Water Potential and Component Potentials in Expanded and Unexpanded Leaves of Two Xeric Grasses

Water (ψ), osmotic (ψ_s+ψ_m) and pressure (ψ_p) potentials were measured in three leaf regions of Agropyron dasystachyum and A. smithii grown in the field. Spanner-type thermocouple psychrometers were used to measure ψ and (ψ_m+ψ_m). Absolute water content (AWC) was measured gravimetrically. The ψ and ψ_p were slightly lower in the emerging leaf blade (EBI) than in the last fully emerged leaf blade (FEBI); (ψ_s+ψ_m) and AWC were similar in the two regions. A gradient as large as 0.7 MPa was observed between the EBI and the base of the same emerging leaf (EBs); the latter included the meristematic regions. Although (ψ_s+ψ_m) and ψ_p were generally higher in the EBs, the gradients diminished as the level of stress increased in the shoot. Under moderate water stress the ψ_p of the EBs remained constant relative to the ψ_p in the exposed blades. The large ψ gradient within the growing leaf could have resulted from high resistance imposed by poor vascular development in the intercalary meristem. Ability to maintain a relatively large ψ gradient may be of general significance in buffering the growing region of xeric grass leaves from extreme, short-term fluctuations in water stress that occur in exposed leaf blades.     

Salt tolerance in the halophyte Suaeda maritima L. Dum.

Osmotic potentials and individual epidermal cell turgor pressures were measured in the leaves of seedlings of Suaeda maritima growing over a range of salinities. Leaf osmotic potentials were lower (more negative) the higher the salt concentration of the solution and were lowest in the youngest leaves and stem apices, producing a gradient of osmotic potential towards the apex of the plant. Epidermal cell turgor pressures were of the order of 0.25 to 0.3 MPa in the youngest leaves measured, decreasing to under 0.05 MPa for the oldest leaves. This pattern of turgor pressure was largely unaffected by external salinity. Calculation of leaf water potential indicated that the gradient between young leaves and the external medium was not altered by salinity, but with older leaves, however, this gradient diminished from being the same as that for young leaves in the absence of NaCl, to under 30% of this value at 400 mM NaCl. These results are discussed in relation to the growth response of S. maritima.     

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.     

Salinity resistance and plant growth revisited

This article reconsiders a recent hypothesis concerning the physiology of growth inhibition by salinity and its relevance to the breeding of salt-resistant crops (Munns 1993, Plant, Cell and Environment 16, pp. 15–24). The hypothesis states that the osmotic effects of salinity on water availability will strongly and equally inhibit the growth of related species and varieties. The genotypic diversity needed for breeding increased resistance to growth inhibition by salinity is only expected to appear after weeks or months. Higher rates of salt accumulation in more sensitive varieties then lead to accelerated leaf senescence. This further inhibits new growth, as compared with more resistant varieties. Accordingly, breeders aiming to increase crop growth under salinity should focus efforts on manipulating genes which can decrease rates of salt accumulation. However, the osmotic inhibition of growth by salinity appears to involve regulatory physiogical changes. Thus, some genotypic diversity might be expected. Clear evidence is presented for genotypic diversity in early growth responses to salt or PEG-induced osmotic stress, in several species and varieties. The conclusion is that development of plants with increased resistance to inhibition of growth by the osmotic effects of external salinity (in addition to increased resistance to salt accumulation) is both feasible and desirable.     

Diffusional conductance to CO2 is the key limitation to photosynthesis in salt‐stressed leaves of rice (Oryza sativa)

Salinity significantly limits leaf photosynthesis but the factors causing the limitation in salt‐stressed leaves remain unclear. In the present work, photosynthetic and biochemical traits were investigated in four rice genotypes under two NaCl concentration (0 and 150 mM) to assess the stomatal, mesophyll and biochemical contributions to reduced photosynthetic rate (A) in salt‐stressed leaves. Our results indicated that salinity led to a decrease in A, leaf osmotic potential, electron transport rate and CO₂ concentrations in the chloroplasts (C_c) of rice leaves. Decreased A in salt‐stressed leaves was mainly attributable to low C_c, which was determined by stomatal and mesophyll conductance. The increased stomatal limitation was mainly related to the low leaf osmotic potential caused by soil salinity. However, the increased mesophyll limitation in salt‐stressed leaves was related to both osmotic stress and ion stress. These findings highlight the importance of considering mesophyll conductance when developing salinity‐tolerant rice cultivars.     

