Salinity effects on photosynthesis in isolated mesophyll cells of cowpea leaves

Mesophyll cells from leaves of cowpea (Vigna unquiculata [L.] Walp.) plants grown under saline conditions were isolated and used for the determination of photosynthetic CO₂ fixation. Maximal CO₂ fixation rate was obtained when the osmotic potential of both cell isolation and CO₂ fixation assay media were close to leaf osmotic potential, yielding a zero turgor pressure. Hypotonic and hypertonic media decreased the rate of photosynthesis regardless of the salinity level during plant growth. No decrease in photosynthesis was obtained for NaCl concentrations up to 87 moles per cubic meter in the plant growing media and only a 30% decrease was found at 130 moles per cubic meter when the osmotic potential of cell isolation and CO₂ fixation media were optimal. The inhibition was reversible when stress was relieved. At 173 moles per cubic meter NaCl, photosynthesis was severely and irreversibly inhibited. This inhibition was attributed to toxic effects caused by high Cl⁻ and Na⁺ accumulation in the leaves. Uptake of sorbitol by intact cells was insignificant, and therefore not associated with cell volume changes. The light response curve of cells from low salinity grown plants was similar to the controls. Cells from plants grown at 173 moles per cubic meter NaCl were light saturated at a lower radiant flux density than were cells from lower salinity levels.     

Nonosmotic Effects of Polyethylene Glycols upon Sodium Transport and Sodium-Potassium Selectivity by Rice Roots

Addition of polyethylene glycol (PEG) as an osmotic agent (at −230 kilopascals) dramatically lessened the toxicity of NaCl (at 50 moles per cubic meter) to rice (Oryza sativa L.) seedlings. This was explained by a reduction in the uptake of NaCl. This reduction was much greater than could be accounted for by the lowered transpiration rate resulting from the solute potential changes due to the PEG.

Low concentrations of PEG (−33 kilopascals and less) had no effect upon transpiration rate but reduced sodium uptake (from 10-50 moles per cubic meter NaCl) by up to 80%. PEG (at −33 kilopascals) also reduced chloride uptake but had no effect upon the uptake of potassium from low (0.5-2.0 moles per cubic meter) external concentrations. However, the increased uptake of potassium occurring between 2 and 10 moles per cubic meter external concentration was abolished by PEG. Similar concentrations of mannitol had no effect upon sodium uptake in rice. PEG, in similar conditions, had much less effect upon sodium uptake by the more salt-resistant species, barley.

²²Na studies showed that PEG reduced the transport of sodium from root to shoot, but had a long half time for maximal effect (several days).

¹⁴C-labeled PEG was shown to bind to microsomal membranes isolated from rice roots; it is suggested that this is due to multipoint attachment of the complex ions of PEG which exist in aqueous solutions. It is argued that this reduces passive membrane permeability, which accounts for the large effect of PEG on sodium influx in rice and the different effects on sodium influx and (carrier-dependent) potassium influx.

     

Diffusion and Electric Mobility of Ions within Isolated Cuticles of Citrus aurantium: Steady-State and Equilibrium Values

We report a new method for measuring cation and anion permeability across cuticles of sour orange, Citrus aurantium, leaves. The method requires the measurement of two electrical parameters: the diffusion potential arising when the two sides of the cuticle are bathed in unequal concentrations of a Cl⁻ salt; and the electrical conductance of the cuticle measured at a salt concentration equal to the average of that used in the diffusion-potential measurement. The permeabilities of H⁺, Li⁺, Na⁺, K⁺, and Cs⁺ ranged from 2 × 10⁻⁸ to 0.6 × 10⁻⁸ meters per second when cuticles were bathed in 2 moles per cubic meter Cl⁻ salts. The permeability of Cl⁻ was 3 × 10⁻⁹ meters per second. The permeability of Li⁺, Na⁺, and K⁺ was about five times less when measured in 500 moles per cubic meter Cl⁻ salts. We also report an asymmetry in cuticle-conductance values depending on the magnitude and the direction of current flow. The asymmetry disappears at low current-pulse magnitude and increases linearly with the magnitude of the current pulse. This phenomenon is explained in terms of transport-number effects in a bilayer model of the cuticle. Conductance is not augmented by current carried by exchangeable cations in cuticles; conductance is rate limited by the outer waxy layer of the cuticle.     

