Changes in Osmotic Pressure and Mucilage during Low-Temperature Acclimation of Opuntia ficus-indica

Opuntia ficus-indica, a Crassulacean acid metabolism plant cultivated for its fruits and cladodes, was used to examine chemical and physiological events accompanying low-temperature acclimation. Changes in osmotic pressure, water content, low molecular weight solutes, and extracellular mucilage were monitored in the photosynthetic chlorenchyma and the water-storage parenchyma when plants maintained at day/night air temperatures of 30/20°C were shifted to 10/0°C. An increase in osmotic pressure of 0.13 megapascal occurred after 13 days at 10/0°C. Synthesis of glucose, fructose, and glycerol accounted for most of the observed increase in osmotic pressure during the low-temperature acclimation. Extracellular mucilage and the relative apoplastic water content increased by 24 and 10%, respectively, during exposure to low temperatures. These increases apparently favor the extracellular nucleation of ice closer to the equilibrium freezing temperature for plants at 10/0°C, which could make the cellular dehydration more gradual and less damaging. Nuclear magnetic resonance studies helped elucidate the cellular processes during ice formation, such as those revealed by changes in the relaxation times of two water fractions in the chlorenchyma. The latter results suggested a restricted mobility of intracellular water and an increased mobility of extracellular water for plants at 10/0°C compared with those at 30/20°C. Increased mobility of extracellular water could facilitate extracellular ice growth and thus delay the potentially lethal intracellular freezing during low-temperature acclimation.     

Parenchyma-chlorenchyma water movement during drought for the hemiepiphytic cactus Hylocereus undatus

BACKGROUND AND AIMS: Hylocereus undatus, a hemiepiphytic cactus cultivated in 20 countries for its fruit, has fleshy stems whose water storage is crucial for surviving drought. Inter-tissue water transfer during drought was therefore analysed based on cell volumes and water potential components. METHODS: In addition to determining cell dimensions, osmotic pressures and water potentials, a novel but simple procedure leading to an external water potential of zero was devised by which cells in thin sections were perfused with distilled water. The resulting volume changes indicated that the parenchyma-chlorenchyma water movement was related to more flexible cell walls in the water-storage parenchyma with its lower internal turgor pressure (P) than in the chlorenchyma. KEY RESULTS: Under wet conditions, P was 0.45 MPa in the chlorenchyma but only 0.10 MPa in the water-storage parenchyma. During 6 weeks of drought, the stems lost one-third of their water content, becoming flaccid. About 95 % of the water lost came from cells in the water-storage parenchyma, which decreased by 44 % in length and volume, whereas cells in the adjacent chlorenchyma decreased by only 6 %; the osmotic pressure concomitantly increased by only 10 % in the chlorenchyma but by 75 % in the water-storage parenchyma. CONCLUSIONS: The concentrating effect that occurred as cellular volume decreased indicated no change in cellular solute amounts during 6 weeks of drought. The ability to shift water from the parenchyma to the chlorenchyma allowed the latter tissue to maintain a positive net CO2 uptake rate during such a drought.     

Diel Patterns of Water Potential Components for the Crassulacean Acid Metabolism Plant Opuntia ficus-indica when Well-Watered or Droughted

Under well-watered conditions, chlorenchyma acidity in cladodes of Opuntia ficus-indica increased substantially at night, fully accounting for the 0.26-megapascal nocturnal increase in osmotic pressure in the outer 2 millimeters. Osmotic pressure in the inner part of the chlorenchyma and in the water-storage parenchyma did not change significantly over 24-hour periods. Three months of drought decreased nocturnal acid accumulation by 73% and essentially abolished transpiration; also, 27% of the chlorenchyma water and 61% of the parenchyma water was lost during such drought, but the average tissue osmotic pressure was little affected. Turgor pressure was maintained in the chlorenchyma after 3 months of drought, although it decreased sevenfold in the water-storage parenchyma compared with the well-watered condition. Moreover, the nocturnal increases in turgor pressure of about 0.08 megapascal in the outer part of the chlorenchyma was also unchanged by such drought. The water potential magnitudes favored water movement from the parenchyma to the chlorenchyma at the end of the night and in the reverse direction during the late afternoon. Experiments with tritiated water support this pattern of water movement, which is also in agreement with predictions based on electric-circuit analog models for Crassulacean acid metabolism plants.     

