Expressed sequence tag analysis and development of gene associated markers in a near-isogenic plant system of Eragrostis curvula

Eragrostis curvula (Schrad.) Nees is a forage grass native to the semiarid regions of Southern Africa, which reproduces mainly by pseudogamous diplosporous apomixis. A collection of ESTs was generated from four cDNA libraries, three of them obtained from panicles of near-isogenic lines with different ploidy levels and reproductive modes, and one obtained from 12 days-old plant leaves. A total of 12,295 high-quality ESTs were clustered and assembled, rendering 8,864 unigenes, including 1,490 contigs and 7,394 singletons, with a genome coverage of 22%. A total of 7,029 (79.11%) unigenes were functionally categorized by BLASTX analysis against sequences deposited in public databases, but only 37.80% could be classified according to Gene Ontology. Sequence comparison against the cereals genes indexes (GI) revealed 50% significant hits. A total of 254 EST-SSRs were detected from 219 singletons and 35 from contigs. Di- and tri- motifs were similarly represented with percentages of 38.95 and 40.16%, respectively. In addition, 190 SNPs and Indels were detected in 18 contigs generated from 3 to 4 libraries. The ESTs and the molecular markers obtained in this study will provide valuable resources for a wide range of applications including gene identification, genetic mapping, cultivar identification, analysis of genetic diversity, phenotype mapping and marker assisted selection.     

Water stress effects on photosynthesis in different mulberry cultivars

The effect of water stress on photosynthesis was determined in five mulberry cultivars (Morus alba L. cv. K-2, MR-2, BC2-59, S-13 and TR-10). Drought was imposed by withholding water and the plants were maintained at different water potentials ranging from 0.5 -MPa to 2.0 -MPa. Photosynthetic rates, activities of ribulose-1,5-bisphosphate carboxylase and sucrose phosphate synthase, photosystem II activity and chlorophyll content were used as key parameters to assess photosynthetic performance. There was a marked variation in the photosynthetic rates and ribulose-1,5-bisphosphate carboxylase activity among the five mulberry cultivars subjected to water stress. Photosystem II (PSII) and sucrose phosphate synthase activities were also severely reduced as measured by drought conditions. Of the five mulberry cultivars, S-13 and BC2-59 showed higher photosynthetic rates, ribulose-1,5-bisphosphate carboxylase activity, high sucrose phosphate synthase activity and photochemical efficiency of PSII compared to the other varieties.     

Esterase isozyme variation in theEragrostis curvula complex (Poaceae)

The biosystematic relationships of the apomictic complexEragrostis curvula s. lato, is investigated by disc electrophoresis of seed extracts to obtain esterase patterns of 23 accessions representing the morphological variants of this complex: curvula, conferta, robusta, chloromelas and lehmanniana. The zymograms thus obtained were classified into four groups on the basis of the presence of certain bands taken as characteristic and constant markers. Within each group variations were found in strict accordance with the morphological and cytogenetic data available on the complex. Cluster analysis showed similarity levels between the strains studied, representing different genomic groups. The esterase pattern proved useful as an additional criterion for identifying the individual taxa making up the complex and for evaluating their reciprocal relationships.     

Use of simultaneous analysis of gas-exchange and chlorophyll fluorescence quenching for analysing the effects of water stress on photosynthesis in apple leaves

Summary A convenient system for the rapid simultaneous measurement of both chlorophyll fluorescence quenching using a modulated light system, and of CO₂, and water vapour exchange by leaves is described. The system was used in a study of the effects of water deficits on the photosynthesis by apple leaves (Malus x domestica Borkh.). Apple leaves were found to have low values of steady-state variable fluorescence, and the existence of significant fluorescence with open traps (F_o) quenching necessitated the measurement and use of a corrected F_o in the calculation of quenching components. Long-term water stress had a marked effect on both gas-exchange and chlorophyll fluorescence quenching. Non-photochemical quenching (qn) in particular was increased in water-stressed leaves, and it was particularly sensitive to incident radiation in such leaves. In contrast, rapid dehydration only affected gas exchange. Relaxation of qn quenching in the dark was slow, taking approximately 10 min for a 50% recovery, in well-watered and in draughted plants, and whether or not the plants had been exposed to high light.     

