivo (turgid coleoptile segments) and in vitro (frozen-thawedsamples). Cessation of coleoptile elongation was correlatedwith a 95% reduction in E_pl9 whereas E_Ql was only slightly affected.Extension kinetics were measured with living and frozen-thawedsegments cut from growing and non-growing coleoptiles. The correspondingstress-strain (load-extension) curves indicate that the cellwall of the growing coleoptile behaves like an elastic-plasticmaterial whereas that of the non-growing organ shows the behaviourof an elastic solid. These data demonstate that E_pl representsa true plastic (irreversible) deformation of the cell wall.It is concluded that cessation of coleoptile growth after emergenceof the primary leaf is attributable to a loss of cell-wall plasticity.Hence, a mechanical stiffening of the cell wall and not a lossof turgor pressure may be responsible for the deceleration ofcell elongation in the rye coleoptile. Key words: Extension growth, rye coleoptile, cell-wall extensibility, turgor pressure     

The Influence of Temperature on Seed Germination Rate in Grain Legumes: II. INTRASPECIFIC VARIATION IN CHICKPEA (Cicer arietinum L.) AT CONSTANT TEMPERATURES

Positive linear relationships were shown between constant temperaturesand the rates of progress of germination to different percentiles,G, for single populations of each of five genotypes of chickpea(Cicer anetinum L.). The base temperature, T_b, at which therate of germination is zero, was 0·0°C for all germinationpercentiles of all genotypes. The optimum temperature, T_o(G),at which rate of germination is most rapid, varied between thefive genotypes and also between percentiles within at leastone population. Over the sub-optimal temperature range, i.e.from T_b to T_o(G), the distribution of thermal times within eachpopulation was normal. Consequently a single equation was appliedto describe the influence of sub-optimal temperatures on rateof germination of all seeds within each population of each genotype.The precision with which optimum temperature, T_b(G), could bedefined varied between populations. In each of three genotypesthere was a negative linear relationship between temperatureabove T_b(G) and rate of germination. For all seeds within anyof these three populations thermal time at supra-optimal temperatureswas constant. Variation in the time taken to germinate at supra-optimaltemperatures was a consequence of normal variation in the ceilingtemperature, T_o(G)—the temperature at or above which rateof progress to germination percentile G is zero. A new approachto defining the response of seed germination rate to temperatureis proposed for use in germplasm screening programmes. In two populations final percentage germination was influencedby temperature. The optimum constant temperature for maximumfinal germination was between 10°C and 15°C in thesepopulations; approximately 15°C cooler than the optimumtemperature for rate of germination. It is suggested that laboratorytests of chickpea germination should be carried out at temperaturesbetween 10°C and 15°C. Key words: Chickpea, seed germination rate, temperature     

Growth and cell wall changes in azuki bean epicotyls I. Changes in wall polysaccharides during intact growth

Measurement of endogenous growth rates and the mechanical propertyof the cell wall in various regions of light-grown azuki beanepicotyls revealed diat the minimum stress-relaxation time (T_o)was the shortest in the upper region (0–30 mm below theapex) of the epicotyl, where vigorous endogenous growth tookplace, and became longer toward the basal region, which wasmature and not growing. In the upper region of the epicotyl, a lower percentage of a-celluloseand a higher percentage of pectic substances than in the lowerregion were found. The percentage of hemicellulose content wasalmost constant over the whole epicotyl. Major components ofnoncellulosic neutral sugars in the cell wall were galactoseand xylose. The percentage of the galactose content to the noncellulosicpolysaccharide was highest in the upper region and lowest inthe basal region of the epicotyl, and a clearly negative correlationbetween the galactose composition and the T_o value was obtained.On the contrary, the percentage of die xylose content was highestin the basal region and lowest in die upper region, and a clearlypositive correlation between die xylose composition and theT_o value was obtained. During die endogenous growth of die intactepicotyl, all die neutral sugars, particularly galactose, increasedin die upper region, whereas in die middle and basal regions,only xylose increased. Similar changes in sugar compositionswere observed during IAA-induced elongation of die segment excisedfrom various regions of die epicotyl. (Received July 27, 1978; )     

The role of temperature in the seed germination of two species of the Solanum nigrum complex

