Ultrastructural changes in rhizome parenchyma of <Emphasis Type="Italic">Polypodium vulgare</Emphasis> during dehydration with or without abscisic acid pretreatment

Common polypody (Polypodium vulgare L.) belongs to desiccation-tolerant ferns. The structure of storage parenchyma of their rhizome was examined by transmission electron microscopy after dehydration and subsequent rewetting. Analysis revealed that treatment with supplemental abscisic acid resulted in protection of cells against ultrastructural damage compared to untreated ones. Dehydration rate appears to modify the ability of rhizome parenchyma to stand water stress.     

Involvement of exopeptidases in dehydration tolerance of spring wheat seedlings

The observed inability of 6-d-old seedlings of spring wheat (Triticum aestivum L.) to tolerate the same water deficit as compared to the 4-d-old seedlings seems to be associated with the higher carboxypeptidase and lower aminopeptidase activities. Free amino acid pools differentiated also the 4-d-old seedlings from the older ones. Dehydration decreased the amino acid content in 4-d-old seedlings, increased it in 6-d-old seedlings and changed composition of amino acid pool. In tolerant phase of wheat seedling growth carboxypeptidase activity increased in response to water deficit and aminopeptidase activity increased in dehydrated seedlings, independently of their age.     

Freezing of dehydrated calli of spring wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) in liquid nitrogen and their morphogenetic potential

A cryopreservation method developed earlier was modified for freezing of calli derived from mature embryos of four spring wheat (Triticum aestivum L.) pure lines. The effects of particular stages of cryopreservation protocol on water content, number of calli recovering growth, and rate of morphogenesis were analyzed. Regrowth was observed in 90.5–100% of calli after dehydration and in 93.3–100% after freezing-thawing. Dehydration, but not freeze-thawing significantly decreased the frequency of morphogenetic variation.     

Cryopreservation of non-encapsulated embryogenic tissue of sweet potato (Ipomoea batatas)

Embryogenic tissue of the sweet potato (Ipomoea batatas (L) LAM) genotype TIB 10 was established from in vitro axillary shoot tips on Murashige and Skoog (1962) medium supplemented with 5 M 2,4-dichlorophenoxyacetic acid. Embryogenic aggregates of fresh mass 9.0–12 mg were subjected to a rapid freezing protocol in liquid nitrogen following sucrose preculture and varying degrees of dehydration. Up to 50% of embryogenic explants survived rapid freezing after preculture on 0.4 or 0.7M sucrose only. Dehydration with silica gel to moisture contents in the range 18–41% improved the survival after cryopreservation of embryogenic tissue. Tissue dehydrated for intermediate periods exhibited poor survival. Following freezing, embryogenic tissue appeared to develop normally, retaining its competence to produce mature embryos and plantlets.Abbreviations BA 6-benzyladenine - 2,4-D 2,4-dichlorophenoxyacetic acid - MS Murashige and Skoog (1962) medium     

Effects of Cytokinin Preparations on the Pools of Pigments and Proteins of Wheat Cultivars Differing in Their Tolerance to Water Stress

Contents of chlorophylls, carotenoids, soluble leaf proteins, and the key enzyme of carbon metabolism—ribulose bisphosphate carboxylase/oxygenase (RuBisCO; EC 4.1.1.39)—in young seedlings and adult leaves of the wheat Triticum aestivum L. cultivars Mironovskaya 808 and Lyutescens 758, contrasting in their water stress tolerances, were compared under conditions of normal available water supply, water deficiency, and subsequent rehydration. It was discovered that compounds displaying a cytokinin activity (6-benzylaminopurine, thidiazuron, kartolin-2, and kartolin-4) reduced the decreases in contents of chlorophylls, carotenoids, soluble leaf proteins, and RuBisCO, progressing with development of water stress, as well as contributed to their more rapid recovery. These compounds with cytokinin activity also accelerated restoration of the compounds studied to their initial concentrations during rehydration. The kartolin preparations caused a maximal protective effect. Water stress had a more pronounced negative effect on the cultivar Lyutescens 758. Dehydration resulted in a more extensive destruction of seedlings compared to leaves of adult plants.     

