Water Relations of Nitella Translucens

Summary

Volume changes in interodal cells of Nitella translucens have been measured by three methods: weight changes, static length changes and kinetic changes. We have obtained consistent results by the different methods which indicate that the volume change between incipient plasmolysis and full turgor is usually in the range 2 1/2–5 per cent. In addition, our confirm the values for the hydraulic conductivity (L_p) of these cells which have previously been obtained in transcellular osmosis experiments, and we have observed an apparent polarity of L_p similar to that found in these experiments.     

Salinity and Salicornia bigelovii Torr. Seedling Establishment. Water Relations

Seedling establishment of Salicornia bigelovii was inhibitedwhen seeds were sown in vermiculite watered with 0 ppt NaCl(control treatment) while the presence of 10 or 30 ppt NaClhad no inhibitory effect. During most of the experimental periodthe rate of water absorption in both cotyledons and embryo-axisof the salt treatments was at least twice that of the control.The organic material decreased in the cotyledons at the sametime that it was increasing in the embryo-axis. Salinity didnot show any apparent effect on reserve mobilization from thecotyledons to the embryo-axis. Ions accumulated with the increasedsalinity of the root environment in both the cotyledons andthe embryo-axis. The former accumulated more than the latterexcept in the control treatment. After the 8th day from sowingthe cotyledon was —1•38, —1•95 and —2•74MPa for the 0, 10 and 30 ppt treatments, respectively. The gradients between external solution and the cotyledons were(0•80, 1•19 and 1•15 MPa for the 0, 10 and 30ppt, respectively. The _p of the control treatment was very low(0•02 MPa) when compared to the ones for the 10(0•53MPa) and the 30 ppt (0•99 MPa) treatments. Glycinebetaineand soluble amino nitrogen accounted for over 90% of the organiccomponent of the tissue osmolality with soluble sugars beingresponsible for the remainder. In the control the inorganicand organic components were of the same magnitude while in the10 and 30 ppt treatments the inorganic was 8•6 and 14•1fold greater than the organic component, respectively. Watermovement into the control plants was apparently inhibited bythe lack of inorganic solutes and as a consequence the cotyledonsfrom these plants did not expand and seedling growth was severelyreduced. Key words: Halophyte, seedling establishment, tissue osmolality components     

The Water Relations of Pinus sylvestris III. Diurnal and Seasonal Patterns of Water Potential

The water potential (ψ) in twigs from four clones of Scots pine (Pinus sylvestris L.) growing in close proximity to each other was measured in a pressure chamber during five selected periods between May 1972 and March 1973. Diurnal and seasonal patterns of ψ are presented in relation to irradiance, air temperature and vapour pressure deficit (VPD) for one cloudy and one clear day in each experimental period. Significant differences in ψ were found amongst most of the clones. Larger amplitudes and earlier day-time minima in ψ were found during the summer than during the winter. Noon values of ψ of less than ?12 bars were regularly found during the summer months. Plots of ψ against irradiance or VPD during a day, showed marked hysteresis as the result of the simultaneous influence on ψ of several environmental factors. Close linear relations resulted when ψ was plotted against potential evaporation rate (calculated from the Penman-Monteith formula). The slopes of these regression lines, essentially the flow resistance, showed marked seasonal variations. At both high and low evaporation rates, the clone that grew the fastest had the lowest values of ψ, and the clone that grew the slowest had the highest values of ψ, while the other two clones had intermediate values. Differences in solute potential or in stomatal, plant or soil resistance are discussed as possible explanations of the more or less constant differences between the clones during the year of observations.     

The Water Relations of Pinus sylvestris II. Comparative Field Studies of Water Potential and Relative Water Content

Comparative field studies of water potential and relative water content in needles of Scots pine, Pinus sylvestris L., were carried out in September-October 1965, in June 1966 and in July-August 1968. The sample trees were grafts, planted in 1946 and belonging to two clones, growing in close proximity and under the same environmental conditions. The main subject of the investigation was to determine whether differences in water potential and/or relative water content existed between these two clones, and if these differences could be correlated to the growth differences and thus aid in the development of selection criteria. The results obtained demonstrate such differences in water potential but not in relative water content. The differences were not consistent through the experimental periods. The clone which had the highest water potentials in June 1966, had the lowest in September-October 1965 and in July-August 1968. The results revealed that the clone which showed the fastest total growth, normally had the lowest water potentials when irradiated. In 1968 the current and the previous season's needles were separately investigated. The water potential and relative water content were always higher in the current season's needles. Highly significant negative correlations between water potential or relative water content and irradiance, temperature, and vapour pressure deficit were found.     

