Search

Cold and frost are stress factors of widespread occurrence for plants. They damage woody plants due to chilling temperatures in the tropics, by freezing sensitive plants in regions with episodic frosts and temperatures down to -10°C, and by freezing of even tolerant plants in regions with cold winters below -10 and -40°C. The wide range of frost resistances in plants from different climatic zones indicates that the ability to become hardened to freezing has evolved in a stepwise process. There are opinions how an evolutionary acquisition of freezing tolerance could have taken place in the following way: First there is a differentiation of better adapted ecotypes and in the long term a series of functional plant groups develop along climatic gradients. Frequent climatic stress and climatic changes stimulate the selection of new genotypes resistant to climatic extremes. Furthermore, plants exposed to multiple stress factors in their habitat acquire a general cross-tolerance to climatic constraints.The objective of this review is the understanding of low temperature resistance in woody plants, e.g. stabilization of biomembranes by lipid composition, deep supercooling, and ability to become freezing tolerant during winter dormancy. An over a hundred year long history of investigating frost stress and survival of different plant species has brought about an important understanding of basic processes. Recently antifreeze proteins which slow the freezing processes, and proteins, which protect the protoplasm against low temperature, frost and dehydration have been discovered. Findings that help to explain the mechanisms for coping with low temperature will be applicable to agriculture, horticulture, forestry and bioclimatology.

AZA, 5-Acetamido-1,3,4-thiadiazole-2-sulphonamide CA, carbonic anhydrase DIC, dissolved inorganic carbon Hepes, A-(2-hydroxyethyl)-1 piperazine-ethane sulfonic acid IC, inorganic carbon PAR, photosynthetic active radiation PATAg, periodic acid-thiosemicarbazide-silver proteinate Tris, tris (hydroxymethyl)-aminomethane The structural and physiological strategies developed by the leaves of the freshwater macrophyte Ranunculus trichophyllus to adapt to submersed life were studied. Photosynthesis is carried out mainly by the epidermis cells of the numerous segments into which the leaf is finely dissected. In these cells, containing most of the chloroplasts, a peculiar organization of the wall has been identified by cytochemical tests. A thin compact outer region covers the cell surface and splits up forming large lacunae between adjacent cells. Below it, a thick and loose inner region rich in hydrophilic pectic acids occurs, which grows in along the cell sides giving rise to wide transfer areas. In this latter cell wall region, in which the cell/environment contact and exchanges are amplified, the systems for inorganic carbon supply to photosynthetic cells operate. The leaves of R. trichophyllus can rely on environmental CO2 and HCO3– as sources of inorganic carbon for photosynthesis. A mechanism for bicarbonate utilization seems to involve its conversion to CO2 by an apoplastic carbonic anhydrase, whose activity gains importance as the availability of environmental CO2 decreases. Interestingly, it has been demonstrated that in this species CO2 can also be obtained from HCO3– by a photodependent increase in plasmamembrane H+-ATPase activity in the transfer areas of the epidermis cells. This is the first time that such a mechanism has been noted in a nonpolar leaf of a submerged macrophyte.

Light, fluorescence and electron microscopical analysis of the rooted freshwater plant Ranunculus trichophyllus revealed a peculiar anatomical feature. In addition to the true endodermis encircling the root stele, endodermis-like sheaths occurred around each vascular bundle of the leaf segments and of the eustelic stem with its large central cavity, which assumed an anatomical feature resembling that of some pteridophyte stems. These impermeable sheaths, whose cells differentiate suberized walls, can play a major role in hampering the apoplastic leakage of the pressurized water solution which flows throughout the plant in xylem vessels and contains the mineral nutrients taken up by the roots from the sediment. Moreover, these sheaths can function in preventing flooding of the aerenchymatic cavities of the submerged organs. In this way the endodermis-like sheaths preserve the correct circulation of gas and nutrient solution through the entire organism and assume great significance as a structural mechanism evolved by this species to survive and grow underwater.

Characteristic of a high mountain climate are lower temperatures, frequency and intensity of wind and a more irregular distribution of precipitation. High mountain climate is defined by small-scale, terrain-dependent and short-term changeability.