Water-storage capacity ofThuja,Tsuga andAcer stems measured by dehydration isotherms

Water-storage capacity was measured inThuja occidentalis L.,Tsuga canadensis (L.) Carr., andAcer saccharum Marsh. during the dehydration of stem segments 1.5–2.5 cm in diameter. Stem water potential was measured with a temperature-corrected stem hygrometer and cavitations were detected acoustically. Water loss was measured by weight change. Dehydration isotherms consistently displayed three phases. The first phase, from water potential (Ψ) 0 to about −0.2 MPa, had a high capacitance (C>0.4kg water lost· (1 of tissue)⁻¹· MPa⁻¹) and we have attributed this high C to capillary water as defined by Zimmermann (1983, Xylem structure and the ascent of sap, Springer-Verlag). The second phase from Ψ=−0.5 to about −2.0 had the lowest C values (<0.02 kg·l⁻¹·MPa⁻¹) and was accompanied by a few cavitation events. This phase may have been a transition zone between capillary storage and water released by cavitation events as well as water drawn from living cells of the bark. The third phase also had a high C (about 0.07–0.22kg·l⁻¹·MPa⁻¹) and was associated with many cavitation events while Ψ declined below about −2.5 MPa; we presume the high capacitance was the consequence of water released by cavitation events. We discuss the ecological adaptive advantage of these three phases of water-storage in trees. In moist environments, water withdrawn from capillary storage may be an important fraction of transpiration, but may be of little adaptive advantage. For most of the growth season trees draw mainly on elastic storage, but stem elastic storage is less than leaf elastic storage and therefore unlikely to be important. In very dry environments, water relased by cavitation events might be important to the short-term survival of trees.     

Seasonal patterns of tissue water relations in three Mediterranean shrubs co‐occurring at a natural CO2 spring

Seasonal changes in tissue water relations of Erica arborea L., Myrtus communis L. and Juniperus communis L., grown in a Mediterranean environment, were analysed under field conditions over a 12 month period by comparing plants grown in the proximity of a natural CO₂ spring (about 700 μ mol mol ^{? 1} atmospheric CO₂ concentration, [CO₂]) with plants in ambient conditions. Tissue water relations varied in response to changes in water availability, but the seasonal course of tissue water relations parameters was also related to ontogeny. Tissue water relations of these co‐occurring shrubs were not alike. Osmotic potentials and saturated mass/dry mass ratio were lowest during peak drought stress periods. Diurnal changes in osmotic potential at the point of turgor loss were least early in the season, maximal in mid‐season, and decreased again in autumn. Turgor potentials decreased as drought progressed and were highest in late fall and mid‐winter. Symplastic water fraction was highest in mid‐spring for E. arborea and M. communis and decreased during the summer, while the opposite was observed for J. communis. Common to all species, under elevated [CO₂], was an increase of turgor pressure, particularly during the summer months. Other parameters showed species‐specific responses to long‐term elevated [CO₂]. In particular, exposure to elevated [CO₂] increased osmotic potentials in E. arborea under drought, while the opposite was the case for J. communis. Site differences in predawn to midday shifts were not strong in any of the species. Differences in tissue water relations suggest that the coexistence of these shrubs in the same environment with similar water availability are partially based on differential water relations strategies and water use patterns. Regardless of the mechanisms, growth of these shrubs in elevated [CO₂] may be either less, similarly or more affected by drought stress than plants in ambient [CO₂] depending on the species and season.     

Effects of Water Deficit during Germination of Wheat Seeds

Germinating seeds of spring wheat (Triticum aestivum L.) were tolerant to dehydration up to the 4^th day following imbibition and from the 5^th day the seedling survival decreased. Dehydration also inhibited the rate of seed dry mass depletion and seedling dry matter accumulation and increased the content of soluble sugars both in grain and seedlings. Glucose supplied either to dry seeds or to 4-d-old seedlings increased survival of dehydrated seedlings. In contrast, exogenously supplied non-readily metabolizable sorbose and mannose suppressed seedling survival.     