Growth, water relations and photosynthesis of seedlings and resprouts after fire

Seasonal patterns of growth, water relations, photosynthesis and leaf characteristics were compared between obligate seeders (Cistus monspeliensis and Cistus ladanifer) and resprouters (Arbutus unedo and Pistacia lentiscus) from the first to the second year after fire. We hypothesized that seedlings would be more water-limited than resprouts due to their shallower root systems. Regarding water use strategies, Cistus species are drought semi-deciduous and A. unedo and P. lentiscus are evergreen sclerophylls, therefore, comparisons were based on the relative deviation from mature conspecific plants. Seedlings and resprouts had higher shoot elongation and leaf production than mature plants, and over an extended period. Differences from mature plants were larger in resprouts, with two-fold transpiration, leaf conductance and photosynthesis in late spring/early summer. Seedlings of C. monspeliensis exhibited higher transpiration and leaf conductance than mature plants, while those of C. ladanifer only exhibited higher water potential. Growth increments and ameliorated water relations and photosynthesis after fire were attributed to an increase in water and nutrient availability. The small differences in water relations and photosynthesis between seedlings and mature conspecifics are in accordance with the prediction of seedlings experiencing higher water limitation than resprouts. We attribute these results to differences in root systems: resprouters benefited from an increase in root/shoot ratios and the presence of deep roots whereas Cistus seedlings relied on very shallow roots, which cannot provide assess to deep water during summer. Nevertheless, seedlings did not show evidence of experiencing a more severe water limitation than mature conspecifics, which we attributed to the presence of efficient mechanisms of avoiding and tolerating water stress. The results are discussed in relation to post-fire demography of seeders and resprouters in Mediterranean communities.     

Combined effects of water stress and high temperature on photosynthesis, nitrogen metabolism and lipid peroxidation of a perennial grass Leymus chinensis

Drought and high-temperature stresses have been extensively studied; however, little is known about their combined impact on plants. In the present study, we determined the photosynthetic gas exchange, chlorophyll fluorescence, nitrogen level, and lipid peroxidation of the leaves of a perennial grass (Leymus chinensis (Trin.) Tzvel.) subjected to three constant temperatures (23, 29 and 32°C), and five soil-moisture levels (75–80%, 60–65%, 50–55%, 35–40% and 25–30% of field capacity, respectively). High temperature significantly decreased plant biomass, leaf green area, leaf water potential, photosynthetic rate (A), maximal efficiency of PSII photochemistry (F _v/F _m), actual PSII efficiency (Φ_PSII), the activities of nitrate reductase (NR; EC 1.6.6.1) and glutamine synthetase (GS; EC 6.3.1.2), but markedly increased the ratio of leaf area to leaf weight (SLA), endopeptidase (EP; EC 3.4.24.11) activity, and malondialdehyde (MDA) content, especially under severe water stress conditions. The A and F _v/F _m were significantly and positively correlated with leaf-soluble protein content, and the activities of NR and GS. However, both photosynthesis parameters were significantly and negatively correlated with EP activity and MDA content (P < 0.05). It is suggested that high temperature, combined with severe soil drought, might reduce the function of PSII, weaken nitrogen anabolism, strengthen protein catabolism, and provoke lipid peroxidation. The results also indicate that severe water stress might exacerbate the adverse effects of high temperature, and their combination might reduce the plant productivity and distribution range of L. chinensis in the future.     