Summary Leaves of Trachycarpus fortunei from palms growing in warm temperate regions of Europe and Japan as outdoor ornamentals were cooled down to low subfreezing temperatures, and the process of ice formation was followed using differential thermal analysis (DTA). Inactivation of photosynthetic function was determined by in vivo chlorophyll fluorescence, and the progress of injuries during freezing was assayed by viability tests after thawing. Ultrastructure of mesophyll cells was investigated by transmission electron microscopy. Silica incrustation of cell walls was documented by means of qualitative energy dispersive X-ray analysis (EDAX) on samples of Trachycarpus fortunei, Washingtonia filifera, and of the bamboo Pseudosasa japonica. DTA revealed typical freezing patterns, with a high temperature exotherm at about -6 to -8°C and a broad low temperature exotherm rising from about -13 to -14°C. As soon as the tissues froze, characteristic changes appeared in the kinetics of in vivo fluorescence. No irreversible inactivation of photosynthesis or damage was detectable at temperatures above appearance of the low exotherm. Within a short time after attainment of the exotherm peak, the leaf segments were found to be lethally damaged. Both of in vivo fluorescence and viability tests clearly indicated that Trachycarpus leaves remain in the persistent supercooled state down to quite low temperatures. The tensile state of water at threshold supercooling temperatures of Trachycarpus leaves was calculated according to the equation of Rajashekar and Burke (1982) and found to be about -14 MPa. An explanation for the permanent maintenance of such a large displacement from thermodynamic equilibrium could be that silica incrustation, as revealed by EDXA, favours supercooling by increasing the rigidity of the cell walls and/or resistance to the growth of ice crystals.

Summary The development of the photosynthetic apparatus was studied in seedlings of Picea abies and Larix decidua grown both in darkness and in light. In light the pattern of differentiation was similar in the two species, and mature chloroplasts showed a well developed thylakoid system arranged into granal and intergranal regions. In the dark, etio-chloroplasts were formed that had different lamellar and tubuliform membrane amounts in the two species. Spruce chloroplasts contained small prolamellar bodies and many thylakoids; in L. decidua chloroplasts, voluminous prolamellar bodies and only a few thylakoids were formed. Fluorescence emission spectra of cotyledons at 77 K as well as pigment analyses showed different behaviour of L. decidua compared with P. abies when grown in the dark. When etiolated seedlings were exposed to the light, the rate of phototransformation of the etio-chloroplast and the achievement of photosynthetic functionality was fast in spruce and slow in larch. Spruce cotyledons attained normal values of in vivo chlorophyll fluorescence characteristics, i.e. Fv/Fm and quenching coefficients qP and qNP within 1.5 hours of light exposure; with larch, the control value of qP was reached much more quickly (ca 3 h) than that of Fv/Fm (ca 20 h), indicating that PS I becomes fully active before PS II. After exposure to light, etiolated larch seedlings behave like those gymnosperms lacking the ability to synthesize chlorophyll in the dark.

Summary Cowpea [Vigna unguiculata (L.) Walp. var. IT 82 D-889] plants were exposed to a double stress whereby salinization (4–6 weeks at 100, 150 or 200 mM NaCl in the substrate) was the predisposing, and low and high temperatures were the additional constraints. Stress responses were assessed by measuring deviations from normal photosynthetic function by in vivo chlorophyll fluorescence. Salinization alone resulted in considerable changes in morphology, ion concentration and osmolality of the cellular sap of leaves, but had remarkably little effect on fluorescence characteristics at 20 °C. Decreasing and increasing temperatures caused marked changes in the fluorescence quenching kinetics and revealed that temperatures of 5–10 °C are suboptimal, and those around 40 °C, supraoptimal. In non-salinized plants, severe temperature stress at 3–6 °C and 45–50 °C was reflected by break-points in the temperature courses of the quenching coefficients at steady state, of Fv/FM, of (Fv)Mdark/(Fv′)Mlight, and of the heat-induced rise of Fo. Mild superimposed temperature stress enhanced the unfavourable effect of saline stress on the kinetics of qN, indicating an increased impairment of the photosynthetic process in response to the combined stress. With severe superimposed temperature stress, however, the salinized plants appeared to be less susceptible than the control plants, since the threshold temperatures for abrupt deviations of Fv/FM and (Fv)Mdark/(Fv′)Mlight, and for injury indicators (chilling symptoms, tissue freezing temperatures, Tc) shifted by 2–3K in the direction of the stressing temperature. We discuss possible mechanisms for these contrasting effects, i.e. the enhancement of disturbances at sub- and superoptimal temperatures but coadaptive adjustments at extreme stress temperatures. The results demonstrate the value of employing superimposed stressors for intensifying stress expression and for providing more information on interactions between environmental factor combinations as they exist in nature.

