A freeze-etch study of the effects of extracellular freezing on cellular membranes of wheat

Seedlings of Triticum aestivum L. cv. Lennox were grown in different environments to obtain different hardiness. Pieces of laminae and leaf bases were slowly cooled to sub-zero temperatures and the damage caused was assessed by an ion-leakage method. Comparable pieces of tissue were slowly cooled to temperatures between 2° and-14°C and were then freeze-fixed and freeze-etched. Membranes generally retained their lamellar structures indicated by the abundance of typical membrane fracture faces in all treatments, and some membrane fracture faces had patches which lacked the usual scattering of intramembranous particles (IMP). These IMP-free areas were present in the plasma membrane of tissues given a damaging freezing treatment, but were absent from the plasma membrane of room-temperature controls, of supercooled tissues, and of tissues given a non-damaging freezing treatment. The frequency of IMP-free areas and the proportion of the plasma membrane affected increased with increasing damage. In the most damaged tissue (79% damage; leaf bases exposed to-8°C), 20% of the plasma membrane was IMP-free. The frequencies of IMP at a distance from the IMP-free areas were unaffected by freezing treatments. There was a patchy distribution of IMP in other membranes (nuclear envelope, tonoplast, thylakoids, chloroplast envelope), but only in the nuclear envelope did it appear possible that their occurrence coincided with damage. The IMP-free areas of several membranes were sometimes associated together in stacks. Such membranes lay both to the outside and inside of the plasma membrane, indicating that at least some of the adjacent membrane fragments arose as a result of membrane reorganization induced by the damaging treatment. Occasional views of folded IMP-free plasma membrane tended to confirm this conclusion. The following hypothesis is advanced to explain the damage induced by extracellular freezing. Areas of plasma membrane become free of IMP, probably as a result of the freezing-induced cellular dehydration. The lipids in these IMP-free patches may be in the fluid rather than the gel phase. The formation of these IMP-free patches, especially in the plasma membrane, initiates or involves proliferation and possibly fusion of membranes, and during or following this process, the cells become leaky.Abbreviations EF exoplasmatic fracture face - IMP intramembranous particles - PF protoplasmatic fracture face     

The Supramolecular Organization of Red Algal Vacuole Membrane Visualized by Freeze-fracture

The supramolecular organization of the vacuole membrane (orof the membranes of mucilage sacs) in 27 species of red algaeis studied in replicas of rapidly frozen and fractured cells.Intramembranous particle complexes composed of four particles('tetrads' with average diameters between 8·5 and 14·5have been observed in the protoplasmic fracture (PF) face butmost clearly and more frequently in the exoplasmic fracture(EF) face of the vacuole membrane of all red algae investigated.The tetrads lie individually within the vacuole membrane orform clusters in several species and are randomly distributed.In the species Ceramium diaphanum var. strictum and Laurenciaobtusa the intramembranous particle complexes ('tetrads') havebeen observed both in the EF and PF faces of the vacuole membrane;the 'membrane tetrads' at least as regards these two speciesseem to span both the outer and inner leaflets of the vacuolemembrane ('transmembrane particles'). The occurrence of particletetrads in the plasma membrane is probably due to exocytosiseither of the Golgi vesicles or of the mucilage sacs. Tetradfrequency in the EF face of the vacuole membranes of the investigatedred algae varies between 2 and 87 µm^-2, while that ofsingle particles varies between 102 and 695 µm^-2. ThePF face of the vacuole membrane is characterized by a higherparticle density than the EF face. The particle densities ofthe PF and EF faces of the plasma membrane for a given speciesare higher than those of the corresponding fracture faces ofthe vacuole membrane. Some members of Bangiophycidae bear smallerprotein particles (diameter between 8·5 and 10·5nm) in comparison with those of Florideophycidae (diameter between10·5 and 14·5 nm). It is suggested, based uponthe particle tetrads lying in depressions of the vacuole membraneand the origin of vacuoles (mucilage sacs) from ER, that theparticle tetrads originate from the ER or the Golgi complex.Since vacuoles (mucilage sacs) in red algae, along with theGolgi complex, are involved in the synthesis and export of cellsurface polysaccharides, it could be assumed that the 'membrane-tetrads'within the vacuole membrane represent a membrane-bound multienzymecomplex, participating in the synthesis of amorphous extracellularmatrix polysaccharides.Copyright 1993, 1999 Academic Press Red algae, freeze-fracture, vacuole membrane, mucilage sacs, membrane tetrads, supramolecular organization     