Field water relations of three temperate vines

Summary A two year comparative field water relations study was conducted in central Maryland on three sympatric temperate lianas, Lonicera japonica, Vitis vulpina and Parthenocissus quinquefolia. Seasonal physiological activity was longer by approximately 9 weeks in the evergreen L. japonica, while peak rates of stem elongation were 4–10 fold higher in the two deciduous species. There were marked differences in vascular anatomy and water use patterns among the three species, however all three evidenced varying degrees of stomatal closure in response to decreasing soil water availability and increasing atmospheric evaporative demand. The range of leaf water potentials measured in these species was quite narrow in comparison to other temperate woody species. Two of the species showed no alterations in their tissue water release properties in response to decreased soil water availability, while V. vulpina showed a limited capacity in this regard. Most significant among the species differences in water relations were the conservative water use patterns of P. quinquefolia, and the midday maxima of transpirational water loss measured in L. japonica compared to the morning peaks in traspiration for the two deciduous species. The differences found in anatomy, leaf phenology, climbing mechanics, water relations and canopy development among these three sympatric vines implies a spatial and temporal partitioning of light and water resources and emphasizes the diversity of morphological-physiological suites of characters present among species co-occurring in the same macrohabitat.     

Increased photosynthesis and water potentials in Silphium integrifolium galled by cynipid wasps

Interactions between drought, insect herbivory, photosynthesis, and water potential play a key role in determining how plants tolerate and defend against herbivory, yet the effects of insect herbivores on photosynthesis and water potential are seldom assessed. We present evidence that cynipid wasp galls formed by Antistrophus silphii on Silphium integrifolium increase photosynthesis (A), stomatal conductance (g), and xylem water potential (). Preliminary data showed that in drought-stressed plants galled shoots had 36% greater A, and 10% greater stem than ungalled shoots, while in well-watered plants leaf gas exchange was not affected by galls. We hypothesized that 1) galled shoots have higher , g, and A than ungalled shoots, but this differences diminishes if plant drought stress is reduced, and 2) galls can reduce decreases in A and g if water availability decreases. A field experiment testing the first hypothesis found that galls increased g and , but that differences between galled and ungalled shoots did not diminish after plants were heavily watered. A laboratory test of the second hypothesis using potted Silphium found that galled plants had smaller drops in A and g over a 4-day dry-down period. A vs g and A vs intercellular CO₂ concentration relationships were consistent with the explanation that increased allows galls to increase A by reducing stomatal limitation of A, rather than by altering sink-source relationships or by removing low- limitations on non-stomatal components of A. Our working hypothesis is that galls increase and A by reducing the shoot: root ratio so that the plant is exploiting a greater soil volume per unit leaf area. We argue that increased A is an ineffective way for Silphium to compensate for negative effects of gall insect attack. Instead, increased and A may protect gall insects from variation in resource availability caused by periodic drought stress, potentially reducing negative effects of drought on plant quality and on gall insect populations.     

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     

Gas exchange and water relations of two Rocky Mountain shrub species exposed to a climate change manipulation

Gas exchange and water relations responses to warming were compared for two shrub species, Artemisia tridentata spp. vaseyana (Asteraceae), a widely distributed evergreen species of the Great Basin and the western slope of the Rocky Mountains, and Pentaphylloides floribunda (Rosaceae), a deciduous shrub limited in distribution to moist, high-elevation meadows. Plants were exposed to an in situ infrared (IR) climate change manipulation at the Rocky Mountain Biological Laboratory, near Crested Butte, CO. Measurements of gas exchange and water relations were made on the two species in July and August, 1993 from plants growing in situ in infrared-heated and control plots. Carbon dioxide uptake, water loss, leaf temperature, water use efficiency, and water potential were compared to test the hypothesis that leaf and soil responses to IR will cause leaf level changes in photosynthesis. Photosynthetic CO₂ uptake and water use efficiency increased for A. tridentata (2.9 vs. 1.9 mol m^–2 s^–1 and 1.2 vs. 0.7 mmol C/mol H₂O) in the heated plots compared to the controls, while water potential was significantly lower in the heated plots (–1.1 vs. –0.5 MPa). The heating treatment decreased rates of photosynthesis for P. floribunda, but not significantly so. For A. tridentata, the results are consistent with the community-level changes observed with heating. Taken together, the evidence suggests that global warming is likely to result in increasing dominance of A. tridentata in subalpine meadow habitat now dominated by forbs.     