Photosynthesis and ion content of leaves and isolated chloroplasts of salt-stressed spinach

Spinach (Spinacia oleracea) plants were subjected to salt stress by adding NaCl to the nutrient solution in increments of 25 millimolar per day to a final concentration of 200 millimolar. Plants were harvested 3 weeks after starting NaCl treatment. Fresh and dry weight of both shoots and roots was decreased more than 50% compared to control plants but the salt-stressed plants appeared healthy and were still actively growing. The salt-stressed plants had much thicker leaves. The salt-treated plants osmotically adjusted to maintain leaf turgor. Leaf K⁺ was decreased but Na⁺ and Cl⁻ were greatly increased.

The potential photosynthetic capacity of the leaves was measured at saturating CO₂ to overcome any stomatal limitation. Photosynthesis of salt-stressed plants varied only by about 10% from the controls when expressed on a leaf area or chlorophyll basis. The yield of variable chlorophyll a fluorescence from leaves was not affected by salt stress. Stomatal conductance decreased 70% in response to salt treatment.

Uncoupled rates of electron transport by isolated intact chloroplasts and by thylakoids were only 10 to 20% below those for control plants. CO₂-dependent O₂ evolution was decreased by 20% in chloroplasts isolated from salt-stressed plants. The concentration of K⁺ in the chloroplast decreased by 50% in the salt-stressed plants, Na⁺ increased by 70%, and Cl⁻ increased by less than 20% despite large increases in leaf Na⁺ and Cl⁻.

It is concluded that, for spinach, salt stress does not result in any major decrease in the photosynthetic potential of the leaf. Actual photosynthesis by the plant may be reduced by other factors such as decreased stomatal conductance and decreased leaf area. Effective compartmentation of ions within the cell may prevent the accumulation of inhibitory levels of Na⁺ and Cl⁻ in the chloroplast.

     

Photosynthetic and Respiratory Activity of Fruiting Forms within the Cotton Canopy

The supply of photosynthates by leaves for reproductive development in cotton (Gossypium hirsutum L.) has been extensively studied. However, the contribution of assimilates derived from the fruiting forms themselves is inconclusive. Field experiments were conducted to document the photosynthetic and respiratory activity of cotton leaves, bracts, and capsule walls from anthesis to fruit maturity. Bracts achieved peak photosynthetic rates of 2.1 micromoles per square meter per second compared with 16.5 micromoles per square meter per second for the subtending leaf. However, unlike the subtending leaf, the bracts did not show a dramatic decline in photosynthesis with increased age, nor was their photosynthesis as sensitive as leaves to low light and water-deficit stress. The capsule wall was only a minor site of ¹⁴CO₂ fixation from the ambient atmosphere. Dark respiration by the developing fruit averaged −18.7 micromoles per square meter per second for 6 days after anthesis and declined to −2.7 micromoles per square meter per second after 40 days. Respiratory loss of CO₂ was maximal at −158 micromoles CO₂ per fruit per hour at 20 days anthesis. Diurnal patterns of dark respiration for the fruit were age dependent and closely correlated with stomatal conductance of the capsule wall. Stomata on the capsule wall of young fruit were functional, but lost this capacity with increasing age. Labeled ¹⁴CO₂ injected into the fruit interior was rapidly assimilated by the capsule wall in the light but not in the dark, while fiber and seed together fixed significant amounts of ¹⁴CO₂ in both the light and dark. These data suggest that cotton fruiting forms, although sites of significant respiratory CO₂ loss, do serve a vital role in the recycling of internal CO₂ and therein, function as important sources of assimilate for reproductive development.     