Cold-Induced Alterations in Plasma Membrane Proteins That are Specifically Related to the Development of Freezing Tolerance in Cold-Hardy Winter Wheat

The objective of this study was to identify plasma membraneproteins that are specifically induced by cold acclimation inwheat (Triticum aestivum L.). Two cultivars with a marked differencein the genetic ability to cold-acclimate, namely, spring wheat(cv. Chinese Spring) and winter wheat (cv. Norstar), were usedas the experimental material. After four weeks of growth ina cold chamber, the freezing tolerance in the shoots of winterwheat increased to –18°C, whereas it increased onlyto –8°C in the shoots of spring wheat. In the caseof roots from both cultivars, freezing tolerance increased onlyslightly after the growth in the cold environment. Cold acclimationinduced remarkable changes in the electrophoretic patterns ofplasma membrane proteins which depended on both the cultivarand the tissue examined. Levels of polypeptides with molecularmasses from 22 to 31 kDa decreased in both the root and shootplasma membranes from both cultivars. Among these polypeptides,levels of those of 28 and 26 kDa decreased abruptly after oneweek of cold acclimation. By contrast, levels of polypeptidesof 89, 83, 52, 23, 18 and 17 kDa increased specifically in theshoots of winter wheat. The increases in the levels of the 23-,18- and 17-kDa polypeptides were proportional to the developmentof freezing tolerance. Freeze-fracture electron microscopy ofplasma membranes from shoot cells revealed that the number ofintramembrane particles on the fracture faces decreased markedlyin winter wheat after cold acclimation, but to a lesser extentin spring wheat. These results suggest that the plasma membranesmight undergo molecular reorganization during cold acclimation. ¹Contribution no. 3709 from the Institute of Low TemperatureScience, Hokkaido University.     

Lipids,amino acids,sugars, hardiness and growth of Festuca arundinacea

Tall fescue (Festuca arundinacea Schreb. cv S.170) plants were grown in environments differing only in temperature: 6/4, 16/14 or 21/19°. The content of total and individual sugars and amino acids in leaf laminae and roots did not relate closely to the hardiness of the organ. The unsaturation of lipid fatty acids alone was clearly unrelated to hardiness because the difference in unsaturation was greater in the neutral lipids, glycolipids and phospholipids from roots than from leaves but only the latter differed in hardiness. Total amounts of lipids could have been related to hardiness but phospholipids and glycolipids in the roots were not. At least some of these changes may be related to adaptation of growth to temperature.     

The ultrastructure of plasma and thylakoid membrane vesicles from the fresh water cyanobacteriumAnacystis nidulans adapted to salinity

Summary Photoautotrophically growing cultures of the fresh water cyanobacteriumAnacystis nidulans adapted to the presence of 0.4–0.5 M NaCl (about sea water level) with a lag phase of two days after which time the growth rate reassumed 80–90% of the control. Plasma and thylakoid membranes were separated from cell-free extracts of French pressure cell treatedAnacystis nidulans by discontinuous sucrose density gradient centrifugation and purified by repeated recentrifugation on fresh gradients. Identity of the plasma and thylakoid membrane fractions was confirmed by labeling of intact cells with impermeant protein markers prior to breakage and membrane isolation. Electron microscopy revealed that each type of membrane was obtained in the form of closed and perfectly spherical vesicles. Major changes in structure and function of the plasma membranes (and, to a much lesser extent, of the thylakoid membranes) were found to accompany the adaptation process. On the average, diameters of plasma membrane vesicles from salt adapted cells were only one-third of the diameters of corresponding vesicles from control cells. By contrast, the diameters of thylakoid membrane vesicles were the same in both cases.Freeze-etching the cells and counting the number of membrane-intercalating particles on both protoplasmic and exoplasmic fracture faces of plasma and thylakoid membranes indicated a roughly 50% increase of the particle density in plasma membranes during the adaptation process while that in thylakoid membranes was unaffected. Comparison between particle densities on isolated membranes and those on corresponding whole cell membranes permitted an estimate as to the percentage of inside-out and right-side-out vesicles. Stereometric measurement of particle sizes suggested that two distinct sub-populations of the particles in the plasma membranes increased during the adaptation process, tentatively correlated to the cytochrome oxidase and sodium-proton antiporter, respectively. The effects of salt adaptation described in this paper were fully reversed upon withdrawal of the additional NaCl from the growth medium (deadaptation). Moreover, they were not observed when the NaCl was replaced by KCl.Abbreviations CM cytoplasmic or plasma membrane - ICM intracytoplasmic or thylakoid membrane - EF exoplasmic fracture face - PF protoplasmic fracture face - DABS diazobenzosulfonate; Hepes N-2-hydroxyethylpiperazine-N-2-ethane-sulfonate - PMSF phenylmethylsulfonylfluoride Dedicated to the memory of Professor Oswald Kiermayer     