Diel water movement between parenchyma and chlorenchyma of two desert CAM plants under dry and wet conditions

Abstract. Electric-circuit analogue models of the water relations of crassulacean acid metabolism (CAM) succulents such as Agave deserti and Ferocactus acanthodes have predicted diel movement of water between the water-storage parenchyma and the photo-synthetic chlorenchyma. Injection of tritiated water into either tissue in the laboratory confirmed substantial and bidirectional water movements, especially under conditions of wet soil. For A. deserti , water movement from the water-storage parenchyma to the chlorenchyma increased at night as the chlorenchyma osmotic pressure increased. Although nocturnal osmotic pressure increases and transpiration for both species were minimal in the field under dry conditions, diel changes in the deuterium: hydrogen ratio (expressed as ΔD) were similar for the water-storage parenchyma and the chlorenchyma. Such indication of [substantial mixing of water between the tissues over a 24-h cycle was more evident under wet conditions in the field. For A. deserti , ΔD then increased by 32%o from the afternoon to midnight and was essentially identical in the water-storage parenchyma and the chlorenchyma. For F. acanthodes , the diel changes in ΔD were one-third those of A. deserti , and ΔD was always slightly higher for the chlorenchyma than for the water-storage parenchyma, apparently reflecting the lower surface-to-volume ratio of A. deserti. In summary, data obtained using radioactive and stable isotopes strongly supported model predictions concerning diel cycles of internal water distribution for these CAM species.     

Effects of drought on water relations and nonstructural carbohydrates in cladodes of Opuntia ficus-indica

Nocturnal acid accumulation, water content, osmotic pressure (π), and nonstructural carbohydrates were determined in the chlorenchyma and the water-storage parenchyma of Opuntia ficus-indica (L.) Miller for well-watered plants and those subjected to drought for 15 weeks. During the 15-week drought, total cladode water content decreased by 57%, the water-storage parenchyma losing a greater fraction of water than the chlorenchyma, which most likely helped maintain nocturnal acid accumulation in the latter tissue. Despite the preferential water loss from the water-storage parenchyma, it had a lower π than the chlorenchyma over the 15 weeks of drought, suggesting a substantial decrease in osmotically active solutes in the water-storage parenchyma. Also, the measured π increases of both tissues were much less than those predicted based on the loss of water during drought and the initial content of osmotically active solutes under well-watered conditions. A decrease in the amount of soluble sugars (glucose. fructose and sucrose) occurred in plants subjected to drought. accounting for 46% and 81% of the difference between the measured and the predicted increases in π of the chlorenchyma and the water-storage parenchyma. respectively. The decrease in soluble sugars was associated with an equivalenl increase in polysaccharides, presumably starch, in the water-storage parenchyma. but not in the chlorenchyma.     

Water flow and water storage in Agave deserti: osmotic implications of crassulacean acid metabolism

Abstract Water flow and water storage were investigated for Agave deserti, a desert succulent showing crassulacean acid metabolism (CAM). The anatomy and water relations of the peripheral chlorenchyma, where CAM occurs, and the central water-storage parenchyma were investigated for its massive leaves so that these tissues could be incorporated as discrete elements into an electrical-circuit analogue of the whole plant. The daily cycling of osmotic pressure was represented by voltage sources in series with the storage capacitors. With soil water potential and leaf transpiration rate as input variables, axial water flow through the vascular bundles and radial flows into and out of storage during the day/night cycle were determined. The predominantly nocturnal transpiration was coincident with increases in cell osmotic pressure and in titratable acid of the leaf chlorenchyma. In the outer layers of the chlorenchyma, water potential was most negative at the beginning of the night when transpiration was maximum, while the water-storage parenchyma reached its minimal water potential 9 h later. The roots plus stem contributed 7% and the leaves contributed 50% to the total water flow during maximal transpiration; peak water flow from the soil to the roots occurred at dawn and was only 58% of the maximal transpiration rate. Over each 24-h period, 39% of the water lost from the plant was derived from storage, with flow into storage occurring mainly during the daytime. Simulations showed that the acid accumulation rhythm of CAM had little impact on water uptake from the soil under the conditions employed. In the outer chlorenchyma, water potential and water flows were more sensitive to the day/night changes in transpiration than in osmotic pressure. Nevertheless, cell osmotic pressure had a large influence on turgor pressure in this tissue and determined the extent to which storage was recharged during the latter part of the night.     