Environmental effects on circadian rhythms in photosynthesis and stomatal opening

Persistent circadian rhythms in photosynthesis and stomatal opening occurred in bean (Phaseolus vulgaris L.) plants transferred from a natural photoperiod to a variety of constant conditions. Photosynthesis, measured as carbon assimilation, and stomatal opening, as conductance to water vapor, oscillated with a freerunning period close to 24 h under constant moderate light, as well as under light-limiting and CO₂-limiting conditions. The rhythms damped under constant conditions conducive to high photosynthetic rates, as did rates of carbon assimilation and stomatal conductance, and this damping correlated with the accumulation of carbohydrate. No rhythm in respiration occurred in plants transferred to constant darkness, and the rhythm in stomatal opening damped rapidly in constant darkness. Damping of rhythms also occurred in leaflets exposed to constant light and CO₂-free air, demonstrating that active photosynthesis and not simply light was necessary for sustained expression of these rhythms. This is CIWDPB Publication No. 1142 This research was supported by National Science Foundation grant BSR 8717422 (C.B.F.) and a U.S. Department of Agriculture training grant to Stanford University (T.L.H.).     

Photosynthesis and water relations and the role of anatomy in Umbilicariaceae (lichenes) from Central Spain

Summary The response of net photosynthesis and dark respiration in eight species of Umbilicariaceae (lichenes) to temperature (-5, 0, 5, 10, 15, 20, 25, 30°C) and irradiance (55, 110, 220, 400, 620 mol photons m^-2 s^-1 PAR) was studied. The samples were collected in montane and alpine localities of the Spanish Sistema Central. The species differed widely in their net photosynthetic rates. The optimal temperature for net photosynthesis in alpine species was significantly lower than in montane species. Montane species were more photophytic than alpine ones. Water saturation and water loss rate were dependent on morphology and particularly anatomy of the thallus. The physiological and structural data are useful in the interpretation of the ecology and altitudinal distribution of the Umbilicariaceae. No adaptation could be linked to particularities of the mediterranean climate.     

Biomass production,photosynthesis, and leaf water relations of <Emphasis Type="Italic">Spartina alterniflora</Emphasis> under moderate water stress

The perennial smooth cordgrass, Spartina alterniflora, has been successfully introduced in salty ecosystems for revegetation or agricultural use. However, it remains unclear whether it can be introduced in arid ecosystems. The aim of this study was to investigate the physiological response of this species to water deficiency in a climate-controlled greenhouse. The experiment consisted of two levels of irrigation modes, 100 and 50% field capacities (FC). Although growth, photosynthesis, and stomatal conductance of plants with 50% FC were reduced at 90 days from the start of the experiment, all of the plants survived. The water-stressed plants exhibited osmotic adjustment and an increase in the maximum elastic modulus that is assumed to be effective to enhance the driving force for water extraction from the soil with small leaf water loss. An increase in the water use efficiency was also found in the water-stressed plants, which could contribute to the maintenance of leaf water status under drought conditions. It can be concluded that S. alterniflora has the capacity to maintain leaf water status and thus survive in arid environment.     

