Plasma membrane and cell wall properties of an aspen hybrid (Populus tremula × tremuloides) parenchyma cells under the influence of salt stress

The effect of salinity on cell turgor, plasma membrane permeability and cell wall elasticity has been measured in petioles of an aspen hybrid using the cell pressure probe. Control plants were grown in soil without the addition of NaCl and treated plants were grown in soil with 50 mM of NaCl for 1, 2, 3 and 4 weeks. In parenchyma cells from Populus tremula × tremuloides petioles with an increased level of NaCl in the soil: (a) turgor pressure was reduced after 1 week of treatment but afterward it was similar to untreated plants, (b) the value of elastic modulus of the cell walls increased, and (c) hydraulic conductivity of the plasma membrane of treated plants decreased in comparison to untreated ones. No histological differences and distribution of JIM5 antibody between the petioles of plants grown under salinity and the untreated were found. In cell walls of parenchyma and collenchyma from plants grown under salinity, the presence of pectic epitopes recognized by JIM7 antibodies was increased in comparison to the control plants. The obtained results indicate that under salt stress the permeability of water through plasma membrane is disturbed, cell walls became more rigid but the turgor pressure did not change.     

The formation of ice in plant tissues

Summary The formation of ice in the petioles ofSolanum acaule andSolanum tuberosum has been studied by light microscopy and by continuous recordings of latent heat production. Frost hardy material ofS. acaule, when slightly turgor deficient, freezes in two distinct stages. The first short freezing is due to the crystallisation of liquid in the vascular tissues and adjacent intercellular spaces. The second major phase shows the gradual formation of layers of ice in the sub-hypodermal regions of the material after the withdrawal of fluid from the cells of the interior tissues. InS. tuberosum, a frost susceptible plant, the two stages of freezing are less well defined, and are confluent; during the second stage of freezing the ice is laid down at scattered loci through the tissues of the petiole. The behaviour of the petiole ofBrassica oleracea was similar to that ofS. acaule, whilstBegonia rex, an extremely frost sensitive plant, froze similarly toS. tuberosum, but without any indication of a normal double cooling curve.It is proposed that during the freezing process of hardy plants an extensive thaw, initiated by a release of solutes from the protoplasts, occurs. The thaw, which may take place in two stages, is to be seen during the second stage of freezing when petioles ofS. acaule are frozen. The release of solutes and a lowering of turgor pressure within the cells may account for the subsequent formation of the extensive layers of ice formed in the outer regions of hardy plants. The release would be favoured by the high permeabilities of resistant cells. In frost susceptible plants the extent of thawing appears to be too small to permit a wholesale movement of fluid to the outer regions of the tissues and, consequently, ice forms at random points within the material and probably causes a greater degree of mechanical injury than is found in hardy tissues.

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Zusammenfassung Die Eisbildung im Blattstiel vonSolanum acaule undS. tuberosum-Pflanzen ist bei laufender lichtmikroskopischer Beobachtung und dauerndem Messen der Kristallisationswärme untersucht worden. Frostresistente Gewebe, die schwach ausgetrocknet sind, frieren in zwei deutlichen Phasen. Die erste kurze Gefrierphase beschreibt die Erstarrung der Flüssigkeit in den Gefäßen und Intercellularräumen. In der zweiten Phase tritt als Folge einer Flüssigkeitsbewegung von den inneren Zellen her eine allmähliche Eisbildung unter dem Hypoderm des Blattstiels ein. Im frostempfindlichenS. tuberosum fließen die zwei Phasen des Gefrierens zusammen: Eisbildung findet in vereinzelten Stellen der Blattstielgewebe statt. Das Verhalten des Blattstiels vonBrassica oleracea war ähnlich dem vonS. acaule. Begonia rex, eine sehr empfindliche Pflanze, verhält sich gleichartig wieS. tuberosum, aber ohne jeglichen Hinweis auf eine normale doppelt gekniete Gefrierkurve.Wahrscheinlich findet in frostresistenten Pflanzen in der zweiten Gefrierphase ein zeitweiliges Tauen statt als Folge einer plötzlichen Abgabe gelöster Stoffe durch das Protoplasma. Dieses Tauen, das vermutlich in zwei Schritten vor sich geht, kann während der zweiten Gefrierphase in vielen Abkühlungskurven unter gleichzeitiger mikroskopischer Untersuchung besonders beiS. acaule beobachtet werden. Die Abgabe von gelösten Stoffen und die dadurch bewirkte Senkung des Turgors in den Zellen trägt zur Erklärung der Bildung von ausgedehnten Eisschichten bei, die in frostharten Pflanzen gefunden werden. Die hohe Durchlässigkeit der frostresistenten Zellen würde solch eine Entbindung begünstigen. In frostempfindlichen Pflanzen, wo Eisbildung an vereinzelten Stellen der Gewebe stattfindet, scheint der Grad des Auftauens zu gering, um eine völlige Bewegung der Flüssigkeit zum äußeren Teil der Gewebe zu erlauben; daher ist die mechanische Schädigung größer als in frostresistenten Pflanzen.