Leaf anatomy,water relations and crassulacean acid metabolism in the chlorenchyma and colourless internal water-storage tissue of Carpobrotus edulis and Senecio ?mandraliscae

Both Carpobrotus edulis and Senecio ?mandraliscae possess leaves with a peripheral chlorenchyma and colourless internal water-storage tissue. Water stress in C. edulis growing under semi-natural conditions resulted in the induction of weak Crassulacean acid metabolism (CAM) whereas well-watered plants of S. ?mandraliscae exhibited a similar degree of CAM. Titratable acidity in the separated water-storage tissue was substantially lower than in the chlorenchyma in both species but, nevertheless, increased during the night and decreased during the day either when sampled from the intact plant or from incubated tissue slices. Indeed, the increase in nocturnal titratable acidity produced by the water-storage tissue in situ accounted for approx. 30% of total acidification on a per-leaf basis. It appears that during the night the water-storage tissue in these species is able to fix CO₂ which is subsequently released during the day to enter the photosynthetic carbon-reduction cycle of the chlorenchyma. Diurnal rhythms of water potential (Ψ) and osmotic potential (Ψs) were measured in separated chlorenchyma and water-storage tissue by thermocouple psychrometry. Both parameters increased during the latter part of the daytime and initial nocturnal period and decreased during the rest of the night and into the post-dawn period. The chlorenchyma of water-stressed plants of C. edulis appeared to possess a marked negative turgor pressure (as determined from Ψ-Ψs) but this was caused by a severe underestimation in the measurement of the chlorenchyma Ψ. It is suggested that this artefact arose from release of colloidal polysaccharide mucilage, or possibly tannins, from broken tannin cells producing a lowering of water activity when measured using thermocouple psychrometry.     

Extreme temperatures and thermal tolerances for seedlings of desert succulents

Summary Extreme temperatures near the soil surface, which can reach 70°C at the main study site in the northwestern Sonoran Desert, markedly affect seedling survival. Computer simulations indicated that for the rather spherical barrel cactus Ferocactus acanthodes (Lem.) Britt. & Rose the maximum surface temperature decreased 8°C and the minimum temperature increased 3°C as the seedling height was increased from 1 mm up to 50 mm. Simulated changes in shortwave and longwave irradiation alone showed that shading could decrease the maximum temperature by about 5°C for the common desert agave, Agave deserti Engelm., and raise the minimum 1°C. Actual field measurements on seedlings of both species, where shading would affect local air temperatures and wind speeds in addition to irradiation, indicated that shading decreased the average maximum surface temperature by 11°C in the summer and raised the minimum temperature by 3°C in winter.Seedlings grown at day/iight air temperatures of 30°C/20°C tolerated low temperatures of about -7°C and high temperatures of about 56°C, as measured by the temperature where stain uptake by chlorenchyma cells was reduced 50%. Seedling tolerance to high temperatures increased slightly with age, and F. acanthodes was more tolerant than A. deserti. Even taking the acclimation of high temperature tolerance into account (2.7°C increase per 10°C increase in temperature), seedlings of A. deserti would not be expected to withstand the high temperatures at exposed sites, consistent with previous observations that these seedlings occur only in protected microhabitats. Based primarily on greater high temperature acclimation (4.3°C per 10°C), seedlings of F. acanthodes have a greater high temperature tolerance and can just barely survive in exposed sites. Wide ranges in photoperiod had little effect on the thermal sensitivities of either species. When drought increased the chlorenchyma osmotic pressure from about 0.5 MPa to 1.3 MPa, seedlings of both species became about 2°C less tolerant of high temperatures, which would be nonadaptive in a desert environment, and 2°C more tolerant of low temperatures, which also occurs for other species.In conclusion, seedlings of A. deserti and F. acanthodes could tolerate tissue temperatures over 60°C when acclimated to high temperatures and below -8°C when acclimated to low temperatures. However, the extreme environment adjacent to desert soil requires sheltered microhabitats to protect the plants from high temperature damage and also to protect them from low temperature damage at their upper elevational limits.     

