Photosynthetic response of Ulva rotundata to light and temperature during emersion on an intertidal sand flat

Summary We have investigated the diurnal response of photosynthesis and variable photosystem II (PSII) chlorophyll fluorescence at 77 K for thalli of the chlorophyte macroalga, Ulva rotundata, grown in outdoor culture and transplanted to an intertidal sand flat in different seasons. The physiological response in summer indicated synergistic effects of high PFD and aerial exposure, the latter probably attributable to temperature, which usually increased by 8 to 10° C during midday emersion. Except at extreme emersed temperatures in summer (38° C), the light-saturated photosynthesis rate (P_m) did not decline at midday. In contrast, light-limited quantum yield of photosynthetic O₂ exchange () and the ratio of variable to maximum fluorescence yield (F_v/F_m) reversibly declined during midday low tides in all seasons. Shade-grown thalli exhibited a fluorescence response suggestive of greater photodamage to PSII, whereas sun-grown thalli had greater photoprotective capacity. The fluorescence decline was smaller when high tide occurred at midday, and was delayed during morning cloudiness. These results suggest that the diurnal response to PFD in this shallow water species is modified by tidal and meteorological factors. U. rotundata has a great capacity for photoprotection which allows it to tolerate and even thrive in the harsh intertidal environment.Abbreviations F_o instantaneous yield of chlorophyll fluorescence - F_m maximum yield of fluorescence - F_v variable yield (F_m–F_o) of fluorescence - PFD photon flux density (400–700 nm) - P_m light-saturated rate of photosynthesis - PSH photosystem II - Q_A electron acceptor of PSII - light-limited quantum yield of photosynthesis     

Irradiance and temperature effects on photosynthesis of tussock tundra Sphagnum mosses from the foothills of the Philip Smith Mountains,Alaska

Summary Photosynthetic characteristics of three species of Sphagnum common in the foothills of the Brooks Range on the North Slope of Alaska were investigated. Generally, light-saturated rates of net photosynthesis decreased in the order S. squarrosum, S. angustifolium, and S. warnstorfii when plants were grown under common growth chamber conditions. For field-grown S. angustifolium, average light compensation point at 10°C was 37 mol m^-2s^-1 photosynthetic photon flux density (PPFD), and light saturation occurred between 250 and 500 mol m^-2 s^-1. At 20°C, compensation point increased to 127 mol m^-2s^-1 and the PPFD required for light saturation increased to approximately 500 mol m^-2s^-1, while maximum rates of CO₂ uptake increased only slightly. Light response curves of chamber-grown plants exhibited substantially lower compensation points and higher light-saturated rates of CO₂ assimilation than field-grown material, due perhaps to a higher percentage of green, photosynthetically competent tissue. All three species exhibited broad responses to temperature, with optima near 20°C, and maintained at least 75% of maximum assimilation between approx. 13° and 30°C. Rates at 5°C were approx. 50% of maximum. Studies of the microclimate of Sphagnum at the field research site suggest that CO₂ uptake should occur at near light-saturated rates during the day in open tussock tundra but that PPFD may often be limiting under Salix and Betula canopies in a water track drainage. Simulations using a simple model provided a seasonal estimate of 0.78 g dry weight (DW) of S. angustifolium produced from each initial g of photosynthetic tissue under willow canopies, assuming no water limitations. Although the simulation model suggests that production would be 66% higher in open tussock tundra, S. angustifolium is rarely found in this potentially more stressful habitat. To explain the relative abundance of Sphagnum in shaded water track areas as compared to open tussock tundra, we postulate that the vascular plant canopies provide protection from adverse effects of high temperatures, excess irradiance and reduced water availability. Under conditions of normal water availability, removal of the vascular plant cover did not affect the tissue water content of S. squarrosum, but resulted in a strong decrease in photosynthetic capacity, accompanied by chlorophyll bleaching. These results suggest that photoinhibition may limit production under certain conditions.     

Comparative effects of salt and alkali stresses on growth, osmotic adjustment and ionic balance of an alkali-resistant halophyte Suaeda glauca (Bge.)

