Adaptation to Water Stress in Wheat

Three experiments were designed to investigate to what extent adaptation to water stress take place. Wheat (Triticum aestivum L. cv. Kolibri) was grown in water culture at constant temperature, air humidity, and light intensity. When the plants were 16 days old, the potential of the root medium (ψ_r) was lowered by 1 bar every second day by means of polyethyleneglycol 1500 down to ?4 or ?7 bar and then remained at these levels. As a control one experiment was grown at ?0.7 bar. By regression it was found that when ψ_r was lowered by I bar, osmotic potential in leaf (ψ_π) decreased 1.46 bar, and leaf water potential (ψ_t) 0.68 bar, which mean an increase of turgor of 0.78 bar. At the same time the leaf water content did fall 0.30 g per g dry matter. Specific transpiration rate increased significantly after ψ_r was kept constant, but the increase in area of fresh leaves was strongly reduced due to wilting of old leaves. After an “adaptation” period during which ψ_r remained at ?0.7, ?4, and ?7 bar, respectively, for at least 1 week. ψ_r was altered so as to cover the range from 0 to ?14 bar and ψ_π, ψ_r, transpiration and diffusion resistance in stomata (r_s) were measured. The levels of ψ_π and ψ₁ were lower (more negative) and turgor potential higher in plants grown at low ψ_r. The transpiration in pre-stressed plants showed less sensitivity to the alteration of ψ_r than in the non-stressed plants. The values of ψ_r at which r_s increased greatly, were found to be about ?13, ?15, and ?18 bar for plants grown at ?0.7, ?4, and ?7 bar, respectively.     

Measurement of restricted rotational diffusion of fluorescent lipids in supported planar phospholipid monolayers using angle-dependent polarized fluorescence photobleaching recovery

A theory describing the shapes of polarized fluorescence photobleaching recovery (PFPR) curves for a population of fluorophores undergoing restricted rotational diffusion in two-dimensional systems such as planar membranes has been developed. In this model, restricted rotational diffusion of the fluorophores is described by using reflective boundary conditions, in which the fluorophores are assumed to diffuse freely but only within an angular space of width 2ω. The magnitude and apparent rate of the PFPR postbleach fluorescence curves are a function of both ω and the angle between the bleaching and observation beam polarizations ψ. It is shown that estimates of the degree of rotational restriction ω may be obtained from changes in the ψ-dependent postbleach fluorescence intensities. Using angle-dependent PFPR, slow rotational reorientations of the fluorescent lipid analogue 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine in distearoylphosphatidylcholine Langmuir–Blodgett monolayers deposited on octadecyltrichlorosilane-treated fused quartz were measured. As theoretically predicted for a rotationally restricted fluorophore population, both the initial F_ψ(0) and final F_ψ(∞) postbleach fluorescence intensities varied as a function of ψ, and no measurable change in the postbleach fluorescence intensities was observed for ψ = 45°. Using the theory for restricted rotational motion, the ψ-dependent variations of the final fluorescence intensities F_ψ(∞) obtained at two bleaching intensities gave an average apparent ω ≈? 52°. However, to adequately fit the F_ψ(0) data, inclusion of the theoretical effects of rapid (faster than the duration of the photobleaching pulse) fluorophore dynamics was also required. Best fits of the F_ψ(0) and F_ψ(∞) data were obtained when the fluorophores were assumed to rapidly wobble within a cone of semiangle δ ≈? 30°–50° while slowly rotating within an angular space defined by semiangle ω ≈? 35°–60°. Subsequent analysis of the time- and ψ-dependent changes in the postbleach fluorescence curves F_ψ(t) gave apparent diffusion coefficients ranging from D ≈? 10^?3 s^?1 to 4 × 10^?2 s^?1. © 1993 John Wiley & Sons, Inc.     