FLEXURAL RIGIDITY OF CHIVE AND ITS RESPONSE TO WATER POTENTIAL

The biomechanical relationship between the ability of a plant organ to resist bending and the extent to which tissues are hydrated is illustrated for the cylindrical leaves of chive (Allium schoenoprasnum var. schoenoprasnum L.). The flexural rigidity (EI), which measures the ability to resist bending, is maximum when leaves are fully turgid and decreases monotonically as a function of water potential (r² = 0.99). Dehydration results in a reduction in the elastic modulus (E) of leaves. Reductions in E are correlated with geometric distortion in the transverse geometry of leaves which influences their second moment of inertia (I). The traditional theory of elastic stability (developed on the basis of the mechanical behavior of nonbiological systems) is shown to be inadequate to distinguish the behavior of E as plant organs geometrically distort during dehydration. This inadequacy results from the violation of a principal assumption made by the theory (= uniform cross-sectional geometry). A derivation is presented that accommodates the localized geometric distortions in cylindrical plant organs and permits a valid estimate of reductions in E as tissues dehydrate. Based on this derivation, the Young's modulus of chive leaves just before mechanical failure due to buckling is shown to be less than 50% of that calculated for fully turgid leaves.     

The Pool of Chlorophyllous Pigments in Barley Seedlings of Different Ages under Heat Shock and Water Deficit

The effects of a high temperature (3 h, 40°C) and water deficit (45 h on 3% PEG 6000) on the pool of chlorophyllous pigments in the leaves of 4-, 7-, and 11-day-old barley (Hordeum vulgare L.) seedlings were studied. Heating resulted in a decrease in the total content of chlorophylls (Chl) (a + b) in 4-day-old plants but not in the older leaves. Water deficit induced an increase in the pigment content in young seedlings but reduced it in the leaves of 11-day-old plants. In young seedlings, hyperthermia and dehydration affected similarly Chl (a + b) degradation, leading to a marked inhibition of the chlorophyllase (Chlase) activity hydrolyzing Chl to chlorophyllides and phytol. In old leaves, an activation of this enzyme was observed. The stress factors under study affected different stages of pigment biosynthesis. High temperature inhibited the activity of dark and light stages of Chl(a + b) biosynthesis. Dehydration did not change markedly the resynthesis of protochlorophyllide, while the enzymes of the light stage of Chl biosynthesis were activated in young but inhibited in old barley leaves. The results thus obtained allowed us to conclude that heat treatment and dehydration specifically affected the Chl biosynthesis. At the same time, the Chlase response was nonspecific.     

Foliar dehydration tolerance of twelve deciduous tree species

The potential for foliar dehydration tolerance and maximum capacity for osmotic adjustment were compared among 12 temperate, deciduous tree species, under standardized soil and atmospheric conditions. Dehydration tolerance was operationally defined as lethal leaf water potential (); the of the last remaining leaves surviving a continuous, lethal soil drying episode. Nyssa sylvatica Marsh., and Liriodendron tulipifera L. were most sensitive to dehydration, having lethal leaf of -2.04 and -2.38 MPa, respectively. Chionanthus virginicus L., Quercus prinus L., Acer saccharum Marsh., and Quercus acutissima Carruthers withstood the most dehydration, with leaves not drying until leaf dropped to -5.63 MPa or below. Lethal leaf (in MPa) of other, intermediate species were: Quercus rubra L. (-3.34), Oxydendrum arboreum (L.) D.C. (-3.98), Halesia carolina L. (-4.11), Acer rubrum L. (-4.43), Quercus alba L. (-4.60), and Cornus florida L. (-4.88). Decreasing lethal leaf was significantly correlated with increasing capacity for osmotic adjustment. C. virginicus and Q. acutissima showed the most osmotic adjustment during the lethal soil drying episode, with osmotic potential at full turgor declining by 1.73 and 1.44 MPa, respectively. Other species having reductions in osmotic potential at full turgor exceeding 0.50 MPa were (in MPa) Q. prinus (0.89), A. saccharum (0.71), Q. alba (0.68), H. carolina (0.67), Q. rubra (0.60), and C. florida (0.52).     