The germination behaviour of S. nigrum L. and S. physalifoliumRusby var. nitidibacatum (Bitter) Edmonds is compared, basedon temperature requirements during imbibition. Three seed lotsof S. nigrum had base temperatures (T_b) between 7.5C and 10C,showing a lower T_b when the period of freezing days, duringwhich each population was collected, was reduced. S. physalifoliumhas a higher value for T_b at constant temperatures (21C) thatcan be interpreted as a dormancy constraint. This constraintis released by alternating temperatures at amplitudes exceeding5C and with the high temperature above 21C by apparently reducingT_b to 12.5C. This implies that for S. physaiifolium temperature has a dualeffect on germination. It is the driving force for changes indormancy, but germination also depends on the temperature. Therole of temperature for S. nigrum is simpler: each populationcollected showed differences in the thermal time required forgermination that could be related to the temperature regimenof the original environment. Key words: Dormancy, S. nigrum, S. physalifolium, thermal time     

Effects of hypergravity on growth and cell wall properties of cress hypocotyls

Elongation growth of etiolated hypocotyls of cress (Lepidiumsativum L.) was suppressed when they were exposed to basipetalhypergravity at 35 g and above. Acceleration at 135 g causeda decrease in the mechanical extensibility and an increase inthe minimum stress-relaxation time of the cell wall. Such changesin the mechanical properties of the cell wall were prominentin the lower regions of hypocotyls. The amounts of cell wallpolysaccharides per unit length of hypocotyls increased underthe hypergravity condition and, in particular, the increasein the amount of cellulose in the lower regions was conspicuous.Hypergravity did not influence the neutral sugar compositionof either the pectin or the hemicellulose fraction. The amountof lignin was also increased by hypergravity treatment, althoughthe level was low. The data suggest that hypergravity modifiesthe metabolism of cell wall components and thus makes the cellwall thick and rigid, thereby inhibiting elongation growth ofcress hypocotyls. These changes may contribute to the plants'ability to sustain their structures against hypergravity. Key words: Cell wall extensibility, cellulose, hypergravity, Lepidium sativum L., lignin     

Development of Epidermal Cell Wall Peroxidases along the Mung Bean Hypocotyl: Possible Involvement in the Cell Wall Stiffening Process

Goldberg, R., Liberman, M., Mathieu, C, Pierron, M. and Catesson,A. M. 1987. Development of epidermal cell wall peroxidases alongthe mung bean hypocotyl: possible involvement in the cell wallstiffening process.—J. exp. Bot. 38: 1378–1390. Ultrastructural investigation showed that in the epidermis ofmung bean hypocotyls, cell wall peroxidatic activities couldbe detected mainly below the maximal elongation zone. In theepidermis the peroxidatic activities were preferentially locatedin the radial cell walls. Cell wall peroxidases were then isolatedfrom epidermal strips and further characterized. The possiblepresence of a H₂O₂-generating system in the epidermis of mungbean hypocotyls was also investigated. When whole segments wereprocessed for electron microscopy, H₂O₂ could be detected cytochemicallyin the cell walls with the CeCl₃ technique. A positive reactionwas obtained in the same location when specimens were incubatedin a 3-3'-diaminobenzidine medium for peroxidases in which H₂O₂was replaced by its possible precursors (NADH or NAD + malate).However, isolated epidermal cell walls could not generate H₂O₂at the expense of NADH although they were able to oxidize thereduced nicotinamide-adenine-dinucleotide. The possible relationshipsbetween peroxidase activities, H₂O₂, and Ca²⁺ ions are discussedwith respect to their involvement in the cell wall stiffeningprocess. Key words: Epidermis, cell wall, elongation, peroxidases     

