Effects of water stress on the gas exchange,the activities of some enzymes of carbon and nitrogen metabolism,and on the pool sizes of some organic acids in maize leaves

Water-stressed maize (Zea mays L.) leaves showed a large decrease in leaf conductance during photosynthesis. Net CO₂ uptake and evaporation declined fast at mild stress (=–0.6 to –1.0 MPa) and slower at more severe stress (=–1.0 to -1.2 MPa), whereas the CO₂ concentration in the intercellular spaces (C_i) did not drop to the CO₂ compensation point. The activities of the enzymes of photosynthetic carbon metabolism tested in this study dropped by approx. 30% at =-1.2 MPa. Glutamine synthetase activity was unaffected by water stress, whereas the activity of nitrate reductase was almost completely inhibited. The decline of enzyme activities in relation to was correlated with a concomitant decrease in the content of total soluble protein of the stressed leaves. The total leaf pools of malate, pyruvate and oxaloacetate decreased almost linearly in relation to , thus obviously contradicting the almost constant C_i. In comparison to the controls (=0.6 MPa) the content of citrate and isocitrate increaed markedly at =-0.9 MPa and decreased again at =-1.2 MPa.Abbreviations PCR photosynthetic carbon reduction cycle - PCO photosynthetic carbon oxidation cycle - PEP phosphoenolypyruvate - RuBP ribulose-1,5-bisphosphate     

Phlorizin inhibits hexose transport across the plasmalemma of Riccia fluitans

The effect of phlorizin has been tested on hexose transport and hexose-induced changes of electrical potential (_m) and conductance (g _m) across the plasmalemma of rhizoid and thallus cells of the aquatic liverwort Riccia fluitans. The decrease of _m (depolarization) and g _m induced by 1 mM 3-oxymethyl-D-glucose (3-OMG) is substantially inhibited by simultaneous addition of 2 mM phlorizin, whereas no significant response was observed when phlorizin was added alone or several minutes after the sugar. Current-voltage data show that the 3-OMG-generated electrical inward current of 0.016 A m^-2 drops to 0.010 A m^-2 when phlorizin is present. Uptake as well as efflux of [¹⁴C]-3-OMG is strongly and reversibly inhibited by phlorizin between 0.2 and 5 mM. The results are consistent with our hypothesis that the hexose carrier has one binding site with competitive inhibition of glucose uptake by phlorizin (k _i=0.08 mM). The electrical data indicate that phlorizin affects an _m step of the carrier transport cycle.Abbreviation 3-OMG 3-oxymethyl-D-glucose     

Effect of Growth Regulators on Photosynthesis, Transpiration and Related Parameters in Water Stressed Cotton

Gas exchange in Gossypium hirsutum L. cv. H-777 as affected by water deficit and growth regulators (IAA, GA₃, BAP, ABA, ethrel) was examined. Sixty days after sowing, growth regulators in concentration 50 µM were applied as foliar spray and irrigation was withheld to get desired (moderate and severe) water deficit. All the parameters were measured on the third leaf from the top between 10:00 and 11:00. Net photosynthetic rate (P_N), transpiration rate (E), stomatal conductance (g_s), carboxylation efficiency (CE), and water potential (_w) decreased significantly with the increasing water stress, however, water use efficiency (WUE) was unaffected. Foliar spray with IAA, GA₃ and BAP partially counteracted the effect of water deficit on the above parameters except _w, which became more negative. ABA and up to some extent ethrel increased WUE and maintained higher _w, however, caused further decrease in P_N, E, and g_s.     

Leaf water potential,component potentials and relative water content in a xeric grass,Agropyron dasystachyum (Hook.) Scribn.

