Apoplastic Water Fraction Research Articles

Water relations and root growth of two populations of Gutierrezia sarothrae

We hypothesise that genotypic differences in transpiration and root growth in the southern and northern populations of Gutierrezia sarothrae are driven by growing season vapour pressure deficit (VPD) and that ecotypic differentiations are linked to corresponding variations in tissue and leaf water relations. Seedlings from an Idaho (ID) and a Texas (TX) seed source were grown either in an open nursery (full sunlight) or under shade. There were no population differences in transpiration, root growth and tissue water relation parameters when the plants were grown under the shade. However, significant population differences were observed in the plants grown in the open where VPD was substantially higher. Transpiration in the TX population increased twice as rapidly as the ID population in response to rising potential evapotranspiration (PET). In addition, the TX plants grew longer and larger lateral roots than the ID plants when both populations were grown in the open. Moreover, the TX plants had more elastic cell walls (lower bulk cell elastic modulus, E) and were able to maintain leaf turgor at lower relative water content than the ID plants when grown in the open. Similar changes in tissue water relation parameters were observed in the New Mexico (NM) and the ID seedlings subjected to soil water deficit. Cell elastic modulus ( E), relative water content at zero turgor (RWC 0) and apoplastic water fraction ( W a) were significantly lower in the NM population than in the ID population in response to soil water deficit, suggesting a higher turgor maintenance ability in the NM population. RWC 0 was highly correlated with E in the NM ( r=0.92, P<0.0001) and the TX ( r=0.91, P<0.05) plants, but poorly correlated with osmotic potential at full turgor (Π 100), indicating significance of cell wall elasticity in turgor maintenance. Rigid cell walls and poor turgor maintenance ability in the ID plants subjected to drought may lead to restricted growth, which reflected hydraulic limitation of plant root systems. Higher water use efficiency (WUE) and higher apoplastic water fraction in the ID population reflected conservative water use at the leaf and tissue levels, which was consistent with the water use pattern at the canopy level.

Water relations in Azospirillum-inoculated wheat seedlings under osmotic stress

Azospirillum has been shown to improve coleoptile growth in seedlings growing in darkness under osmotic stress. However, the changes in water relations that may occur in this experimental system have not yet been studied. Two-centimetre long Triticum aestivum cv. Buck Pucará and Triticum durum cv. Balcarceño-INTA seedlings were inoculated with viable or autoclaved (control) Azospirillum brasilense Sp. 245 bacteria, at approximately 108 cells per seedling. Three days after inoculation, seedlings were exposed to osmotic stress by immersing their roots in 20% polyethylene glycol 6000 for up to 72 h. Germination and seedling growth were at 20°C in darkness. Shoots were excised after 72 h of stress, and water-status parameters were determined through pressure-volume analyses. While osmotic potential at full turgor remained constant, Azospirillum-stimulated growth in Buck Pucará seedlings was accompanied by significant decreases in osmotic potential and relative water content at zero turgor, in volumetric cell wall modulus of elasticity, and in absolute symplastic water volume and by a significant rise in apoplastic water fraction parameters. Except for a constant volumetric cell wall modulus of elasticity, similar results were obtained with Balcarceño-INTA seedlings. However, bacterial growth promotion was evident only in the less tolerant cv. Buck Pucará. Turgor at low water potential was higher in inoculated seedlings in both wheat cultivars under osmotic stress. These results are consistent with a better water status in Azospirillum-inoculated wheat seedlings under water stress, where both effects on cell wall elasticity and (or) apoplastic water are evident.Key words: Azospirillum, drought, seedlings, water status, wheat.

Cut‐shoot vs root rehydration: Effects on pressure‐volume analysis of shoots and fascicles ofPinus resinosaseedlings

Two methods of rehydrating red pine (Pinus resinosaAit.) shoots for pressure‐volume (PV) analysis were compared to clarify the effects of rehydration on estimated tissue water relations of shoots and fascicles. The commonly employed cut‐shoot method was compared to rehydration by means of water uptake through the roots of intact plants. Cut‐shoot rehydration and increased duration of rehydration significantly decreased estimates of tissue elasticity and relative water content at zero turgor for both shoots and fascicles. Rehydration of cut shoots for 2 days significantly increased the slope of the linear region of shoot pressure‐volume (PV) curves and decreased estimates of the apoplastic water fraction. Changes in these and estimates of other water relations attributes were correlated with increased initial water content during rehydration. Estimated apoplastic water fraction was higher for needle fascicles than shoots despite the large amount of woody stem tissue contained in shoots. Fascicle water status strongly influenced shoot water relations, in part due to apparent apoplastic loading with water of fascicles during rehydration.