Effects of salt and alkali stresses on growth, osmotic adjustment and ionic balance of Suaeda glauca (Bge.), an alkali-resistant succulent halophyte, were compared. The results showed that alkali stress clearly inhibited the growth of S. glauca. Moreover, the concentrations of Na⁺ and K⁺ both increased with increasing salinity under both stresses, suggesting no competitive inhibition between absorptions of Na⁺ and K⁺. The mechanism underlying osmotic adjustment during salt stress was similar to alkali stress in shoots. The shared essential features were that organic acids, betaine and inorganic ions (dominated by Na⁺) mostly accumulated. On the other hand, the mechanisms governing ionic balance under both stresses were different. Under salt stress, S. glauca accumulated organic acids and inorganic anions to maintain the intracellular ionic equilibrium, but the anion contribution of inorganic ions was greater than that of organic acids. However, the concentrations of inorganic anions under alkali stress were significantly lower than those under salt stress of the same intensity, suggesting that alkali stress might inhibit uptake of anions, such as NO₃ ^– and H₂PO₄ ^–. Under alkali stress, organic acids were the dominant factor in maintaining ionic equilibrium. The contribution of organic acids to anions was 74.1%, while that of inorganic anions was only 25.9%. S. glauca enhanced the synthesis of organic acids, dominated by oxalic acid, to compensate for the shortage of inorganic anions.     

Inhibition of photosynthesis by chilling in moderate light: a comparison of plants sensitive and insensitive to chilling

Photosynthetic activity, in leaf slices and isolated thylakoids, was examined at 25° C after preincubation of the slices at either 25° C or 4° C at a moderate photon flux density (PFD) of 450 mol·m^–2·s^–1, or at 4° C in the dark. The plants used wereSpinacia oleracea L.,Cucumis sativus L. andNerium oleander L. which was acclimated to growth at 20° C or 45° C. The plants were grown at a PFD of 550 mol·m^–2·s^–1. Photosynthesis, measured as CO₂-dependent O₂ evolution, was not inhibited in leaf slices from any plant after preincubation at 25° C at a moderate PFD or at 4° C in the dark. However, exposure to 4° C at a moderate PFD induced an inhibition of CO₂-dependent O₂ evolution within 1 h inC. sativus, a chilling-sensitive plant, and in 45° C-grownN. oleander. The inhibition in these plants after 5 h reached 80% and 40%, respectively, and was independent of the CO₂ concentration but was reduced at O₂ concentrations of less than 3%. Methyl-viologen-dependent O₂ exchange in leaf slices from these plants was not inhibited. There was no photoxidation of chlorophyll, in isolated thylakoids, or any inhibition of electron transport at photosystem (PS)II, PSI or through both photosystems which would account for the inhibition of photosynthesis. The conditions which inhibit photosynthesis in chilling-sensitive plants do not cause inhibition inS. oleracea, a chilling-insensitive plant, or in 20° C-grownN. oleander. The CO₂-dependent photosynthesis, measured at 5° C, was reduced to about 3% of that recorded at 25° C in chilling-sensitive plants but only to about 30% in the chilling-insensitive plants. Methyl-viologen-dependent O₂ exchange, measured at 5° C, was greater than 25% of the activity at 25° C in all the plants. The results indicate that the mechanism of the chilling-induced inhibition of photosynthesis does not involve damage to PSII. That inhibition of photosynthesis is observed only in the chilling-sensitive plants indicates it is related, in some way, to the disproportionate decrease in photosynthetic activity in these plants at chilling temperatures.Abbreviations Chl chlorophyll - DPIPH reduced form of 2,6-dichlorophenol-indophenol - DMQ 2,5-dimethyl-p-benzoquinone - MV methyl viologen - 20°-oleander Nerium oleander grown at 20° C - 45°-oleander N. oleander grown at 45° C - PFD photon flux density (photon fluence rate) - PSI and PSII photosystem I and II, respectively     

Influence of oxidative and osmotic stresses on the structure of the cell wall mannan of Candida albicans serotype A

In this study, we investigated the structural changes in the cell wall mannan of Candida albicans serotype A strain cells cultured under various stress conditions, that is, oxidative stress of 3.5 mM H₂O₂, osmotic stress of 1.5 M NaCl, and heat stress at 37 °C, compared with the normal condition of 30 °C in yeast extract-added Sabouraud liquid medium (YSLM). Based on the ¹H nuclear magnetic resonance (NMR) and fluorophore-assisted carbohydrate electrophoresis (FACE) analyses of the mannans, we showed that the proportion of the terminal β-1,2-linked mannose side chain unit in the mannan increased in the cell proliferation process under both the normal condition and the oxidative stress condition. The osmotic stress induced a slight decrease in the proportion of the β-1,2-linked mannose unit in the acid-labile fraction. The heat stress induced a significant decrease in the proportions of the β-1,2-linked mannose unit in both the acid-labile and acid-stable fractions. Based on these results, we propose that C. albicans significantly changes the mannan structures under various stress conditions and that sufficient attention to the cultural conditions is needed to perform an accurate diagnosis of candidiasis.     