Analyse conformationnelle de la N-méthylamide de l'acide L-pyroglutamique par diffusion Rayleigh depolarisée,spectroscopie de vibration et calcul de l'energie conformationnelle

Conformational analysis of N-methylamide of pyroglutamic acid has been performed by theoretical energy calculations and experimental physical techniques, namely, laser Raman spectroscopy and depolarized Rayleigh scattering. The two theoretically predicted conformations are evidenced in crystalline state (ψ₁ = +169°) and in aqueous solution (ψ₁ ? ?20°). This study confirms the interest of a careful vibrational analysis of peptides and N-deuterated derivatives for providing an estimate of the dihedral angle ψ. The relationship between amide III frequency and ψ values is emphasized.     

Hydraulic architecture and water flow in growing grass tillers (Festuca arundinacea Schreb.)

The water relations and hydraulic architecture of growing grass tillers (Festuca arundinacea Schreb.) are reported. Evaporative flux density, E (mmol s^?1 m^?2), of individual leaf blades was measured gravimetrically by covering or excision of entire leaf blades. Values of E were similar for mature and elongating leaf blades, averaging 2·4 mmol s^?1 m^?2. Measured axial hydraulic conductivity, K_h (mmol s^?1 mm MPa^?1), of excised leaf segments was three times lower than theoretical hydraulic conductivity (K_t) calculated using the Poiseuille equation and measurements of vessel number and diameter. K_t was corrected (K_t*) to account for the discrepancy between K_h and K_t and for immature xylem in the basal expanding region of elongating leaves. From base to tip of mature leaves the pattern of K_t* was bell‐shaped with a maximum near the sheath–blade joint (≈ 19 mmol s^?1 mm MPa^?1). In elongating leaves, immature xylem in the basal growing region led to a much lower K_t*. As the first metaxylem matured, K_t* increased by 10‐fold. The hydraulic conductances of the whole root system, (mmol s^?1 MPa^?1) and leaf blades, (mmol s^?1 MPa^?1) were measured by a vacuum induced water flow technique. and were linearly related to the leaf area downstream. Approximately 65% of the resistance to water flow within the plant resided in the leaf blade. An electric‐analogue computer model was used to calculate the leaf blade area‐specific radial hydraulic conductivity, (mmol s^?1 m^?2 MPa^?1), using , K_t* and water flux values. values decreased with leaf age, from 21·2 mmol s^?1 m^?2 MPa^?1 in rapidly elongating leaf to 7·2 mmol s^?1 m^?2 MPa^?1 in mature leaf. Comparison of and values showed that ≈ 90% of the resistance to water flow within the blades resided in the liquid extra‐vascular path. The same algorithm was then used to compute the xylem and extravascular water potential drop along the liquid water path in the plant under steady state conditions. Predicted and measured water potentials matched well. The hydraulic design of the mature leaf resulted in low and quite constant xylem water potential gradient (≈ 0·3 MPa m^?1) throughout the plant. Much of the water potential drop within mature leaves occurred within a tenth of millimetre in the blade, between the xylem vessels and the site of water evaporation within the mesophyll. In elongating leaves, the low K_t* in the basal growth zone dramatically increased the local xylem water potential gradient (≈ 2·0 MPa m^?1) there. In the leaf elongation zone the growth‐induced water potential difference was ≈ 0·2 MPa.     

Seasonal Changes in Water Potential and Turgor Maintenance in Sorghum and Maize under Water Stress

Leaf water (Ψ) and solute (ψ) potential were measured in field sorghum and maize under well irrigated (I) and dryland (NI) conditions throughout a season. Despite decreases in ψ due to slow soil water depletion and to apparent increases in liquid phase plant resistance, midday leaf turgor (ψ_p) in the NI sorghum was maintained at similar levels as in the I treatment throughout the season due to concomitant decreases in ψ_s. Osmotic adjustment was also observed in maize, although ψ_p was significantly lower in the NI treatment as compared to I during the final stages of grain filling. A seasonal shift in the ψ vs. relative water content relation of NI sorghum leaves was observed, more water being retained by the older leaf at any particular ψ. The major factor for turgor maintenance was a net increase in solutes per unit of tissue. The role played by increases in the proportion of tissue volume occupied by cell wall was also evaluated. No stomatal closure due to water stress was found in NI sorghum even though leaf ψ reached —20 bars late in the season. Under similar conditions, stomata closed at —14 to —16 bars in younger plants where water stress was made to develop much faster.     