The Plasma Membrane and Peripheral Cytoplasm of the Green Algal Flagellate, Gloeomonas kupfferi

A morphological analysis of the plasma membrane and peripheralendomembrane components of the unicellular chlamydomonad flagellate,Gloeomonas kupfferi, was performed. Conventional fixation, freezesubstitution, and rapid freeze-deep etch processing protocolsfor electron microscopic analyses revealed the following. Theplasma membrane is highlighted by distinct infoldings whichdo not appear to be sensitive to changes in osmotic or cellcycle conditions. These infoldings are irregularly-spaced androughly 200-225 nm apart. Each elliptical infolding is 400-440nm long and 90-115 nm wide. The exoplasmic face (EF) of eachinfolding is highlighted by an aggregation of 130-160 nm intramembranousparticles (imps) that are 9.1-10·5 nm in size. Theseinfoldings are associated with the peripheral endoplasmic reticulumnetwork and microtubular network internally and the inner walllayer externally. It is suggested that these infoldings maybe associated with cell wall maintenance.Copyright 1994, 1999Academic Press Gloeomonas, plasma membrane, infoldings, freeze fracture     

Electrical Impedance Analysis in Plant Tissues8

Electrical impedance spectra (100 Hz–800 kHz) were measuredin leaves of Peperomia obtusifolia L. (a succulent) and Brassicaoleracea L. (cabbage). By measuring impedances at three or moreinter-electrode distances in a single leaf, electrode impedanceand specific tissue impedance were separated. Analysis of impedance data from B. oleracea leaves in relationto an equivalent circuit model showed that leaf developmentwas accompanied by increases in extracellular resistance, cytoplasmicresistance and vacuole interior resistance, together with decreasesin plasma membrane capacitance and tonoplast capacitance. AfterB. oleracea leaves were subjected to a –6 °C freeze-thawstress, extracellular resistance, cytoplasmic resistance andvacuole interior resistance decreased, but plasma membrane capacitanceand tonoplast capacitance did not change. These results indicatethat useful measurements of leaf parameters can be obtainedby this technique. Examination of the electrode impedance spectrum showed thatelectrode insertion produced a damaged collar, 0·4–0·5mm wide, around the electrode. This was confirmed by visualobservation of the damage in P. obtusifolia leaf. Key words: Peperomia obtusifolia L., Brassica oleracea L. (cabbage), electrical impedance, equivalent circuit, electrode polarization     

Comparison Between Growth Chamber and Field-Grown Zea mays Plants for Photosynthetic Carboxylase Activities and .Other Physiological Characteristics with Respect to Leaf Position

Baldy, P. 1986. Comparison between growth chamber and field-grownZea mays plants for photosynthetic carboxylase activities andother physiological characteristics with respect to leaf position.—J.exp. Bot. 37: 309–314. The lengths and fresh weights of leaves, and the amounts ofchlorophyll and protein per leaf, were higher in maize grownin a controlled environment than for field-grown maize harvestedat a similar stage of growth with the seventh leaf just emerging.However, the fresh weight and chlorophyll per unit leaf areawere not different and more protein was present per Unit leafarea in maize cultivated in the field, particularly in the 4thleaf. Except in the seventh leaf the youngest present, the PEP carboxylaseactivity was 2-fold higher in field-grown maize than in plantsfrom the controlled environment. The RuBP carboxylase activitywas not significantly affected by growth conditions. The maximumactivities of the carboxylases were found in the 5th leaf formaize grown in the controlled environment and the 4th leaf forfield maize; in these two leaves the ratios of the PEP: RuBPcarboxylasc activities were 3·0 and 4·4 respectively. The results are used to justify the choice of the fully expanded4th leaf of l9-d-old plants grown in a controlled environmentfor studies of the enzymes involved in the photosynthetic carbonmetabolism of this C₄ plant. Key words: Growth conditions, PEP carboxylase/RuBP carboxylase activity ratio, Zea mays leaf position     