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With 6 Figures in the Text     

狭叶柴胡各器官结构与其分泌道的分布规律

采用石蜡切片法对狭叶柴胡全株不同器官的结构及各器官中分泌道的分布特征进行了解剖学观察研究.结果表明:(1)狭叶柴胡的主根从外向内由周皮、中柱鞘薄壁细胞环和次生维管组织组成,其中柱鞘薄壁细胞环是不同于一般双子叶植物根的一种结构;茎从外到内由表皮、皮层和维管柱组成;叶为等面叶,其上、下表皮内都具栅栏组织;花主要由花瓣、雄蕊...     

Roles of the plasma membrane and the cell wall in the responses of plant cells to freezing

In an effort to clarify the responses of a wide range of plant cells to freezing, we examined the responses to freezing of the cells of chilling-sensitive and chilling-resistant tropical and subtropical plants. Among the cells of the plants that we examined, those of African violet ( Saintpaulia grotei Engl.) leaves were most chilling-sensitive, those of hypocotyls in mungbean [ Vigna radiata (L.) R. Wilcz.] seedlings were moderately chilling-sensitive, and those of orchid [ Paphiopedilum insigne (Wallich ex Lindl.) Pfitz.] leaves were chilling-resistant, when all were chilled at -2 degrees C. By contrast, all these plant cells were freezing-sensitive and suffered extensive damage when they were frozen at -2 degrees C. Cryo-scanning electron microscopy (Cryo-SEM) confirmed that, upon chilling at -2 degrees C, both chilling-sensitive and chilling-resistant plant cells were supercooled. Upon freezing at -2 degrees C, by contrast, intracellular freezing occurred in Saintpaulia leaf cells, frost plasmolysis followed by intracellular freezing occurred in mungbean seedling cells, and extracellular freezing (cytorrhysis) occurred in orchid leaf cells. We postulate that chilling-related destabilization of membranes might result in the loss of the ability of the plasma membrane to act as a barrier against the propagation of extracellular ice in chilling-sensitive plant cells. We also examined the role of cell walls in the response to freezing using cells in which the plasma membrane had been disrupted by repeated freezing and thawing. In chilling-sensitive Saintpaulia and mungbean cells, the cells with a disrupted plasma membrane responded to freezing at -2 degrees C by intracellular freezing. By contrast, in chilling-resistant orchid cells, as well as in other cells of chilling-resistant and freezing-resistant plant tissues, including leaves of orchard grass ( Dactylis glomerata L.), leaves of Arabidopsis thaliana (L.) Heynh. and cortical tissues of mulberry ( Morus bombycis Koids.), cells with a disrupted plasma membrane responded to freezing by extracellular freezing. Our results indicate that, in the chilling-sensitive plants cells that we examined, not only the plasma membrane but also the cell wall lacked the ability to serve as a barrier against the propagation of extracellular ice, whereas in the chilling-resistant plant cells that we examined, not only the plasma membrane but also the cell wall acted as a barrier against the propagation of extracellular ice. It appears, therefore, that not only the plasma membrane but also the cell wall greatly influences the freezing behavior of plant cells.     

Analysis of the distribution of copper amine oxidase in cell walls of legume seedlings