A new stem hygrometer, corrected for temperature gradients and calibrated against the pressure bomb

Abstract A simple stem hygrometer for attachment to a bared section of sapwood or a cross-sectional cut end of a shoot is described. Two welded chromelconstantan thermocouples inside the chamber, one touching the sample and the other in the chamber air, allowed measurement of and correction for the temperature gradient between the sample and the dewpoint measuring junction. The instrument was attached to the cut end of an apical shoot of Thuja occidentalis L. protuding from a Scholander-Hammel pressure bomb. Cut-end water potential (ψ_hyg), measured using the stem hygrometer, was compared to xylem pressure potential (ψ_xp) while the latter was manipulated in the pressure bomb. After an initial equilibration time of 3–4 h, hygrometer equilibrium values were achieved within 1.5–4.0 min of changing ψ_xp in the pressure bomb. The half-time (ψ_1/2) for vapour pressure equilibration was 15–40 s. Stable temperature gradients between the sample and dewpoint measuring junction of 0.01–0.1°C were measured. Correcting ψ_hyg for the temperature gradient resulted in excellent agreement with ψ_xp.     

Evidence for Ideal and Non-Ideal Equilibrium Freezing of Leaf Water in Frosthardy Ivy (Hedera helix) and Winter Barley (Hordeum vulgare)

Temperature dependences of leaf water potentials (ψ_leaf) of frozen leaves of frosthardy ivy and winter barley were determined psychrometrically and found to coincide with the respective water potentials of ice which were obtained using the same technique. The water potentials of ice showed good agreement with theoretically established data. Analysis of the components of ψ of frozen leaves of Hedera helix revealed ideal equilibrium freezing, i.e. the governing of the relative content of liquid (or frozen) water solely by the osmotic potential. In winter barley, by contrast, a negative pressure potential was demonstrated to contribute to ψ_leaf. even under conditions of moderate frost. This reduced the degree of protoplast dehydration and the extent to which the concentrations of the cellular solutes rose. Such a freezing behavior is termed non-ideal equilibrium freezing. Depending on the original content of leaf water, the volume increments of liquid water due to the negative pressure potential amounted up to 10% at ?6 °C and even more at a lower temperature. In addition to the experimental data, a theoretical treatment of psychrometry at subzero temperatures is presented.     

Water relations and leaf chemistry of Chrysothamnus nauseosus ssp. consimilis (Asteraceae) and Sarcobatus vermiculatus (Chenopodiaceae)

At Mono Lake, California, we investigated field water relations, leaf and xylem chemistry, and gas exchange for two shrub species that commonly co-occur on marginally saline soils, and have similar life histories and rooting patterns. Both species had highest root length densities close to the surface and have large tap roots that probably reach ground water at 3.4-5.0 m on the study site. The species differed greatly in leaf water relations and leaf chemistry. Sarcobatus vermiculatus had a seasonal minimum predawn xylem pressure potential (ψ_pd) of -2.7 MPa and a midday potential (ψ_md) of -4.1 MPa. These were significantly lower than for Chrysothamnus nauseosus, which had a minimum ψ_pd of -1.0 MPa and ψ_md of -2.2 MPa. Sarcobatus had leaf Na of up to 9.1 % and K up to 2.7 % of dry mass, and these were significantly higher than for Chrysothamnus which had seasonal maxima of 0.4% leaf Na and 2.4 % leaf K. The molar ratios of leaf K/Na, Ca/Na, and Mg/Na were substantially lower for Sarcobatus than for Chrysothamnus. Xylem ionic contents indicated that both species excluded some Na at the root, but that Chrysothamnus was excluding much more than Sarcobatus. The higher Na content of Sarcobatus leaves was associated with greater leaf succulence, lower calculated osmotic potential, and lower xylem pressure potentials. Despite large differences in water relations and leaf chemistry, these species maintained similar diurnal patterns and rates of photosynthesis and stomatal conductance to water vapor diffusion. Sarcobatus ψ_pd may not reflect soil moisture availability due to root osmotic and hydraulic properties.     