Growth of Plants in Different Oxygen Concentrations

Dwarf french beans (Phaseolus vulgaris var. Canadian Wonder)were grown in chambers at 25?C with the roots aerated at 20per cent oxygen and tops variously maintained at: T₁ O₂ 0.21;CO₂ 270?10^–6: T₂; O₂ 0.05, CO₂, CO₂ 270?10^–6: T₃;O₂ 0.21; CO₂ 550?10^–6. Experiment 1 (T₁ and T₂) lasted2 weeks: Experiment 2 (T₁ T₂ and T₃) only one week. Hourly estimatesof CO₂ uptake were made by gas analysis and weekly estimatesof fresh weight, dry matter in tops and roots, and leaf area,by sampling. Light intensity was 80 W m^–2 of photosyntheticallyactive radiation. An attempt was made to explain the results in terms of a simplelight absorption model such that where dV/dt is the rate of CO₂ uptake per plant, ßis the photosynthetic efficiency, I₀ is the incident light intensity,f is the fraction of incident light absorbed by unit leaf layerand L is the leaf area index. The analysis showed that ß(T₂)was at least double ß(T₁), whilst f(T₂) was smallerthan f(T₁) at a given leaf area. The results also required thatthroughout the period of the experiment, fL(T₁)=fL(T₂) at anygiven time, i.e. the treatment with the larger leaf area (T₂)has the smaller value of f, and therefore intercepts less lightper unit leaf area. This could be advantageous for plant growth,but requires further experiments. The photosynthetic rates per unit leaf are about 40 per centgreater in T₂ than T₁. Over the relatively short period of the experiment the resultsare adequately described by U=btⁿ, where U is the accumulatedcarbon dioxide uptake, b is related to the photosynthetic efficiency(different for the differing treatments), and n is a constant(similar for all treatments). This relationship with time isbelieved to be a relationship with accumulated radiation, forthe light was constant throughout the experiments. Comparisons of carbon fixed (measured gas uptake) and dry matteraccumulation (sampling) show great scatter with an average valueof 0.43. The first week's results were generally smaller thanthis value and the second week's greater. Energy fixation as a fraction of photosynthetically active radiationon the ground area covered by the plants ranged from 3.5 to10 per cent. The results from treatment T₃ were similar to T₂ suggestingthat increasing CO₂ concentration decreases the growth inhibitionat 21 per cent O₂.     

Studies on the Development of the Air Pores and Air Chambers of Marchantia paleacea: 1. Light Microscopy

The ontogeny of the air pores and air chambers of Marchantiapaleacea begins with the schizogenous development of protodermalintercellular spaces of the initial apertures, and is completedwith the formation of the air pores and giant sub-epidermalair chambers bearing numerous photosynthetic filaments. Intercellularspace formation commences from the thallus surface and proceedsinwards to the first internal layer of cells. The cells amongwhich spaces develop do not originate from one mother cell.Spaces are formed only in the regions of the intersection ofthe anticlinal walls of three, four, or sometimes more successivederivatives of S₁, S₃ and S₄ segments of the apical cell, oneor two of which have been divided periclinally and the restanticlinally. Protodermal intercellular spaces appear in mostor all the corners of these cells, the anticlinal walls of whichexhibit an opposite disposition. The S₁, S₂, S₃ and S₄ segmentsare produced by definite divisions of a five-sided apical celland by a series of divisions give rise to initial cells of theinternal layers of the thallus and initial cells of the protodermaland sub-protodermal layers. The concept of a quiescent apicalcell cannot be accepted, since dividing apical cells have beenobserved, and the pattern of wall disposition of the thallusapex cannot be explained without the active participation ofthe apical cell. The air chambers are apparently of exogenous origin. They resultfrom the broadening of the bottom of the initial apertures bythe coordination of the rate of anticlinal divisions and growthof the protodermal and sub-protodermal cells surrounding theintercellular spaces of the initial apertures. The ontogenyof the pore rings starts at an advanced stage of air chamberformation not from a mother cell but from the cells which surroundthe closed entrance of the air chamber, by a shift of the planeof division from anticlinal to periclinal. Before the periclinaldivisions a new axis of growth perpendicular to the thallussurface is established in the mother cells of the pore. By a polarized growth into the air chamber followed by periclinaldivisions, the cells of the floor form initial cells of thephotosynthetic filaments. The latter divide again to form singleor branched photosynthetic filaments. Marchantia paleacea, air pore, air chamber     

The Influence of Temperature on Seed Germination Rate in Grain Legumes: I. A COMPARISON OF CHICKPEA, LENTIL, SOYABEAN AND COWPEA AT CONSTANT TEMPERATURES