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     

Clonal variation of tea [Camellia sinensis (L.) O. Kuntze] in countering water deficiency

Various clones of tea [Camellia sinensis (L.) O. Kuntze] such as TTL-1, TTL-2, TTL-4, TTL-5, TTL-6, UPASI-2 and UPASI-3 planted in the field were subjected to soil moisture stress conditions by withholding irrigation. A control set of the same clones were maintained by watering regularly. The soil water content of the irrigated and non irrigated plants was monitored through the soil moisture status. The extent of effect of drought on tea plants were monitored through various physiological parameters such as shoot weight, leaf water potential, chlorophyll and carotenoid content, chlorophyll fluorescence (Fv/Fm), net photosynthetic rate, transpiration rate, stomatal conductance and biochemical parameters such as extent of proline accumulation and free radical generation. These parameters were studied on the 30 d of non irrigation and on the 5 d during recovery from drought. The plants recovered when re-irrigated after 30 d of non-irrigation, which suggests that permanent wilting did not occur due to non-irrigation up to 30 d. On the 30 d of non-irrigation the clones TTL-1, TTL-6 and UPASI-2 showed lesser reduction of shoot weight, leaf water potential, chlorophyll fluorescence, photosynthetic rate, transpiration rate and stomatal conductance and increased proline and lesser lipid peroxidation as compared to the other clones. From these results it can be concluded that the clones TTL-1, TTL-6 and UPASI-2 are comparatively more drought tolerant than the clones TTL-2, TTL-4, TTL-5 and UPASI-3.     

Effect of water stress on lupin stem protein analysed by two-dimensional gel electrophoresis

Lupinus albus plants can withstand severe drought stress and show signs of recovery 24 h after rewatering (RW). Two-dimensional gel electrophoresis was used to evaluate the effect of water deficit (WD) on the protein composition of the two components of the lupin stem (stele and cortex). This was performed at three distinct stress levels: an early stage, a severe WD, and early recovery. Protein characterisation was performed through mass spectrometric partial sequencing. Modifications in the protein expression were first noticed at 3 days of withholding water, when the plant water status was still unaffected but some decrease in the relative soil water content had already occurred. An increase in serine proteases, possibly associated with WD sensing, was an early alteration induced by WD. When the stress severity increased, a larger number of stem proteins were affected. Immunophilin, serine protease and cysteine protease (well-known components of animal sensing pathways) were some of these proteins. The simultaneous expression of proteases and protease inhibitors that reacted differently to the stress level and to RW was found. Although the level of protease inhibitors was significantly raised, RW did not cause de novo expression of proteins. Many amino acid sequences did not match known sequences of either protein or expressed sequence tag databases. This emphasises the largely unknown nature of stem proteins. Nevertheless, some important clues regarding the way the lupin plant copes with WD were revealed.     

Effects of water stress on cryptoendolithic cyanobacteria from hot desert rocks

Four strains of Chroococcidiopsis and one Chroococcus, all isolated from extreme arid desert rocks, and one marine Chroococcus, were subjected to water stress using both matric and osmotic control methods. For all Chroococcidiopsis strains, photosynthetic rates decreased with decreasing water potential. After 24h preincubation the decrease was linear but after 72h there was a sharp drop below-3400 kPa (a _w0.976). In contrast, the two Chroococcus strains showed optimum photosynthesis between-3000 and-4000 KPa. It appears, therefore, that Chroococcidiopsis in deserts may have a different survival strategy in response to aridity than Chroococcus (rare in deserts).Absolute rates of ¹⁴CO₂ uptake were higher in matric than in osmotic control systems. It is suggested that, in a matric experimental system, the water status is more representative of the natural conditions in arid environments.The consistent differences between different strains in their response to water stress suggest that this character in Cyanobacteria may be of taxonomic significance.     

Comparative responses of three fuel wood yielding plants to PEG-induced water stress at seedling stage

Three species of fast growing fuel wood yielding plants locally available (Acacia holosericea, Bauhinia variegata and Cassia siamea) were characterized in respect of their responses to water stress. Seedlings (25 days) of these species, exposed to two levels of water stress (−0.5 and −1.0 MPa) induced by PEG-6000 for 24 h, were analysed for relative water content (RWC) and the contents of chlorophyll, protein, soluble sugars and proline in leaves along with activities of catalase, peroxidase and superoxide dismutase (SOD). RWC was lower in stressed compared to the unstressed seedlings. However, stress-induced decline in RWC was lowest in B. variegata. Chlorophyll and protein contents declined with increasing levels of water stress, decline being least in B. variegata. Soluble sugar and proline contents increased under water stress particularly in B. variegata. The enzyme activity of catalase (EC-1.11.1.6), peroxidase (EC-1.11.1.7) and SOD (EC-1.15.1.1) decreased with increased levels of water stress. Such decline in the activity of these enzymes was least in B. variegata. Apparently, B. variegata is potentially the species most tolerant to water stress among these three fuel wood-yielding plants.     