The winter photosynthetic activity (quantified by net CO(2) assimilation rates and chlorophyll (Chl) a fluorescence parameters) of 20 plant species (including two lichens and two mosses) of a Hungarian temperate semi-desert sand grassland was determined on one occasion per year in 1984, 1989 and 1994. Throughout winter, the overwintering green shoots, leaves or thalli were regularly exposed to below zero temperatures at night and daytime temperatures of 0-5 degrees C. In situ tissue temperature varied between -2.1 and +6.9 degrees C and the photosynthetic photon flux density (PPFD) between 137 and 351 micromol m(-2)s(-1). Under these conditions 18 of the grassland species exhibited photosynthetic CO(2) uptake (range: vascular plants ca. 0.2-3.8 micromol m(-2)s(-1), cryptogams 0.3-2.79 micromol kg(-1)s(-1)) and values of 0.9-5.1 of the Chl fluorescence decrease ratio R(Fd). In 1984, Festuca vaginata and Sedum sexangulare had net CO(2) assimilation at leaf temperatures of -0.85 to -1.2 degrees C. In 1989, all species except Cladonia furcata showed net CO(2) assimilation at tissue temperatures of 0 to +3.3 degrees C, with the highest rates observed in Poa bulbosa and F. vaginata. The latter showed a net CO(2) assimilation saturation at a PPFD of 600 micromol m(-2)s(-1) and a temperature optimum between +5 and +18 degrees C. At the 1994 measurements, the photosynthetic rates were higher at higher tissue water contents. The two mosses and lichens had a net photosynthesis (range: 1.1-2.79 micromol CO(2)kg(-1)s(-1)) at 2 degrees C tissue temperature and at 4-5 degrees C air temperature. Ca. 80% of the vascular grassland plant species maintained a positive C-balance during the coldest periods of winter, with photosynthetic rates of 1.5-3.8 micromol CO(2)m(-2)s(-1). In an extremely warm beginning March of the relatively warm winter of 2006/2007, the dicotyledonous plants had much higher CO(2) assimilation rates on a Chl (range 6-14.9 micromol g(-1)Chl s(-1)) and on a dry weight basis (9-48 micromol kg(-1)dw s(-1)) than in the cold winter of 1994. However, the assimilation rates of the three investigated cryptogams (Tortula and two Cladonia) and the two grasses Festuca and Poa were not affected by this increase. The results indicate that the photosynthetic activity of temperate semi-desert sand grassland species can help somewhat in slowing the general CO(2) rise in winter and function as a potential carbon sink of the investigated semi-desert Hungarian grassland species.

Summary Leaves and stems of Elodea canadensis Michx have been characterized morphologically and ultrastructurally. Significant features, such as chloroplast structure and distribution in the different tissues, the transfer cell-like organization of the lower leaf cells, the gas-lacunae of the stem cortical region and the hydrophobic material present in the cell walls, have been described and specially considered in relation to their possible role in fulfilling the photosynthetic demands of the two organs. In order to verify the possibility that the two green organs may adopt different strategies to supply themselves with inorganic C, the ability of leaves and stems to carry out photosynthesis in different environmental conditions, such as different water pH and dissolved inorganic carbon (DIC) availability, has also been studied in this submerged plant. The results show that the leaves in Elodea canadensis are the -truly submerged. organs, well adapted to underwater life. They have ultrastructural features corresponding to an enhancement of the water-cell exchanges and can utilize with high efficiency both the environmental CO2 and HCO-3 as inorganic C sources. The stem, on the contrary, behaves as a «terrestrial organ, whose photosynthetic ability strictly depends on the availability of CO2 entrapped in the wide intercellular spaces of its tissues.