Copper-containing amine oxidase (CuAO) has been proposed to play a role in H2O2 production in plant cell walls during cell development and in response to pathogen attack. We have compared the localisation of CuAO in pea (Pisum sativum L.), lentil (Lens culinaris M.) and chick pea (Cicer arietinum L.) grown under different light conditions, using both immuno- and histochemical techniques. The enzyme was detected by indirect immunofluorescence in the cell walls of parenchyma tissues of etiolated pea and lentil plants and was particularly abundant at intercellular spaces. Upon de-etiolation, CuAO largely disappeared from cortical cell walls except in the region of intercellular spaces. In the apical internode of light-grown seedlings, CuAO occurred mainly in cortical cell walls and, to some extent, in cell walls of xylem vessels. In both the elongation zone and mature regions of roots, CuAO was restricted to cortical cell walls and some cell junctions close to the meristem. Extensin epitopes co-localised to intercellular spaces of the cortex in de-etiolated pea, indicating that CuAO may have a role in cell wall strengthening at intercellular spaces. In chick pea, the localisation of the enzyme varied between different cultivars that have differing susceptibility to the fungus Ascochyta rabiei. In a susceptible cultivar Calia, immunogold labelling localised CuAO to cell walls of the cortex, as in lentil and pea, while in a resistant cultivar Sultano, it was most abundant in xylem vessels and, in light-grown plants, in the epidermis. These expression patterns are discussed with regard to the possible functions of amine oxidase in cell growth, cell differentiation and pathogen resistance.     

Activity of C<Subscript>4</Subscript> enzymes in C<Subscript>3</Subscript>-type herbaceous plants

The activity of enzymes characteristic for C₄-type photosynthesis was determined in different organs of two herbaceous plants: Reynoutria japonica Houtt. and Helianthus tuberosus L. The activity of phosphoenolpyruvate carboxylase (PEPC) was usually higher in the roots, some of the stem tissues and petioles in comparison to the leaf blades. The highest activity of malic enzymes (NAD-ME, NADP-ME) and phosphoenolpyruvate carboxykinase (PEPCK) was in the petioles and stem tissues of both plants and the lowest in the leaf blades and the pith of Helianthus tuberosus L.     

Proteins accumulate in the apoplast of winter rye leaves during cold acclimation

During cold acclimation, winter rye ( Secale cereale L.) plants develop the ability to tolerate freezing temperatures by forming ice in intercellular spaces and xylem vessels. In this study, proteins were extracted from the apoplast of rye leaves to determine their role in controlling extracellular ice formation. Several polypeptides in the 15 to 32 kDa range accumulated in the leaf apoplast during cold acclimation at 5°C and decreased during deacclimation at 20°C. A second group of polypeptides (63, 65 and 68 kDa) appeared only when the leaves were maximally frost tolerant. Ice nucleation activity, as well as the previously reported antifreeze activity, was higher in apoplastic extracts from cold-acclimated than from nonacclimated rye leaves. These results indicate that apoplastic proteins exert a direct influence on the growth of ice. In addition, freezing injury was greater in extracted cold-acclimated leaves than in unextracted cold-acclimated leaves, which suggests that the proteins present in the apoplast are an important component of the mechanism by which winter rye leaves tolerate ice formation     

白花前胡营养器官结构及其分泌道分布规律的研究

运用石蜡切片方法,观察白花前胡营养器官的显微结构及其分泌道的分布特征,以明确营养器官内分泌结构的分布规律,为揭示白花前胡次生代谢产物的积累提供解剖学依据。结果表明,(1)白花前胡成长根从外到内由周皮、中柱鞘薄壁组织和次生维管组织组成,而且中柱鞘薄壁组织不同于一般双子叶植物根的结构;茎从外到内由表皮、皮层和维管柱组成;叶为异面叶结构。(2)白花前胡根、茎及叶中均有分泌道存在,分泌道在根中分布于中柱鞘薄壁组织和次生韧皮部中,茎中分布于皮层和髓中,叶中分布于维管束上下两侧的薄壁组织中。     

Infection of Subterranean Clover (Trifolium subterraneum) by Kabatiella caulivora

Kabatiella caulivora is a serious pathogen of clover ( Trifolium ) spp. Subterranean clover ( T. subterraneum ) cv. Woogenellup was inoculated with K. caulivora , to study the attachment and germination of conidia, germ-tube penetration of the plant surface, and histochemistry and ultrastructure of changes in the host associated with lesion development. The foliar architecture caused the conidia to concentrate at the base of leaflets and on the petiolules (between the leaflets and petioles). Epidermal cells immediately beneath conidia and, occasionally, also adjacent cells developed a yellow-brown discoloration 1 day post-inoculation. Penetration appeared to be directly through the cuticle, characterized by constricted hyphae at the point of entry. No appressoria were observed. In leaves, invasion was restricted to the area proximal to the petiolule and leaf mid-rib. In petioles and petiolules, the hyphae initially remained between the epidermal cells and first layer of mesophyll cells before moving intercellularly through the mesophyll tissue towards phloem tissues. The cuticle was occasionally degraded in petiole and petiolule infections, the loss of epidermal and mesophyll cell wall components was detected, and chloroplasts and starch grains were disrupted. Plants developed macroscopic symptoms 10–11 days post-inoculation with necrotic lesions occurring on leaves, petioles and petiolules. Sporulation occurred approximately 15–18 days post-inoculation when affected plants collapsed. This information may be useful for breeding programmes aimed at selecting varieties with improved resistance to the clover scorch disease.     