Summary Leaf water potential ( _l ), osmotic potential ( _s ), pressure potential ( _p , turgor pressure), relative water content (R) and their interrelationships were determined for a xeric grass (Agropyron dasystachyum) found in the grasslands of Canada. Thermocouple psychrometers were used to measure _l and _s ; _p was obtained by subtraction. _l dropped from near 0 bars to about-28 bars as R went from 90% to 75%. R greater than 90% was not observed, perhaps because of a systematic error in determination of turgid water content. R remained relatively high in A. dasystachyum, even at low _l . The slope of the _l -R relationship was similar to other species which are generally considered to be drought tolerant. _p as high as 14 bars was observed. Most of the decrease in _l was accounted for by a decline in _p . The ability of A. dasystachyum to adjust to fluctuating water stress over the growing season is probably as much related to changes in tissue structure and turgor relationships as to simple changes in osmotic potential.     

The mitochondrial membrane potential (deltapsi(m)) in apoptosis; an update

Mitochondrial dysfunction has been shown to participate in the induction of apoptosis and has even been suggested to be central to the apoptotic pathway. Indeed, opening of the mitochondrial permeability transition pore has been demonstrated to induce depolarization of the transmembrane potential (_m), release of apoptogenic factors and loss of oxidative phosphorylation. In some apoptotic systems, loss of _m may be an early event in the apoptotic process. However, there are emerging data suggesting that, depending on the model of apoptosis, the loss of _m may not be an early requirement for apoptosis, but on the contrary may be a consequence of the apoptotic-signaling pathway. Furthermore, to add to these conflicting data, loss of _m has been demonstrated to not be required for cytochrome c release, whereas release of apoptosis inducing factor AIF is dependent upon disruption of _m early in the apoptotic pathway. Together, the existing literature suggests that depending on the cell system under investigation and the apoptotic stimuli used, dissipation of _m may or may not be an early event in the apoptotic pathway. Discrepancies in this area of apoptosis research may be attributed to the fluorochromes used to detect _m. Differential degrees of sensitivity of these fluorochromes exist, and there are also important factors that contribute to their ability to accurately discriminate changes in _m.     

Water relations and growth of shrubs before and after fire in a semi-arid woodland

Summary Plant water relations and shoot growth rate of shrubs resprouting after fire or unburnt were measured in a semi-arid poplar box (Eucalyptus populnea) shrub woodland of eastern Australia. In vegetation unburnt for about 60 years, the dawn xylem water potential (_x) of the dominant shrub species was about-1.0 MPa when the soil was wet and-8.0 MPa when the soil was very dry. At any one time, the dominant shrub species,Eremophila mitchellii, E. sturtii, Geijera parviflora andCassia nemophila, were similar in _x butAcacia aneura andDodonaea viscosa were consistently higher in _x than this group when the soil was moist and lower when the soil was dry. The dominant tree species,Eucalyptus populnea andE. intertexta, appeared to have access to additional water beneath the hardpan which is located 60–80 cm below the surface. When shrubs were under extreme water stress (_x of-8 MPa), the trees had a _x of-3 to-3.6 MPa. Following a fire, both _x and leaf stomatal conductance (g _s) of resprouting shrubs were higher for about 5 years than comparable-aged unburnt vegetation, with relative differences in _x increasing with drought stress. Elongation rate of resprouts was positively linked to prefire shrub height in 3 of 4 species. However, shrubs resprouting after high intensity fires had substantially higher rates of shoot elongation than after low intensity fires which were in turn higher than for foliar expansion of unburnt shrubs. It is concluded that the growth rate of resprouting shrubs is primarily determined by physiological/ morphological factors associated with plant size but is also assisted by greater availability of water and possibly nutrients for a period after fire.     

Effects of water deficit on N2(C2H2) fixation in cowpea and groundnut

The effect of water deficit on nodulation, N₂ fixation, photosynthesis, and total soluble sugars and leghemoglobin in nodules was investigated in cowpea and groundnut. Nitrogenase activity completely ceased in cowpea with a decrease in leaf water potential ( _leaf) from –0.4 MPa to –0.9 MPa, while in groundnut it continued down to –1.7 MPa. With increasing water stress, the acetylene reduction activity (ARA) declined very sharply in cowpea, but ARA gradually decreased in groundnut. Even with mild water stress ( _leaf of 0.2 MPa), nodule fresh weight declined 50% in cowpea partly due to a severe nodule shedding whereas nodule fresh weight declined in groundnut only when _leaf decreased by 1.0 MPa. No nodule shedding was noticed even at a higher stress level in groundnut. Photosynthesis and stomatal conductance were also more stable in groundnut than in cowpea under water stress. There was a sharp increase in total soluble sugars and leghemoglobin in the nodules of groundut with water stress, but no definite trend could be found in cowpea.     