The flowering time of lilac is a frequently used bioindicator for temperature driven developmental processes in spring time. In the present study the influence of altitude on the flowering dates of lilac was investigated for 24 sites in the Northern and Central Alps, and 10 sites in the Southern Alps. The onset of flowering dates in these areas of the Alps are closely related to the mean air temperatures in April. A temperature decrease by 1 � C postpones the onset of flowering by 6.6 days. This results in a gradient of lilac flowering times of 4.2–4.6 days per 100 m in the Central Alps between 600 and 1500 m a.s.l. and a gradient of 3.7 days per 100 m a.s.l. in the Southern Alps between 90 and 1100 m a.s.l. Over the 20th century a deviation from the mean onset of flowering dates of about � 12 days was seen due to climatic fluctuations in the Northern and Central Alps. On the Southern rim of the Alps the thermal conditions are most favourable for the development of lilac and therefore, there are no remarkable time shifts to be seen.

The photosynthetic activity of leafless twigs and buds of Fagus sylvatica was determined by in vivo chlorophyll fluorescence from November to May. Measurements were made on the day of sampling, and during exposure to warm temperatures until reactivation was attained. Under the same conditions, bud development and growth were forced by exposure of cut twigs to 25/18° C at long-day conditions, and bud swelling and bud burst were monitored. Winter inactivation of photosynthesis results in a reduction of the photochemical efficiency of PS II, as indicated by lowering of FV/FM, from January through March. The greatest reduction is in cortical chlorenchyma, the least in folded leaflets and primordia of buds. Restoration of photosynthetic activity, brought about by warming, needed 3–4 weeks in cortex and 1–2 weeks in buds during the coldest period of winter. Frequency distributions based on three types of chlorophyll fluorescence transients, defined by quantitative fluorescence parameters, have proved to be a valuable method for a differentiated expression of the unequal functional activation states of parallel samples. The seasonal course of winter inactivation of photosynthesis did not correspond entirely with the depth of bud dormancy as revealed by the forcing treatments; inactivation of photosynthesis may be more closely synchronized with changes in frost hardiness; possible causes are discussed. We suggest, therefore, that a distinction should be made between inactivation of metabolic processes and depth of dormancy, even though these processes are inherently interrelated.

Photosynthetic mechanisms have been compared in leaves and, separately, in stems of Egeria densa Planch. In order to correlate the structural and functional characteristics of the two organs (1) the ultrastructural features of leaves and stems have been studied and (2) their photosynthetic activity has been evaluated by measuring in vivo both oxygen evolution and the kinetics of chlorophyll fluorescence. The results confirm the aquatic behaviour of the leaf which is able to utilize inorganic C supplied both as CO2 and HCO 3 − . In this respect, the different wall organization found in the two cell layers of the leaf is particularly interesting, since it could be related to the known polar mechanism of inorganic-C uptake. The stem, by contrast, behaves rather as an aerial organ, needing very high CO2 concentrations in the aquatic environment in order to carry out photosynthesis. In the stem, the aerenchyma plays a role in supplying the green cells with gaseous respiratory CO2, thus facilitating the photosynthetic activity of the submerged stems.

Du point de vue des botanistes, le stress peut etre decrit comme un etat dans lequel l'augmentation des demandes faites vis-a-vis du vegetal conduit a un debut de destabilisation des fonctions, suivi d'une normalisation et d'une amelioration de la resistance. Si les limites de la tolerance sont depassees et les capacites d'adaptation surchargees, des dommages permanents peuvent apparaitre allant jusqu'a la mort

Summary Suction tension (depression of water potential) and osmotic pressure of young and mature sclerophyllous leaves of Quercus ilex were determined during desiccation until drought injuries occurred. The suction tension was measured with the Wescor Sample Chamber Psychrometer, the osmotic pressure by cryoscopy; from measured values of the osmotic pressure and the suction tension the turgor pressure was calculated. In leaves of Quercus ilex the suction tension does not fall to zero when the twigs are saturated with water, but remains at a level of about 4 bar (young leaves) to 9 bar (mature leaves). At water saturation deficits between the point of turgor loss and the appearance of the first drought injuries, the suction tension rises above the level of the osmotic pressure, thus indicating the effectiveness of negative turgor pressure. The higher the drought resistance of the leaves, the higher the values of negative turgor pressure and suction tension achieved.

Gradations in cold resistance of plants with different latitudinal and altitudinal distribution, and the various categories of chilling and freezing resistance are hypothesized to be evolutionary steps of adaptive responses to increasing low temperature stress and annual seasonality. The gradual lowering of the critical phase transition temperature of biomembrane lipids, the capacity of persistent supercooling of tissues, and the development of a dormancy linked freezing-tolerance are considered to be essential mechanisms resulting in improved acclimatation to low temperature climates.