Water Relations of Tropical Epiphytes: II. PERFORMANCE DURING DROUGHTING

Relative water content R, water potential and leaf diffusiveconductance C₅ were monitored while five tropical epiphyteswere subjected to an extended period of drought. The speciesstudied were: ferns Pyrrosia adnascens (Forst.) Ching and Pyrrosiaangustata (Sw.) Ching, family Polypodiaceae; orchids: Eria velutinaLindl., Dendrobium tortile Lindl. and Dendrobium crumenatumSw. The ferns reached zero turgor rapidly and after only small declinesin relative water content (R) and . Beyond this point stomatalmovement seemed strongly suppressed, but the leaves continuedto lose water vapour until very low values of R were reached.Nevertheless, on re-watering water potential (), R and diffusiveconductance (C₅) returned to pre-stress levels within 3 d. The orchids showed a more gradual decline in R and , stomatalactivity was not so strongly suppressed, and night opening ofstomata was observed under stress. The relationship between and R was found for each species,curves being fitted to the data points by non-linear regression.From these analyses it was concluded that these species hadvery dilute cell sap, and consequently the change in for agiven decline in R was smaller than for most other species recordedin the literature. Key words: Orchid, Fern, Drought, Relative water content, Diffusive conductance, Water potential     

Water Relations of Seed Development and Germination in Muskmelon (Cucumis melo L.) : V. Water Relations of Imbibition and Germination

The initiation of radicle growth during seed germination may be driven by solute accumulation and increased turgor pressure, by cell wall relaxation, or by weakening of tissues surrounding the embryo. To investigate these possibilities, imbibition kinetics, water contents, and water (Ψ) and solute (ψ_s) potentials of intact muskmelon (Cucumis melo L.) seeds, decoated seeds (testa removed, but a thin perisperm/endosperm envelope remains around the embryo), and isolated cotyledons and embryonic axes were measured. Cotyledons and embryonic axes excised and imbibed as isolated tissues attained water contents 25 and 50% greater, respectively, than the same tissues hydrated within intact seeds. The effect of the testa and perisperm on embryo water content was due to mechanical restriction of embryo swelling and not to impermeability to water. The Ψ and ψ_s of embryo tissues were measured by psychrometry after excision from imbibed intact seeds. For intact or decoated seeds and excised cotyledons, Ψ values were >−0.2 MPa just prior to radicle emergence. The Ψ of excised embryonic axes, however, averaged only −0.6 MPa over the same period. The embryonic axis apparently is mechanically constrained within the testa/perisperm, increasing its total pressure potential until axis Ψ is in equilibrium with cotyledon Ψ, but reducing its water content and resulting in a low Ψ when the constraint is removed. There was no evidence of decreasing ψ_s or increasing turgor pressure (Ψ-ψ_s) prior to radicle growth for either intact seeds or excised tissues. Given the low relative water content of the axes within intact seeds, cell wall relaxation would be ineffective in creating a Ψ gradient for water uptake. Rather, axis growth may be initiated by weakening of the perisperm, thus releasing the external pressure and creating a Ψ gradient for water uptake into the axis. The perisperm envelope contains a cap of small, thin-walled endosperm cells adjacent to the radicle tip. We hypothesize that weakening or separation of cells in this region could initiate radicle expansion.     

The Water Relations of Pinus sylvestris

Comparative field studies of transpiration rate and drying-transpiration in Scots pine were carried out in the autumns of 1964 and 1965 and in the early summer of 1966. The experimentl material was nineteen to twinty year old grafts, of the same clone but growing on different types of ground, or of different clones but growing in close proximity and under the same environmental conditions. Transpiration rates were measured on detached shoots. The results indicated that the type of ground did not very much modify the responses of the shoots to illumination-darkness treatment. Drying-transpiration is here defined as the transpiration occurring when detached shoots are prevented from water uptake. Measurements were made of the rate of loss of weight and the amount of water given off from illuminated shoots. Defferences were found both in the rate of weight loss, and in the amount of water given off, between pine trees of the same clone but growing on different types of ground, and between pine trees of different clones but growing on the same type of ground.     