Osmotic adjustment in leaves ofLycopersicon esculentum andL. pennellii in response to saline water irrigation

Two tomato species (Lycopersicon esculentum andL. pennellii) were grown under unheated plastic greenhouse and irrigated with 0 or 140 mM NaCl. Salinity induces a more important reduction in predawn leaf water potential (ψ_pd) inL. esculentum than inL. pennellii. In both species the osmotic adjustment was achieved by active solute accumulation. The leaf water potential at turgor loss point (ψ_tlp) seemed to be controlled by leaf osmotic potential (ψ_os). The results revealed the existence of limits to the accumulation of osmotic solutes in leaf tissues and the existence of an ontogenetic effect on the solute accumulation. In both species, but essentially inL. pennellii the inorganic solutes contribution especially Na⁺ and Cl^? accumulation to ψ_os was higher than the organic solutes. Therefore, wild species save energy more markedly.     

Diffusive conductance of rice (Oryza sativa) leaves during gradually induced soil water stress

The diffusive conductance (C_s) of rice (Oryza sativa cvs Jaya and Bala) leaves was measured during a soil drying cycle from flooding to decreasing soil water potential (φ_s) in a controlled-environment chamber. Plants were grown continuously under 5 cm submergence up to 69 days after transplanting and thereafter were subjected to gradual soil drying for a period of 17 days in the vegetative growth stage. In both the cultivars, the values of C_s were generally more on adaxial than abaxial leaf surfaces. This response of stomata during the period of soil drying was independent of leaf rolling. Further, the slopes of the curves (C_s, vs φ_s) also did not differ significantly (P= 0·05). The total C_s, of both cultivars during flooding was almost equal (0·60 cm s^-1) but at the end of the soil drying cycle, the values of total C_s, were 0·11 cm s^-l at ψ_s of -1·3 MPa and 0·08 cm s^-1 at ψ_s, of -0·8 MPa in cvs Jaya and Bala, respectively. For total C_s, slopes differed significantly (P = 0·05). A close relationship between total C_s, and ψ_s, in both cultivars (C_s, = 0·58-0·40 ψ_s, for cv. Jaya and C_s= 0·46-0·56 ψ_s, for cv. Bala) indicated that stomata were sensitive to increasing soil water deficit.     

Photosynthetic performance and resistance to photoinhibition of Zea mays L. leaves grown at sub-optimal temperature

The performance of the photosynthetic apparatus was examined in the third leaves of Zea mays L. seedlings grown at near-optimal (25 °C) or at suboptimal (15 °C) temperature by measuring chlorophyll (ChI) a fluorescence parameters and oxygen evolution in different temperature and light conditions. In leaf tissue grown at 25 and 15 °C, the quantum yield of PSII electron transport (ψ_PSII) and the rate of O₂ evolution decreased with decreasing temperature (from 25 to 4 °C) at a photon flux density of 125 μmol m^?2 s^?1. In leaves grown at 25 °C, the decrease of ψ_PSII correlated with a decrease of photochemical ChI fluorescence quenching (q_p), whereas in leaves crown at 15 °C q_p was largely insensitive to the temperature decrease. Compared with leaves grown at 25 °C, leaves grown at 15 °C were also able to maintain a higher fraction of oxidized to reduced Q_A (greater q_p) at high photon flux densities (up to 2000 μmol m^?2 s^?1), particularly when the measurements were performed at high temperature (25 °C). With decreasing temperature and/or increasing light intensity, leaves grown at 15 °C exhibited a substantial quenching of the dark level of fluorescence F₀ (q₀) whereas this type of quenching was virtually absent in leaves grown at 25 °C. Furthermore, leaves grown at 15 °C were able to recover faster from photo inhibition of photosynthesis after a photoinhibitory treatment (1200 μmol m^?2 s^?1 at 25, 15 or 6 °C for 8 h) than leaves grown at 25 °C. The results suggest that, in spite of having a low photosynthetic capacity, Z. mays leaves grown at sub optimal temperature possess efficient mechanisms of energy dissipation which enable them to cope better with photoinhibition than leaves grown at near-optimal temperature. It is suggested that the resistance of Z. mays leaves grown at 15 °C to photoinhibition is related to the higher content of carotenoids of the xanthophyll cycle (violaxanthin + antheraxanthin + zeaxanthin) measured in these leaves than in leaves grown at 25 °C.     