Disentangling evolutionary,environmental and morphological drivers of plant anatomical adaptations to drought and cold in Himalayan graminoids

Understanding what determine plants ability to survive drought and cold is crucial for predicting how plants may respond to ongoing climate change. Plant survival strategies are usually characterized by morphological and physiological adaptations, while their underlying anatomical settings are largely unknown. Woody angiosperms and herbaceous dicots have repeatedly evolved small water transporting conduits and large storage parenchyma tissues at colder or drier places to cope with freezing‐ and drought‐induced damages. However, whether these adaptations are also valid for graminoids remains unclear. Here we show that stem anatomical variations in grasses, sedges and rushes dominating in western Himalayan grasslands are driven by elevation and soil moisture via control over aboveground plant stature and belowground clonal growth, while phylogenetic constraints have only a weak effect. Phylogenetic comparative analyses controlling for confounding factors showed that the elevation‐related cooling controls the conductive system through reduced vessel diameter and extended assimilatory and storage tissues with more chlorenchyma and less sclerenchyma around vessels. The soil moisture deficit, on the other hand, determines stabilization structures by promoting short‐rhizomatous turf graminoids with hollow stems, thicker epidermis and deep adventitious roots in dry steppes and semi‐deserts. Saline wetlands and moist alpine pastures promote long‐rhizomatous short‐stature plants with lower need for mechanical support (absence of hollow stem) and exposure to high evaporative forcing (thinner epidermis). Observed trends of decreasing vessel sizes and lignification rate with elevation supports the existing knowledge that narrower vessels and extensive parenchyma assist plants to grow in cold environments by avoiding freezing‐induced cavitation. Our results bring novel information on ecological drivers influencing the evolution of anatomical adaptations in high mountain graminoids. Distinct grassland types, covering elevations from 2650 to 6150 m, harbor unrelated species with different evolutionary histories that have converged towards similar anatomical structures.     

Occurrence of endodermis with a casparian strip in stem and leaf

It is well known that an endodermis with casparian strip always occurs in roots, but few people are aware that it also occurs in stems and leaves of some vascular plants. The rather sparse literature on endodermis in aerial organs was last included in a review in 1943. The present compilation, which does not consider hydathodes, nectaries, or other secretory structures, emphasizes distribution of cauline and foliar endodermis with casparian strip. It occurs unevenly among major taxa: quite common in rhizomes and leaves among pteridophyte groups, with exceptions; absent in gymnosperm stems but found in leaves at least among some conifers; in stems of at least 30 mostly herbaceous angiosperm families, but far less common in leaves, where it is mostly reported from petioles. Etiolation can induce casparian strips in stems and petioles of some herbaceous plants, but results from leaf blades are questionable. There are recent reports of an endodermis with casparian strip in leaves of both woody and herbaceous taxa. The physiological function, if any, of a casparian strip in aerial organs remains unknown.     

Cytology of Potato Callus Cells in Relation to their Frost Hardiness

Structure of callus cells of frost-sensitive and frost-tolerant Solanum species and a frost-tolerant cell line (D20-1), selected from S. tuberosum cv. Desirée callus, was studied. Like frost-tolerant species S. commersonii, cells of the frost-tolerant cell line contained starch grains in their plastids. The cells of this frost-tolerant line also possessed an increased number of microbodies containing protein crystals which suggests the involvement of proteins in frost tolerance but the mechanism may differ from that in frost-tolerant species.     

Phylogenetic analyses in cornus substantiate ancestry of xylem supercooling freezing behavior and reveal lineage of desiccation related proteins

The response of woody plant tissues to freezing temperature has evolved into two distinct behaviors: an avoidance strategy, in which intracellular water supercools, and a freeze-tolerance strategy, where cells tolerate the loss of water to extracellular ice. Although both strategies involve extracellular ice formation, supercooling cells are thought to resist freeze-induced dehydration. Dehydrin proteins, which accumulate during cold acclimation in numerous herbaceous and woody plants, have been speculated to provide, among other things, protection from desiccative extracellular ice formation. Here we use Cornus as a model system to provide the first phylogenetic characterization of xylem freezing behavior and dehydrin-like proteins. Our data suggest that both freezing behavior and the accumulation of dehydrin-like proteins in Cornus are lineage related; supercooling and nonaccumulation of dehydrin-like proteins are ancestral within the genus. The nonsupercooling strategy evolved within the blue- or white-fruited subgroup where representative species exhibit high levels of freeze tolerance. Within the blue- or white-fruited lineage, a single origin of dehydrin-like proteins was documented and displayed a trend for size increase in molecular mass. Phylogenetic analyses revealed that an early divergent group of red-fruited supercooling dogwoods lack a similar protein. Dehydrin-like proteins were limited to neither nonsupercooling species nor to those that possess extreme freeze tolerance.     