The Water Relations of Pinus sylvestris

The water potential (β) in ten-year-old Scots pine trees (Pinus sylvestris L.) from four different latitudinal provenances ranging from 57° N to 67° N, growing in close proximity to each other, was measured in a pressure chamber during five selected periods between May 1972 and March 1973. Diurnal and seasonal patterns of φ are presented in relation to irradiance, air temperature and vapour pressure deficit (D) for one cloudy and one clear day in each experimental period. The largest daily amplitude in φ was found at the beginning and end of summer, indicating a larger resistance to water flow from soil to needles. As the soil water potentials (measured as pre-dawn values) were, at the same time, the highest for the year, it is suggested that these changes in resistance from period to period mainly take place in the trees. Plots of φ against D during clear days, showed marked hysteresis as the result of the simultaneous influence on φ of several environmental factors. Close linear relations resulted when φ was plotted against potential evaporation rate (calculated from the Pennman-Monteith formula). The slopes of these regression lines, essentially the flow resistance, showed marked seasonal variations, with the largest resistance found at the beginning and end of summer. In most periods the water relations of the trees from different provenances were strikingly similar. A continuous change in the water relations of the remote provenances towards the situation for the “home-provenance” is indicated by the experimental results. It is concluded that trees from different latitudes after ten years of growth have about the same chance as the home provenance to survive periods during which their water balance might become critical.     

The Effects of Root Pruning on the Water Relations of Helianthus annuus L.

Four-week-old Helianthus annuus plants, grown in both soil andliquid culture, were root pruned at the point of root attachmentto the stem. Transpiration, leaf water potential and leaf conductivitywere monitored for several days after pruning. Pruning loweredtranspiration and leaf conductivity in amounts proportionalto the amount of pruning. In some experiments pruning causeda slight lowering of leaf water potential, while in others nopruning effect could be found. The effects of pruning varieddepending upon culture conditions, with greater effects beingfound in soil and unaerated liquid culture than in aerated liquidculture. Soil water potential did not appear to have a stronginfluence on the magnitude of the pruning effect. The effectsof root pruning are less than would be predicted by an Ohm'sLaw analysis of flow; possible reasons for this are discussed.     

Water Relations of Seed Development and Germination in Muskmelon (Cucumis melo L.) : I. Water Relations of Seed and Fruit Development

Total water potential (ψ), solute potential, and turgor potential of field-grown muskmelon (Cucumis melo L.) fruit tissue (pericarp) and seeds were determined by thermocouple psychrometry at 5-day intervals from 10 to 65 days after anthesis (DAA). Fruit maturity occurred between 44 and 49 DAA, and seed germination ability developed between 35 and 45 DAA. Pericarp ψ was essentially constant at approximately −0.75 megapascal (MPa) from 10 to 25 DAA, then decreased to a minimum value of −1.89 MPa at 50 DAA before increasing to −1.58 MPa at 65 DAA. Seed ψ remained relatively constant at approximately −0.5 MPa from 10 to 30 DAA then decreased to −2.26 MPa at 50 to 60 DAA before increasing to −2.01 MPa at 65 DAA. After a rapid increase to 20 DAA, seed fresh weight declined until 30 DAA due to net water loss, despite continuing dry weight gain. As fruit and seed growth rates decreased, turgor potential initially increased, then declined to small values when growth ceased. A disequilibrium in ψ was measured between seeds and pericarp both early and late in development. From 20 to 40 DAA, the ψ gradient was from the seed to the tissue, coinciding with water loss from the seeds. From 50 to 65 DAA, seed ψ decreased, causing a reversal of the ψ gradient and a slight increase in seed water content. The partitioning of solutes between symplast and apoplast may create and maintain ψ gradients between the pericarp and seed. The low solute potential within the pericarp due to solute accumulation and loss of cellular compartmentation during ripening and sensecence may be involved in prevention of precocious germination of mature seeds.     

Comparative Water Relations of Phaseolus vulgaris L. and Phaseolus acutifolius Gray

Leaf area expansion, dry weight, and water relations of Phaseolus vulgaris L. and P. acutifolius Gray were compared during a drying cycle in the greenhouse to understand the characteristics which contribute to the superior drought tolerance of P. acutifolius. Stomates of P. acutifolius closed at a much higher water potential than those of P. vulgaris, delaying dehydration of leaf tissue. P. acutifolius had a more deeply penetrating root system, which also contributes to its drought tolerance. Root-shoot ratios did not differ between the two species either under well watered or water stressed conditions. Leaf osmotic potential was also similar in the two species, with no apparent osmotic adjustment during water stress. These results indicate that P. acutifolius postpones dehydration and suggest that sensitive stomates and a deeply penetrating root system are characteristics which, if incorporated into cultivated beans, might increase their drought tolerance.     

Some Theoretical Considerations of Cell Water Relations

An analysis of the forces exerted during the osmotic changesin volume of plant cells shows that the outwardly directed turgorpressure and the inwardly directed wall pressure are numericallyequal under dynamic as well as static conditions. The suggestionthat the term 'potential' should replace 'pressure' in all casesexcept where reference is to a hydrostatic pressure, is stronglysupported.     

Xylem Hydraulic Characteristics, Water Relations and Wood Anatomy of the Resurrection PlantMyrothamnus flabellifoliusWelw.