Metabolite levels in the stroma of spinach chloroplasts exposed to osmotic stress: Effects of the pH of the medium and exogenous dihydroxyacetone phosphate

The levels of stromal photosynthetic intermediates were measured in isolated intact spinach (Spinacia oleracea L.) chloroplasts exposed to reduced osmotic potentials. Stressed chloroplasts showed slower rates of metabolite accumulation upon illumination than controls. Relative to other metabolites sedoheptulose-1,7-bisphosphate (SBP) and fructose-1,6-bisphosphate (FBP) accumulated in the stroma in the stressed treatments. Under these conditions 3-phosphoglycerate (3-PGA) efflux to the medium was restricted. Chloroplasts previously incubated with [³²P]KH₂PO₄ and [³²P]dihydroxyacetone phosphate ([³²P]DAP) in the dark were characterized by very high FBP and SBP levels prior to illumination. Metabolism of these pools upon illumination increased with increasing pH of the medium but was consistently inhibited in osmotically stressed chloroplasts. The responses of stromal FBP and SBP pools under hypertonic conditions are discussed in terms of both inhibited light activation of fructose-1,6-bisphosphatase (EC 3.1.3.11) and sedoheptulose-1,7-bisphosphatase (EC 3.1.3.37), and likely increases in stromal ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) active-site concentrations.Abbreviations and symbols DAP dihydroxyacetone phosphate - FBP fructose-1,6-bisphosphate - PGA 3-phosphoglycerate - RuBP ribulose-1,5-bisphosphate - SBP sedoheptulose-1,7-bisphosphate - _s osmotic potential     

Recovery of maize seedling growth, development and photosynthetic efficiency after initial growth at low temperature

In order to investigate the mechanisms of maize adaptation to temperate climate, we studied photosynthetic efficiency, as evaluated by means of phiPSII and chloroplast ultrastructure, as well as growth and development of two inbred lines (the chilling-tolerant KW 1074 and the chilling-sensitive CM 109) under laboratory conditions. Plants were grown from seed to the 3rd leaf stage at a suboptimal temperature (14 degrees C/ 12 degrees C) and then the temperature was increased to 24 degrees C/22 degrees C. To verify the results obtained with the two model lines, twelve inbred lines were tested under both laboratory and field conditions. Initial growth at low temperature affected the chloroplast ultrastructure and photosynthetic efficiency, and this was more pronounced in CM 109 than in KW 1074 plants. The differences between the two lines were particularly pronounced in leaf 5. One week after the onset of favourable conditions, mesophyll chloroplast grana in the CM 109 line were small and thylakoids were developed only poorly. Also, thylakoids in bundle sheath chloroplasts were less frequent in CM 109 than in KW 1074. However, two weeks after the temperature increase, the ultrastructure of chloroplasts of the 5th leaf no longer differed distinctly between the two lines. One should note that in both lines, only the 7th and younger leaves reached a chloroplast ultrastructure and phiPSII indistinguishable from those of control plants. In general, the recovery of photosynthetic efficiency followed the development of leaves. It was delayed in the CM 109 more than in the KW 1074 inbred line relative to control plants grown continuously at the optimal temperature. The growth difference of 2-3 days between the two lines persisted even after the growth temperature was elevated. This suggested that the primary factor responsible for the different chilling-sensitivities of the two model lines was leaf development and the differences in development of the photosynthetic apparatus had only a secondary role. The delay in leaf development appeared as early as the stage of the 1st leaf. The same delay was observed when only the shoot apex was cooled. The importance for further recovery of the early stages of morphogenesis was confirmed by a correlation of Laboratory and field data that were obtained using a set of 12 inbred lines. Our results suggest that early stages of shoot morphogenesis determine the duration of the vegetative phase in cool regions, since the delay in growth at a low temperature cannot be compensated for during later growth at a higher temperature.     