Water translocation between ramets of strawberry during soil drying and its effects on photosynthetic performance

To explore the mechanisms underlying water regulation in clonal plants and its effects on carbon assimilation under water stress, we studied the responses of water status, gas exchange and abscisic acid (ABA) contents to water stress in leaves of pairs of strawberry ramets that consist of mother and daughter ramets. There was a greater decrease in photosynthetic rates (P_n) and stomatal conductance (G_s) in the disconnected mother ramets than the connected mother ramets upon exposure to water stress, indicating that water stress in mother ramets was alleviated by water translocation from the well‐watered daughter ramets. Conversely, the connected mother ramets displayed enhanced symptoms of water stress when the connected daughter ramets were exposed to water deficit. The mother ramets had lower water potential (ψ_w) due to their stronger osmotic adjustment than in well‐watered daughter ramets; this resulted in water flow from the connected daughter ramets to mother ramets, thus alleviating water stress of mother ramets. During soil drying, there was a striking increase in ABA concentrations in leaves of the disconnected mother ramets, whereas leaf bulk ABA was much lower in the connected and water‐stressed mother ramets than that in the drought‐affected mother ramets in the disconnected group. In this study, though G_s was linearly correlated with leaf bulk ABA and ψ_w, G_s in water‐stressed mother ramets in disconnected group exhibited less sensitivity to the variation in leaf bulk ABA and ψ_w than that in connected and water‐stressed mother ramets. Taken together, these results indicate that: (1) the flux of water translocation between the connected ramets is determined by a water potential gradient; (2) water translocation between connected ramets helps to keep sensitivity of G_s to ABA and ψ_w in drought‐affected ramets, thereby benefit to effectively maintain the homeostasis of leaf water status and (3) the improvements in P_n in water‐stressed ramets due to water translocation from well‐watered ramets suggest the advantages of physiological integration in clonal plants in environments with heterogeneous water distribution.     

Sap flow characteristics and responses to summer rainfall for Pinus tabulaeformis and Hippophae rhamnoides in the Loess hilly region of China

As a major driving element of the structure and function of arid and semiarid ecosystems, rainfall is the essential factor limiting plant biological processes. To clarify the characteristics of transpiration and responses to summer rainfall, sap flow density (F_d) of Pinus tabulaeformis and Hippophae rhamnoides was monitored using thermal dissipation probes. In addition, midday leaf water potential (ψ_m) and leaf stomatal conductance (G_s) were also analyzed to determine water use strategies. The results indicated that the diurnal variation in the normalized F_d values exhibited a single‐peak curve for P. tabulaeformis, while H. rhamnoides showed multiple peaks. The normalized F_d for P. tabulaeformis remained relatively stable regardless of rainfall events. However, there was also a significant increase in the normalized F_d for H. rhamnoides in response to rainfall in June and August (p < .05), although no significant differences were observed in July. The normalized F_d values for P. tabulaeformis and H. rhamnoides fitted well with the derived variable of transpiration, an integrated index calculated from the vapor pressure deficit and solar radiation (R_s), using an exponential saturation function. The differences in fitting coefficients suggested that H. rhamnoides showed more sensitivity to summer rainfall (p < .01) than P. tabulaeformis. Furthermore, during the study period, P. tabulaeformis reduced G_s as soil water decreased, maintaining a relatively constant ψ_m; while H. rhamnoides allowed large fluctuations in ψ_m to maintain G_s. Therefore, P. tabulaeformis and H. rhamnoides should be considered isohydric and anisohydric species, respectively. And more consideration should be taken for H. rhamnoides in the afforestation activities and the local plantation management under the context of the frequently seasonal drought in the loess hilly region.     