The Response of Petioles of Glechoma hederacea to a Dynamic Light Climate

Abstract: In herbaceous vegetation patterns of light distribution may change over time. Prostrate plants growing in such a dynamic light environment may benefit from petioles that respond plastically to changing light conditions. In an experiment, the response of petioles of Glechoma hederacea to changing light conditions was analyzed. Treatments included continuous shade, continuous high light, a shift from shade to high light and from high light to shade when the plants had formed 10 ramets. In all four treatments, even petioles that had apparently ceased growing, were still able to elongate slightly but the extent of elongation decreased with the age of the petiole. In the oldest petioles relative extension rates were higher in shade than in high light. In plants that were exposed to full daylight in the second half of the experiment, even newly formed petioles were longer than those in plants that grew in full daylight continuously though they had elongated over a shorter period. In plants that were shaded in the second half of the experiment, only the youngest 4 to 5 petioles reached lengths similar to that in continuous shade. This mechanism may enable plants to keep young (productive) leaves in the upper layers of the canopy while other less productive leaves remain at lower levels of the vegetation.     

The Role of the Epidermis in the Control of Elongation Growth in Stems and Coleoptiles

The peripheral cell wall(s) of stems and coleoptiles are 6 to 20 times thicker than the walls of the inner tissues. In coleoptiles, the outer wall of the outer epidermis shows a multilayered, helicoidal cellulose architecture, whereas the walls of the parenchyma and the outer wall of the inner epidermis are unilayered. In hypocotyls and epicotyls both the epidermal and some subepidermal walls are multilayered, helicoidal structures. The walls of the internal tissues (inner cortex, pith) are unilayered, with cellulose microfibrils oriented primarily transversely. Peeled inner tissues rapidly extend in water, whereas the outer cell layer(s) contract on isolation. This indicates that the peripheral walls limit elongation of the intact organ. Experiments with the pressure microprobe indicate that the entire organ can be viewed as a giant, turgid cell: the extensible inner tissues exert a pressure (turgor) on the peripheral wall(s), which bear the longitudinal wall stress of the epidermal and internal cells. Numerous studies have shown that auxin induces elongation of isolated, intact sections by loosening of the growth-limiting peripheral cell wall(s). Likewise, the effect of light on reduction of stem elongation and cell wall extensibility in etiolated seedlings is restricted to the peripheral cell layers of the organ. The extensible inner tissues provide the driving force (turgor pressure), whereas the rigid peripheral wall(s) limit, and hence control, the rate of organ elongation.     

Modifications of abscisic acid level in winter oilseed rape leaves during acclimation of plants to freezing temperatures

An almost twofold increase in abscisic acid (ABA) content was observed in the leaves of winter oilseed rape plants (Brassica napus L., var. oleifera L., cv. Jantar) grown in the cold (>0°C). This ABA increase took place during the first three days of cold treatment. After 6 days of plant growth in the cold, the level of ABA started to decline or remained constant, depending on the calculation basis: dry weight or disc area units, respectively. The exposure of cold-acclimated plants to night frost (–5°C for 18 h) induced a further increase (65%) in the ABA level, which begun during the first few hours after thawing. The comparison of time courses of frost resistance increments and ABA content changes showed that modifications of ABA level in the cold-treated leaves preceded those of frost resistance, whereas in the frost-pretreated tissues the ABA increase occurred later than that of frost tolerance. Possible interrelations between ABA content, frost tolerance and tissue water potential modifications in the low temperature-affected tissues are discussed.     

VASCULAR BUNDLES IN PETIOLES OF SOME HERBACEOUS AND WOODY DICOTYLEDONS

Structure and distribution of vascular bundles (VB) have been investigated in petioles of 26 herbaceous and woody dicotyledons. No single pattern of vascular bundle distribution could be found for both groups of plants. Both groups of plants had VB patterns ranging from a cylinder to separate bundles arranged in a variety of patterns, e.g., distorted cylinder, various configurations of a crescent, and other types. The total number of xylem vessels in herbaceous and woody plant petioles varied between 110 and 1756.     

