Effects of chloride and sodium on gas exchange parameters and water relations of Citrus plants

Gas exchange parameters, water relations and Na⁺/Cl^- content were measured on leaves of one-year-old sweet orange ( Citrus sinensis [L.] Osbeck cv. Hamlin) seedlings grown at increasing levels of salinity. Different salts (NaCl, KCl and NaNO₃) were used to separate the effects of Cl⁻ and Na⁺ on the investigated parameters. The chloride salts reduced plant dry weight and increased defoliation. Accumulation of Cl⁻ in the leaf tissue caused a sharp reduction in photosynthesis and stomatal conductance. By contrast, these parameters were not affected by leaf Na⁺ concentrations of up to 478 m M in the tissue water. Leaf water potentials reached values near −1.8 MPa at high NaCl and KCl supplies. This reduction was offset by a decrease in the osmotic potential so that turgor was maintained at or above control values. The changes in osmotic potential were closely correlated with changes in leaf proline concentrations. Addition of Ca²⁺ (as calcium acetate) increased growth and halved defoliation of salt stressed plants. Furthermore, calcium acetate decreased the concentration of Cl⁻ and Na⁺ in the leaves, and increased photosynthesis and stomatal conductance. Calcium acetate also counteracted the reductions in leaf water and osmotic potentials induced by salinity. In addition, calcium acetate inhibited the accumulation of proline in the leaves which affected the reduction in osmotic potential. These results indicate that adverse effects of salinity in Citrus leaves are caused by accumulation of chloride.     

Water Stress in Leaves of Pbaseolus vulgaris Infected with Xanthomonas campestris pv. phaseoli

Infection of bean leaves ( Phaseolus vulgaris ) by Xanthomonas campestris pv. phaseoli in the field frequently resulted in the appearance of isolated flaccid areas in green leaf tissue adjacent to necrotic and chlorotic lesions. The flaccid leaf areas had significantly higher stomatal resistances compared to nearby turgid areas on the same leaf, and the turgid areas had stomatal resistances that were the same or only moderately elevated compared to those of healthy leaves. The flaccid tissues also had significantly lower relative water contents than turgid tissues on the same leaf demonstrating that pathogeninduced water stress was localized. The levels of free proline, another indicator of water stress, were directly correlated (r²= 0.556) with disease severity. The change in free proline content implied that water stress increased in direct proportion with the amount of tissue infected. Water stress may be due to the disruption of xylem elements by the invasion of X. c. phaseoli from nearby lesions. One result of xylem invasion could have been severe water deficits which were sufficient to cause stomatal closure and leaf flaccidity; however, this effect was highly localized and the remainder of the diseased leaf was either significantly less water stressed or not affected.     

Photosynthesis as related to xylem water potential and carbon dioxide concentration in Sitka spruce

Current-year shoots of Sitka spruce ( Picea sitchensis (Bong.) Carr.) were removed from the forest canopy. After steady-state rates of net photosynthesis were obtained in a leaf chamber, the shoots were excised in air and removed at different times to establish a relationship between net photosynthesis and xylem water potential. The experiment was repeated at five ambient carbon dioxide concentrations.
Net photosynthesis remained constant over a wide range of xylem water potential and increased linearly with ambient carbon dioxide concentration between 20 and 300 cm³ m⁻³. At low water potential net photosynthesis declined at each ambient carbon dioxide concentration and there was little difference in the potential (±0.05 MPa) at which zero photosynthesis was observed.
There was a small increase in the CO₂ compensation concentration at low xylem water potentials, but calculated mesophyll conductance still declined at low water potential after correction for this change in compensation concentration. Mesophyll conductance reached zero within the same range of water potential as net photosynthesis. The results suggested that the non-stomatal contribution to the decline of photosynthesis was approximately 30% until almost complete stomatal closure occurred.     