Zusammenfassung Im Rahmen der photosynthetischen Stoffproduktion spielt die Zeit auf mehrfache Weise eine bedeutende Rolle. Sowohl die Photosynthese und Atmung und die damit verbundene CO2-Diffusion als auch die Assimilatverteilung sind Vorgange, denen eine bestimmte, von Art zu Art und nach dem jeweiligen Aktivitatszustand bezeichnende Reaktionsgeschwindigkeit zukommt. Ferner laufen alle diese Vorgange zu verschiedenen Zeitpunkten (zu verschiedenen Tageszeiten, Jahreszeiten oder Lebensaltern) mit unterschiedlicher Intensitat ab. Bei integraler Betrachtung der pflanzlichen Stoffproduktion kommt der Zeit schlieslich eine ertragsbestimmende bzw. ertragsbegrenzende Rolle zu: Der endgultige Reinertrag der Photosynthese hangt weitestgehend von der Zeitdauer ab, wahrend der eine nennenswerte Stoffproduktion moglich ist. Es wird vorgeschlagen, fur diese Zeitdauer Bezeichnungen wie „Produktivitatszeit” oder „Produktionsperiode” und nicht die Bezeichnung „Vegetationsperiode” zu verwenden.

Wahrend sich in der kaltgemasigten Zone alle einigermasen frost empfindlichen Holzpflanzen durch Abwerfen des Laubes der Winter kalte und -durre entziehen, sind die Immergrunen im Mittelmeergebiet vollbelaubt der Frosteinwirkung ausgesetzt. Da experimentelle Unter suchungen uber die winterliche Kalteresistenz der Blatter von Pflanzen aus dem mediterranen Florengebiet noch vollig fehlen, wurde versucht, diese Lucke wenigstens zum Teil auszufullen und daruber hinaus folgende Fragen zu klaren: 1. Gibt es einen Jahresgang der Frostharte, wie er von Immergrunen Mittelund Nordeuropas bekannt ist, auch bei jenen aus dem Mittel meerraum ? Wenn ja, 2. wieweit ist dieser durch den jahreszeitlichen Klimawechsel, bzw. wieweit ist er auch endogen geregelt ?

Zusammenfassung An funfjahrigen eingetopften Tannen (Abies alba Mill.) und Bergahornpflanzen (Acer pseudoplatanus L.) wurde festgestellt, ob und wie stark extreme Temperaturen, die gerade noch keine Gewebeschaden verursachen, Atmung und Photosynthesevermogen nachwirkend beeinflussen konnen. Dabei ergab sich, das nach Kalteeinwirkung die Dunkelatmung einen kurzfristigen Anstieg zeigt, der um so hohere Werte erreicht und umso langer anhalt, je naher man an den Schadigungsbereich herankommt. Diese Beobachtung kann ausgenutzt werden, um die Resistenzanalyse auch auf den vornekrotischen Bereich auszudehnen. Sowohl Hitze als auch Kalte blockieren fur eine gewisse Zeit das Photosynthesevermogen schon in einem Temperaturbereich, in dem irreversible Gewebeschaden noch nicht auftreten. Unsere vorlaufigen Ergebnisse deuten darauf hin, das mit dem Erreichen sowohl des Temperaturminimums als auch des Temperaturmaximums der Nettophotosynthese nicht nur eine momentane Sistierung des Nettokohlenstofferwerbs, sondern auch eine (wenigstens wenige Stunden andauernde) nachwirkende Behinderung der Kohlenstoffassimilation verbunden ist. Diese Beobachtung vermag die stoffwechselokologische Bedeutung der beiden Kardinalpunkte zu unterstreichen. Der Osterreichische Forschungsrat unterstutzte diese Untersuchung durch Beistellung eines Gasanalysators und einer Tiefkuhltruhe. Die Abteilung Forstwesen der Vorarlberger Landesregierung (Dipl.-Ing. H. Mahr) uberlies uns kostenlos die Tannen-Versuchspflanzen, wofur auch an dieser Stelle gedankt sei.

