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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.

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.

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.

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.

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.