For a single seed population of each of four species of grainlegume positive linear relationships were shown between temperatureand rate of germination for different fractions (G) of eachpopulation, from a base temperature, T_b(G), at which germinationrate is zero, to an optimum temperature, T_o(G) at which germinationrate is maximal. At constant temperatures warmer than T_o(G)there were negative relations (probably linear) between temperatureand rate of germination to the maximum temperature for germination,T_m(G), Within each population T_b(G) did not differ, but it didvary between species, viz.0.0?C, 0.25?C, 4.and 8.5?C for chickpea(Cicer arietinum L.), lentil (Lens culinaris Medic.), soyabean(Glycine max [ Merr.) and cowpea (Vigna unguiculata [L.] Walp.),respectively. In contrast, T_o(G) varied both within each populationand also between the four species: 80% of seeds in each populationhad T_o(G) values within the range 31.8?C to 33.8 ?C, 24.0?Cto 24.4?C, 34.0?C to 34.5?C and 33.2?C to >40?C, respectively.Values of T_m(G) were much more vanable: the 80% population rangewas 48 .0?C to 60.8?C for chickpea, 31.8?C to 34.4?C for lentiland 46.8?C to 55.2?C for soyabean; reliable estimates couldnot be made for cowpea, but the results suggest higher and morevariable values of T_m(G) than in the other three species. Atsub-optimal temperatures the distribution of thermal time forthe different fractions of each population was normal, exceptfor lentil where it was log-normal. A single equation is proposedto describe the influence of sub-optimal temperatures on ratesof germination for whole seed populations. At supra-optimaltemperatures, variation in thermal time for the different fractionsof each population was only slight. The implications of thesefindings for the adaptation of grain legume crops to differentenvironments, and for the screening of germplasm, are discussed. Key words: Seed germination rate, temperature, grain legumes     

The Influence of Temperature on Seed Germination in Cultivars of Common Bean

Common bean (Phaseolus vulgaris L.) is grown over a wide rangeof environments, including sites with low or high soil temperaturesat sowing time. To describe the temperature responseof seedgermination, 20 bean genotypes were evaluated using a rolledpaper towel system with 11 constant temperatures ranging from12 to 34 °C. Germination response was characterized by fittingcumulative counts using a maximum-likelihood analysis. Rateof germination increased from abase temperature (T_b) typicallynear 8 °C to an optimal development temperature (T_o) of29 to 34 °C. T_b did not differ among common bean genotypes.Mesoamerican germplasm showed slightlyhigher T_o than Andeangermplasm, but there was large variation in T_o within each ofthe two gene pools. The single accession of tepary bean (P.acutifolius) evaluated appeared to be the mosttolerant to highgermination temperatures. Key words: Common bean, seed germination rate, temperature     

Formation of Isodityrosine by Peroxidase Isozymes

Fry, S. C. 1987. Formation of isodityrosine by peroxidase isozymes.—J.exp. Bot. 38: 853–862. Tyrosine residues of extensin are oxidatively coupled in vivoto form isodityrosine bridges, whereas treatment of purifiedextensin with H₂O₂+ peroxidase in vitro yields only dityrosine.Two explanations for the correct mode of coupling in vivo weretested. The first, that the pH of the cell wall is lower thanthat (pH 9-0) at which in vitro experiments have been conducted,provided part of the answer since treatment of L-tyrosine withH₂O₂+peroxidase in vitro at pH 37–5 yielded some isodityrosine.The second, that the wall contains other isozymes of peroxidasethan the basic isozyme usually studied in vitro, appeared unlikelybecause several sharply contrasting isozymes yielded similarisodityrosine: dityrosine ratios from L-tyrosine+ H₂O₂ at anygiven pH. The isozymes were also similar in their ability tooxidize tyrosine-dimers further to higher polymers. It is concludedthat the formation of isodityrosine in vivo is dictated by neighbouringwall molecules, possibly ionically-bound pectins, which modifythe local environment of the tyrosine residues of extensin. Key words: Isodityrosine, peroxidase isozymes, extensin     

Effect of indole-3-acetic acid on cell wall loosening: Changes in mechanical properties and noncellulosic glucose content of Avena coleoptile cell wall

The effect of indole-3-acetic acid on cell wall loosening andchemical modifications of noncellulosic components of the cellwall in Avena coleoptile segments was studied and the followingresults were obtained. (1) Auxin decreased both the minimum stress-relaxation time(T_o) and the noncellulosic glucose content of the cell wall. (2) Decreases were observed in the absence or presence of mannitolsolution at concentrations lower than 0.20 M which osmoticallysuppressed auxin-induced extension, while at concentrationshigher than 0.25 M, there was little auxin effect, indicatingthat it is turgor-dependent. (3) The decrease in T_o of the cell wall and that in the noncellulosicglucose content caused by auxin in the presence of mannitolsolutions of various concentrations paralleled each other (thecorrelation coefficient was 0.897). (4) Both decreases in T_o and glucose content caused by auxinwere inhibited by nojirimycin (5-amino-5-deoxy-D-glucopyranose)in the presence of mannitol. The results suggest that auxin-induced cell wall loosening iscaused by the degradation of noncellulosic rß-glucanin the cell wall. (Received December 24, 1976; )