Comparison of salt tolerance and osmotic adjustment of low-sodium and high-sodium subspecies of the C4 halophyte, Atriplex canescens

The relationship between Na⁺ accumulation and salt tolerance was tested by comparing subspecies of the halophyte, Atriplex canescens (fourwing saltbush), that differed markedly in Na⁺ content and Na:K ratios. Above ground tissues of one low-sodium and two high-sodium subspecies were compared with respect to cation accumulation, osmotic adjustment and growth along a salinity gradient in greenhouse trials. Plants of each subspecies were grown for 80 d on 2.2, 180, 540 and 720 mol m^?3 NaCl. At harvest, A. canescens ssp. canescens had significantly lower Na⁺ levels, higher K⁺ levels and lower Na:K ratios in leaf and stem tissues than A. canescens ssp. macropoda and linearis over the salinity range (P < 0.05 or 0.01). Na:K ratios in leaves of the latter two, high-sodium, subspecies were approximately 2 on the lowest salinity treatment and ranged from 5 to 10 on the more saline solutions. By contrast, Na:K ratios in leaves of the low-sodium subspecies canescens, were only 0.4 on the lowest salinity and ranged narrowly from 1.7 to 2.3 at higher salinities. However, despite different patterns of Na⁺ and K⁺ accumulation, all three subspecies exhibited equally high salt tolerance and had similar osmotic pressures in their leaves or stems over the salinity range. Contrary to expectations, high salt tolerance was not necessarily dependent on high levels of Na⁺ accumulation in this species.     

Effect of high salinity on Atriplex portulacoides: Growth,leaf water relations and solute accumulation in relation with osmotic adjustment

Atriplex (Halimione) portulacoides is a halophyte with potential interest for saline soil reclamation and phytoremediation. Here, we assess the impact of salinity reaching up to two-fold seawater concentration (0–1000 mM NaCl) on the plant growth, leaf water status and ion uptake and we evaluate the contribution of inorganic and organic solutes to the osmotic adjustment process. A. portulacoides growth was optimal at 200 mM NaCl but higher salinities (especially 800 and 1000 mM NaCl) significantly reduced plant growth. Na⁺ and Cl⁻ contents increased upon salt exposure especially in the leaves compared to the roots. Interestingly, no salt-induced toxicity symptoms were observed and leaf water content was maintained even at the highest salinity level. Furthermore, leaf succulence and high instantaneous water use efficiency (WUE_i) under high salinity significantly contributed to maintain leaf water status of this species. Leaf pressure–volume curves showed that salt-challenged plants adjusted osmotically by lowering osmotic potential at full turgor (Ψ_π¹⁰⁰) along with a decrease in leaf cell elasticity (values of volumetric modulus elasticity (ε) increased). As a whole, our findings indicate that A. portulacoides is characterized by a high plasticity in terms of salt-response. Preserving leaf hydration and efficiently using Na⁺ for the osmotic adjustment especially at high salinities (800–1000 mM NaCl), likely through its compartmentalization in leaf vacuoles, are key determinants of such a performance. The selective absorption of K⁺ over Na⁺ in concomitance with an increase in the K⁺ use efficiency also accounted for the overall plant salt tolerance.     