A Freeze-Fracture Study of Membranes of Rapidly Drought-Stressed Leaf Bases of Wheat

Pearce, R. S. 1985. A frceze-fracture study of membranes ofrapidly drought-stressed leaf bases of wheat.—J. exp.Bol. 36: 1209-1221. Bases of expanding leaves were taken from well-watered or drought-hardenedwheat seedlings, and were progressively dehydrated (over ?–9h or, more slowly, for 24 h or 36 h) to between 76% and 5% ofthe water content of the turgid tissue. Damage was assessedby an ion-leakage test. The dehydrated tissues were freeze-fixedwithout rehydration. Patches free from intramembraneous particles(IMP) occurred in the plasma membrane, tonoplast and chloroplastenvelope of all the damaged leaf bases, and were mostly absentfrom undamaged tissues and controls. 15% of these patches appearedto have an ordered sub-structure. Lamellae with few or no IMP,were associated with some IMP-free patches of plasma membrane.Sometimes IMP-free patches and lamellae were associated withIMP-free folds. Groups of IMP-free lamellae occurred in thecytoplasm of the most severely stressed material. Vesicles andmembraneous sacs accumulated just below the plasma membranein some cells from stressed drought-hardened leaf bases. Depressions,‘lesions’ (mainly unusual circular discontinuities),and associated IMP-free patches, occurred in some plasma membranes,mostly in the stressed hardened tissues, including in non-damagedtissue. The results are related to an hypothesis previouslysuggested to explain damage due to extracellular freezing inwheat tissues: the stress causes cell dehydration and this inducesIM P-free patches leading to membrane reorganization (here expressedas IMP-free lamellae and folds) which results in leakage. Thepresent results confirm the role of cytoplasmic dehydrationin the formation of IMP-free patches and in other membrane changes. Key words: Drought stress, freezing stress, plasma membrane     

Ultrastructural Changes Related to Frost Hardiness in the Cortical Parenchyma Cells from Mulberry Twigs

Ultrastructural changes in the cortical parenchyma cells ofmulberry (Morus bombyciz Koidz. cv. ‘Gorogi’) twigswere related closely to the seasonal cycle of frost hardiness.Changes in hardiness from summer to winter and vice versa tookplace particularly in the plasmamembrane. During stages that result in a rapid increase in hardiness,the sequence of cytological changes for the replacement of theplasmamembrane was marked invaginations of the plasmamembrane;pinching off into vesicles in the peripheral cytoplasm; engulfmentof these vesicles into vacuoles; generation of numerous microvesiclesby the Golgi apparatus; the appearance of these microvesicleswith the plasmamembrane. Similar cytological changes that areassociated with increased hardiness occur during artificialhardening at 0°C for 5 to 20 days in both the autumn andspring. In the midwinter cells, however, the plasmamembranehas a fairly smooth structure. Thus, the highly folded structureof the plasmamembrane is not always characteristic of extremelyhardy cells. Ultrastructural changes associated with hardinessare discussed in terms of membrane renewal. ¹Contribution No. 2267 from the Institute of Low TemperatureScience. (Received October 11, 1980; Accepted December 5, 1980)     

Changes in the Root System of Wheat Seedlings Following Root Anaerobiosis I. Anatomy and Respiration in Triticum aestivum L.

Anatomical changes in roots of wheat seedlings (Triticum aestivumL. cv. Hatri) following oxygen deficiency in the rooting mediumwere investigated. The response of the plant to stress was testedat a very early developmental stage when the first adventitiousroots had just emerged. In order to analyze the adaptation ofdifferent roots, respiration rates of the roots 1–3 and4–n were compared with the respiration rates of the totalroot system. Oxygen deficiency was induced either by flushingnutrient solution with nitrogen or flooding of sand. In contrast to plants grown in well aerated media, both stressvariants led to a significant increase of the intercellularspace of the root cortex in seminal and first adventitious roots.Radial cell enlargement of cortical cells near the root tip,cell wall thickenings in flooded sand cultures and an increasein phloroglucinol-stainable substances were found to be furtherindicators of low oxygen supply. The roots 4–n which were promoted in growth under hypoxiashowed higher respiration rates; hence the total root respirationwas not restricted. Triticum aestivum L. cv. Hatri, wheat, roots, anatomy, anaerobiosis, stress, root respiration, intercellular space     