Acclimation of CO(2) Assimilation in Cotton Leaves to Water Stress and Salinity

Cotton (Gossypium hirsutum L. cv Acala SJ2) plants were exposed to three levels of osmotic or matric potentials. The first was obtained by salt and the latter by withholding irrigation water. Plants were acclimated to the two stress types by reducing the rate of stress development by a factor of 4 to 7. CO₂ assimilation was then determined on acclimated and nonacclimated plants. The decrease of CO₂ assimilation in salinity-exposed plants was significantly less in acclimated as compared with nonacclimated plants. Such a difference was not found under water stress at ambient CO₂ partial pressure. The slopes of net CO₂ assimilation versus intercellular CO₂ partial pressure, for the initial linear portion of this relationship, were increased in plants acclimated to salinity of −0.3 and −0.6 megapascal but not in nonacclimated plants. In plants acclimated to water stress, this change in slopes was not significant. Leaf osmotic potential was reduced much more in acclimated than in nonacclimated plants, resulting in turgor maintenance even at −0.9 megapascal. In nonacclimated plants, turgor pressure reached zero at approximately −0.5 megapascal. The accumulation of Cl⁻ and Na⁺ in the salinity-acclimated plants fully accounted for the decrease in leaf osmotic potential. The rise in concentration of organic solutes comprised only 5% of the total increase in solutes in salinity-acclimated and 10 to 20% in water-stress-acclimated plants. This acclimation was interpreted in light of the higher protein content per unit leaf area and the enhanced ribulose bisphosphate carboxylase activity. At saturating CO₂ partial pressure, the declined inhibition in CO₂ assimilation of stress-acclimated plants was found for both salinity and water stress.     

Studies of the Uptake of Nitrate in Barley: I. Kinetics of NO(3) Influx

¹³NO₃⁻ was used to investigate patterns of NO₃⁻ influx into roots of barley plants (Hordeum vulgare L. cv Klondike) previously grown with (`induced') or without (`uninduced') a source of external NO₃⁻ ([NO₃⁻]₀). In both induced and uninduced plants, ¹³NO₃⁻ influx was biphasic in the range from 0.005 to 50 moles per cubic meter [NO₃⁻]₀. In the low concentration range (<1 mole per cubic meter for induced plants and <0.3 mole per cubic meter for uninduced plants), influx was saturable and V_max and K_m values for influx either increased or decreased according to NO₃⁻ pretreatment. By contrast, ¹³NO₃⁻ influx in the high concentration range revealed a strictly linear concentration dependence. These fluxes appeared to be mediated by a constitutive, rather than an inducible, transport system.     

Studies of the Uptake of Nitrate in Barley : II. Energetics

Q₁₀ values for ¹³NO₃⁻ influx were determined in `uninduced' (NO<sub>3</sub><sup>−</sup>-starved) and `induced' (NO₃⁻-pretreated) roots of barley (Hordeum vulgare L.) plants at various concentrations of external NO₃⁻ ([NO₃⁻]₀). At 0.02 mole per cubic meter [NO₃⁻]₀, Q₁₀ values for influx were from 3 to 4 between 5 and 10°C. As [NO₃⁻]₀ increased Q₁₀ values decreased, reaching values of 1.2 and 2.0, respectively, at 20 moles per cubic meter in uninduced and induced plants. The metabolic dependence of ¹³NO₃⁻ influx at low and high [NO₃⁻]₀ (0.1 and 20.0 moles per cubic meter, respectively) in uninduced and induced plants was probed by the use of various inhibitors. These experiments confirmed the findings of the Q₁₀ studies, demonstrating that at low [NO₃⁻]₀¹³NO₃⁻ influx was extremely sensitive to metabolic inhibition. By contrast, at high [NO₃⁻]₀, influx was relatively insensitive to the presence of inhibitors.     