Combined effect of NaCl-salinity and hypoxia on growth,photosynthesis, water relations and solute accumulation in Phragmites australis plants

Aim of the present study was to investigate the effects of two key environmental factors of estuarine ecosystems, salinity and hypoxia, on the physiological attributes in reed plants (Phragmites australis (Cav.) Trin. ex Steudel). Growth, leaf gas exchange, water (and ion) relations, and osmotic adjustment were determined in hydroponically grown plants exposed to hypoxia at varying NaCl-salinity concentrations (0, 50, 100, and 200 mM). Plants grew well under hypoxia treatment with standard nutrient solution without added salt and at NaCl concentrations up to 100 mM. Reed plants were able to produce and allocate phytomass to all their organs even at the highest salt level (200 mM NaCl). In plants subjected to hypoxia at various water potentials no clear relationships were found between growth and photosynthetic parameters except for g_s, whereas growth displayed a highly significant correlation with plant–water relations. A and g_s of reed plants treated with hypoxia at varying water potential of nutrient solutions were positively correlated and the former variable also had a strong positive relationship with E. Leaf Ψ_w and Ψ_π followed a similar trend and declined significantly as water potential of watering solutions was lowered. Highly significant positive correlations were identified between leaf Ψ_w and photosynthetic parameters. At all NaCl concentrations, the increase in total inorganic ions resulted from increased Na⁺ and Cl⁻ while K⁺, Ca²⁺, and Mg²⁺ concentrations decreased with increasing osmolality of nutrient solutions. Common reed has an efficient mechanism of Na⁺ exclusion from the leaves and exhibited a high leaf K⁺/Na⁺ selectivity ratio over a wide range of salinities under hypoxia treatment. In Phragmites australis grown in 200 mM NaCl, K⁺ contributed 17% toΨ_π, whereas Na⁺ and Cl⁻ accounted for only 11% and 6%, respectively. At the same NaCl concentration, the estimated contribution of proline to Ψ_π was less than 0.2%. Changes in leaf turgor occurred with a combined effect of salinity and hypoxia, suggesting that reed plants could adjust their water status sufficiently.     

The effect of ionic stress on photosynthesis in Dunaliella tertiolecta

A comparison of the effects of ionic stress and an uncoupler on long-term fluorescence transients (the Kautsky effect) in the green alga Dunaliella tertiolecta indicated that the large quenching induced by ionic stress was caused by a pH gradient across the thylakoid membrane. This possiblity was given support by the increase in the slow phase of 3-(3,4-dichlorophenyl)-1,1-dimethylurea-induced fluorescence relaxation in algae subjected to ionic stress. Low-temperature fluorescence emission spectra indicated that salt stress enhanced photosystem-I emission in the dark, and a comparison of simultaneous emissions at 695 and 720 nm at room temperature indicated a further increase in photosystem-I emission during the fluorescence transients. Taken together with the decrease in the fast phase of 3-(3,4-dichlorophenyl)-1,1-dimethylurea-induced fluorescence relaxation in stressed algae, our results indicate that ionic stress stimulates cyclic electron flow, and that non-cyclic flow is inhibited. The effect of sucrose-induced osmotic stress was similar to, but less marked than, the effects of NaCl and KCl; the effect of decreasing the external salinity was small.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - FCCP carbonylcyanide p-trifluoromethoxyphenylhydrazone - PSI, II photosystem I, II