Photosynthetic parameters and carbon reserves of a resurrection plant <Emphasis Type="Italic">Reaumuria soongorica</Emphasis> during dehydration and rehydration

The effects of dehydration and subsequent rehydration on photosynthetic parameters and carbon reserves were investigated in the resurrection plant Reaumuria soongorica. Dehydration was imposed by withholding water and covering plants with a PVC sheet when it rained, over a period of 53 days, by which time all leaves had been shed. Thereafter, plants were watered at 7-day intervals. The diurnal course of the net photosynthetic rate (Pn) was bimodal under well-watered conditions. After a period of withholding water, the second peak disappeared, and Pn, instantaneous water use efficiency (WUE), stomatal conductance (gs) and intercellular CO₂ concentration (Ci) decreased, but sugar, starch and non-structural carbohydrate (NSC) reserves increased. It was concluded that under the conditions of high temperature and dehydration, the reduction of Pn should be mainly attributed to gs. On rehydration Pn, gs, Ci and WUE increased slightly in the stem. Accompanying new leaf production, carbon reserves in the stem decreased. This indicated that carbon reserves in the stem have two important ecological roles, survival during dormant periods and support of vegetative regrowth following rehydration.     

SEASONAL FLUCTUATIONS IN TISSUE NITROGEN FOR FIVE SPECIES OF PERENNIAL MACROALGAE IN RHODE ISLAND SOUND1

Tissue nitrogen was assessed monthly for 16 months in five species of perennial macroalgae representing three phyla at one location in Rhode Island Sound. The species showed a remarkable similarity in their pattern of seasonal fluctuation in both nitrate and total nitrogen. The period of greatest accumulation (January through March) coincided with the period of highest concentration of inorganic nitrogen in the water, and for most of these algae it was also the time of-least growth. Conversely, the period of lowest tissue nitrogen (50% of the winter value, May through July) coincided with the period of lowest inorganic nitrogen in the water and highest algal growth. The greatest accumulation of nitrate was found in Laminaria saccharina (L.) Lamour. (80 μmol·g dry wt.^?1), four times as much as that measured simultaneously in the other species and 560 times the ambient concentration. By April the concentration of internal nitrate had dropped to nearly undetectable levels, but in August it began to accumulate again—a pattern that was repeated in Chondrus crispus Stackh. In Ascophyllum nodosum (L.) Le Jolis, Fucus vesiculosus L. and Codium fragile subsp. tomentosoides (Van Goor) Silva, the period of negligible internal nitrate level extended from March to December. The greatest concentration of total tissue nitrogen was measured in C. crispus (4.8% dry wt.), double the maximum in L. saccharina (2.3% dry wt.).     

Tissue-water relations of two co-occurring evergreen Mediterranean species in response to seasonal and experimental drought conditions

Tissue-water relations were used to characterize the responses of two Mediterranean co-occurring woody species (Quercus ilex L. and Phillyrea latifolia L.) to seasonal and experimental drought conditions. Soil water availability was reduced 15% by partially excluding rain throughfall and lateral flow (water runoff). Seasonal and experimental drought elicited physiological and morphological adaptations other than osmotic adjustment: both species showed large increases in cell-wall elasticity and decreased saturated-to-dry-mass ratio. Increased elasticity (lower elastic modulus) resulted in concurrent decreases in relative water content at turgor loss. In addition, P. latifolia showed significant increases in apoplastic water fraction. Decreased saturated-to-dry-mass ratio and increased apoplastic water fraction were accompanied by an increased range of turgor maintenance, which indicates that leaf sclerophyllous traits might be advantageous in drier scenarios. In contrast, the degree of sclerophylly (as assessed by the leaf mass-to-area ratio) was not related to tissue elasticity. An 15% reduction in soil water availability resulted in significant reductions in diameter growth when compared to control plants in both species. Moreover, although P. latifolia underwent larger changes in tissue water-related traits than Q. ilex in response to decreasing water availability, growth was more sensitive to water stress in P. latifolia than in Q. ilex. Differences in diameter growth between species might be partially linked to the effects of cell-wall elasticity and turgor pressure on growth, since Q. ilex showed higher tissue elasticity and higher intrinsic tolerance to water deficit (as indicated by lower relative water content at turgor loss) than P. latifolia.     