Salinity Induced Limitations on Photosynthesis in Prunus salicina, a Deciduous Tree Species

The response of photosynthetic CO₂ assimilation to salinization in 19 year old Prunus salicina was evaluated under field conditions for a 3 year period. The observed decline in CO₂ assimilation capacity was apparently related to increasing leaf chloride (Cl⁻) content, and independent of changes in leaf carbohydrate status. The response of net CO₂ assimilation (A) to leaf intercellular CO₂ partial pressure (C_i) indicated that the reduction in the capacity for A with Cl⁻ was not the result of decreased stomatal conductance but a consequence of nonstomatal inhibition. The nonstomatal limitations to CO₂ assimilation capacity, as determined by the response of A to C_i and biochemical assay, were related to a decline in the activity of ribulose 1,5-bisphosphate carboxylase (Rubpcase) and the pool size of triose phosphate, ribulose 1,5-bisphosphate (Rubp) and phosphoglycerate with increasing salinity. Lack of agreement between the initial slope of the A to C_i response curve and Rubpcase activity suggests the occurrence of heterogeneous stomatal apertures with the high salinity treatment (28 millimolar). Prolonged exposure to chloride salts appeared to increase the Rubp or Pi regeneration limitation, decrease Rubpcase activity and reduce leaf chlorophyll content. Observed changes in the biochemical components of CO₂ fixation may, in turn, affect total leaf carbohydrates, which also declined with time and salinity. The reduction in Rubpcase activity was apparently a consequence of a reduced Rubpcase protein level rather than either a regulatory or inhibitory effect.     

Water Deficit and Associated Changes in Some Photosynthetic Parameters in Leaves of ;Valencia' Orange (Citrus sinensis [L.] Osbeck)

Photosynthetic CO₂ assimilation, transpiration, ribulose-1,5-bisphosphate carboxylase (RuBPCase), and soluble protein were reduced in leaves of water-deficit (stress) `Valencia' orange (Citrus sinensis [L.] Osbeck). Maximum photosynthetic CO₂ assimilation and transpiration, which occurred before midday for both control and stressed plants, was 58 and 40%, respectively, for the stress (−2.0 megapascals leaf water potential) as compared to the control (−0.6 megapascals leaf water potential). As water deficit became more severe in the afternoon, with water potential of −3.1 megapascals for the stressed leaves vs. −1.1 megapascals for control leaves, stressed-leaf transpiration declined and photosynthetic CO₂ assimilation rapidly dropped to zero. Water deficit decreased both activation and total activity of RuBPCase. Activation of the enzyme was about 62% (of fully activated enzyme in vitro) for the stress, compared to 80% for the control. Water deficit reduced RuBPCase initial activity by 40% and HCO₃⁻/Mg²⁺-saturated activity by 22%. However, RuBPCase for both stressed and control leaves were similar in K_cat (25 moles CO₂ per mole enzyme per second) and K_m for CO₂ (18.9 micromolar). Concentrations of RuBPCase and soluble protein of stressed leaves averaged 80 and 85%, respectively, of control leaves. Thus, reductions in activation and concentration of RuBPCase in Valencia orange leaves contributed to reductions in enzyme activities during water-deficit periods. Declines in leaf photosynthesis, soluble protein, and RuBPCase activation and concentration due to water deficit were, however, recoverable at 5 days after rewatering.     