Comparative water relations of four Mediterranean oak species

The water relations and responses of two evergreen (Quercus ilex L. and Q. suber L.) and two deciduous (S. afares Pomel. and Q. faginea Will.) Quercus species were studied under experimental conditions. Two-year old seedlings grown in 30 l. pots were subjected to a drying period during which stomatal conductance, pre-dawn potential and minimum foliar potential were measured.The results shows that, for all species, the daily course of stomatal conductance agrees with the patterns proposed by Hinckley et al. (1978 & 1983). Concurrent with the species responses to short-term variation in water availability, it was found that pre-dawn leaf water potential controlled the maximum daily leaf conductance. There was a strong correlation between pre-dawn leaf potential and maximum daily conductance as described by the reciprocal function g_{srmax for}=(-0.47+2.61._p)^-1 the evergreen oaks and g_{srmax for}=(-1.94+7.39._p)^-1 for the deciduous species. These differences between the two groups may partialy explain their geograhic distributions, and suggest general questions concerning the mechanisms which optimize water-use efficiency in Mediterranean oak species.     

Measurements of electrical potential differences across yeast plasma membranes with microelectrodes are consistent with values from steady-state distribution of tetraphenylphosphonium inPichia humboldtii

Summary Electrical potential differences across the plasma membrane () of the yeastPichia humboldtii were measured with microelectrodes (filled with 0.1m KCl) inserted into cells immobilized in microfunnels. The registered signals were reproducible and stable for several minutes. On attainment of stable reading for the specific membrane resistanceR _sp was determined by applying square-current pulses to the preparation. Both andR _sp were pH dependent and displayed equal but opposite deflection, reaching its maximal value of –88±9 mV (n=13) andR _sp its minimal value of 10 k·cm² (maximal conductance) at pH 6.5. Uncouplers and the polyene antibiotic nystatin depolarized the cells, decreasing to –21±15 mV (n=10) with concomitant decrease ofR _sp. Comparison of values from microelectrode measurements with those calculated from the steady-state distribution of tetraphenylphosphonium ions agreed within 10 mV under all physiological conditions tested, except at pH values above 7.0. During microelectrode insertion transient voltage signals (a few msec long) were detected by means of an oscilloscope. These voltage signals were superimposed on the stable recordings described above. These short voltage signals disappeared in uncoupled cells. The closely related values obtained by two independent methods (direct measurements with microelectrodes and calculation from steady-state distribution of a lipophilic cation) provide evidence that these values reffect the true membrane potential of intact cells.     

Shoot water relations and gas exchange of western hemlock and western red cedar seedlings during establishment on a reforestation site

Summary Shoot water relations, summer gas exchange response and morphological development of western hemlock [Tsuga heterophylla (Raf.) Sarg.] and western red cedar (Thuja plicata Donn) seedlings were monitored over the first growing season on a coastal reforestation site in British Columbia. In March, osmotic potential (_s) at saturation [_s(sat)] was –1.98 MPa and turgor loss point [_s(tlp)] –2.38 MPa for western hemlock, while western red cedar had –1.45 MPa _s(sat) and –1.93 MPa _s(tlp). Seasonally _s increased through June and then decreased through September, with western hemlock –0.15 to –0.50 MPa lower than western red cedar. Maximum bulk modulus of elasticity (_max) for western hemlock was 29.3 MPa in March, decreased to 15.0 MPa in June and increased to 25.0 MPa from July through September, while western red cedar _max was 10.6 MPa in March and around 8.0 MPa thereafter. Utilized turgor (T _util) for western hemlock was <40% from March through May, 69 to 78% from June through August and 96% in September, while western red cedar T _util was 68 to 73% during March and April, 84 to 96% from May through August and 100% in September. Maximum CO₂ assimilation rate (A) of western red cedar was more than double western hemlock, and for both species A declined in a linear fashion with increasing vapour pressure deficit (D). Maximum foliage conductance (g _wv) declined in a concave manner as D increased in both species, with western red cedar values 50 to 67% greater than western hemlock. Maximum daily g _wv declined in a concave manner as predawn shoot water potential (_pd) decreased, with maximum daily g _wv 1.8 to 3.6 times greater in western red cedar than western hemlock, when _pd was –0.25 and –1.4 MPa, respectively. Western red cedar, compared to western hemlock, had a greater increase in A as g _wv increased. Eight months after planting, western red cedar seedlings had twice the root growth, measured as root dry weight and root number, of western hemlock.     