Electrical properties of hydrated collagen. I. Dielectric properties

The effect of water on the low-frequency (10²-10⁵ H_z) complex permittivitv of native, sold-state collagen has been investigated experimentally. Measurements at ambient temperature show that dry collagen exhibits essentially no frequency or temperature dependence. As water is absorbed, both dielectric constant and loss factor increase simultaneously and rise sharply upward at a hydration level which may be associated with the completion of the primary absorption layer as determined from independent water absorption studies. The behaviour is qualitatively identical to that observed for other proteins and related materials. Temperature-dependent measurements made under vacuum conditions in the range ?196°C to +100°C are characteristic of the dielectric properties of the water in the sample. Dehydration produced by successive temperature recycling to the maximum temperature effectively eliminates any temperature or frequency dependence. A maximum in the temperature-dependent curves is found at about +40°C and is explained as the superposition of two processes: (1) the transition of water molecules from bound to free states, and (2) the difffusion of water molecules out of the system. The dielectric constant of dry collagen, after desorption at ambient temperature, is about 4.5. Desorption at elevated temperatures reduced the room temperature value to about 2.3 and the liquid nitrogen temperature value to a number indistinguishable from the optical value of n² = 2.16.     

The effect of prolonged flooding on the bark of mangrove trees

Growth and physiological response of woody plants to flooding have been analyzed in detail; however, relatively few studies have been oriented towards the effects of water immersion on cambial activity and wood and bark anatomy of trees that are growing in prolonged flooding conditions. These studies are important to understand the possible effects of predicted sea level rising in mangroves as a consequence of global warming. We studied five species growing in a mangrove forest, sampling three to six trees of each species, in sites that have the longest flooding period. Differences in bark appearance and phloem structure between the submerged stem portion and the portion of the stem above the water surface exist in all species. Although aerenchyma formation and stem hypertrophy are the most common events related to flooding, each type of tissue responded differently. Annona glabra L., Laguncularia racemosa (L.) Gaertn f. and Hibiscus tiliaceus L. developed rythidome. Avicennia germinans (L.) Stearn developed rythidome only in the submerged stem portion. Phyllanthus elsiae Urb., developed one periderm in both stem portions. Species that developed rythidome also developed aerenchyma between periderms and in the phellem. H. tiliaceus and P. elsiae, showed the highest values for anatomical phloem and periderm characters below water surface, while an inverse tendency was observed in A. glabra and L. racemosa, suggesting that prolonged flooding modifies vascular cambium and phellogen differently. Results indicate that sea level rising would affect distribution of the species according to their specific flooding tolerance.     

INSECT PHENOLOGY MEDIATED BY HOST-PLANT WATER RELATIONS

Water relations of host plants modify Enchenopa binotata life histories by mediating the termination of egg dormancy, thereby promoting synchronization of egg hatch. Dormant eggs must undergo dehydration and subsequent hydration to begin development. Dehydration of eggs is brought about in the field by declining water levels in branches during the fall and by prolonged cold. Hydration of eggs occurs when sap begins to rise in early spring. Since the ascent of sap occurs at different times in the six species of Enchenopa host plants, the phenology of egg hatch and adult maturation are allochronic. Shifts to novel host plants differing in phenology promote asynchrony of Enchenopa life histories among host-plant species. Thus, the host plant acts as an extrinsic disruptive factor that may promote genetic divergence and temporal reproductive isolation in Enchenopa.     

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.     

Nuclear magnetic resonanceimaging of membrane permeability changes in plants during osmoticstress

The cell water balance of maize (Zea mays L., cv LG 11) andpearl millet (Pennisetum americanum L., cv MH 179) duringosmotic stress was studied non‐invasively using ¹H nuclearmagnetic resonance (NMR) microscopy. Single NMR parameter imagesof (i) the water content (ii) the transverse relaxation time (T₂)and (iii) the apparent diffusion coefficient (D_app)were used to follow the water status of the stem apical region duringosmotic stress. During stress there are hardly any changes in watercontent or T₂ of the stem region of maize. Incontrast, the apical tissue of pearl millet showed a ～ 30% decreaseof T₂ within 48 h of stress, whereasthe water content and D_app did not change. Thesechanges can be explained by an increase of the membrane permeabilityfor water. This conclusion is supported by results from scanningelectron microscopy, relaxation measurements of sugar solutionsand numerical simulations of the relaxation and (apparent) diffusionbehaviour of water in a plant cell.