Carbon gain and photosynthetic response of chrysanthemum to photosynthetic photon flux density cycles

Most models of carbon gain as a function of photosynthetic irradiance assume an instantaneous response to increases and decreases in irradiance. High- and low-light-grown plants differ, however, in the time required to adjust to increases and decreases in irradiance. In this study the response to a series of increases and decreases in irradiance was observed in Chrysanthemum × morifolium Ramat. “Fiesta” and compared with calculated values assuming an instantaneous response. There were significant differences between high- and low-light-grown plants in their photosynthetic response to four sequential photosynthetic photon flux density (PPFD) cycles consisting of 5-minute exposures to 200 and 400 micromoles per square meter per second (μmol m⁻²s⁻¹). The CO₂ assimilation rate of high-light-grown plants at the cycle peak increased throughout the PPFD sequence, but the rate of increase was similar to the increase in CO₂ assimilation rate observed under continuous high-light conditions. Low-light leaves showed more variability in their response to light cycles with no significant increase in CO₂ assimilation rate at the cycle peak during sequential cycles. Carbon gain and deviations from actual values (percentage carbon gain over- or underestimation) based on assumptions of instantaneous response were compared under continuous and cyclic light conditions. The percentage carbon gain overestimation depended on the PPFD step size and growth light level of the leaf. When leaves were exposed to a large PPFD increase, the carbon gain was overestimated by 16 to 26%. The photosynthetic response to 100 μmol m⁻² s⁻¹ PPFD increases and decreases was rapid, and the small overestimation of the predicted carbon gain, observed during photosynthetic induction, was almost entirely negated by the carbon gain underestimation observed after a decrease. If the PPFD cycle was 200 or 400 μmol m⁻² s⁻¹, high- and low-light leaves showed a carbon gain overestimation of 25% that was not negated by the underestimation observed after a light decrease. When leaves were exposed to sequential PPFD cycles (200-400 μmol m⁻² s⁻¹), carbon gain did not differ from leaves exposed to a single PPFD cycle of identical irradiance integral that had the same step size (200-400-200 μmol m⁻² s⁻¹) or mean irradiance (200-300-200 μmol m⁻² s⁻¹).     

Effects of Irradiance during Growth on Adaptive Photosynthetic Characteristics of Velvetleaf and Cotton

We grew velvetleaf (Abutilon theophrasti Medic.) and cotton (Gossypium hirsutum L. var. Stoneville 213) at three irradiances and determined the photosynthetic responses of single leaves to a range of six irradiances from 90 to 2000 μeinsteins m⁻²sec⁻¹. In air containing 21% O₂, velvetleaf and cotton grown at 750 μeinsteins m⁻²sec⁻¹ had maximum photosynthetic rates of 18.4 and 21.9 mg of CO₂ dm⁻²hr⁻¹, respectively. Maximum rates for leaves grown at 320 and 90 μeinsteins m⁻²sec⁻¹ were 15.3 and 10.3 mg of CO₂ dm⁻²hr⁻¹ in velvetleaf and 12 and 6.7 mg of CO₂ dm⁻²hr⁻¹ in cotton, respectively. In 1 O₂, maximum photosynthetic rates were 1.5 to 2.3 times the rates in air containing 21% O₂, and plants grown at medium and high irradiance did not differ in rate. In both species, stomatal conductance was not significantly affected by growth irradiance. The differences in maximum photosynthetic rates were associated with differences in mesophyll conductance. Mesophyll conductance increased with growth irradiance and correlated positively with mesophyll thickness or volume per unit leaf area, chlorophyll content per unit area, and photosynthetic unit density per unit area. Thus, quantitative changes in the photosynthetic apparatus help account for photosynthetic adaptation to irradiance in both species. Net assimilation rates calculated for whole plants by mathematical growth analysis were closely correlated with single-leaf photosynthetic rates.     