Evidence for the involvement of nitric oxide and reactive oxygen species in osmotic stress tolerance of wheat seedlings: Inverse correlation between leaf abscisic acid accumulation and leaf water loss

Nitric oxide (NO) and reactive oxygen species (ROS) play important roles in both abscisic acid (ABA) signaling and stress-induced ABA accumulation. However, little is known about their physiological roles in the whole plant. In this study, the effects of NO and ROS on leaf water control and the roles of ABA were determined using wheat (Triticum aestivum L.) seedlings. As compared with the control, osmotic stress reduced leaf water loss (LWL) while it increased leaf ABA content. The effects of osmotic stress on LWL and ABA contents were partially reversed by NO scavengers or NO synthase (NOS) inhibitors. Furthermore, sodium nitroprusside (SNP) at concentrations between 0.01 and 10 mM all reduced LWL efficiently and induced ABA accumulation in a dose-dependent manner. When ABA synthesis was inhibited by fluridone or actidione, the effects of SNP on LWL were partially reversed. These results suggest that NO is involved in leaf water maintenance of wheat seedlings under osmotic stress, and one of the possible mechanisms is by stimulating ABA synthesis. The ROS scavengers used in our experiments had no effects on either LWL or ABA accumulation induced by osmotic stress. However, all ROS induced LWL reduction and ABA accumulation significantly. Hydrogen peroxide had the same effects as SNP on LWL and induced ABA accumulation in a dose-dependent manner but had a maximal effect at 1 mM. Fluridone reversed the effects of H₂O₂ on both LWL reduction and ABA accumulation, while actidione had no effect. These results suggest that ROS are also involved in leaf water maintenance of wheat seedlings by stimulating ABA biosynthesis, but with a different mechanism to that of NO. The ABA-independent mechanism in NO/ROS regulation of leaf water balance is discussed, in relation to our results.     

Photoinhibition of photosynthesis in intact kiwifruit (Actinidia deliciosa) leaves: Changes in susceptibility to photoinhibition and recovery during the growth season

Kiwifruit (Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson) plants grown in an outdoor enclosure were exposed to the natural conditions of temperature and photon flux density (PFD) over the growing season (October to May). Temperatures ranged from 14 to 21° C while the mean monthly maximum PFD varied from 1000 to 1700 mol · m^–2 · s^–1, although the peak PFDs exceeded 2100 mol · m^–2 · s^–1. At intervals, the daily variation in chlorophyll fluorescence at 692 nm and 77K and the photon yield of O₂ evolution in attached leaves was monitored. Similarly, the susceptibility of intact leaves to a standard photoinhibitory treatment of 20° C and a PFD of 2000 mol · m^–2 · s^–1 and the ability to recover at 25° C and 20 mol · m^–2 · s^–2 was followed through the season. On a few occasions, plants were transferred either to or from a shade enclosure to assess the suceptibility to natural photoinhibition and the capacity for recovery. There were minor though significant changes in early-morning fluorescence emission and photon yield throughout the growing season. The initial fluorescence, F_o, and the maximum fluorescence, F_m, were, however, significantly and persistently different from that in shade-grown kiwifruit leaves, indicative of chronic photoinhibition occurring in the sun leaves. In spring and autumn, kiwifruit leaves were photoinhibited through the day whereas in summer, when the PFDs were highest, no photoinhibition occurred. However, there was apparently no non-radiative energy dissipation occurring then also, indicating that the kiwifruit leaves appeared to fully utilize the available excitation energy. Nevertheless, the propensity for kiwifruit leaves to be susceptible to photoinhibition remained high throughout the season. The cause of a discrepancy between the severe photoinhibition under controlled conditions and the lack of photoinhibition under comparable, natural conditions remains uncertain. Recovery from photoinhibition, by contrast, varied over the season and was maximal in summer and declined markedly in autumn. Transfer of shade-grown plants to full sun had a catastrophic effect on the fluorescence characteristics of the leaf and photon yield. Within 3 d the variable fluorescence, F_v, and the photon yield were reduced by 80 and 40%, respectively, and this effect persisted for at least 20 d. The restoration of fluorescence characteristics on transfer of sun leaves to shade, however, was very slow and not complete within 15 d.Abbreviations and Symbols F_o, F_m, F_v initial, maximum, variable fluorescence - F_i F_v at t = 0 - F F_v at t = - PFD photon flux density - PSII photosystem II - leaf absorptance ratio - (_a photon yield of O₂ evolution (absorbed basis) - _{i a} at t = 0 - _a at t = We thank Miss Linda Muir and Amanda Yeates for their technical assistance in this study.     