Extracellular ice and cell shape in frost-stressed cereal leaves: A low-temperature scanning-electron-microscopy study

Low-temperature scanning electron microscopy was used to examine transverse fracture faces through cereal leaf pieces subjected to frost. Specimens were studied before and after sublimation of the ice. The position of extracellular ice in the leaf was inferred from the difference between the specimen before and after sublimation and from ridges and points which occurred in the extracellular ice during sublimation. Steps in the fracture surface indicated that the fracture plane passed through the extracellular ice crystals as well as through cells and also helped identify extracellular ice. The cells in controls were turgid and extracellular ice was absent. Leaf pieces from hardened rye were excised and frost-stressed to-3.3°,-21° and-72°C, cooling at 2–12°·h^-1. Cell collapse and extracellular ice were evident at-3.3°C and increased considerably by-21° C. At-21° and-72°C the leaf pieces were mainly filled with extracellular ice and there were few remaining gas spaces. The epidermal and mesophyll cells were laterally flattened, perpendicular to their attachment to adjacent cells, and phloem and vascular sheath cells were more irregularly deformed. Leaf pieces from tender barley were cooled at 2°C·min^-1 to-20° C; they were then mainly filled with extracellular ice, and the cells were highly collapsed as in the rye. In rye leaves frozen to-3.6° C before excision, ice crystals occurred in peri-vascular, sub-epidermal and intervening mesophyll spaces. In rye leaf pieces frozen to-3.3° C after excision or to-3.6° C before excision, mesophyll cells were partly collapsed even when not covered by ice, indicating that collapse of the cell wall, as well as the enclosed protoplast, was driven by dehydration. No gas or ice-filled spaces were found between wall and the enclosed protoplast. It is suggested that this can be explained without invoking chemical bonding between cell wall and plasma membrane: when the wall pores are filled by water, the pore size would reduce vapour pressure so making penetration of the wall by ice or gas less likely.Abbreviations SEM scanning electron microscopy     

The role of petioles in light acquisition by Hydrocotyle vulgaris L. in a vertical light gradient

In natural herbaceous vegetation plants are exposed to a vertical light gradient. In experiments, however, morphogenetic responses of stoloniferous plants to shade have nearly always been tested under homogeneous shade conditions. In this study we simulated a vertical light gradient and found that the response of Hydrocotyle vulgaris in this gradient differed considerably from the responses to homogenous shade. Petioles grew longer while at the same time the specific weight of petioles increased. The elongated petioles raised leaf-blades into better-lit places resulting in higher biomass. Though leaves in the light gradient started their growth under low-light conditions, the size of the leaf-blade was the same as in high light. Internodes were longer than in homogeneous shade conditions but specific weight decreased, probably due to increased allocation to the fast-growing petioles. Received: 2 October 1997 / Accepted: 24 July 1998     

Effects of Hormones on Morphogenesis and Cold Resistance in Berseem Clover (Trifolium alexandrinum L.)

Plants of berseem clover (Trifolium alexandrinum L.) cv. Taborwere raised under conditions inhibiting the acquisition of coldhardiness (non-hardened) or inducing cold hardiness (hardened).All non-hardened plants developed an elongated shoot and exhibitedconsiderable frost sensitivity, as measured by the extent ofthe reduction in yield of variable chlorophyll fluorescenceafter exposure to sub-zero temperature. Hardened plants developeda shorter shoot, with fewer leaves and a greater percentageof dry matter in the root system. These parameters were associatedwith a marked increase in frost resistance. Exogenous applicationof ABA to plants effected similar morphological modificationsin both hardening and non-hardening temperature regimes; plantsdeveloped a shorter primary shoot axis and leaves exhibiteda marked increase in frost hardiness. In berseem clover ABAcan thus substitute, at least partially, for the low temperaturetreatment required to induce cold hardiness. Spraying plantsraised under hardening conditions with gibberellic acid reversedthe effects of the hardening treatment, since they developedan elongated shoot and exhibited frost sensitivity comparableto non-treated plants grown under non-hardening conditions.It is concluded that these endogenous hormones are directlyinvolved in triggering changes in morphogenesis which accompanyphysiological and metabolic events associated with the inductionof plant cold hardiness. Key words: Frost resistance, morphogenesis, abscisic acid, giberellic acid, Trifolium alexandrinum