Photosynthetic limitations of a halophyte sea aster (<Emphasis Type="Italic">Aster tripolium</Emphasis> L) under water stress and NaCl stress

To understand the mechanisms of salt tolerance in a halophyte, sea aster (Aster tripolium L.), we studied the changes of water relation and the factors of photosynthetic limitation under water stress and 300 mM NaCl stress. The contents of Na⁺ and Cl^- were highest in NaCl-stressed leaves. Leaf osmotic potentials (Ψ _s) were decreased by both stress treatments, whereas leaf turgor pressure (Ψ _t) was maintained under NaCl stress. Decrease inΨ _s without any loss ofΨ _t accounted for osmotic adjustment using Na⁺ and Cl^- accumulated under NaCl stress. Stress treatments affected photosynthesis, and stomatal limitation was higher under water stress than under NaCl stress. Additionally, maximum CO₂ fixation rate and O₂ evolution rate decreased only under water stress, indicating irreversible damage to photosynthetic systems, mainly by dehydration. Water stress severely affected the water relation and photosynthetic capacity. On the other hand, turgid leaves under NaCl stress have dehydration tolerance due to maintenance of Ψ _t and photosynthetic activity. These results show that sea aster might not suffer from tissue dehydration in highly salinized environments. We conclude that the adaptation of sea aster to salinity may be accomplished by osmotic adjustment using accumulated Na⁺ and Cl^-, and that this plant has typical halophyte characteristics, but not drought tolerance. Electronic Publication     

Solute Accumulation in Tobacco Cells Adapted to NaCl

Cells of Nicotiana tabacum L. var Wisconsin 38 adapted to NaCl (up to 428 millimolar) which have undergone extensive osmotic adjustment accumulated Na⁺ and Cl⁻ as principal solutes for this adjustment. Although the intracellular concentrations of Na⁺ and Cl⁻ correlated well with the level of adaptation, these ions apparently did not contribute to the osmotic adjustment which occurred during a culture growth cycle, because the concentrations of Na⁺ and Cl⁻ did not increase during the period of most active osmotic adjustment. The average intracellular concentrations of soluble sugars and total free amino acids increased as a function of the level of adaptation; however, the levels of these solutes did not approach those observed for Na⁺ and Cl⁻. The concentration of proline was positively correlated with cell osmotic potential, accumulating to an average concentration of 129 millimolar in cells adapted to 428 millimolar NaCl and representing about 80% of the total free amino acid pool as compared to an average of 0.29 millimolar and about 4% of the pool in unadapted cells. These results indicate that although Na⁺ and Cl⁻ are principal components of osmotic adjustment, organic solutes also may make significant contributions.     

Spinach Leaf Chloroplast CO(2) and NO(2) Photoassimilations Do Not Compete for Photogenerated Reductant: Manipulation of Reductant Levels by Quantum Flux Density Titrations

Potential competition between CO₂ and NO₂⁻ photoassimilation for photogenerated reductant (e.g. reduced ferredoxin and NADPH) was examined employing isolates of mesophyll cells and intact chloroplasts derived from mature `source' spinach leaves. Variations in the magnitude of incident light energy were used to manipulate the supply of reductant in situ within chloroplasts. Leaf cell and plastid isolates were fed with saturating CO₂ and/or NO₂⁻ to produce the highest demand for reductant by CO₂ and/or NO₂⁻ assimilatory processes (enzymes). Even in the presence of CO₂ fixation, NO₂⁻ reduction in intact leaf cell isolates as well as plastid isolates was maximal at light energies as low as 50 to 200 microeinsteins per second per square meter. Simultaneously, 500 to 800 microeinsteins per second per square meter were required to support maximal CO₂ assimilation. Regardless of the magnitude of the incident light energy, CO₂ assimilation did not repress NO₂⁻ reduction, nor were these two processes mutually repressed. These observations have been interpreted to mean that reduced ferredoxin levels in situ in the plastids of mature source leaf mesophyll cells were adequate to supply the concurrent maximal demands exerted by enzymes associated with CO₂ as well as with inorganic nitrogen photoassimilation.     