Aufbauend aud die in einer vorausgegangenen Arbeit (Larcher 1961) veroffentlichten Ergebnisse einer eingehenden Untersuchung des Assimilations und Respirationsvermogens vonOlea europaea, Quercus ilex undQuercus pubescens aus dem nordlichen Gardaseegebiet (Mt. Brione bei Riva) wird versucht, auf dem Wege einer Uberschlagungsrechnung zu erfahren, wie weit mediterrane Hartlaubpflanzen an der Nordgrenze ihres Vorkommens den Winter fur den Kohlenstofferwerb ausnutzen konnen und unter welchen klimatischen Voraussetzungen dort immergrune Baume gegenuber sommergrunen im Vorteil sein mogen. Der Winter kann von den Immergrunen des Mittelmeergebiets sehr wohl in den durch Lichtangebot und jeweils herrschende Temperatur gesetzten Grenzen fur die Stoffproduktion ausgenutzt werden, wobeiQuercus ilex wahrend des ganzen Winters und besonders zur Zeit der grosten Kalte im Januar dem Olbaum deutlich uberlegen ist (vgl. Tabelle 2 und die Abbildung). Bleibt die Wasserversorgung stets optimal, dann vermogen Zweige vonQuercus pubescens in den 6 Monaten ihrer Vegetationstatigkeit pro Trockengewichtseinheit mehr CO2 zu binden als Zweige beider Hartlaubholzer im Laufe des ganzen Jahres (vgl. Tabelle 3); auch diese wurden — immer unter Voraussetzung unnaturlich regelmasiger Wasserversorgung — den groseren Teil (Olea etwa 64%,Quercus etwa 61%) ihrer Jahresausbeute im warmen Halbjahr Mai bis Oktober erarbeiten. Daraus ergibt sich, das die immergrunen Baume die laubabwerfenden im Jahresstoffertrag nur dort ubertreffen, wo eineEntsprechend lange und strenge sommerliche Durrezeit die Assimilationstatigkeit der Baume stort.

Zusammenfassung Im ersten Teil unserer Arbeit war von den unteren Temperaturgrenzen (Temperaturminimum) der Netto-Photosynthese im Zusammenhang mit dem Gefrieren und der Kalteresistenll der Blatter reprasentativer Bewohner verschiedener thermisch charakterisierter Gegenden (Nordrand des Gardasees, Tal-, Berg- und Hochgebirgslagen Nordtirols) die Rede. Im vorliegenden Teil wird erganzend uber Temperaturmaxima und Hitzeresistenz derselben Pflanzen berichtet. Die Beleuchtungsstarke bei der Bestimmung der oberen Grenztemperatur der Netto-Photosynthese war 10 Kilolux, die Versuchdauer betrug eine Stunde. Ebenso lange wirkte jede Warmestufe bei der Ermittlung der Resistenz auf die Pflanzen ein. Die CO2- Bilanz der untersuchten vier immergrunen Pinaceen und die der sommergrunen Larix, aber auch jene der hochalpinen Staude Geum reptans schlagt am Licht in CO2-Ausscheidung urn, wenn die Assimilationsorgane je nach Art auf 36 °C bis 38 °C erwarmt werden. Genauso verhalten sich die beiden nordisch-hochalpinen Arten Oxyria digyna und Ranunculus glacialis bei 39 °C, Leucojum und Taxus bei 41 °C. Im engen Spielraum von 42 °C und 44 °C drangen sich die Temperaturmaxima heimischer wie sudlicher sommer- und immergruner breitblattriger Holzpflanzen. Aber auch Helianthus annuus ist nach fruherer Bearbeitung hier zu nennen und erste, orientierende Versuche mit Futtergrasern und Krautern der Wiesen weisen diese in dasselbe Spatium ein. Auf 45 °C und daruber als Grenztemperatur gewinnbringender Photosynthese bringen es ausschlieslich sudliche Holzpflanzen wie Pittosporum, Quercus ilex und Olea (48 °C). Das Laub dieser zuletzt genannten Arten, dazu jenes der sommergrunen submediterranen Quercus pubescens und von Laurus ist zugleich gegen Hitze sehr widerstandsfahig (erst 50°C bis 55 °C toten etwa die Halfte der gesamten exponierten Blattflache), woraus sich gegenuber dem Temperaturmaximum ein Abstand von 7 bis 10 Graden ergibt, der zur Produktion nicht genutzt wird. Ahnlich gros sind die Intervalle bei den Koniferen, bei denen sie aber aus tiefem Temperaturmaximum bei mittlerer Resistenz resultieren. Fast bis zur Grenze ihrer (bescheidenen) Widerstandsfahigkeit nut zen Leucojum und Alnus viridis die Warme.