Responses to water stress of apoplastic water fraction and bulk modulus of elasticity in sunflower (Helianthus annuus L.) genotypes of contrasting capacity for osmotic adjustment

The responses to water stress of the bulk modulus of elasticity (e) and the apoplastic water fraction were examined using six sunflower cultivars of differing capacity for osmotic adjustment (OA). Water stress did not affect the partitioning of water between apoplastic (ca. 20%) and symplastic fractions in leaves which expanded during the exposure to stress in any genotype. Hence, no genotype-linked effects on either the buffering of cell water status during stress or on the estimates of bulk leaf osmotic potential could be expected. Genotypes differed in the degree of change in e (estimated from pressure/volume [P/V] curves) and OA (estimated using both ln RWC/ ln Ψo plots and P/V curves) induced by exposure to stress. In three genotypes e increased significantly (p=0.05) as a consequence of stress, in another three change were small. OA was the only attribute of the three examined that could have contributed to turgor maintenance under stress. There was a strong negative association between leaf expansion and degree of OA across genotypes (r=−0.91) and a strong positive one between OA and e (r=0.94). However all genotypes evidenced some degree of OA. These results are consistent with part of the genotype differences in OA being attributable to variations in leaf expansion during exposure to stress.

Drought-induced changes in water relations in broom snakeweed ( Gutierrezia sarothrae) under greenhouse- and field-grown conditions

Pressure-volume analyses were done on a half-shrub broom snakeweed ( Gutierrezia sarothrae) subjected to different soil water regimes under pot- and field-grown conditions. Under pot and field conditions, water deficit decreased water and osmotic potentials. In the severely droughted pot-grown, osmotic potential at full turgor (П 100) decreased by 0.44 MPa due to reduced symplastic volume, the result of a decreased leaf turgid to dry weight ( TW/DW) ratio. A reduction of 0.86 MPa in П 100 was observed in the field-grown plants subjected to the first drought cycle, but increased cell wall elasticity occurred in the second drought cycle. The lowered П 100 was attributed to osmotic adjustment and reduced symplastic volume. High cell wall elasticity in the second drought cycle led to a greater water loss and tissue desiccation. It is concluded that osmotic adjustment, increased apoplastic water fraction and more rigid tissue are important dehydration-avoidance mechanisms in broom snakeweed.

Osmotic Adjustment in Water Stressed Grapevine Leaves in Relation to Carbon Assimilation

The response of grapevine plants to severe water deficit (predawn leaf water potential of - 1.13 MPa), imposed at a rate of about 0.16 MPa day-1 was studied in terms of leaf water relations characteristics, stomatal behaviour and gas exchange. Carbohydrate status of leaves was also analysed in order to assess the contribution of soluble sugars as osmotic solutes during drought. Pressure/volume analysis showed an active osmotic adjustment in water-stressed leaves, which decreased osmotic potential at full turgor by 0.45 MPa and the apoplastic water fraction showed a reduction of 19% as compared to the well- watered plants. Cell wall elasticity was not significantly affected by water stress, and turgor loss point in stressed leaves was reached at lower water potential and relative water content values than in the well-watered controls. Photosynthesis was markedly reduced in water-stressed plants. However, well-watered and water-stressed leaves had similar concentrations of glucose and fructose. The concentrations of sucrose and starch decreased in water-stressed leaves. This accounted for a marked decrease in the ratio of leaf dry weight to area in droughted plants. The changes in concentrations of soluble carbohydrates could not account for the difference in osmotic potential between water-stressed and well-watered leaves.

Acremonium Endophyte Effects on Tall Fescue Drought Tolerance

Tall fescue (Festuca arundinacea Schreb.) is grown as a forage, turfgrass, and conservation grass from the Gulf Coast to Canada in the eastern USA. The presence of endophytes in grasses can confer resistance to insect pests, enhance growth, and improve tall fescue persistence under high temperature and drought stress. Most tall fescue is infected with Acremonium coenophialum Morgan‐Jones &amp; Gams, a fungal endophyte. Our objectives were to determine the effects of endophyte infection on water relations and survival of water stressed (i.e., drought‐stressed) tall rescue under greenhouse conditions. Endophyte‐free and infected plants of three tall fescue selections were vegetatively propagated and grown together in 38‐L weighing lysimeters to allow interplant competition for available soil moisture. Irrigation water and fertilizer were applied for 12 wk after planting before irrigation water was withheld for 8 wk. Water‐relation characteristics including full‐turgor osmotic potential (ψπ100), water potential at zero turgot (ψL0), relative water content at zero turgot (RWC0), apoplastic water fraction (β), bulk modulus tissue elasticity (ϵ), and the turgid weight to dry weight ratio (TW/DW) were determined before and after water stress. Osmotic adjustment (Δψπ) was also determined. Endophytein fection did not affect ψπ100, ψL0, RWC0, and β either before or after water stress and did not affect Δψπ. Endophyte infection did affect ϵ and the TW/DW ratio, but this effect was not sufficient to alter ψL0. Total tillers, tiller survival, plant survival, and recovery weights were similar for endophyte‐free and ‐infected plants. No evidence for endophyte‐mediated drought tolerance was observed in this study. Based on this work, endophyte‐mediated drought resistance may be due to alterations in drought avoidance.