Responses of Ribulose-1,5-Bisphosphate Carboxylase,Protein Content,and Stomatal Conductance to Water Deficit in Maize,Tomato, and Bean

We compared responses of maize, tomato, and bean plants to water stress. Maize reached a severe water deficit (leaf water potential –1.90 MPa) in a longer period of time as compared with tomato and bean plants. Maize stomatal conductance (g _s) decreased at mild water deficit. g _s of tomato and bean decreased gradually and did not reach values as low as in maize. The protein content was maintained in maize and decreased at low water potential (_w); in tomato it fluctuated and also decreased at low _w; in bean it gradually decreased. Ribulose-1,5-bisphosphate carboxylase/oxygenase activity remained high at mild and moderate stress in maize and tomato plants; in bean it remained high only at mild stress.     

Sodium-coupled amino acid and sugar transport byNecturus small intestine

Summary Necturus small intestine actively absorbs sugars and amino acids by Na-coupled mechanisms that result in increases in the transepithelial electrical potential difference ( ^ms ) and the short-circuit current (I _sc) which can be attributed entirely to an increase in the rate of active Na absorption. Studies employing conventional microelectrodes indicate that the addition of alanine or galactose to the mucosal solution is followed by a biphasic response. Initially, there is a rapid depolarization of the electrical potential difference across the apical membrane ( ^ms ) which reverses polarity (i.e. cell interior becomes positive with respect to the mucosal solution) and a marked decrease in the ratio of the effective resistance of the mucosal membrane to that of the serosal membrane (R ^m /R ^s ); these events do not appear to be dependent on the availability of metabolic energy. These initial, rapid events are followed by a slow increase in (R ^m /R ^s ) toward control values which is paralleled by a repolarization of ^ms and increases in ^ms andI _sc; this slow series of events is dependent upon the availability of metabolic energy.The results of these studies indicate that: (i) the Na-coupled mechanisms that mediate the entry of sugars and amino acids across the apical membrane are rheogenic (conductive) and result in a decrease inR ^m and a depolarization of ^ms ; and (ii) the subsequent increase in (R ^m /R ^s ) and repolarization of ^ms are the results of a decrease inR ^s which is associated with an increase in the activity of the Na pump at the basolateral membrane.The physiologic implications of these findings are discussed and an equivalent electrical circuit model for rheogenic Na-coupled solute transport processes is analyzed.     

Gas exchange and photosynthesis of Eucalyptus camaldulensis seedlings inoculated with different ectomycorrhizal symbionts

Eucalyptus camaldulensis Dehnh. seedlings inoculated with Pisolithus tinctorius (Pers.) Coker & Couch and Thelephora terrestris Ehrl. per Fr. were grown in well watered soil (_s –0.03 MPa) or subjected to a long-term soil water stress of up to –1.0 MPa over 13-week period in a glasshouse. After 13 weeks, all seedling containers were watered to field capacity and then water was withheld from the E. camaldulensis seedlings to induce a short-term drought. Diurnal measurements of seedling photosynthesis rate (A), leaf stomatal conductance (g) and leaf water potential (_p) were completed before, during, and after the short term drought. Although they were growing in an equal soil volume, photosynthesis rate (A), leaf stomatal conductance and leaf water potential (_p) of larger seedlings with P. tinctorius ectomycorrhizae were similar to those of smaller seedlings colonized with T. terrestris during the short-term drought period. Seedlings inoculated with Pisolithus tinctorius maintained higher photosynthesis rates over the course of the short-term drought. Thus, P. tinctorius ectomycorrhizae appear to be more efficient than those of T. terrestris in assisting seedlings to maintain gas exchange and photosynthesis under limited soil moisture conditions.     