Salinity and drought tolerance of mannitol-accumulating transgenic tobacco

Tobacco plants (Nicotiana tabacum L.) were transformed with a mannitol-1-phosphate dehydrogenase gene resulting in mannitol accumulation. Experiments were conducted to determine whether mannitol provides salt and/or drought stress protection through osmotic adjustment. Non-stressed transgenic plants were 20–25% smaller than non-stressed, non-transformed (wild-type) plants in both salinity and drought experiments. However, salt stress reduced dry weight in wild-type plants by 44%, but did not reduce the dry weight of transgenic plants. Transgenic plants adjusted osmotically by 0.57 MPa, whereas wild-type plants did not adjust osmotically in response to salt stress. Calculations of solute contribution to osmotic adjustment showed that mannitol contributed only 0-003-0-004 MPa to the 0.2 MPa difference in full turgor osmotic potential (π_o) between salt-stressed transgenic and wild-type plants. Assuming a cytoplasmic location for mannitol and that the cytoplasm constituted 5% of the total water volume, mannitol accounted for only 30–40% of the change in π_o of the cytoplasm. Inositol, a naturally occurring polyol in tobacco, accumulated in response to salt stress in both transgenic and wild-type plants, and was 3-fold more abundant than mannitol in transgenic plants. Drought stress reduced the leaf relative water content, leaf expansion, and dry weight of transgenic and wild-type plants. However, π_o was not significantly reduced by drought stress in transgenic or wild-type plants, despite an increase in non-structural carbohydrates and mannitol in droughted plants. We conclude that (1) mannitol was a relatively minor osmolyte in transgenic tobacco, but may have indirectly enhanced osmotic adjustment and salt tolerance; (2) inositol cannot substitute for mannitol in this role; (3) slower growth of the transgenic plants, and not the presence of mannitol per se, may have been the cause of greater salt tolerance, and (4) mannitol accumulation was enhanced by drought stress but did not affect π_o or drought tolerance.     

Na and k accumulation and salt tolerance of Atriplex canescens (Chenopodiaceae) genotypes

Sixteen accessions of the xerohalophyte, Atriplex canescens (Pursh.) Nutt., differing in tendency to accumulate Na or K in leaf tissues, were compared for salt tolerance in a greenhouse study. Plants were grown along a salinity gradient from 72 to 2017 mol/m³ NaCl measured in the root zone. Growth rates (RGR) were negatively affected by salinity for all accessions. Initial leaf levels of Na (measured before exposing plants to saline solutions) were positively correlated with subsequent RGR's of accessions on the salinity gradient (r = 0.60 - 0.88, P < 0.05 across salinity levels), whereas initial leaf K levels were negatively correlated (r = -0.68 to -0.85, P < 0.01 across salinity levels). Varieties linearis (S. Wats.) Munz and grandidentatum Stutz & Sanderson had greater tendency for Na accumulation, lower tendency for K, and higher growth rates on saline solutions than var. occidentalis (Torr. & Frem.) Welsh & Stutz accessions. Within var. occidentalis accessions, RGRs were negatively correlated with initial leaf levels of K but not Na. Postexposure leaf Na and K levels were not strongly correlated with RGR's. All accessions responded to salinity by increasing their uptake of Na, which is the primary mechanism of osmotic adjustment to salinity in this species. It is suggested that differences in tendency to accumulate Na or K among A. canescens genotypes are related to their specialization for saline or xeric habitats, respectively.     

REGULATION OF NaCl IN JAUMEA CARNOSA (ASTERACEAE), A SALT MARSH SPECIES,AND ITS EFFECT ON LEAF SUCCULENCE

A salt marsh species, Jaumea carnosa, was used in hydroponic experiments to test the effects of increasing NaCl concentrations on leaf succulence and plant accumulations of K, Ca, Mg, Na and Cl. A nested experimental design was used with four salinity levels. Plants were grown in full Hoagland's solution plus different amounts of NaCl (0.0–1.2 osmoles). Leaf succulence was measured as percent water content as well as vertical elongation of mesophyll cells. There were no corresponding increases in leaf succulence with increasing concentrations of NaCl in the root zone. Plants receiving aerosol spray (40 mg/dm²/day) did not show significant increases in leaf succulence. Leaf succulence was significantly increased when the plants were removed from the NaCl solutions and placed in non-salinized Hoagland's solution. Osmotic concentrations of cell sap in leaf tissues showed significant increases as NaCl concentrations increased in the root zone. The concentrations of K, Ca and Mg were higher in plants grown without NaCl than in those grown with NaCl. The accumulations of K in the root tissues were always higher than those of the shoot tissues. Although there was a two-fold difference in NaCl concentrations at the highest levels, the concentrations of Na in the shoot tissues were relatively similar. The results of the Cl analyses of shoot tissues showed a similar pattern of regulation of uptake. This regulation of salt uptake may be important in preventing injury by limiting accumulations of salt in plant tissues when growing in soils of high osmotic potentials.     