Analysis of Pressure-volume Data Using Segmented, Nonlinear Regression Algorithms

where Ra is the apoplastic water fraction at saturation and is equal to 1 – X. When using the modified exponential equation for ψ p, differentiatio n of E will give: E = a + a&. The maximum E (determined at full turgor) can be easily calculated using the differentiated equation for E and by substituting A SAS program, with the general model using the aforementioned

Evaluation of contrasting cellular-level acclimation responses to leaf water deficits in three wheat genotypes

Osmotic adjustment, increased apoplastic water fraction (AW) and increased cell wall elasticity have been cited as cell physiological responses to leaf water deficits which could allow acclimation to water stress. All three cell responses to stress should allow the maintenance of greater turgor (Ψp) at low water potential (Ψw). The physiological significance of drought-induced changes in these cell water relations parameters were evaluated in plants of three wheat ( Triticum sp.) genotypes exposed to successive water stress episodes using results from pressure/volume curve analysis. Maintenance of Ψp at low leaf Ψw during subsequent water stress episodes was taken as a measure of plant acclimation to stress. Net photosynthesis was used as a measure of cell metabolic function during the stress episodes. Results indicated that stress-induced increases in AW and cell wall elasticity were not associated with acclimation to water deficits, while osmotic adjustment did result in acclimation and enhanced photosynthesis at low Ψw. It was concluded that among these wheat genotypes osmotic adjustment, and not the other cell responses to water deficits, is an important acclimation mechanism which can allow for the maintenance of relatively greater metabolic function at low leaf Ψw.

Nitrogen, Potassium, and Irrigation Effects on Water Relations of Kentucky Bluegrass Leaves

Transpiration and expansive growth in leaves are turgor‐dependent processes. Solute concentration and osmotic potential are inextricably linked to turgor maintenance. An empirical equation predicting stomatal resistance (Rs) from bulk leaf turgor (ψt) would be useful in developing computer simulations for turfgrass management. A growth‐chambers study was conducted to quantify the relationship between Rs and ψt in Kentucky bluegrass (Poa pratensis L. cv. A‐34). The effects of N, K, and irrigation frequency on bulk leaf osmotic potential at full turgor (ψπ100) and apoplastic water fraction (AWF) also were examined, using psychrometric techniques. Treatments consisted of two rates of N and K and two irrigation frequencies. An exponential model was used to describe the relationship between Rs and ψt. The least squares prediction equation was Rs = 581 + 2500 exp(−6,99ψt), r2 = 0.69, where ψt and Rs. are expressed in units of MPa and s m−1, respectively. Increasing the time between rewatering containers to −0.02 MPa from 1 to 5 d did not influence ψπ100. For plants watered dally, increasing the amount of N supplied every 30 d from 35 to 175 kg ha−1 increased ψπ100 0.22 MPa, while increasing K from 17.5 to 175 kg ha−1 for the same application interval caused ψπ100 to decline 0.20 MPa. Altering the supply of N or K did not affect ψπ100 when the containers were watered every 5 d. Increasing the irrigation interval from 1 to 5 d caused AWF to decline from 22 to 12%; however, AWF estimates were highly variable. Results indicate the combined influences of N and K fertility practices can have a significant impact on the concentration of osmotically active solutes within Kentucky bluegrass bulk leaf tissue.

Derivation of Pressure-Volume Curves by a Non-Linear Regression Procedure and Determination of Apoplastic Water

Data from pressure-volume (PV) analysis may be submitted to transformation I [i.e. leaf water potential (ψ1) versus inverse relative water content (1/R)] or to transformation II (i.e. 1/ψ1 versus R). This may cause an essential distortion of the error structure especially in transformation II due to the relatively large range which is to be covered by the 1/ψ1 ratio. Similarly, logarithmic transformation of leaf turgor potential (ψP) when deriving the sensitivity factor of elasticity (β) by linear regression from values of In ψp and 1/R may distort the error structure. In order to investigate the magnitude of the distortion effect on parameters derived from PV analysis by regression a non-linear regression procedure was compared with the common linear procedure when calculating ψp from β in the turgid region and leaf osmotic potential (ψP) in both the turgid and non-turgid region. As test plants we used field grown species of spring barley (Hordeum distichum L., cvs Gunnar and Alis). The results show that transformations and application of linear regression procedures distort the error structure of ψp more than the error structure of ψπ', which was only slightly affected. However, we recommend the use of the non-linear procedure in both cases. Furthermore, from PV analysis, obtained by thermocouple hygrometry on living and killed leaf tissue, respectively, we derived the mathematical basis for calculating the apoplastic water fraction (Ra). Ra was 0.15 at R= 1 and decreased with dehydration. The equations describing the relation between ψπ and R and between ψp and R were extended to take into account the apoplastic water fraction.