Sodium partitioning within the shoot of soybean

Uptake and partitioning of Na⁺ and Cl⁻ in plants of soybean ( Glycine max L. Merr. cv. Hodgson) exposed to moderate NaCl concentrations were studied over an 8-day period. Plants showed marked retention of Na⁺ in the stems and low transport to laminae of young leaves. The xylem sap ascending the main axis was progressively depleted in Na⁺. The oldest leaf greatly contributed to Na⁺ depletion of the sap flowing to younger leaves. These results in combination with estimates of phloem recirculation indicated that Na⁺ accumulation in the young leaf was prevented both by depletion of Na⁺ from the xylem stream, and by a high recirculation of Na⁺ via the phloem. However, this protection of young leaves was effective only for very mild salinity treatment.     

Relations between water content, potential and permeability in stems of conifers

Abstract. The influence of sapwood water content on the conductivity of sapwood to water was measured on stem sections of Pinus contorta. A reduction in relative water content from 100 to 90% caused permeability to fall to about 10% of the saturated value.
Pressure–volume curves of branchwood and stem sapwood of Pinus contorta and Picea sitchensis have been analysed to definè the tissue capacitance and the time constant and resistance for water movement between stored water and the functional xylem as functions of tissue water potential. Three phases in water loss were discernible. In the initial phase at high water potentials (> –0.5 MPa), the capacitance was large, the time constant long and the resistance to flow large in comparison with intermediate water potentials (−0.5 to −1.5 MPa). At still lower water potentials (−1.5 to −3.0 MPa), the time constant and resistance declined still further but the capacitance had a tendency to increase again, especially in the stemwood of Sitka spruce. Typical values in the second phase were for the time constant 5 s, for the resistance 4 × 10⁻¹³ N s m⁻⁵ and for the capacitance (change in relative water content per unit change in potential) 1×10⁻¹¹ m³ Pa⁻¹. These parameters define the availability of stored water and are being used in a dynamic model of water transport in trees.     

Effect of high external NaCl concentrations on ion transport within the shoot of Lupinus albus. I. Ions in xylem sap

Abstract. Xylem sap was collected from individual leaves of intact transpiring lupin plants exposed to increasing concentrations of NaCl by applying pneumatic pressure to the roots. Concentrations of Na⁺ and Cl⁻ in the xylem sap increased linearly with increases in the external NaCl concentration, averaging about 10% of the external concentration. Concentrations of K⁺ and NO₃⁻, the other major inorganic ions in the sap, were constant at about 2.5 and 1.5 mol m⁻³, respectively. There was no preferential direction of Na ⁺ or Cl⁻ to either young or old leaves: leaves of all ages received xylem sap having similar concentrations of Na⁺ and Cl⁻, and transpiration rates (per unit leaf area) were also similar for all leaves. Plants exposed to 120–160 mol m⁻³ NaCl rapidly developed injury of oldest leaves; when this occurred, the Na⁺ concentration in the leaflet midrib sap had increased to about 40 mol m⁻³ and the total solute concentration to 130 osmol m⁻³. This suggests that uptake of salts from the transpiration stream had fallen behind the rate of delivery to the leaf and that salts were building up in the apoplast.     

Diurnal water storage in the stems of Picea sitchensis (Bong.) Carr.

Abstract. Two models of the relationship between diurnal variation in shoot water potential and transpiration in 14-year-old Picea sitchensis (Bong.) Carr. were compared. The first model was a physiologically based resistance-capacitance (R-C) analogue with its associated differential equations. The second was a non-physiological discrete-difference (D-D) or stochastic transfer function model. The RC model included only the effect of water storage in the phloem and bark while the D-D model implicity included all storage mechanisms. The R-C and D-D models explained similar fractions (62% and 68% respectively) of the variation in shoot water potential due to diurnal changes in transpiration rate. However, the D-D model had fewer parameters than the R-C model. The results from the D-D model showed that the resistance to flow from soil to shoots along the trunk, (RT), was 5 × 10³ MPa kg^-1s and the capacitance of the phloem and bark treated as a single store, (C_s), was 1.6 kg MPa^-1. It is suggested that the resistance to flow into storage (R_s) is much greater than R_T and can be disregarded. A non-linear version of the D-D model suggested [hat resistance to flow in the trunk increases with increasing transpiration rate.     