Cell-wall tension of the inner tissues of the maize coleoptile and its potential contribution to auxin-mediated organ growth

Plant organs such as maize (Zea mays L.) coleoptiles are characterized by longitudinal tissue tension, i.e. bulk turgor pressure produces unequal amounts of cell-wall tension in the epidermis (essentially the outer epidermal wall) and in the inner tissues. The fractional amount of turgor borne by the epidermal wall of turgid maize coleoptile segments was indirectly estimated by determining the water potential ^* of an external medium which is needed to replace quantitatively the compressive force of the epidermal wall on the inner tissues. The fractional amount of turgor borne by the walls of the inner tissues was estimated from the difference between -^* and the osmotic pressure of the cell sap (_i) which was assumed to represent the turgor of the fully turgid tissue. In segments incubated in water for 1 h, -^* was 6.1–6.5 bar at a _i of 6.7 bar. Both -^* and _i decreased during auxin-induced growth because of water uptake, but did not deviate significantly from each other. It is concluded that the turgor fraction utilized for the elastic extension of the inner tissue walls is less than 1 bar, i.e. less than 15% of bulk turgor, and that more than 85% of bulk turgor is utilized for counteracting the high compressive force of the outer epidermal wall which, in this way, is enabled to mechanically control elongation growth of the organ. This situation is maintained during auxin-induced growth.Abbreviations and Symbols _i osmotic pressure of the tissue - ₀ external water potential - ^* water potential at which segment length does not change - IAA indole-3-acetic acid - ITW longitudinal inner tissue walls - OEW outer epidermal wall - P turgor Supported by Deutsche Forschungsgemeinschaft (SFB 206).     

Effect of drought on ear and flag leaf photosynthesis of two wheat cultivars differing in drought resistance

We investigated net photosynthetic rate (P_N) of ear and two uppermost (flag and penultimate) leaves of wheat cultivars Hongmangmai (drought resistant) and Haruhikari (drought sensitive) during post-anthesis under irrigated and non-irrigated field conditions. The P_Nof ear and flag leaf were significantly higher and less affected by drought in Hongmangmai than in Haruhikari. The rate of reduction in stomatal conductance (g_s) was similar for the two cultivars, but intercellular CO₂concentration (C_i) in the flag leaf of Hongmangmai was lower than that of Haruhikari in non-irrigated treatment. No differences were observed in leaf water potential (₁) and osmotic adjustment of the flag leaf of the cultivars. These results imply that differences in photosynthetic inhibition on the flag leaf at low leaf ₁between the cultivars were primarily due to non-stomatal effects. Hence the main physiological factor associated with yield stability of Hongmangmai under drought stress may be attributed to the capacity for chloroplast activity in the flag leaf, which apparently allows sustained P_Nof flag leaf during grain filling under drought stress. The higher P_Nof ear in Hongmangmai under drought could also be related to its drought resistance.This revised version was published online in March 2005 with corrections to the page numbers.     