Water stress in Eucalyptus pauciflora: comparison of effects on stomatal conductance with effects on the mesophyll capacity for photosynthesis,and investigation of a possible involvement of photoinhibition

Seedlings of Eucalyptus pauciflora Sieb. ex Spreng., grown in 4-1 pots, were stressed by withholding water while relationships between net assimilation rate (A) and intercellular partial pressure of CO₂ (p_i) in selected leaves were obtained repeatedly throughout the stress cycle. Water stress at first caused stomatal closure without any decline in the A(p_i) relationship. As stress became more severe, the A(p_i) relationship was affected as well. This always affected assimilation rate at both high and low intercellular partial pressures of CO₂. It was then tested whether water-stressed leaves were more prone to photoinhibition than unstressed ones. Plants were water-stressed while at the same time subjected to strong photon flux area density (2000 mol quanta·m^-2·s^-1). A possible light-induced inhibition was assessed by comparing quantum yields of photosynthesis with light directed onto one or the other surface of the leaf. A decline in quantum yield was observed, and the decline on the previously irradiated side was more pronounced than on the previously shaded side, but the effect was small and disappeared entirely within 1 d of rewatering the plants. It is concluded that photoinhibition can play a role, but not an important one, in the effect of water stress on the A(p_i) relationship in leaves of E. pauciflora.Abbreviations and symbols RuBP ribulose-1,5-bisphosphate - A net assimilation rate - p_i intercellular partial pressure of CO₂ - quantum yield of photosynthesis (net assimilation or RuBP-regeneration rate) - w difference in water content between air saturated at leaf temperature and the actual vapor content of the air, expressed as mole fraction     

Growth response of barley and tomato to nitrogen stress and its control by abscisic acid,water relations and photosynthesis

Barley (Hordeum vulgare L.) and tomato Lycopersicon esculentum Mill.) were grown hydroponically and examined 2, 5, and 10 d after being deprived of nitrogen (N) supply. Leaf elongation rate declined in both species in response to N stress before there was any reduction in rate of dryweight accumulation. Changes in water transport to the shoot could not explain reduced leaf elongation in tomato because leaf water content and water potential were unaffected by N stress at the time leaf elongation began to decline. Tomato maintained its shoot water status in N-stressed plants, despite reduced water absorption per gram root, because the decline in root hydraulic conductance with N stress was matched by a decline in stomatal conductance. In barley the decline in leaf elongation coincided with a small (8%) decline in water content per unit area of young leaves; this decline occurred because root hydraulic conductance was reduced more strongly by N stress than was stomatal conductance. Nitrogen stress caused a rapid decline in tissue NO ₃ ^- pools and in NO ₃ ^- flux to the xylem, particularly in tomato which had smaller tissue NO ₃ ^- reserves. Even in barley, tissue NO ₃ ^- reserves were too small and were mobilized too slowly (60% in 2 d) to support maximal growth for more than a few hours. Organic N mobilized from old leaves provided an additional N source to support continued growth of N-stressed plants. Abscisic acid (ABA) levels increased in leaves of both species within 2 d in response to N stress. Addition of ABA to roots caused an increase in volume of xylem exudate but had no effect upon NO ₃ ^- flux to the xylem. After leaf-elongation rate had been reduced by N stress, photosynthesis declined in both barley and tomato. This decline was associated with increased leaf ABA content, reduced stomatal conductance and a decrease in organic N content. We suggest that N stress reduces growth by several mechanisms operating on different time scales: (1) increased leaf ABA content causing reduced cell-wall extensibility and leaf elongation and (2) a more gradual decline in photosynthesis caused by ABA-induced stomatal closure and by a decrease in leaf organic N.Abbreviation and symbols ABA abscisic acid - c_i leaf internal CO₂ concentration - L_p root hydraulic conductance     