Leaf xylem embolism, detected acoustically and by cryo-SEM, corresponds to decreases in leaf hydraulic conductance in four evergreen species

Hydraulic conductance of leaves ( K _leaf) typically decreases with increasing water stress. However, the extent to which the decrease in K _leaf is due to xylem cavitation, conduit deformation or changes in the extra-xylary pathway is unclear. We measured K _leaf concurrently with ultrasonic acoustic emission (UAE) in dehydrating leaves of two vessel-bearing and two tracheid-bearing species to determine whether declining K _leaf was associated with an accumulation of cavitation events. In addition, images of leaf internal structure were captured using cryo-scanning electron microscopy, which allowed detection of empty versus full and also deformed conduits. Overall, K _leaf decreased as leaf water potentials ( Ψ _L) became more negative. Values of K _leaf corresponding to bulk leaf turgor loss points ranged from 13 to 45% of their maximum. Additionally, Ψ _L corresponding to a 50% loss in conductivity and 50% accumulated UAE ranged from −1.5 to −2.4 MPa and from −1.1 to −2.8 MPa, respectively, across species. Decreases in K _leaf were closely associated with accumulated UAE and the percentage of empty conduits. The mean amplitude of UAEs was tightly correlated with mean conduit diameter ( R ² = 0.94, P  = 0.018). These results suggest that water stress-induced decreases in K _leaf in these species are directly related to xylem embolism.     

Field productivity of a CAM plant, Agave salmiana, estimated using daily acidity changes under various environmental conditions

Nocturnal acidity increases of the Crassulaceae acid metabolism succulent Agave salmiana Otto ex Salm. ssp. crassispina (Trel.) Gentry were determined for various photosynthetically active radiation (PAR) levels in the field and various day/night temperatures and soil water potentials in the laboratory. When a particular factor was optimal for nocturnal acid accumulation, it was assigned an index value of unity. The product of the three indices was termed the environmental productivity index (EPI), which summarizes the effect of PAR, temperature, and water status on nocturnal acid accumulation. A monthly value for EPI was determined using microclimatic conditions at the field site near Salinas de Hidalgo, San Luis Potosf, Mexico. EPI was highly correlated with the number of leaves unfolding monthly from plants in the field (r²= 0.95), where monitoring of such leaf unfolding is a non-destructive technique for assessing plant productivity. By using the measured leaf area index for four groups of A. salmiana and a relationship between acid accumulation and net CO₂ up-take, the EPI acidity data were converted to dry weight gain per unit ground area. This measure of productivity closely agreed with the 1.05 kg m⁻² year⁻¹ determined by conventional harvesting techniques. The productivity is also in agreement with studies on other desert agaves, but about 10-fold higher than that generally considered for desert ecosystems.     

Ion supply capacity of roots in relation to rejuvenation of primary leaves in vivo

Removal of the shoot above the primary node (detopping) of 3-week-old bean plants ( Phaseolus vulgaris L. cv. Contender) altered the metabolism and development of the remaining leaves. An increase in levels of chlorophyll, protein, stomatal opening, photosynthesis and growth, i.e. rejuvenation of primary leaves, was established within 7 days of detopping. These levels were maintained while the primary leaves of equivalent intact plants senesced.
The flux of xylem solution (mineral ions, cytokinins and water) into leaves is related to the leaf area to be supplied and root supply capacity; it has been suggested that detopping leads to an increased availability of root-supplied solutes and hence rejuvenation of the remaining leaves. This assumes however that root output of solutes is not decreased by the defoliation treatment.
We found that root output of ions (electrical conductivity of passive xylem exudate) in detopped plants was 30% lower than in intact plants after 24 h and 60% lower after 7 days. The output of Ca²⁺, Mg²⁺ and K⁺ were similarly reduced 7 and 14 days after detopping as were fresh and dry weights of roots. Furthermore, neither the calculated xylem flux of ions nor directly measured levels of Ca²⁺, Mg²⁺ and K⁺ were significantly increased in leaves of detopped plants during their rejuvenation. We therefore conclude that root output is tightly coupled to shoot demand and that the apparent rejuvenation of primary leaves caused by detopping bean plants is not a consequence of increased xylem flux of mineral ions into the leaves.     