Effect of temperature,hydrogen ion concentration and osmotic potential on oospore germination of fivePythium spp. isolated from pond water

Oospore germination occurred over a temperature ranging of 15–35°C forPythium coloratum, 10–35°C forP. diclinum, 15–30°C forP. dissotocum, 7–30°C forP. monospermum, and 10–30°C forP. pleroticum. Optimum temperature was 25°C for all species tested. In case of pH, oospore germination occurred over a range of 4.76–8.55 with an optimum of 6.40–7.40. The least germination occurred at pH 4.76 forP. coloratum, P. diclinum, P. monospermum andP. pleroticum, whileP. dissotocum germinated from pH 5.02. Oospores of the all tested pythia were able to germinate at –0.13 to –1.65 MPa and could not germinate at –3.40 MPa, with the highest germination rate at –0.27 to –0.47 MPa. The effect of temperature, pH and osmotic potential on oospore germination was discussed in relation to pollution of pond water.     

Changes in leaf ultrastructure and carbohydrates inArabidopsis thaliana L. (Heyn) cv. Columbia during rapid cold acclimation

Summary We studied cell ultrastructure and carbohydrate levels in the leaf tissue ofArabidopsis thaliana L. (Heyn) cv. Columbia during rapid cold acclimation. Freezing tolerance of the leaves from 26 day old plants was determined after 48 h and 10 days at 4°C. Acclimation treatment of 48 h decreased the lethal freezing temperature from –5.7°C to –9.4°C. Freezing tolerance was not altered further by acclimation at 4 °C for 10 days. Ultrastructural changes in the parenchyma cells were evident after 6 to 24 h of cold acclimation. The plasma membrane showed signs of extensive turnover. Evidence of membrane invaginations and sequestering of membrane material was observed. In addition, numerous microvesicles, paramural bodies, and fragments of endoplasmic reticulum were noticed in the vicinity of plasma membrane. Modifications in the structure of cell membranes were evident after 5 days of exposure to low temperature. Small, darkly stained globules were seen on the plasma membrane, tonoplast, chloroplast envelope membrane, mitochondrion outer membrane, dictyosome cisternae membrane, and microvesicle membrane. As far as we are aware, this type of membrane modification has not been described previously in plant cells exposed to low temperature. We propose to call these structures membraglobuli. Acclimation treatment also increased the concentrations of soluble sugars and starch. These observations suggest that cold acclimation inA. thaliana induces changes in both plasma membrane properties and carbohydrate composition.     

Characterization of the bacterial flora of mass cultivated Brachionus plicatilis

Bacterial density and composition in association of mass cultivated rotifers (Brachionus plicatilis, SINTEF-strain) was investigated, during experimental conditions identical to the procedures used for preparing rotifers as live food for marine cold water fish larvae. These procedures include cultivation, enrichment with squid meal and acclimation to low temperature by storage of the rotifer culture at 6 °C. Large variations were observed in the number of rotifer associated (1.8–7.6 · 10³ colony forming units per rotifer^–1) and free-living (0.6–25 10⁷ cells·ml^–1) bacteria. An increase of 50–150% in the bacterial number was normally observed after feeding the rotifer with squid meal, but after three days of acclimation at 6 °C, the bacterial numbers decreased to the initial level.After enrichment of the cultures with squid meal, the similarity in the composition of the bacterial flora between the rotifers and water was reduced. However, acclimation of the culture at 6 °C resulted in better agreement of the rotifer associated flora and that in water. Enrichment of the cultures induced a shift in the bacterial composition from Cytophaga/Flavobacterium dominance to Pseudomonas/Alcaligenes dominance. The bacterial flora of the rotifer cultures are dominated by presumably opportunistic species after enrichment, which may have detrimental effects when rotifers are fed as live food to marine fish larvae.     