Potential-sensitive response mechanism of diS-C3-(5) in biological membranes

Summary The potential-sensitive response mechanism of 3,3-dipropylthiodicarbocyanine iodide (diS-C₃-(5)) was examined based on our previous model of diS-C₃-(5) interaction with brush border membrane vesicles (BBMV) in the absence of a membrane potential. The model contained binding (6 msec), reorientation (30 msec), dimerization (<10 nsec), and translocation (1 sec) reaction steps (Cabrini & Verkman, 1986.J. Membrane Biol. 90:163–175). Transmembrane potentials () were induced in BBMV by K⁺ gradients and valinomycin. Steady-state diS-C₃-(5) fluorescence (excitation 622 nm, emission 670 nm) increased linearly with . The reorientation and translocation reaction steps were resolved by the stopped-flow technique as a biexponential decrease in fluorescence following mixture of diS-C₃-(5) with BBMV at varying . The fractional amplitude of the faster exponential increased from 0.36 to 0.73 with increasing (–17 to 87 mV); the time constant for the faster exponential (30–35 msec) was independent of . There were single exponential kinetics (0.5–1.5 sec) for diS-C₃-(5) fluorescence response to a rapid (<2 msec) change in in the absence of a diS-C₃-(5) concentration gradient. These results, and similar findings in placental brush border vesicles, red cell vesicles, and phosphatidylcholine vesicles, support a translocation mechanism for diS-C₃-(5) response, where induced membrane potentials drive diS-C₃-(5) redistribution between sites at the inner and outer membrane leaflets, with secondary effects on diS-C₃-(5) dimerization and solution/membrane partitioning. Fluorescence lifetime and dynamic depolarization measurements showed no significant change in diS-C₃-(5) rotational characteristics or in the polarity of the diS-C₃-(5) environment with changes in . Based on the experimental results, a mathematical model is developed to explain the quantitative changes in diS-C₃-(5) fluorescence which accompany changes in at arbitrary dye/lipid ratios.     

Structural and Functional Rearrangements of Mesophyll as a Probable Basis for the Cytokinin-Dependent Assimilate Translocation in Detached Leaves

The effects of benzyladenine (BA) on the mesophyll functioning, such as osmotic potential (), the effect of the inhibitors of ⁺-ATPase on the influx of ¹⁴C-sucrose, the direction of carbon metabolism, and the rate of dark respiration, were followed in the detached leaves of pumpkin (Cucurbita pepo L.) and broad beans (Vicia faba L.). BA elevated and established a gradient of (_p) between the treated and untreated leaf regions. The inhibitors of H⁺-ATPase did not affect the BA-induced influx of ¹⁴C-sucrose. The changes were accompanied with the elevated synthesis of starch and other polymeric compounds and the diminished synthesis of the substances of relatively low molecular weight. The stimulation of dark respiration was short and inconsiderable. The author concludes that the BA-induced transport was a passive process related to a increase. Leaf expansion accompanied by the synthesis of high-molecular-weight substances essential for cell growth and by starch synthesis apparently increased the sink capacity of the BA-treated detached leaves. The diminished efflux from the leaf blade was probably related to a lowered level of the transportable carbon compounds restricting their entry into the phloem. The influx induction could result from the activation of growth and metabolic processes, the decline in the number of organic molecules per cell volume unit, and the development of _p between the source and sink leaf regions.     

Ecophysiological analysis of woody species in contrasting temperate communities during wet and dry years

This study employed an intensive sampling regime in which leaf gas exchange and tissue-water relations were measured simultaneously on the same leaf at midday on 19 tree species from three distinct forest communities during wet (1990) and dry (1991) growing seasons. The study sites were located on a xeric barrens, a misic valley floor, and a wet-mesic floodplain in central Pennsylvania, United States. The xeric, mesic, and wetmesic sties had drought-related decreases in gravimetric soil moisture of 53, 34 and 27%, respectively. During the wet year, xeric and mesic communities had high seasonal mean photosynthetic rates (A) and stomatal conductance of water vapor (g _wv) and low midday leaf water potential (), whereas the wet-mesic community had low A and g _wv and high midday . The mesic and wet-mesic communities had dry year decreases in predawn , g _wv and A with the greatest drought effect occurring in the mesic community. Regression analysis indicated that species from each site that exhibited high wet-year A and g _wv tended to have low midday . This trend was reversed only in the mesic community in the drought year. Despite differences in midday , all three communities had similar midday leaf turgor pressure (_p) in the wet year attributable to lower osmotic potential at zero turgor ( ⁰ ) with increasing site droughtiness. Lower wet year ⁰ in the xeric community was due to low symplast volume rather than high solute content. Species with the lowest ⁰ in the wet year often did not have the lowest ¹⁰⁰ possibly related to differences in tissue elasticity. Moreover, increased elasticity during drought may have masked osmotic adjustment in ¹⁰⁰ but not in ⁰ , via dilution of solutes at full hydration in some species. Despite the sampling regime used, there were no relationships between gas exchange and osmotic and elastic parameters that were consistently significant among communities or years. This result questions the universal, direct effect of osmotic and elastic adjustments in the maintenance of photosynthesis during drought. By including a large number of species, this study provided new insight to the ecophysiology of contrasting forest communities, and the community-wide impact of drought on contrasting sites.     