Translocation of NH4+ in oilseed rape plants in relation to glutamine synthetase isogene expression and activity

Translocation of NH₄⁺ was studied in relation to the expression of three glutamine synthetase (GS, EC 6.3.1.2) isogenes and total GS activity in roots and leaves of hydroponically grown oilseed rape ( Brassica napus ). The concentration of NH₄⁺ in the stem xylem sap of NO₃⁻-fed plants was 0.55–0.70 m M , which was ≈60% higher than that in plants deprived of external nitrogen for 2 days. In NH₄⁺-fed plants, xylem NH₄⁺ concentrations increased linearly both with time of exposure to NH₄⁺ and with increasing external NH₄⁺ concentration. The maximum xylem NH₄⁺ concentration was 8 m M , corresponding to 11% of the nitrogen translocated in the xylem. In the leaf apoplastic solution, the NH₄⁺ concentration increased from 0.03 m M in N-deprived plants to 0.20 m M in N-replete plants. The corresponding values for leaf tissue water were 0.33 and 1.24 m M , respectively. The addition of either NO₃⁻ or NH₄⁺ to N-starved plants induced both cytosolic gs isogene expression and GS activity in the roots. In N-replete plants, gs isogene expression and GS activity were repressed, probably due to carbon limitations, thereby protecting the roots against the excessive drainage of photosynthates. Repressed gs isogene expression and GS activity under N-replete conditions caused enhanced NH₄⁺ translocation to the shoots.     

The effect of water stress on photosynthesis and related parameters in Pinus halepensis

Net photosynthesis, transpiration, dark respiration rates and stomatal and mesophyll resistances were studied in young potted seedlings of Pinus halepensis Mill. under gradually decreasing soil and leaf water potentials. Stomatal resistance under non-limiting xylem water potentials was 6–7 times higher than mesophyll resistance. Stomata started to close at threshold xylem water potentials of −0.8 MPa, whereas mesophyll resistance started to increase at about −1.4 MPa. Decreasing xylem water potentials increased the CO₂ compensation point and decreased the water use efficiency (expressed by the photosynthesis to transpiration ratio) and dark respiration rate. It is concluded that at least part of the drought resistance characteristics of P. halepensis are associated with a sensitive stomatal mechanism which enables an efficient control of water loss.     

Low carbon dioxide concentrations can reverse stomatal closure during water stress

Leaf water potentials below threshold values result in reduced stomatal conductance (g_s). Stomatal closure at low leaf water potentials may serve to protect against cavitation of xylem. Possible control of g_s by leaf water potential or hydraulic conductance was tested by drying the rooting medium in four herbaceous annual species until g_s was reduced and then lowering the [CO₂] to determine whether g_s and transpiration rate could be increased and leaf water potential decreased and whether hydraulic conductance was reduced at the resulting lower leaf water potential. In all species, low [CO₂] could reverse the stomatal closure because of drying despite further reductions in leaf water potential, and the resulting lower leaf water potentials did not result in reductions in hydraulic conductance. The relative sensitivity of g_s to internal [CO₂] in the leaves of dry plants of each species averaged three to four times higher than in leaves of wet plants. Two species in which g_s was reputed to be insensitive to [CO₂] were examined to determine whether high leaf to air water vapor pressure differences (D) resulted in increased stomatal sensitivity to [CO₂]. In both species, stomatal sensitivity to [CO₂] was indeed negligible at low D, but increased with D, and low [CO₂] partly or fully reversed closure caused by high D. In no case did low leaf water potential or low hydraulic conductance during drying of the air or the rooting medium prevent low [CO₂] from increasing g_s and transpiration rate.     

Endogenous abscisic acid levels are linked to decreased growth of bush bean plants treated with NaCl