Photosynthesis of <Emphasis Type="Italic">Rehmannia glutinosa</Emphasis> subjected to drought stress is enhanced by choline chloride through alleviating lipid peroxidation and increasing proline accumulation

Rehmannia glutinosa seedlings were pretreated with choline chloride (CC) in concentrations of 0, 0.7, 2.1 and 3.5 mM, and then subjected to drought and rewatering treatment to study the effects of CC on the generation of reactive oxygen species (O₂⁻, H₂O₂), lipid peroxidation, proline accumulation, water status and photosynthesis. The results showed that pretreatment with CC alleviated the inhibition of SOD and APX activity caused by drought stress, and therefore, the rate of O₂⁻ production and H₂O₂ concentration were reduced and lipid peroxidation decreased in pretreated plants. CC pretreatment also accelerated accumulation of proline, maintained higher Ψw and RWC, deferred leaf water loss during drought stress and retarded the drop in proline concentration after rewatering. Consequently, drought-induced decreases in F_m/F₀, F_v/F_m, Φ_PS2, q_P, and A and increase in q_NP were inhibited and the recovery of photosynthesis after rewatering was quicker in pretreated plants. Although differences in F_v/F_m, Φ_PS2 and q_P between treatments were not significant, there was a general trend that the effects of CC increased with the rise of its concentrations. The data suggested that 2.1 mM of CC be suitable for alleviating lipid peroxidation, promoting proline accumulation, retarding leaf water loss and improving photosynthesis of R. glutinosa seedlings under drought stress.     

Photoinhibition of photosynthesis in intact kiwifruit (Actinidia deliciosa) leaves: effect of growth temperature on photoinhibition and recovery

Intact leaves of kiwifruit (Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson) from plants grown in a range of controlled temperatures from 15/10 to 30/25°C were exposed to a photon flux density (PFD) of 1500 μmol·m⁻²·s⁻¹ at leaf temperatures between 10 and 25°C. Photoinhibition and recovery were followed at the same temperatures and at a PFD of 20 μmol·m⁻²·s⁻¹, by measuring chlorophyll fluorescence at 77 K and 692 nm, by measuring the photon yield of photosynthetic O₂ evolution and light-saturated net photosynthetic CO₂ uptake. The growth of plants at low temperatures resulted in chronic photoinhibition as evident from reduced fluorescence and photon yields. However, low-temperature-grown plants apparently had a higher capacity to dissipate excess excitation energy than leaves from plants grown at high temperatures. Induced photoinhibition, from exposure to a PFD above that during growth, was less severe in low-temperature-grown plants, particularly at high exposure temperatures. Net changes in the instantaneous fluorescence,F ₀, indicated that little or no photoinhibition occurred when low-temperature-grown plants were exposed to high-light at high temperatures. In contrast, high-temperature-grown plants were highly susceptible to photoinhibitory damage at all exposure temperatures. These data indicate acclimation in photosynthesis and changes in the capacity to dissipate excess excitation energy occurred in kiwifruit leaves with changes in growth temperature. Both processes contributed to changes in susceptibility to photoinhibition at the different growth temperatures. However, growth temperature also affected the capacity for recovery, with leaves from plants grown at low temperatures having moderate rates of recovery at low temperatures compared with leaves from plants grown at high temperatures which had negligible recovery. This also contributed to the reduced susceptibility to photoinhibition in low-temperature-grown plants. However, extreme photoinhibition resulted in severe reductions in the efficiency and capacity for photosynthesis.     

Determination of carbon-reduction-cycle intermediates in leaves of Arbutus unedo L. suffering depressions in photosynthesis after application of abscisic acid or exposure to dry air

Gas exchange and contents of photosynthetic intermediates of leaves of Arbutus unedo L. were determined with the aim of recognizing the mechanisms of inhibition that were responsible for the midday depression of photosynthesis following exposure to dry air, and the decline in photosynthetic capacity following application of abscisic acid (ABA). Rapidly killed (<0.1 s) leaf samples were taken when gas analysis showed reduced CO₂ assimilation. Determination of the contents of 3-phosphoglyceric acid (PGA), ribulose 1,5-bisphosphate (RuBP), triose phosphates, fructose 1,6-bisphosphate and hexose phosphates in the samples showed that significant variation occurred only in the level of PGA. As a result, the ratio PGA/RuBP decreased with increasing inhibition of photosynthesis, particularly when application of ABA had been the cause. A comparison of metabolite patterns did not bring out qualitative differences that would have indicated that effects of ABA and of dry air had been caused by separate mechanisms. Depression of photosynthesis occurred in the presence of sufficient RuBP which indicated that the carboxylation reaction of the carbon-reduction-cycle was inhibited after application of ABA or exposure to dry air.Abbreviations and symbols ABA abscisic acid - C _a partial pressure of CO₂ in the ambient air - C _i partial pressure of CO₂ in the intercellular spaces - I quantum flux - PGA 3-phosphoglyceric acid - RuBP ribulose 1,5-bisphosphate - I _L leaf temperature - w water-vapor pressure difference between leaf and air     