The effect of membrane potential on the redox state of cytochromeb 561 in antimycin-inhibited submitochondrial particles

The oxidation of cytochromeb ₅₆₁ by ATP was measured in submitochondrial particles inhibited by antimycin. The redox potential of the bulk (M phase) was controlled by the ratio of fumarate:succinate, and the oxidation of cytochromeb was calculated and expressed as a change in redox potential (E _h) measured in millivolts. The oxidation of cytochromeb ₅₆₁ is an energy-driven reaction affected only by the component of the proton motive force. The oxidation (measured in millivolts) is a function of the phosphate potential, reaching a maximal value of 40 mV at G_ATP<–12 kcal/mole. The maximal measured value of ATP-dependent was 100 mV. Thus only a fraction of the membrane potential effects the redox state of cytochromeb ₅₆₁. In contrast to the ATP-induced oxidation of cytochromeb ₅₆₁, cytochromeb ₅₆₆ is in redox equilibrium with fumarate succinate either in the presence or in the absence of ATP. The selective oxidation ofb ₅₆₁ is explained within the term of theQ cycle as a reflection of on the electron electrochemical potential. The positive electric potential of theC phase causes cytochromeb ₅₆₆ to act as oxidant with respect to cytochromeb ₅₆₁. In the presence of antimycin cytochromeb ₅₆₁ cannot equilibrate with the quinone and undergoes oxidation, while cytochromeb ₅₆₆ reequilibrates with the quinone and thus regains redox equilibrium with the fumarate succinate redox buffer.Abbreviations used: ETP_H, phosphorylating submitochondrial particles; TMPD,N ₁ N ₁ NN-tetramethyl-p-phenylenediamine; FCCP, carbonylcyanidep-trifluoromethoxyphenylhydrazone; Mes, 2-(N-morpholino) ethanesulfonic acid.     

Sensitivity of Nostoc commune UTEX 584 (Cyanobacteria) to water stress

Cells of Nostoc commune UTEX 584 from liquid cultures expressed an upshift in nitrogenase activity when immobilised on inert supports and exposed to matric water potentials between -1.10 and -99.5 MPa. Cells incubated at 0.10 MPa (a_w=c 1.0) maintained increased activity for at least 48 h following immobilization. At water potentials below -23.1 MPa (a_w=0.85), the upshift was transitory. Nitrogenase activity decreased rapidly when immobilised cells were incubated at lower values of _m.Desiccated cells stored at -99.5 MPa (a_w=0.50) underwent an upshift in nitrogenase activity, and in the size of the intracellular ATP pool, when rewetted with either distilled water or liquid MB_o medium (_o =-0.18 MPa). The upshift in nitrogenase activity was chloramphenicol-sensitive and was preceeded by a lag. The duration of the lag depended on the time taken to equilibrate cells to-99.5 MPa, the time desiccated, and the conditions of storage and rewetting. Cells that had no, or very low, nitrogenase activity when rewetted in air, showed a marked stimulation of nitrogenase activity in the presence of 5% v/v CO₂ under both aerobic and anerobic conditions.When rewetted in the presence of 1% w/v glucose (_o =-0.14 MPa), vegetative cells remained intact, but heterocysts underwent autolysis and nitrogenase activity was not detected, even in the presence of 5% v/v CO₂.Abbreviations TTC 2,3,5-triphenyl-2-tetrazolium chloride - _m matric water potential - _o osmotic water potential - a_w water activity