Estimating contribution of anthocyanin pigments to osmotic adjustment during winter leaf reddening

The association between plant water stress and synthesis of red, anthocyanin pigments in leaves has led some plant biologists to propose an osmotic function of leaf reddening. According to this hypothesis, anthocyanins function as a solute in osmotic adjustment (OA), contributing to depression of osmotic potential (Ψ_π) and maintenance of turgor pressure during drought-stressed conditions. Here we calculate the percent contribution of anthocyanin to leaf Ψ_π during OA in two angiosperm evergreen species, Galax urceolata and Gaultheria procumbens. Both species exhibit dramatic leaf reddening under high light during winter, concomitant with declines in leaf water potential and accumulation of solutes. Data previously published by the authors on osmotic potential at full turgor (Ψ_π,100) of G. urceolata and G. procumbens leaves before and after leaf reddening were used to estimate OA. In vivo molar concentrations of anthocyanin, glucose, fructose, and sucrose measured from the same individuals were converted to pressure equivalents using the Ideal Gas Law, and percent contribution to OA was estimated. Estimated mean OA during winter was −0.7 MPa for G. urceolata and −0.8 MPa for G. procumbens. In vivo concentrations of anthocyanin (3–10 mM) were estimated to account for ∼2% of OA during winter, and comprised <0.7% of Ψ_π,100 in both species. Glucose, fructose, and sucrose combined accounted for roughly 50 and 80% of OA for G. urceolata and G. procumbens, respectively, and comprised ∼20% of Ψ_π,100. We observed that a co-occurring, acyanic species (Vinca minor) achieved similar OA without synthesizing anthocyanin. We conclude that anthocyanins represent a measurable, albeit meager, component of OA in red-leafed evergreen species during winter. However, due to their low concentrations, metabolic costliness relative to other osmolytes, and striking red color (unnecessary for an osmotic function), it is unlikely that they are synthesized solely for an osmoprotectant role.     

CARBOHYDRATE AVAILABILITY,RESPIRATION, AND THE GROWTH OF KENAF (HIBISCUS CANNABINUS) UNDER MODERATE SALT STRESS

Salt stress may impose osmotic and respiratory costs on nonhalophytes that limit the availability of carbohydrates for growth. This was examined in kenaf (Hibiscus cannabinus L.) by the analysis of soluble carbohydrates, starch, and respiration rates in mature and expanding leaves from plants exposed to moderate salt stress. Plants were grown for 35 days in solution culture at 1, 37, and 75 mM NaCl under greenhouse conditions. Total carbohydrates increased in mature and expanding leaves with increasing salinity. The majority of this increase was as starch. Mature leaf respiration also increased under salt stress. The net accumulation of non-osmotically active carbohydrates in expanding leaves suggests that growth was not limited by the generation or availability of carbohydrates but rather by the ability of the plant to effectively utilize this substrate in osmotic adjustment and growth.     

Does turgor limit growth in tall trees?

The gravitational component of water potential contributes a standing 0.01 MPa m^?1 to the xylem tension gradient in plants. In tall trees, this contribution can significantly reduce the water potential near the tree tops. The turgor of cells in buds and leaves is expected to decrease in direct proportion with leaf water potential along a height gradient unless osmotic adjustment occurs. The pressure–volume technique was used to characterize height‐dependent variation in leaf tissue water relations and shoot growth characteristics in young and old Douglas‐fir trees to determine the extent to which growth limitation with increasing height may be linked to the influence of the gravitational water potential gradient on leaf turgor. Values of leaf water potential (Ψ_l), bulk osmotic potential at full and zero turgor, and other key tissue water relations characteristics were estimated on foliage obtained at 13.5 m near the tops of young (approximately 25‐year‐old) trees and at 34.7, 44.2 and 55.6 m in the crowns of old‐growth (approximately 450‐year‐old) trees during portions of three consecutive growing seasons. The sampling periods coincided with bud swelling, expansion and maturation of new foliage. Vertical gradients of Ψ_l and pressure–volume analyses indicated that turgor decreased with increasing height, particularly during the late spring when vegetative buds began to swell. Vertical trends in branch elongation, leaf dimensions and leaf mass per area were consistent with increasing turgor limitation on shoot growth with increasing height. During the late spring (May), no osmotic adjustment to compensate for the gravitational gradient of Ψ_l was observed. By July, osmotic adjustment had occurred, but it was not sufficient to fully compensate for the vertical gradient of Ψ_l. In tall trees, the gravitational component of Ψ_l is superimposed on phenologically driven changes in leaf water relations characteristics, imposing potential constraints on turgor that may be indistinguishable from those associated with soil water deficits.     