Effect of water stress on growth, Na+ and K+ accumulation and water use efficiency in relation to osmotic adjustment in two populations of Atriplex halimus L.

The effect of water stress on growth, Na⁺ and K⁺ accumulation and water utilization was investigated in plants of two populations of Atriplex halimus L. originating from Kairouan (Tunisia) and Tensift (Morocco). Water deficit was applied by withholding water for 22 days. All plants remained alive until the end of the treatment although growth was strongly reduced in both populations. Water stress decreased CO₂ assimilation in saturating conditions, mainly in the population obtained from Kairouan, suggesting an impact of drought on the dark phase of photosynthesis, beside a decrease in stomatal conductance which was recorded mainly in the population obtained from Tensift. The two studied populations did not differ in their water consumption, as indicated by similar soil gravimetric water content and plant transpiration. However, water use efficiency increased under stress conditions in the population from Tensift but not in the population from Kairouan. Thelatter population displayed a larger capacity for osmotic adjustment. A drought-induced specific increase in Na⁺ concentration was also reported in both populations. It is concluded that in A. halimus, water stress resistance estimated in terms of biomass production, could be associated with higher WUE rather than with with a greater osmotic adjustment and that sodium may assume a specific physiological function in this xerohalophytic C₄ species.     

A simple procedure for polymerase chain reaction of the PSBA gene in algae: application to the screening of mutations conferring atrazine resistance and discrimination of natural populations of Porphyra linearis

A simplified procedure is described for polymerase chain reaction (PCR) of a partial sequence (bp 601–893) of the plastid gene psbA in the rhodophyte Porphyra linearis and the diatoms Haslea ostreria and Skeletonema costatum. This procedure involves the use of all tissues of P. linearis and live cell suspensions of H. ostreria or S. costatum as DNA templates, without any further purification of DNA. As in the case of PCR with DNA extracts, a single major band of the expected size (292 bp) was obtained after PCR for the three species. Sequences of the amplified fragments were aligned, confirming that the amplified products were part of the psbA gene. The method was then used to screen mutations in partial psbA genes of 23 samples of P. linearis collected at four different stations along the mid-Atlantic coast of France. An alignment was obtained indicating the existence of mutations, though not in codons known for herbicide resistance. All mutations found were silent. However, genetic polymorphism discriminated between samples collected from two stations. The method employed allows rapid amplification of the herbicide target gene and simplifies the procedure for screening mutations or populations in algae. Its application to other genes and species is considered.     

Control of light-induced bean leaf expansion: Role of osmotic potential,wall yield stress,and hydraulic conductivity

The role of three-turgor-related cellular parameters, the osmotic potential ( _s), the wall yield stress (Y) and the apparent hydraulic conductivity (L'p), in the initiation of ligh-induced expansion of bean (Phaseolus vulgaris L.) leaves has been determined. Although light causes an increase in the total solute content of leaf cells, the water uptake accompanying growth results in a slight increase in _s. Y is about 4 bar; and is unaffected by light. L'p, as calculated from growth rates and isopiestic measurements of leaf water potential, is only slightly greater in rapidly-growing leaves. The turgor pressure of growing cells is lower than that of the controls by about 35%. We conclude that light does not induce cell enlargement in the leaf by altering any of the above parameters, but does so primarily by increasing wall extensibility.Abbreviations and symbols RL red light - WL white light - L'p apparent hydraulic conductivity - OC osmotic concentration - Y wall yield stress - _s osmotic potential     

Movement and compartmentation of abscisic acid in guard cells of Valerianella locusta: Effects of osmotic stress,external H+-concentration and fusicoccin