Photosynthesis, growth, and the role of chloride

Previous studies with isolated chloroplasts have indicated that Cl⁻ is an essential cofactor for photosynthesis. Considerable support for the postulated Cl⁻ requirement in photosynthesis came from the observation that Cl⁻ is essential for growth. Data are presented which show that a 60% reduction in growth which occurred in Cl⁻ -deficient sugar beet (Beta vulgaris L.) was not due to an effect of Cl⁻ on the rate of photosynthesis in vivo (net CO₂ uptake per unit area of attached leaves). The principal effect of Cl⁻ deficiency was to lower cell multiplication rates in leaves, thus slowing down their growth and ultimately decreasing their area. The absence of an effect of Cl⁻ on photosynthesis in vivo was unlikely to have been due to Cl⁻ retention by the chloroplasts because their Cl⁻ concentration (measured after nonaqueous isolation) decreased progressively with decrease in leaf Cl⁻.     

Metabolic evidence for stelar anoxia in maize roots exposed to low o(2) concentrations

This investigation presents metabolic evidence to show that in 4- to 5-day-old roots of maize (Zea mays hybrid GH 5010) exposed to low external O₂ concentrations, the stele receives inadequate O₂ for oxidative phosphorylation, while the cortex continues to respire even when the external solution is at zero O₂ and the roots rely solely on aerenchyma for O₂ transport. Oxygen uptake rates (micromoles per cubic centimeter per hour) declined at higher external O₂ concentrations in excised segments from whole roots than from the isolated cortex; critical O₂ pressures for respiration were greater than 0.26 moles per cubic meter O₂ (aerated solution) for the whole root and only 0.075 moles per cubic meter O₂ for the cortex. For plants with their shoots excised and the cut stem in air, ethanol concentrations (moles per cubic meter) in roots exposed to 0.06 moles per cubic meter O₂ were 3.3 times higher in the stele than in the cortex, whereas this ethanol gradient across the root was not evident in roots exposed to 0 moles per cubic meter O₂. Alanine concentrations (moles per cubic meter) in the stele of roots exposed to 0.13 and 0.09 moles per cubic meter O₂ increased by 26 and 44%, respectively, above the levels found for aerated roots, whereas alanine in the cortex was unchanged; the increase in stelar alanine concentration was not accompanied by changes in the concentration of free amino acids other than alanine. For plants with their shoots intact, alcohol dehydrogenase and pyruvate decarboxylase activities (micromoles per gram protein per minute) in roots exposed to 0.13 moles per cubic meter O₂ increased in the stele by 40 to 50% over the activity in aerated roots, whereas there was no appreciable increase in alcohol dehydrogenase and pyruvate decarboxylase activity in the cortex of these roots. More convincingly, for roots receiving O₂ solely from the shoots via the aerenchyma, pyruvate decarboxylase in the cortex was in an “inactive” state, whereas pyruvate decarboxylase in the stele was in an “active” state. These results suggest that for roots in O₂-free solutions, the aerenchyma provides adequate O₂ for respiration in the cortex but not in the stele, and this was supported by a change in pyruvate decarboxylase in the cortex to an active state when the O₂ supply to the roots via the aerenchyma was blocked.     

Effects of magnesium deficiency on the photosynthesis and respiration of leaves of sugar beet

The effects of Mg deficiency on the photosynthesis and respiration of sugar beets (Beta vulgaris L. cv. F58-554H1) were studied by withholding Mg from the culture solution and by following changes in CO₂ and water vapor exchange of attached leaves. Leaf blade Mg concentration decreased from about 1200 to less than 200 meq kg⁻¹ dry matter without change in the rate of photosynthetic CO₂ uptake per unit leaf area, while from 200 to 50 meq kg⁻¹ the rate decreased to one-third. Rates of photorespiratory evolution of CO₂ into CO₂-free air responded to Mg like those of photosynthetic CO₂ uptake, the rates decreasing to one-half, below 200 meq kg⁻¹. Respiratory CO₂ evolution in the dark increased almost 2-fold in low Mg leaves. Magnesium deficiency had less effect on leaf (mainly stomatal) diffusion resistance (r₁) than on mesophyll resistance (r_m); in Mg-deficient plants r_m increased from 2.9 to 7.1 sec cm⁻¹, whereas r₁ became significantly greater than the control value only in the most severe instances of Mg deficiency.     