Shoots ofQuercus ilex gain more efficient ability to become frosthardened in winter during the course of ageing. The greatest increase in frosthardiness occurs within the first five winters. Full capacity for frost-resistance is not reached until the plant enters its reproductive phase. Roots do not change their resistance pattern with age.

Zusammenfassung An funfjahrigen Tannen (Abies alba Mill.) und ebenso alten Bergahornpflanzen (Acer pseudoplatanus L.) wurde vergleichend der Jahresgang der Hitzeund Kalteresistenz des Kambiums mehrjahriger Sprosachsen und Wurzeln untersucht. Die Mesergebnisse werden in Zusammenhang mit den Standorttemperaturen und der jahreszeitlichen Entwicklung der Pflanzen gebracht Dabei ergab sich, das Hitze- und Kalteresistenz nur dann parallel verliefen, wenn die Hohe der Resistenz gegen beide Noxen ausschlieslich vom Aktivitatszustand des entsprechenden Gewebes abhing. Zu Abweichungen kam es immer dann, wenn eine der beiden Resistenzarten temperaturbedingten Abhartungs- oder Enthartungsvorgangen unterworfen war: So blieb langere Kalte, die zu all den Anderungen im Zellbereich fuhrt, die das Protoplasma frosthart machen, auf die Hitzeresistenz der Sprosachsen wirkungslos, da deren kambiale Teilungstatigkeit im Herbst schon eingestellt wurde, bevor tiefe Temperaturen einwirken konnten. Die starke Temperaturabhangigkeit des Wurzelwachstums konnte eine Erklarung dafur sein, das jahreszeitliche Anderungen der Hitzeresistenz der Wurzeln parallel zu den Minimumtemperaturen im Boden verliefen. Da weiters das Wachstum der Wurzeln erst durch Temperaturen blockiert wurde, die bei der Tanne auch einen reaktiven Anstieg in der Kalteresistenz auslosten, anderten sich Hitze- und Kalteresistenz der Wurzeln wahrend des Jahres weitgehend synchron. Ein unterschiedliches Verhalten von Hitze- und Kalteresistenz ware auch dann zu erwarten, wenn die Hitzeresistenz im Sommer durch eine auf die Kalteresistenz stets wirkungslos bleibende Hitzehartung adaptiv anstiege, wozu es aber in den von uns untersuchten Pflanzen nicht kam. Die Hitzeresistenz durfte bei Tanne und Ahorn in ihrem Jahresverlauf nur auf Anderungen im Aktivitatszustand reagieren

Im Gegensatz zu den immergrunen Coniferen und Ericaceen der Waldgrenze und zum Efeu vertragen die Blatter des Olbaums (Olea europaea) im Sommer und Winter gleiches Mas von Austrocknung, aber verschieden grose Kalte (s. Abb. 2); daher gehen Austrocknungs- und Kalteresistenz beiOlea nicht parallel.

Wo immer es irgendwie moglich ist, sollte man den C02-Gaswechsel an ganzen Pflanzen oder an Blattern und Zweigen im ungestorten Kon takt mit der Pflanze messen. Nur so kommt man zu okologisch vollig einwandfreien Ergebnissen. Wo hingegen nicht so sehr die Assimila tionstatigkeit einer Pflanze unter dem naturlichen Zusammenspiel der Standortsfaktoren erkundet als vielmehr der Einflus einzelner Faktoren

In chilling-sensitive plants (Glycine max, Saintpaulia ionantha, Saccharum officinarum) a sudden reversible drop in chlorophyll fluorescence occurs during photosynthetic induction immediately following saturating light pulses at low temperatures in the range 4 to 8°C. A comparison of two soybean cultivars of different chilling sensitivities revealed that this phenomenon, termed lowwave, indicates specific thresholds of low temperature stress. Its occurrence under controlled chilling can be regarded as a quantitative marker for screening chilling susceptibility in angiosperms.

Acute winter frost drought in Rhododendron ferrugineum at the alpine timberline was provoked by removing the snow that had covered the shrubs and by irradiating and rewarming the twigs in situ in a climatized chamber simulating snow melt conditions. Water potential decreased to -2.3 MPa within three days. After activation of photosynthesis on the 1st day, the COr-uptake decreased during the 2nd day and approached zero at the 3rd day. With advanced desiccation, the internal COr concentration of the leaves increased indicating an impairment of chloroplast functions. The procedure of the disturbances appears to be similar to that of other acute drought events.