Seasonal patterns of leaf water relations in four co-occurring forest tree species: Parameters from pressure-volume curves

Summary Leaf water relationships were studied in four widespread forest tree species (Ilex opaca Ait., Cornus florida L., Acer rubrum L., and Liriodendron tulipifera L.). The individuals studied all occurred on the same site and were selected to represent a range of growth forms and water relationships in some of the principal tree species of the region. The water relations of the species were analyzed using the concept of the water potential-water content relationship. The pressure-volume method was used to measure this relationship using leaf material sampled from naturally occurring plants in the field. Water potential components (turgor, osmotic, and matric) were obtained by analysis of the pressure-volume curves.Initial osmotic potentials (the value of the osmotic component at full turgidity) were highest (least negative) at the start of the growing season. They decreased (becoming progressively more negative) as the season progressed through a drought period. Following a period of precipitation at the end of the drought period, initial osmotic potentials increased toward the values measured earlier in the season.Seasonal osmotic adjustments were sufficient in all species to allow maintenance of leaf turgor through the season, with one exception: Acer appeared to undergo some midday turgor loss during the height of the July drought period.In addition to environmental influences, tissue stage of development played a role; young Ilex leaves had higher early season initial osmotic potentials than overwintering leaves from the same tree.The seasonal pattern of initial osmotic potential in Liriodendron and the observed pattern of leaf mortality suggested a possible role of osmotic potentials in the resistance of those leaves to drought conditions. The fraction of total leaf water which is available to affect osmotic potentials, called the osmotic water fraction in this study, was greatest in young tissue early in the season and declined as the season progressed.The results of this study showed that the water potential-water content relationship represents a dynamic mechanism by which plant internal water relations may vary in response to a changing external water-availability regime. The measured water relationships confirmed the relative positions of the species along a water-availability gradient, with Cornus at the wettest end and Ilex at the driest end of the gradient. Acer and Liriodendron were intermediate in their water relations. The spread of these species along a water-availability gradient on the same site suggested that coexistence is partially based on differential water use patterns.     

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 GROWTH CHARACTERISTICS OF PROSOPIS GLANDULOSA VAR. TORREYANA IN A SIMULATED PHREATOPHYTIC ENVIRONMENT

Recent studies of Prosopis glandulosa have demonstrated a unique system of a deeply rooted species with significant water stress tolerance. Several growth and developmental characteristics have been correlated with water stress and nitrogen availability during field studies. Here we present a lab experiment in which a phreatophytic regime is simulated and the availability of nitrogen and water are varied. Increased ground water salinity caused lower plant water potentials and greater osmotic adjustment without significant increases in leaf Na⁺ concentrations. Leaf conductance was higher in the higher salinity treatments. Low water potential was also associated with reduced leaf size, reduced leaf area per plant and increased root to shoot ratio. Specific leaf weight and the transpiration ratio were unaffected by the low water potentials induced by increased salinity. Increasing nitrogen availability caused increased growth rates but did not influence water use efficiency. Net assimilation rates increased with increasing nitrogen availability but relative growth rates were more dependent on overall plant size than treatment conditions. The responses of P. glandulosa to the simulated phreatophytic environment were similar to those predicted by field measurements.     

Drought effects on photosynthesis of the mangrove, Avicennia germinans, under contrasting salinities

 Drought effects on leaf photosynthesis of A. germinans growing under two contrasting salinities were studied in a Venezuelan fringe mangrove. During both wet and dry seasons, severe chronic-photoinhibition at predawn was not observed but strong down regulation occurred at midday during both seasons. Carbon assimilation rates (A, μmol CO₂ m⁻² s⁻¹) declined during the dry season from 11.9±1.8 to 7.0±1.5 and from 9.6±2.0 to 4.7±2.5 in plants from low and high salinity sites, respectively. Changes in carbon assimilation per unit of chlorophyll (A/Chl, mmol CO₂ mol⁻¹ Chl) were from 31.6±4.7 to 20.5±4.3 and from 21.9±4.7 to 15.2±8.2 in the low and high salinity plants, respectively. Therefore, neither changes in Chl nor seasonal differences in photoprotective down regulation could account fully for the decrease in leaf photosynthesis during drought. A reduction in CO₂ diffusion due to lowered stomatal conductance was not large enough to explain such a dramatic effect of drought on leaf photosynthesis. Stomatal response could be mitigated by the capability of A. germinans for osmotic adjustment under high salinity and/or drought. However, this intracellular salt accumulation may reduce carbon assimilation capacity further by decreasing the metabolism of leaf cells, increasing dark respiration and/or photorespiration. Received: 10 June 1998 / Accepted: 5 October 1998     