Myrothamnus flabellifoliusWelw. is a desiccation-tolerant (‘resurrection’)plant with a woody stem. Xylem vessels are narrow (14 µmmean diameter) and perforation plates are reticulate. This leadsto specific and leaf specific hydraulic conductivities thatare amongst the lowest recorded for angiosperms (k_s0.87 kg m^-1MPa^-1s^-1;k_l3.28x10^-5kg m^-1MPa^-1s^-1, stem diameter 3 mm). Hydraulic conductivitiesdecrease with increasing pressure gradient. Transpiration ratesin well watered plants were moderate to low, generating xylemwater potentials of -1 to -2 MPa. Acoustic emissions indicatedextensive cavitation events that were initiated at xylem waterpotentials of -2 to -3 MPa. The desiccation-tolerant natureof the tissue permits this species to survive this interruptionof the water supply. On rewatering the roots pressures thatwere developed were low (2.4 kPa). However capillary forceswere demonstrated to be adequate to account for the refillingof xylem vessels and re-establishment of hydraulic continuityeven when water was under a tension of -8 kPa. During dehydrationand rehydration cycles stems showed considerable shrinking andswelling. Unusual knob-like structures of unknown chemical compositionwere observed on the outer surface of xylem vessels. These maybe related to the ability of the stem to withstand the mechanicalstresses associated with this shrinkage and swelling.Copyright1998 Annals of Botany Company cavitation, desiccation, hydraulic conductivity, refilling, resurrection plant, root pressure, xylem anatomy,Myrothamnus flabellifolius     

Water Relations of Potato Leaves. II. Pressure--Volume Analysis and Inferences About the Constancy of the Apoplastic Fraction

Water relations characteristics of potato (Solanum tuberosumL. cv. Bintje) leaves were determined from pressure—volumeanalysis using a pressure chamber. Turgor was 077 MPa and thebulk volumetric modulus of elasticity 81 MPa at full turgidity;turgor loss occurred when water potential () had declined to–087 MPa at a relative water content (RWC) of 0912;the apoplastic water fraction (A) was 0235. As is usually found,there was a linear relation between 1/ and RWC beyond turgorloss. This finding supports the assumptions of the constancyof A during leaf dehydration. Beyond turgor loss the difference between and [measured afterfreezing and thawing (_d)] was about 01 MPa. This differencedid not increase as the leaf water content decreased. This resultcontradicts the constancy of A. It was concluded from calculations with a simple model of leafdehydration that analysis of the relation between and _d providesmore insight in the changes in the apoplastic fraction thanthe relation between 1/ and RWC. Research on the size of theapoplastic fraction and its changes with water potential wouldcomplement current understanding of leaf water relations. Solanum tuberosum, L., water potential, pressure chamber, osmotic potential, pressure potential, relative water content, apoplast, symplast     

Water Relations in Pulvini from Samanea saman: I. Intact Pulvini

The movement of Samanea leaflets depends upon changes in the curvature of the pulvinus at the base of each leaflet. Pulvinar bending and straightening, in turn, are driven by the movement of water between opposing (extensor and flexor) sides of the pulvinus. Although water movement depends on water potential (Ψ) and thus on osmotic potential (π) and hydrostatic pressure (P), none of these parameters have been measured in Samanea. In this investigation, Ψ and π were measured and P was calculated for extensor and flexor tissues of excised, whole pulvini that were open in the light and closed in the dark. In fully open pulvini, π in the extensor was generally between 800 and 1000 milliosmol per kilogram and exceeded π in the flexor by 300 to 450 milliosmol per kilogram. In fully closed pulvini the reverse was true, with π in the flexor between 800 and 1000 milliosmol per kilogram, exceeding π in the extensor by 300 to 450 milliosmol per kilogram. To obtain approximate values of Ψ of pulvinar tissues, shallow cuts in extensor and flexor sides of oil-covered pulvini were filled with droplets of polyethylene glycol solutions of known Ψ. Droplets maintaining constant size were assumed to have the same Ψ as the tissue. Extensor and flexor halves of open pulvini had very different Ψ (extensor, about −1.4 MPa; flexor, about −0.3 MPa), but both sides of closed pulvini had similar Ψ (about −0.3 MPa). Measurements of Ψ and π and calculations of P indicate: (a) In open pulvini, P is about the same in extensor and flexor. The large Ψ gradient is caused by a large osmotic gradient. (b) In closed pulvini, P is approximately 50% higher in the flexor than in the extensor. This difference in P compensates for differences in π such that the Ψ gradient is small. (c) Pulvini close as P increases in the flexor and reopen as flexor P decreases; extensor P values are similar in open and closed pulvini.