Complex I Controls Mitochondrial and Plasma Membrane Potentials in Nerve Terminals

Reductions in the activities of mitochondrial electron transport chain (ETC) enzymes have been implicated in the pathogenesis of numerous chronic neurodegenerative disorders. Maintenance of the mitochondrial membrane potential (Δψ_m) is a primary function of these enzyme complexes, and is essential for ATP production and neuronal survival. We examined the effects of inhibition of mitochondrial ETC complexes I, II/III, III and IV activities by titrations of respective inhibitors on Δψ_m in synaptosomal mitochondria. Small perturbations in the activity of complex I, brought about by low concentrations of rotenone (1–50 nM), caused depolarisation of Δψ_m. Small decreases in complex I activity caused an immediate and partial Δψ_m depolarisation, whereas inhibition of complex II/III activity by more than 70% with antimycin A was required to affect Δψ_m. A similarly high threshold of inhibition was found when complex III was inhibited with myxothiazol, and inhibition of complex IV by more than 90% with KCN was required. The plasma membrane potential (Δψ_p) had a complex I inhibition threshold of 40% whereas complex III and IV had to be inhibited by more than 90% before changes in Δψ_p were registered. These data indicate that in synaptosomes, both Δψ_m and Δψ_p are more susceptible to reductions in complex I activity than reductions in the other ETC complexes. These findings may be of relevance to the mechanism of neuronal cell death in Parkinson’s disease in particular, where such reductions in complex I activity are present.

     

The Knotted leaf blade is a mosaic of blade,sheath, and auricle identities

The dominant Knotted-1 mutations in maize alter development of the leaf blade. Sporadic patches of localized growth, or knots, and fringes of ectopic ligule occur along lateral veins of mutant leaf blades. In addition, bundle sheaths do not completely encircle lateral veins on mutant leaf blades. We have compared mutant leaf blades with wild-type leaves to determine the precise nature of the perturbed regions. Our analysis includes characterization of epidermal cell shapes, localization of photosynthetic proteins and histology of the leaf. We show that mutant leaf blades are a mosaic of leaf organ components. Affected regions of mutant leaf blades resemble either sheath or auricle tissue in both external and internal features. This conversion of blade cells represents an acropetal shift of more basal parts of the leaf blade region and correlates with previously identified ectopic expression of the Knotted-1 protein in the leaf blade. We propose that inappropriate expression of Kn1 interferes with the process of establishment of cell identities, resulting in early termination of the normal blade development program or precocious expression of the sheath and auricle development programs. © 1994 Wiley-Liss, Inc.     

Observations on the maintenance and measurement of soil water in simple pot experiments and its effects on seed-borne Fusarium culmorum seedling blight of winter wheat

A simple method for maintaining and measuring soil water and the relationship between soil water and seed-borne Fusarium culmorum seedling blight of wheat was investigated under controlled environmental conditions to develop reproducible assay conditions for epidemiological studies and the testing of fungicide seed treatments. For reproducibility, soil matric potential (ψ_m) was used to define soil water, and a range of watering regimes were tested. Treatments were watered to either a maximum ψ_m or to maintain a mean ψ_m in order to establish which parameter best described the effect that soil water had on the incidence of disease symptoms. The severity of disease symptoms was closely related to the mean soil water status and not the maximum ψ_m value. Watering intervals, ranging from three times a day to once every three days, did not affect this relationship. The percentage of seedlings showing symptoms after emergence (i.e. localised or extensive necrotic lesions) was inversely related to the mean ψ_m. With increasing soil dryness (mean ψ_m from -0.14 MPa to -0.17 MPa) the percentage of seedlings with post-emergence symptoms decreased from 50% to 20%. However, as the mean ψ_m changed from -0.14 MPa to -0.17 MPa the percentage of seedlings dying before emergence increased from 25% to 55% in direct proportion to ψ_m. Overall the incidence of infection as indicated by the total number of seedlings showing symptoms either before or after emergence remained relatively constant, and was not significantly related to mean ψ_m.     