Matric Bound Water of Water Tissue from Succulents

Matric bound water was measured as water retained by frozen and thawed tissue after desorption on a pressure membrane filter under 20 bars nitrogen gas pressure. Central water-storage tissue and peripheral chlorenchyma from leaves or stems of 15 taxonomically diverse non-halophytic succulent species were investigated. Matric bound water as a per cent of the dry weight averaged higher in water storage than in chlorenchyma tissue but lower than values reported for many mesophytic leaves. Matric bound water as a proportion of the total water held, however, was lower in water tissues. Osmotic potentials were generally high (solute contents low). It is concluded that matric or osmotic forces cannot account, in any unique way, for the high water content of water tissues. This appears to depend, instead, on the enormous ability of the thin-walled cells to take up available water and expand.     

Water potentials for developing cladodes and fruits of a succulent plant, including xylem-versus-phloem implications for water movement

Developing cladodes had lower water potentials and developingfruits had higher water potentials than the underlying cladodesof the widely cultivated prickly pear cactus, Opuntia ficus-indica.The 0.06 MPa lower value in 4-week-old daughter cladodes indicateda typical water potential gradient from the underlying clad-odealong the xylem of –0.2 MPa m^–1; the 0.17 MPa highervalue in 4-week-old fruits, which decreased to 0.07 MPa by 10weeks, implicated the phloem as their supplier of water. Thephloem sap of the underlying cladodes had an osmotic pressureof only 0.90 to 0.98 MPa, so the phloem could supply a relativelydilute solution to the photosynthetically dependent fruits (daughtercladodes of O. ficus-indica are photosynthetically independentat 4 weeks). Although the water potentials were similar foradjacent tissues, the osmotic pressures were lower for the water-storagecompared with the photosynthetic tissue; the osmotic pressureswere higher for xylem sap from fruits, for which xylary flowapparently occurred toward the underlying cladodes, than fordaughter cladodes. The relative capacitance (change in relativewater content divided by change in tissue water potential) wasapproximately 0.71 MPa^–1 for the water-storage tissueand the photosynthetic tissue of both daughter cladodes andfruits at 4 weeks of age. When these organs approached maturityat 10 weeks, the relative capacitance increased about 40% fortheir water-storage tissue, but decreased 30% for their photosynthetictissue. As the plant water content decreases during drought,about twice as much water will thus be lost per unit volumeof the water-storage tissue compared with the photosynthetictissue of maturing fruits and cladodes. Key words: Opuntia ficus-indica, phloem, relative water content, water capacitance, water potential     

Water storage and osmotic pressure influences on the water relations of a dicotyledonous desert succulent

Abstract Water storage and nocturnal increases in osmotic pressure affect the water relations of the desert succulent Ferocactus acanthodes, which was studied using an electrical circuit analog based on the anatomy and morphology of a representative individual. Transpiration rates and osmotic pressures over a 24-h period were used as input variables. The model predicted water potential, turgor pressure and water flow for various tissues. Plant capacitances, storage resistances and nocturnal increases in osmotic pressure were varied to determine their role in the water relations of this dicotyledonous succulent. Water coming from storage tissues contributed about one-third of the water transpired at night: the majority of this water came from the nonphotosynthetic, water storage parenchyma of the stem. Time lags of 4 h were predicted between maximum transpiration and maximum water uptake from the soil. Varying the capacitance of the plant caused proportional changes in osmotically driven water movement but changes in storage resistance had only minor effects. Turgor pressure in the chlorenchyma depended on osmotic pressure, but was fairly insensitive to doubling or halving of the capacitance or storage resistance of the plant. Water uptake from the soil was only slightly affected by osmotic pressure changes in the chlorenchyma. For this stem succulent, the movement of water from the chlorenchyma to the xylem and the internal redistribution of water among stem tissues were dominated by nocturnal changes in chlorenchyma osmotic pressure, not by transpiration.     