Growth of Near-isogenic Wheat Lines Differing in Development--Spaced Plants

The relationship between phenological development in wheat (TriticumaestivumL.) and growth was studied to determine if the switchfrom a vegetative to a reproductive apex increases plant growthrate. Plant partitioning and relative growth rates during vegetativeand pre-flowering reproductive periods were determined in twosets of near-isogenic lines differing in phenological development.Spaced plants were grown in two photoperiods (15 and 10 h) toincrease the range of development rates. Lines within each isogenicset and photoperiod treatment did not differ in whole plantgrowth rate despite large differences in developmental rate.In addition, the partitioning of biomass between roots and shootswas also similar. The transition of the apex from vegetativeto reproductive mostly affected the partitioning of shoot biomassinto leaf (blades) and stems (rest of the shoot). A longer timeto reach floral initiation was associated with the productionof more, and larger, leaves as well as more tillers. This resultedin large differences in leaf area between isolines. However,at the whole plant level, all lines accumulated biomass at thesame rate with time. The early flowering lines compensated fortheir reduced leaf area by having a higher net assimilationrate and were thereby able to maintain the same relative growthrate as their later flowering counterparts.Copyright 1998 Annalsof Botany Company Development, growth, partitioning,Triticum aestivumL., wheat, isolines.     

Specific features of source-sink relations in alloplasmic hybrid of winter wheat with alien cytoplasm of goatgrass with emphasis on resistance to low temperature stress

We studied the influence of alien cytoplasm of spring goatgrass Aegilops ovata L. on some physiological parameters in winter wheat (Triticum aestivum L.), Mironovskaya 808, under normal conditions and in the case of modified source-sink relations. Measurements of relative rates of plant dry matter growth and its distribution among organs, CO₂ exchange (photosynthesis upon light saturation and dark respiration), content of sugars (sucrose + glucose + fructose) and their ratio in leaves, frost hardiness, and indices of membrane stability and damage of leaves by frost have shown that, on average, alloplasmic hybrid differed from the initial cultivar by almost all parameters. Reduced frost hardiness, increased index of leaf damage by frost, lowered leaf content of sugars, and reduced sucrose/(glucose + fructose) ratio in the alloplasmic hybrid were combined with higher roots/leaves ratio, relative rate of dry matter growth, and photosynthesis and respiration rates. The alloplasmic hybrid was more tolerant to decreased source strength in source-sink relations as compared to the initial cultivar.     

Growth Analysis of Solution Culture-grown Winter Rye, Wheat and Triticale at Different Relative Rates of Nitrogen Supply

At low nitrogen (N) supply, it is well known that rye has ahigher biomass production than wheat. This study investigateswhether these species differences can be explained by differencesin dry matter and nitrogen partitioning, specific leaf area,specific root length and net assimilation rate, which determineboth N acquisition and carbon assimilation during vegetativegrowth. Winter rye (Secale cereale L.), wheat (Triticum aestivumL.) and triticale (X Triticosecale) were grown in solution cultureat relative addition rates (R_N) of nitrate-N supply rangingfrom 0.03–0.18 d^-1and at non-limiting N supply under controlledconditions. The relative growth rate (R_W) was closely equalto R_Nin the range 0.03–0.15 d^-1. The maximalR_W at non-limitingnitrate nutrition was approx. 0.18 d^-1. The biomass allocationto the roots showed a considerable plasticity but did not differbetween species. There were no interspecific differences ineither net assimilation rate or specific leaf area. Higher accumulationof N in the plant, despite the same relative growth rate atnon-limiting N supplies, suggests that rye has a greater abilityto accumulate reserves of nitrogen. Rye had a higher specificroot length over a wide range of sub-optimal N rates than wheat,especially at extreme N deficiency (R_N=0.03–0.06 d^-1).Triticale had a similar specific root length as that of wheatbut had the ability to accumulate N to the same amount as ryeunder conditions of free N access. It is concluded that thebetter adaptation of rye to low N availability compared to wheatis related to higher specific root length in rye. Additionally,the greater ability to accumulate nitrogen under conditionsof free N access for rye and triticale compared to wheat maybe useful for subsequent N utilization during plant growth.In general, species differences are explained by growth componentsresponsible for nitrogen acquisition rather than carbon assimilation.Copyright 1999 Annals of Botany Company Growth analysis, nitrogen, nitrogen productivity, partitioning, specific root length, Secale cereale L.,Triticum aestivum L., X Triticosecale, winter rye, winter wheat, winter triticale.     