This paper studies the relative importance of endogenous ABA and ion toxicity in the leaf growth inhibition caused by NaCl in salt-adapted and unadapted bush beans. Adaptation to salt-stress was achieved by germination of seeds in 75 m M NaCl, while unadapted plants were germinated in tap water. The adaptation process caused a transitory increase in leaf ABA, Na⁺ and Cl⁻ concentrations, while leaf expansion was inhibited. However, when grown for 8 or 13 days in 75 m M NaCl-containing nutrient solution, primary and first trifoliolate leaves of salt-adapted plants had greater areas than those of unadapted plants. Concentrations of ABA, Na⁺ and Cl⁻ in these leaves were lower in adapted plants, and a strong negative correlation between leaf expansion growth and either leaf Na⁺, Cl⁻ or ABA concentrations could be established. However, in the second trifoliolate leaves only the ABA, but not the Na⁺ or Cl⁻, concentrations were significantly correlated with leaf expansion. Our results suggest that salt-induced inhibition of leaf expansion growth in bush beans is mediated by ABA rather than Na⁺ or Cl⁻ toxicity. Moreover, the increase of ABA, induced by the salt-pretreatment, seems to play an important role in limiting the accumulation of Na⁺ and Cl⁻ in the leaves, leading to adaptation of bush beans to salt-stress.     

Persistent xylem cross-walls reduce the axial hydraulic conductivity in the apical 20 cm of barley seminal root axes: implications for the driving force for water movement

Abstract. Xylem vessels in the apical 25 cm of barley seminal axes were examined by scanning electron microscopy of fractured freeze dried or critical point dried specimens. In the apical 11 cm, there were three cross walls cm⁻¹ root in the central xylem vessel. The frequency then declined with distance but did not become less than 1.0 cm⁻¹ root until the 22–25-cm zone.
Suction was applied to the proximal end of segments of seminal axes whose surfaces had been sealed with wax to prevent radial entry of water. Perfusion of the xylem with solutions of Tinopal CBS-X revealed the conductive xylem vessels by fluorescent staining of their walls. In the apical 20 cm of the axis, only a variable number of smaller xylem vessels conduct water. Beyond this zone, the much larger central vessel becomes functional.
The flow of water (Jv) in the apical zone was very much less for a given presure (△P) than in the proximal zone > 25 cm from the tip, and could be predicted by the Poiseuille equation provided the correct number of functional vessels are known. This information, together with earlier results on water uptake along the root length are used to predict the attenuation of the hydrostatic driving force for water uptake along the root length.
Estimates of K⁺ concentrations in stelar parenchyma and xylem vessels were made by electron microproble X-ray analysis. These results show that [K⁺] in the xylem vessels may be two to three times greater in the zone 1–2 cm from the root tip than in the basal zone. Such a gradient of solute potential may, to some extent, offset the decreasing influence of the leaf water potential in apical zones where xylem is not fully conductive.     

Ontogenetic changes in potassium transport in xylem of tomato

The influence of plant ontogeny on xylem exudate K⁺ concentrations and K⁺ transport to the shoot was studied in both nutrient-solution and field-grown tomato plants ( Lycopersicon esculentum ).
K⁺ concentrations in xylem exudate from decapitated plants decreased during tomato plant development from a high of 12 m M to a low of 5 m M . In the nutrient-solution plants, the most rapid decline occurred during the vegetative growth phase, while in field-grown plants, the xylem K⁺ concentrations remained high during an-thesis and then subsequently declined. The rapid decline in nutrient-solution plants might be related to a decrease in the absorptive efficiency of the root system. In field-grown plants, a reduction in the availability of assimilates to the root might account in part for the decrease in xylem exudate K⁺ concentrations. The volume (ml h⁻¹ plant⁻¹) and the net rates of K⁺ exudation (mmol h⁻¹ plant⁻¹) decreased dramatically as the fruits approached maturity. Since only a small reduction in xylem exudate K⁺ concentrations occurred during fruiting, the hydraulic conductivity of the root system decreased as the tomato plants aged. It is proposed that the ontogenetic changes in xylem transport of K⁺ contribute to a reduction in leaf free space K⁺ concentration which would explain the decline in tomato leaf K⁺ concentrations.     