Epidermal peels of Valerianella locusta were acid-treated for 1 h at pH 3.9 to kill all cells other than guard cells. These guard-cell preparations were used to explore the steady-state one-way fluxes and the cytoplasmic and vacuolar contents of abscisic acid (ABA). The method of compartmental analysis has been applied. The intracellular ABA concentrations were surprisingly high. At an external pH of 5.8 the cytoplasm contained 1.28 mmol·dm^-3 of ABA, twice of the amount which accumulated in the vacuoles (0.57 mmol·dm^-3). The fluxes of ABA at the plasmalemma (_oc=_oc=0.43 fmol · cell ^–1 · h ^–1) were higher than those at the tonoplast (_cv=_vc=0.12 fmol · cell ^–1 · h ^–1). Moderate stress (0.1 and 0.3 mol·dm^-3 sorbitol in the medium) caused a change in the kinetics of ABA movement. The rate constants of the fluxes from the cytoplasm into the vacuole (_cv) and into the apoplast (_co) were increased while the rate constant of the flux from the vacuoles into the cytoplasm (_vc) was decreased. As a consequence the amount of ABA sequestered in the vacuole remained unchanged; the cytoplasmic ABA content, however, was reduced to only 20% of that found in the control treatments (no sorbitol in the medium). Under moderate stress, one Valerianella guard cell released rapidly about 0.36 fmol·cell^-1 to its direct cell-wall space. This surprising result is discussed in regard to rapid stomatal closure under reduced water supply.Abbreviations ABA abscisic acid - FC fusicoccin     

Cancer pagurus (Linnaeus, 1758) physiological responses to simulated live transport: Influence of temperature, air exposure and AQUI-S

Cancer pagurus is a commercially important crab, mostly appreciated in Southern European countries, including Portugal, being usually live transported from UK and France. Once in Portugal, crabs are redistributed across the country in small refrigerated vivier lorries in air or immersed conditions, while some are sent exposed to air to the Azores archipelago with mortality reaching 40-60%. In order to optimise transport conditions and survival, simulated live transport of immersed and air exposed crabs sedated or not with an anaesthetic, AQUI-S^®, was tested at different temperatures. It was found that crabs experienced stress during the experiment (with increased l-lactate, d-glucose and lowered pH), but with different magnitudes according to temperature, treatment and transport duration, resulting in 100% mortalities at 16 °C in immersed conditions and when exposed to air with AQUI-S^®. Results indicate that long duration transport in semi-dry conditions is viable at low temperatures (8 °C), while immersed transport is viable at 12 °C. AQUI-S^® was not an efficient solution at low temperatures in semi-dry conditions, but for short duration transport in immersed conditions at 16 °C it was the only treatment without mortality.     

Contrasting strategies to cope with drought by invasive and endemic species of Lantana in Galapagos

This study compares how Lantana camara, an invasive species, and L. peduncularis, an autochthonous one, cope with drought in Galapagos. Soil surface temperature was the abiotic environmental parameter that best explained variations in photosynthetic stress. Higher soil surface temperatures were recorded in the lowlands and in rain-shadow areas, which were also the driest areas. L. peduncularis, with a shallow root system, behaved as a drought-tolerant species, showing lower relative growth rates, which decreased with leaf water content and higher photosynthetic stress levels in the lowlands and in a northwest rain-shadow area in comparison with higher and wetter locations. Its basal and maximal fluorescences decreased at lower altitudes, reflecting the recorded drops in chlorophyll concentration. In contrast, L. camara with a deep root system behaved as a drought-avoiding species, showing leaf and relative water contents higher than 55% and avoiding permanent damage to its photosynthetic apparatus even in the driest area where it showed very low chlorophyll content. Its relative growth rate decreased more in dry areas in comparison to wetter zones than did that of L. peduncularis, even though it had greater water content. Furthermore, L. camara showed higher water contents, growth rate, and lower photosynthetic stress levels than L. peduncularis in the arid lowlands. Thus, L. peduncularis maintained lower maximum quantum efficiency of photosystem II photochemistry (F _v/F _m) than L. camara even at sunrise, due to higher basal fluorescence values with similar maximal fluorescence, which indicated permanent damage to PSII reaction centres. Our results help to explain the success and limitations of L. camara in the invasion of arid and sub-arid environments.