Growth and solute accumulation in 3-week-old seedlings of Agropyron elongatiun stressed with sodium and potassium salts

Agropyron elongatum [Host. (Beauv.)] [^cv. Arizona Glendale, was grown in liquid medium salinized with either NaCl, KCI, or a 50:50 mixture of these two salts at osmotic potentials ranging from 0 to –1.6 MPa. The amount of growth in 21 days was measured, and extracts were made of the shoots at this time. The extracts were assayed for low-molecular-weight organic compounds (glucose, fructose, sucrose, be-taine, proline) and inorganic solutes (Na⁺, K⁺, Cl^?, P.). The purpose was to determine if there was any correlation between the harmful effect of salinity on growth and the concentrations of solutes in tissues. Growth inhibition of A. elongatum was roughly proportional to the osmotic potential of the growth medium and was independent of the ionic composition of the salinizing salts. Total monovalent cation (the sum of Na⁺ and K⁺) concentrations and the ratio of these two cations in leaves were mainly a function of the ionic compostion of the salt in growth media, and, to a lesser degree, of osmotic potentials. F At an osmotic potential of –0.2 MPa, total monovalent cation in leaves was the same as in non-stressed plants. However, if the salinizing salt contained NaCl, there was an increase in foliar Na⁺ with a balancing decrease in K⁺. At stress levels between –0.4 and –1,6 MPa, and, if the media were salinized with either 100% NaCl or a 50:50 mixture of NaCl and KCI, total monovalent cation concentrations remained constant at a value that was twice that in non-stressed plants. Although total monovalent cation concentrations were equal in plants grown under these two salinity conditions, the K⁺/Na⁺ ratios shifted from a value of 1:2 in plants grown in 100% NaCl to 3:1 in plants subjected to the 50:50 mixture. If 100% KCI was used to salinize media, total monovalent cation was 80% of its concentration in NaCl-treated plants in the range of –0.4 to -1.2 MPa. At –1.6 MPa due to 100% KCI, total monovalent cation was double that in plants subjected to -0.4 MPa. In the range of osmotic potentials from–0.2 to –1.2 MPa, the chloride:cation ratio was 1:2. At –1.6 MPa the ratio changed to 3:4. Proline started accumulating in leaves of A. elongatum when the tissue concentration of total monovalent cation exceeded 200 μ (g fresh weight)^?1. Above this threshold value of total monovalent cation, the proline concentration of leaves was 6% of the amount of total monovalent cation that exceeded 200 umol (g fresh weight)¹.     

Contribution of osmotic adjustment to the dehydration tolerance of water-stressed pigeon pea (Cajanus cajan (L.) millsp.) leaves

Abstract Water-stressed pigeonpea leaves have high levels of osmotic adjustment at low leaf water potentials. The possible contribution of this adjustment of dehydration tolerance of leaves was examined in plants grown in a controlled environment. Osmotic adjustment was varied by withholding water from plants growing in differing amounts of soil, which resulted in different rates of decline of leaf water potential. The level of osmotic adjustment was inversely related to leaf water potential in all treatments. In addition, at any particular water potential, plants that had experienced a rapid development of stress exhibited less osmotic adjustment than plants that experienced a slower development of stress. Leaves with different levels of osmotic adjustment died at water potentials between –3.4 and –6.3 MPa, but all leaves died at a similar relative water content (32%). Consequently, leaves died when relative water content reached a lethal value, rather than when a lethal leaf water potential was reached. Osmotic adjustment delayed the time and lowered the leaf water potential when the lethal relative water content occurred, because it helped maintain higher relative water contents at low leaf water potentials. The consequences of osmotic adjustment for leaf survival in water-stressed pigeonpea are discussed.