Use of Unsaturated Salt Solutions to Generate Leaf Tissue Water‐Release Curves

AbstractOsmotic adjustment and changes in cell wall elastic modulus are important processes that enable plants to maintain physiological activity and prolong leaf survival as plant water deficits increase. This paper describes a simple and practical technique to produce tissue water‐release curves that allows many measurements of osmotic potential (π), elastic modulus (ε), and apoplastic water fraction (A) to be made both rapidly and routinely. Discs, 6 mm in diameter, were punched out of hydrated leaves of wheat (Triticum aestivum L.), barley (Hordeum distichon L.), and oil‐seed rape (Brassica napus and B. campestris) and weighed immediately on a precision balance. Each disc was then suspended above an unsaturated salt solution of known molality in a sealed test tube held at 5°C in a stirred water bath. Eighteen salt solutions were used whose water potentials ranged from −0.09 to −4.67 MPa. After 12 h the discs were reweighed and then oven dried at 65°C for 24 h. Water‐release curves were obtained by plotting paired values of disc relative water content and salt solution water potential. Using leaves from the same stem, π, at full and zero turgor, ε, and A determined from the water‐release curves were compared with pressure chamber measurements made on the next lower leaf. Measurements of wheat, barley, and oil‐seed rape π, ε, and A obtained from water release curves generated using salt solutions were within 0.2 MPa for π, 3.0 MPa for ε, and 0.07 for A, of those obtained using a pressure chamber over a wide range of π and ε. More than 40 water‐release curves could be generated over 2 d.

Evaluation of Peanut Genotypes for Resistance to Water Stress In Situ1

Observations indicate that Spanish‐type peanuts (Arachis hypogaea L.) are more tolerant of dehydration than Virginia types under severe water stress. However, physiological reasons for differences in tolerance to water deficits among peanut genotypes have not been established. The objective of this study was to evaluate peanut genotypes for their response and relative resistance to severe water deficits. A Spanish type (‘Comet’), Virginia type (‘Florunner’), and a Spanish ✕ Virginia selection (OK‐FH15) were grown in situ under irrigated (IRR) and rainfed (RF) conditions. Leaf water potential (ψw), osmotic potential (ψπ), turgor potential (ψp), relative water content (RWC) and its apoplastic water fraction (Aw) were determined weekly between 1300 and 1500 h, and diurnally on selected days. Yields were determined at 149 days after planting (DAP). Ratios (RF/IRR) genotype ψw, ψπ, and RWC showed significant differences among genotypes between 50 and 63 DAP, a critical period of peanut growth and development. The ψw and RWC ratios were maximum for all genotypes at 63 and 70 DAP, respectively. The maximum ψπ ratio occurred on 56 DAP for Comet (1.25), 63 DAP for OK‐FH15 (1.32), and 70 DAP for Florunner (1.18). Regression analysis of 1/ψw vs. RWC showed a biphasic cubic polynomial function best fit the data for all genotypes. The data indicated that as RWC declined, Comet ψw tended to decrease more rapidly than either OK‐FH15 or Florunner. Conservative estimates of ψπ, changes between 95 and 55%R WC were highest for Comet, lowest for Florunner, and OK‐FH15 was intermediate. Aw was calculated for each genotype and plotted against ψp to determine its contribution to hydration maintenance. At high ψp, maximum Aw, was 7.1, 12.1, and 25.2%f or Florunner, OK‐FH15, and Comet, respectively. Florunner Aw, decreased to zero at 0.16 MPa ψp, and the Aw of OK‐FH15 decreased with ψp to the origin. Comet had much greater Aw at low ψp and was the only genotype with appreciable Aw at zero ψp. These results indicate that Aw varies in peanuts and may be genotype specific. Comet showed significantly higher yield, grade, and dollar value than either Florunner or OK‐FH15 under rainfed conditions. The lower ψw, greater change in ψπ, and higher Aw and yield of RF Comet suggests greater resistance to dehydration when high soil moisture deficits and evaporative demand conditions occur.