Formation of multiplex lamellae by equilibrium slow freezing of cortical parenchyma cells of mulberry and its possible relationship to freezing tolerance

Summary Cortical parenchyma cells of mulberry (Morus bombycis Koidz. cv. Goroji) become extremely cold hardy in winter and can tolerate equilibrium freezing below –30 °C and subsequent immersion into liquid nitrogen. We show in this ultrastructural study that, in these extremely cold hardy cortical parenchyma cells of mulberry collected in winter, initiation of freezing at –5 °C resulted in the formation of multiplex lamellae (MPL) that completely covered the area beneath the plasma membrane. The MPL were produced by fusion of pre-existing vesicular endoplasmic reticulum (ER), via a reticular ER network. The completed MPL were composed of a parallel array of sheet-like ER cisternae. This structural reorganization of the ER was completed within 10 min upon freezing at –5 °C and was quickly reversed upon thawing. The same structural reorganization of the ER was produced by osmotic dehydration of the cortical tissues with a 2.7 osmol sorbitol solution at 20 °C. Thus, the structural reorganization of the ER upon freezing was, in fact, produced by dehydration. In winter samples, the formation of MPL with the initiation of freezing completely inhibited close apposition of membranes upon deep freezing that has been reported to be a cause of freezing injury via the production of ultrastructural changes in the plasma membrane. Similar but more or less incomplete MPL were produced by freezing or osmotic dehydration in cortical parenchyma cells collected in spring and autumn, and these MPL partly inhibited close apposition of membranes. MPL were not produced in the cells of mulberry collected in summer and close apposition of membranes occurred upon deep freezing. We speculate that the formation of MPL with the initiation of freezing might play a specific role in inhibiting the close apposition of membranes due to the specific nature of the cisternal membranes and might, consequently, be responsible for the high freezing tolerance of winter cells.     

Hydration-state-responsive proteins link cold and drought stress in spinach

Spinach (Spinacia oleracea L.) seedlings exposed to low nonfreezing temperatures (0–10° C) that promote cold acclimation, synthesize a variety cold-acclimation proteins and at the same time acquire a greater ability to withstand cellular dehydration imposed by the freezing of tissue water. Two of these proteins (160 and 85 kDa) become more abundant over time at low temperature. In addition, a small decline in tissue water status from a maximally hydrated state also appears to be associated with an initiation of the accumulation of these proteins at a noninductive temperature. Imposing a severe water stress on young seedlings grown at 25° C by withholding water leads to substantial accumulation of the 160- and 85-kDa proteins, and maximal induction of freezing tolerance. This evidence implies that responses to cold acclimation and water stress involve common mechanisms, and further establishes the linkage of these two proteins with stresses having an osmotic component.Abbreviations ABA abscisic acid - CAP cold-acclimation protein - kDa kilodaltons We thank T. Sinclair and K. Cline for critical reading and discussions, N. Denslow for assistance with protein sequencing methods, and L. Greene, S. Henry for preparing the monoclonal antibodies. The work was made possible by support from the USDA Competitive Grants Program No. 90-37280-5527, the Institute for Food and Agricultural Sciences, and through access to the protein sequencing and hybridoma facilities of the Interdisciplinary Center for Biotechnology Research at the University of Florida. Florida Agricultural Experiment Station Journal Series R-02399.     

Responsiveness of Orobanche ramosa L. Seeds to GR 24 as Related to Temperature, Oxygen Availability and Water Potential During Preconditioning and Subsequent Germination

Broomrape (Orobanche ramosa L.) is a root holoparasite responsible for important yield losses in numerous crops, particularly in the Mediterranean area. In this paper, the effects of temperature, oxygen concentration and water potential of the medium on broomrape seed germination were investigated. Seeds became able to germinate in the presence of a strigol analogue (GR 24) only after a preincubation period for at least 3 days at 20 °C. Their responsiveness to GR 24 increased with increasing duration of their preconditioning at 20 °C, and was optimal after 2–3 weeks. The preconditioning treatment was effective at temperatures ranging from 10 to 30 °C. At the optimal temperature (20 °C), it required at least 1% oxygen in the atmosphere and remained effective at a water potential of the medium of –2 MPa. A too prolonged preincubation of seeds at sub- or supraoptimal temperatures (5 and 30 °C) resulted in induction of a secondary dormancy. Seeds preconditioned for 14 days at 20 °C germinated in the presence of 1 mg L^–1 GR 24 at temperatures ranging from 10 to 25 °C, and the thermal optimum was the same (20 °C) than that of preconditioning. At 20 °C, seeds were able to germinate in the presence of GR 24 under atmospheres containing at least 3% oxygen and at a water potential of the medium as low as –3 MPa. The differences observed in the effects of environmental factors on preconditioning efficiency and germination of preconditioned seeds suggest that both processes involve different mechanisms. The results obtained might also help to better understand the regulation of O. ramosa spread in temperate areas.     