Development of {beta}-amylase activity and polymorphism in wheat seedling shoot tissues8

Increasing ß-amylase activity in wheat (Triticum aestlvum,var. Star) seedling shoot tissues was consistently accompaniedby the development of a characteristic polymorphism of the enzyme,as shown by electrophoresis employing amylopectin-containingpolyacrylamide gels. Very young shoot tissue contained one principalform of the enzyme (ß1), whereas two other major forms(ß2, ß3) appeared complementary to thisupon further growth. In vitro incubation experiments indicatedthat the polymorphism arose via a probably proteolytic conversionof ß1 into ß2 and ß3. The conversioninvolved neither an activation of ß-amylase nor asignificant modification of ß-amylase component plvalues. The electrophoretic ß-amylase patterns ofsubcellular leaf compartments suggested that ß1 issynthesized in the cytoplasm of leaf mesophyfi cells and thatthe other forms arise upon transfer of this ‘primary’form into the vacuole. The development of shoot ß-amylaseactivity did not require light, but appeared to be under thenegative control of the chloroplast and was stimulated by mineralnutrients. No clear relationship between ß-amylaseactivity and starch metabolism was evident, since the leaf activitywas largely absent from mesophyll protoplasts, could not beunequivocally demonstrated in the mesophyll chioroplasts, anddeveloped regardless of whether the tissues contained significantamounts of starch or not. Key words: Wheat, leaves, ß-amylase, polymorphism, compartmentation     

Wheat tissues freeze-etched during exposure to extracellular freezing: distribution of ice

Pieces excised from leaf bases and laminae of seedlings of Triticum aestivum L. cv. Lennox were slowly frozen, using a specially designed apparatus, to temperatures between 2° and 14° C. These treatments ranged from non-damaging to damaging, based on ion-leakage tests to be found in the accompanying report (Pearce and Willison 1985, Planta 163, 304–316). The frozen tissue pieces were then freeze-fixed by rapidly cooling them, via melting Freon, to liquid-nitrogen temperature. The tissue was subsequently prepared for electron microscopy by freeze-etching. Ice crystals formed during slow freezing would tend to be much larger than those formed during subsequent freeze-fixation. Ice crystals surrounding the excised tissues were much larger in the frozen than in the control tissues (the latter rapidly freeze-fixed from room temperature). Large ice crystals were present between cells of frozen laminae and absent from controls. Intercellular spaces were infrequent in control leaf bases and no ice-filled intercellular spaces were found in frozen leaf bases. Intracellular ice crystals were smaller in frozen tissues than in controls. It is concluded that all ice formation before freeze-fixation was extracellular. This extracellular ice was either only extra-tissue (leaf bases), or extra-tissue and intercellular (laminae). Periplasmic ice was sometimes present, in control as well as slowly frozen tissues, and the crystals were always small; thus they were probably formed during freeze-fixation rather than during slow freezing. The plasma membrane sometimes showed imprints of cell-wall microfibrils. These were less abundant in leaf bases at 8° C than in controls, and were present on only a minority of plasma membranes from laminae. Therefore, extracellular ice probably did not compress the cells substantially, and changes in cell size and shape were possibly primarily a result of freezing-induced dehydration. Fine-scale distortions (wrinkles) in the plasma membrane, while absent from controls, were present, although only rarely, in both damaged and non-damaged tissues; they were therefore ice-induced but not directly related to the process of damage.     

Plasma-membrane rosettes in root hairs of Equisetum hyemale

Particle arrangement in the plasma membrane during cell wall formation was investigated by means of the double-replica technique in root hairs of Equisetum hyemale. Particle density in the protoplasmic fracture face of the plasma membrane was higher than in the extraplasmic fracture face. Apart from randomly distributed particles, particle rosettes were visible in the PF face of the plasma membrane. The rosettes consisted of six particles arranged in a circle and had an outer diameter of approx. 26 nm. No gradient in the number of rosettes was found, which agrees with micrifibril deposition taking place over the whole hair. The particle rosettes were found individually, which might indicate that they spin out thin microfibrils as found in higher-plant cell walls. Indeed microfibril width in these walls, measured in shadowed preparations, is 8.5±1.5 nm. It is suggested that the rosettes are involved in microfibril synthesis. Non-turgid cells lacked microfibril imprints in the plasma membrane and no particle rosettes were present on their PF face. Fixation with glutaraldehyde caused, probably as a result of plasmolysis, the microfibril imprints to disappear together with the particle rosettes. The PF face of the plasma membrane of non-turgid hairs sometimes showed domains in which the intramembrane particles were aggregated in a hexagonal pattern. Microfibril orientation during deposition will be discussed.Abbreviations EF extraplasmic fracture face - PF protoplasmic fracture face     