Survival capacity, i.e. the ability of a plant to withstand adverse weather conditions (Larcher 1973), is a highly complex phenomenon. Although frost survival capacity depends primarily on the specific frost resistance of a plant, various mechanisms are involved which help the plant to mitigate, prevent or escape excessive low temperature stress. Furthermore, recovery from injuries must be taken into consideration: in order to survive unusually severe frost the plant need not necessarily remain completely undamaged, but repair must be still possible (see Sect. 3.4). Therefore, the chance for frost survival of a plant in a given environment consists in mitigating or excluding the freezing risk, in frost resistance and in recovery after frost damage.

Hochgebirge sind extreme Lebensraume, die an die Existenzgrenze des Lebens heran oder uber sie hinausreichen (Franz 1979). Mit zunehmender Meereshohe wird das Warmeangebot geringer, oberhalb der Waldgrenze nimmt die Haufigkeit und Starke des Windes sprunghaft zu und die Niederschlagsverteilung wird ungleichmasiger. Im Hochgebirge ist die Strahlung intensiver als in Tallagen und der Wasserdampfgehalt der Luft niedriger. Vor allem aber zeichnet sich das Hochgebirgsklima durch kleinraumige, vom Gelanderelief abhangige und kurzzeitige Wechselhaftigkeit aus.

Two basically different categories of frost-killing mechanisms in plant cells can be distinguished: 1. Immediate Injury of Freezing-Sensitive Cells. Plant cells and tissues subject to immediate freezing injury, which in the classical descriptive terminology (Molisch 1897) were called “freezing-sensitive”, are killed as soon as intensive ice formation becomes evident (Fig. 3.1a). Damage may result from freezing following heterogeneous nucleation, or, in cells capable of persistent supercooling, following homogeneous nucleation. Under natural conditions, in most tissues sudden lethal freezing sets in after a considerable deviation from thermodynamic equilibrium (nonequilibrium freezing; Olien 1978, 1981). Appreciable quantities of ice appear in freezing-sensitive tissues only when, under the stress of a large water potential difference between cell solutes and the adjacent ice, the semipermeability of the cell membranes is lost, permitting the efflux of fluid from the cells. In determining the specific temperature limit for lethal nonequilibrium freezing the stability and integrity of the biomembranes play a decisive role (Dowgert and Steponkus 1983; Lindstrom and Carter 1985). The sudden freezing can thus be regarded as a consequence of the preceding disorganization and as the final violent event in the process of freezing.

In zones with a seasonal climate, plants undergo periodic transition from a lower to a higher level of resistance. All categories of frost resistance are affected, the tolerance to equilibrium freezing (Fig. 5.1) as well as the ability to undergo deep supercooling (Fig. 5.2). Two types of seasonal acclimation can be distinguished, each being an expression of the different mechanisms by which they achieve their resistance (see Table 4.3): 1. Species Responding Directly to Low Winter Temperatures. Plants growing in regions with a mild winter or in habitats shielded from severe frost acquire greater resistance in the cold season as a direct consequence of the progressive drop in temperature. This enhanced resistance may be due simply to a lowering of Tf and Tsc, or to the acquisition and further development of freezing tolerance. However, if the plants are protected from cold they can remain at a low level of resistance throughout the year. This is the situation in the rhizomes of the cosmopolitan fern Pteris aquilinum, although not in other ferns of the temperate zone (Kappen 1964). A further example is provided by the observation of Larcher (1954) that the leaves of olive trees cultivated in glasshouses exhibit damage at any time of year at −6 °C, whereas under Mediterranean winter conditions they tolerate temperatures down to −10 °C. A similar kind of behaviour is seen in woody plants of the subtropics (e.g. Salix safsaf, S. tetrasperma: Sakai 1978, and Pinus caribaea: Oohata et al. 1981) and in many herbs.

Distribution boundaries of plant groups and biomes are set by a complexity of ecological (climatic, edaphic and biotic) and historical (phytohistorical and anthropogenic) factors which interact with the demands made on its environment by the species involved, and its capacity to withstand stress (Boysen-Jensen 1949; Good 1974; Whittaker 1975; Walter 1984).