Effects of salt stress on the growth,ion content,stomatal behaviour and photosynthetic capacity of a salt-sensitive species,Phaseolus vulgaris L.

Phaseolus vulgaris (cv. Hawkesbury Wonder) was grown over a range of NaCl concentrations (0–150 mM), and the effects on growth, ion relations and photosynthetic performance were examined. Dry and fresh weight decreased with increasing external NaCl concentration while the root/shoot ratio increased. The Cl^- concentration of leaf tissue increased linearly with increasing external NaCl concentration, as did K⁺ concentration, although to a lesser degree. Increases in leaf Na⁺ concentration occurred only at the higher external NaCl concentrations (100 mM). Increases in leaf Cl^- were primarily balanced by increases in K⁺ and Na⁺. X-ray microanalysis of leaf cells from salinized plants showed that Cl^- concentration was high in both the cell vacuole and chloroplast-cytoplasm (250–300 mM in both compartments for the most stressed plants), indicating a lack of effective intracellular ion compartmentation in this species. Salinity had little effect on the total nitrogen and ribulose-1,5-bisphosphate (RuBP) carboxylase (EC 4.1.1.39) content per unit leaf area. Chlorophyll per unit leaf area was reduced considerably by salt stress, however. Stomatal conductance declined substantially with salt stress such that the intercellular CO₂ concentration (C _i) was reduced by up to 30%. Salinization of plants was found to alter the ¹³C value of leaves of Phaseolus by up to 5 and this change agreed quantitatively with that predicted by the theory relating carbon-isotope fractionation to the corresponding measured intercellular CO₂ concentration. Salt stress also brought about a reduction in photosynthetic CO₂ fixation independent of altered diffusional limitations. The initial slope of the photosynthesis versus C _i response declined with salinity stress, indicating that the apparent in-vivo activity of RuBP carboxylase was decreased by up to 40% at high leaf Cl^- concentrations. The quantum yield for net CO₂ uptake was also reduced by salt stress.Abbreviations and symbols A net CO₂ assimilation rate - C _a ambient CO₂ concentration - C _i intercellular CO₂ concentration - RuBP ribulose-1,5-bisphosphate - ¹³C ratio of ¹³C to ¹²C relative to standard limestone     

Abscisic Acid Stimulated Osmotic Adjustment and Its Involvement in Adaptation of Tobacco Cells to NaCl

Osmotic adjustment of cultured tobacco (Nicotiana tabacum L. var Wisconsin 38) cells was stimulated by 10 micromolar (±) abscisic acid (ABA) during adaptation to water deficit imposed by various solutes including NaCl, KCl, K₂SO₄, Na₂SO₄, sucrose, mannitol, or glucose. The maximum difference in cell osmotic potential (Ψπ) caused by ABA treatment during adaptation to 171 millimolar NaCl was about 6 to 7 bar. The cell Ψπ differences elicited by ABA were not due to growth inhibition since ABA stimulated growth of cells in the presence of 171 millimolar NaCl. ABA caused a cell Ψπ difference of about 1 to 2 bar in medium without added NaCl. Intracellular concentrations of Na⁺, K⁺, Cl⁻, free amino acids, or organic acids could not account for the Ψπ differences induced by ABA in NaCl treated cells. However, since growth of NaCl treated cells is more rapid in the presence of ABA than in its absence, greater accumulation of Na⁺, K⁺, and Cl⁻ was necessary for ion pool maintenance. Higher intracellular sucrose and reducing sugar concentrations could account for the majority of the greater osmotic adjustment of ABA treated cells. More rapid accumulation of proline associated with ABA treatment was highly correlated with the effects of ABA on cell Ψπ. These and other data indicate that the role of ABA in accelerating salt adaptation is not mediated by simply stimulating osmotic adjustment.