Critical thermal maxima and acclimation rate of the tropical guppy Poecilla reticulata

Tropical guppies, Poecilia reticulata, collected from the canal of La Laguna Los Patos were acclimated over a four-week period at local water temperatures of 24–33 °C to determine their critical thermal maxima (CTM) and death points (DP), as criteria of thermal tolerance. In addition, individual thermal tolerance times at a lethal temperature of 38.5 °C were measured over 12 days for upward acclimation from 24 to 30 °C and over 16 days for downward acclimation from 30 to 24 °C to determine acclimation rate just before and after changing the acclimation temperatures. The CTM ranged from 38.95 to 40.61 °C and the average DP varied from 41.22 to 42.86 °C. Positive relationships were apparent between thermal tolerance and acclimation temperatures, and thus heat tolerance criteria (CTM and DP) were significantly different among acclimation temperatures. Individual heat tolerance times increased most rapidly during the first 6 hours of upward acclimation after transfer from 24 to 30 °C, continued to increase another 5 days and fluctuated after initial acclimation was completed. The heat tolerance times of fish transferred from 30 to 24 °C declined steadily over times, reaching a minimum at 14–16 days after transfer.     

Structural reorganization of thylakoid systems in response to heat treatment

The structural reorganization of pea thylakoid systems in response to osmotic shock in a wide range of temperatures (36–70°C) was studied. At temperatures 40–46°C, the configuration of thylakoid systems changed from a flattened to a nearly round, whereas thylakoids themselves remained compressed. The percentage of thylakoids stacked into grana at 44°C decreased from 71 % in the control to 40 % in experimental samples, reaching 59 % at 48°C. At 44°C and above, thylakoid systems ceased to respond to the osmotic shock by disordering, in contrast to what happened at lower temperatures (36–43°C) and in the control, and retained the configuration inherent in thylakoid systems at these temperatures. At 50°C and above, the packing of thylakoids in grana systems changed, and thylakoids formed extended strands of pseudograna. Simultaneously, single thylakoids formed a network of anastomoses through local fusions. At temperatures of 60–70°C, thylakoid systems appeared as spherical clusters of membrane vesicles with different degree of separation.This revised version was published online in March 2005 with corrections to the page numbers.     

Pressure-volume analyses on shoots of Picea abies and leaves of Coffea liberica at various temperatures

Shoots from two ecologically different evergreen tree species, Picea abies (L.) Karst and Coffea liberica Hiern, were used to carry out pressure volume (PV) measurements at 5–35°C. For this purpose a pressure chamber was equipped with thermoelectric temperature regulation. The non-linear sections of the resultant PV curves were sigmoidal for both species, with recognizable points of inflexion. These points, at around ψ= 1.12 MPa and relative water content (RWC) = 88.5% for Picea and at ψ= 0.92 MPa and RWC = 95.5% for Coffea , were characterised by a temporary increase in the resistance to water flow of the entire shoot (R₅).
The maximum value of the bulk modulus of elasticity (ɛ max) was also in the region of the point of inflexion. This value was considerably higher in Coffea than in Picea . The osmotic pressures at full water saturation (π₀) and at turgor loss point (π_p) showed a clear temperature dependence between 15 and 35°C differing only slightly from the theoretically expected situation. At 25°C these values were 1.72 and 2.48 MPa. respectively, for Picea and 1.58 and 1.87 MPa. respectively, for Coffea . The turgor loss point occurred at 76–77% RWC in Picea and at 86% RWC in Coffea , the proportion of apoplastic water varied between 22 and 25% in Picea but was only 9–10% in Coffea . The ecological differences between the two species are reflected in their temperature dependence for R₅, which was much steeper for Coffea than for Picea . The energy of activation for the water conductance of the whole shoots was 13.0–14.4 kJ mol⁻¹ in Picea and about 23 kJ mol⁻¹ in Coffea .