Blade life span,structural investment,and nutrient allocation in giant kelp

The turnover of plant biomass largely determines the amount of energy flowing through an ecosystem and understanding the processes that regulate turnover has been of interest to ecologists for decades. Leaf life span theory has proven useful in explaining patterns of leaf turnover in relation to resource availability, but the predictions of this theory have not been tested for macroalgae. We measured blade life span, size, thickness, nitrogen content, pigment content, and maximum photosynthetic rate (P _max) in the giant kelp (Macrocystis pyrifera) along a strong resource (light) gradient to test whether the predictions of leaf life span theory applied to this alga. We found that shorter blade life spans and larger blade areas were associated with increased light availability. In addition, nitrogen and P _max decreased with blade age, and their decrease was greater in shorter lived blades. These observations are generally consistent with patterns observed for higher plants and the prevailing theory of leaf life span. By contrast, variation observed in pigments of giant kelp was inconsistent with that predicted by leaf life span theory, as blades growing in the most heavily shaded portion of the forest had the lowest chlorophyll content. This result may reflect the dual role of macroalgal blades in carbon fixation and nutrient absorption and the ability of giant kelp to modify blade physiology to optimize the acquisition of light and nutrients. Thus, the marine environment may place demands on resource acquisition and allocation that have not been previously considered with respect to leaf life span optimization.     

Effect of hydration level in primary bean leaves on the activity of photosystems 1 and 2 in isolated chloroplasts

Water stress in the leaves was induced by gradual decreasing of substrate moisture in five-day cycles.The hydration level of the leaves was characterized by their water potential (ψ_w), osmotic potential (ψ_s), pressure potential ψ_p and water saturation deficit(ΔW _sat ).The activities of Photosystems 1 and 2 were determined polarographically with Pt/Ag(AgOH) electrode as changes in oxygen concentration in chloroplast suspensions. The shape of light curves of Hill reaction was not influenced by leaf (ψ_w), hence both quantum efficiency and dark phase of this process were affected in a similar manner by water deficit. The activities of both photosystems measured at saturating photon flux density declined with the lowering of leaf (ψ_w) (in the range from -5 to -14 x 10⁵ Pa) and the decrease in activity of Photosystem 2 was more rapid than that of Photosystem 1. The ratio of activities of Photosystems 1 and 2 was mildly enhanced by a lowering of (ψ_w), but it decreased with increasing age. The lowering of ψ_winduced lowering in the chlorophyll a/b ratio thus concealing the usual ontogenetic course of this ratio.     

Influence of salinity on Na+ and K+ accumulation, and gas exchange in Avicennia germinans

We analysed plant growth, ion accumulation, leaf water relations, and gas exchange of Avicennia germinans (L.) L. subjected to a long-term, controlled salinity gradient from 0 to 55 ‰. Growth and leaf area were affected by salinity higher than 10 ‰. As salinity increased, the predawn leaf water potential (Ψ_w) and leaf osmotic potential (Ψ_s) decreased. Leaf Ψ_w was at least −0.32 MPa lower than the Ψ_w of solution. Na⁺ and K⁺ ions explained about 78 % of decrease in Ψ_s. K⁺ tissue water concentration decreased by more than 60 % in all salinity treatments as compared with those grown at 0 ‰. Inversely, Na⁺ concentration in tissue water increased with nutrient solution salinity. The maximum net photosynthetic rate (P _N) and stomatal conductance (g _s) decreased by 68 and 82 %, respectively, as salinity increased from 0 to 55 ‰; the intercellular CO₂ concentration (C _i) followed the same trend. The P _N as a function of C _i showed that both the initial linear slope and upper plateau of the P _N vs. C _i curve were markedly affected by high salinity (40 and 55 ‰).