Effect of competition and leaf age on visible and infrared reflectance in pine foliage

Abstract. The effect of plant competition on spectral reflectance in the 400–2500 nm wavelength region was determined for 3-month-old and 15-month-old leaves of loblolly pine ( Pinus taeda L.). Strong competition decreased water potentials and Mg concentrations, and increased K in young and old leaves. Also, competition decreased Ca and total chlorophyll in young leaves. As measured with a scanning radiometer, reflectance in young leaves at 551 nm decreased from 20 to 14% as water potentials increased from -2.2 to -0.9 MPa (r²= 0.82). For young and old leaves reflectance at 551 nm decreased from 20 to 10% with increasing total chlorophyll (r²= 0.64). Reflectance decreased slightly with increasing K in young leaves (401 nm, r²= 0.55), and with increasing Mg in old leaves (470 nm, r²= 0.57). Increased visible reflectance under strong competition may have resulted primarily from decreased water potentials. Reflectances were much greater in young versus old leaves in the 750–1300 nm range, and were greater in old rather than young leaves from 1400–2500 nm. Infrared reflectances were not, however, significantly affected by competition.     

The importance of xylem constraints in the distribution of conifer species

Vulnerability of stem xylem to cavitation was measured in 10 species of conifers using high pressure air to induce xylem embolism. Mean values of air pressure required to induce a 50% loss in hydraulic conductivity (φ₅₀) varied enormously between species, ranging from a maximum of 14.2±0.6 MPa (corresponding to a xylem water potential of −14.2 MPa) in the semi-arid species Actinostrobus acuminatus to a minimum of 2.3±0.2 MPa in the rainforest species Dacrycarpus dacrydioides . Mean φ₅₀ was significantly correlated with the mean rainfall of the driest quarter within the distribution of each species. The value of φ₅₀ was also compared with leaf drought tolerance data for these species in order to determine whether xylem dysfunction during drought dictated drought response at the leaf level. Previous data describing the maximum depletion of internal CO₂ concentration (c_i) in the leaves of these species during artificial drought was strongly correlated with φ₅₀ suggesting a primary role of xylem in effecting leaf drought response. The possibility of a trade-off between xylem conductivity and xylem vulnerability was tested in a sub-sample of four species, but no evidence of an inverse relationship between φ₅₀ and either stem-area specific (K_a) or leaf-area specific conductivity (K₁) was found.     

Interaction of enriched CO2 and water stress on the physiology of and biomass production in sweet potato grown in open-top chambers

Abstract. The objective of this study was to investigate the effects of water stress in sweet potato ( Ipomoea batatas L. [Lam] 'Georgia Jet') on biomass production and plant-water relationships in an enriched CO₂ atmosphere. Plants were grown in pots containing sandy loam soil (Typic Paleudult) at two concentrations of elevated CO₂ and two water regimes in open-top field chambers. During the first 12 d of water stress, leaf xylem potentials were higher in plants grown in a CO₂ concentration of 438 and 666 μmol mol⁻¹ than in plants grown at 364 μmol mol⁻¹. The 364 μmol mol⁻¹ CO₂ grown plants had to be rewatered 2 d earlier than the high CO₂-grown plants in response to water stress. For plants grown under water stress, the yield of storage roots and root: shoot ratio were greater at high CO₂ than at 364 μmol mol⁻¹; the increase, however, was not linear with increasing CO₂ concentrations. In well-watered plants, biomass production and storage root yield increased at elevated CO₂, and these were greater as compared to water-stressed plants grown at the same CO₂ concentration.     

Reduction of nitrate in the perennial tissues of aerial parts of Alnus glutinosa

In vivo nitrate reductase (NR, EC 1.6.6.1.) activity was measured in leaves, branches and trunk of field-grown Alnus glutinosa (L.) Gaertn. All of the assayed tissues enzymatically reduced nitrate with a decreasing activity [μmol NO₂⁻ (g dry weight)⁻¹ h⁻¹] in the order: leaves > branch bark > inner branch tissues > trunk xylem. The NR activity of the various tissues of excised branches was inhibited by tungstate added to the transpiration stream. Part of the nitrate added to the feeding solution (0.2, 0.5 or 1 m M KNO₃) of excised branches disappeared during its transport via the transpiration stream in the perennial tissues. This disappearance was enzymatic since it was decreased by tungstate.
No evidence was obtained for the presence of nitrate in natural xylem sap nor for a significant correlation between nitrate content of soil and leaf NR activity. These results indicate that in the field-grown black alder, the nitrate not reduced in the roots could be reduced in the perennial tissues of aerial parts. Since the leaf NR activity does not reflect the actual in situ nitrate reduction, the existence of a constitutive NR activity in Alnus leaves is suggested.