Wheat Cultivars Respond Differently to a Drying Top Soil and a Possible Non-Hydraulic Root Signal

Research has shown that when plant roots are exposed to a dryingsoil a non-hydraulic (chemical) signal is produced in the rootand transported to the shoot, causing stomatal closure and growthretardation. This study was designed to reveal genetic diversityin wheat response to soil conditions which elicit a root signal,as the first step in the investigation of the genetic controlof the production of and the response to the root signal. Five spring wheat (Triticum aestivum L.) cultivars were establishedin the growth chamber in soil-filled polyvinyl chloride tubes,120 cm long and of an internal diameter of 10·2 cm. Soilwas well fertilized and wet to field capacity at emergence whentwo treatments were imposed: (1) tubes were watered from thetop as needed to eliminate stress (control); and (2) tubes hada constant water table at a soil depth of 100 to 120 cm, withno applied water. Measurements were performed on five dateson leaf water status and stomatal diffusive resistance. Above-groundbiomass and grain yield per plant were determined at maturity. The water table treatment resulted in dry and hard top soilconditions which were previously indicated to elicit a possibleroot signal. Under these experimental conditions, cultivarsdiffered in their leaf water status, stomatal diffusive resistance(R_s) and plant production. In the control treatment, R_s of cultivarsincreased with reductions in their relative water content (RWC)and leaf water potential (LWP), indicating the expected controlof R_s by leaf water status. Under conditions of a drying topsoil, relative water content (RWC) and leaf water potential(LWP) increased in cultivars that had a higher R_s, indicatingthat stomatal activity was controlling leaf water status. Itwas therefore suggested that the drying top soil elicited aroot signal which caused stomatal closure and reduced plantproduction. Under such conditions, two cultivars (Bethlehemand V748) consistently maintained relatively low R_s and highplant production, despite their relatively lower RWC and LWP,as compared with cvs C97, V747 and V652. Limited observationssuggest that in these two cultivars relatively fewer roots mayhave been exposed to the drying top soil, as compared with theother three cultivars. Key words: Triticum aestivum, cultivars, soil moistrue, drought stress, root, root signal, stomata, relative water content, leaf water potential, biomass, yield     

Morphological Analysis of Leaf and Tiller Number Dynamics of Wheat (Triticum aestivumL.): Responses to Temperature and Light Intensity

In recent literature on Gramineae species, leaf and tiller numberdynamics have been studied by analysing site filling and thephyllochron of the mainstem. However, site filling is influencedby three components: (1) the phyllochron of the mainstem anddaughter tillers; (2) specific site usage (i.e. fraction ofbuds that ultimately develop into a visible tiller at a specificsite); and (3) HS-delay (i.e. difference in Haun Stage (HS)between the parent tiller and daughter tiller above the pointwhere the daughter tiller appears). These three morphologicalcomponents affecting site filling were studied under differentenvironmental conditions in a growth chamber experiment withspring and winter wheat (Triticum aestivumL.). Treatments weretemperature (daily average 10.5, 15.5 or 20.5 °C) and lightintensity (111, 191 or 286 µmol m^-2s^-1). Effects of temperatureand light intensity on phyllochron were well described by equationsalready reported in the literature. Specific site usage washigher at cooler temperatures and greater light intensitiesand was related to tiller position. It is proposed that theseeffects on specific site usage reflect differences in availabilityof local assimilate for tiller appearance. HS-delay of a tillerwas shorter if the expected tiller appearance was later andwas only slightly affected by light intensity or temperature.This new concept, combining HS-delay and specific site usage,can be useful in constructing more general models of the effectsof environmental factors on the dynamics of leaf number andleaf area ofGramineaespecies.Copyright 1998 Annals of BotanyCompany Triticum aestivum; wheat; phyllochron; temperature; light intensity; leaf number; tillering; site filling; site usage.