Turgor Adjustment Research Articles

Ecophysiological Responses of Rice (Oryza sativa L.) to Drought and High Temperature

Global rice crop production is being threatened by a frequent rise in high temperatures and drought. Drought and heat stresses adversely affect the morphological, physiological, and biochemical characteristics of rice, resulting in reduced crop productivity. Heat and drought stresses entail physiological changes in rice plants, such as stomata closure, reduced photosynthesis, loss of turgor adjustment, and reduction in crop productivity. These stresses also cause metabolic changes by increasing the activities of antioxidative enzymes, phytohormones, abscisic acid, reactive oxygen species, and reactive stress metabolites. Among the different growth stages of rice, the reproductive stage is the most sensitive stage to high temperature and drought, resulting in low seed setting and grain yield. Genetic improvement and development of drought and heat-stress-tolerant rice varieties increase seed setting and enhance yield production even under stress conditions. Because of the multigenic nature of traits, the development of drought and high-temperature-tolerant varieties through genetic improvement is the best approach. Here, we summarized the effects of heat and drought stresses on the physiological traits of rice. We focused on different approaches to managing high-temperature and drought stresses, such as an adjustment in cultural practices, genetic improvement through molecular breeding, and the development of transgenics and chemical spray from an agricultural practice perspective.

EFFECT OF DROUGHT ON MORPHOPHYSIOLOGICAL TRAITS OF RICE: A REVIEW

Drought is the condition of prolonged paucity in the water supply, whether atmospheric, surface water, or groundwater. This significant abiotic stress is a severe limiting factor of plant growth and rice productivity, adversely affecting the economic rice production globally. Drought stress has a considerable effect on the rice plant’s morphological and physiological traits preventing it from reaching the maximum yield. Morphological alteration in rice includes inhibition of seed germination, reduced tillers, early maturity, reduced biomass while physiological changes involve restricted cell division and elongation, stomatal closure, loss of turgor adjustment, lower photosynthesis, slower growth rate, yield reduction, etc. The stress further brings about a biochemical change by accumulating stress metabolites, increasing Reactive Oxygen Species (ROS) accumulations, increasing antioxidative enzymes and, Abscisic acid (ABA) accumulation. Various mitigation measures are applied to adhere to the effect of drought on rice plants by selecting drought-tolerant cultivars, planting early varieties, sufficient moisture maintenance, conventional breeding, molecular maintenance, and development of varieties with high yielding attributes. Thus, this review aims to assess the complex responses of rice plants to drought in morpho-physiological characters of rice along with its mitigation strategies in adoption to drought stress.

More sensitive to drought of young tissues with weak water potential adjustment capacity in two desert shrubs

Water is the main limiting factor for survival and growth for desert plants, and plants can alleviate water deficits under drought by adjusting water potential (Ψ). However, the water potential adjustment capacity and water-sensitivity at the tissue level among shrub species remains unclear. The objective of this study was to evaluate water potential adjustment capacity and water-sensitivity of different tissues in Artemisia ordosica and Caragana korshinskii through calculating the water relation parameters from pressure-volume (P-V) curves. The present study found that the sensitivity coefficients, −1/β and −1/b, were gradually decreased with increasing degree of lignification in A. ordosica and C. korshinskii, suggesting that younger tissues with low lignification are more sensitive to water deficit. Additionally, the younger tissues with more negative osmotic potential at full turgor (Ψπ, sat), water potential at turgor loss point (Ψtlp), and lower the bulk modulus of elasticity (ε), the relative water deficit at turgor loss point (RWDtlp), apoplastic water fraction (AWF) and total hydraulic capacitance (Ctotal), which indicated that younger tissues have stronger turgor adjustment capacity compared to osmotic adjustment capacity and them were more easily lose water during times of decreased water potential because of higher cell wall elasticity and weaker water storage capacity. Collectively, the present study highlighted that younger tissues are more sensitive to drought due to their weaker water potential adjustment capacity and provided critical insight into water physiological mechanism or sensitivity of species to drought.

Role of chloride ions in the promotion of auxin-induced growth of maize coleoptile segments.

The mechanism of auxin action on ion transport in growing cells has not been determined in detail. In particular, little is known about the role of chloride in the auxin-induced growth of coleoptile cells. Moreover, the data that do exist in the literature are controversial. This study describes experiments that were carried out with maize (Zea mays) coleoptile segments, this being a classical model system for studies of plant cell elongation growth. Growth kinetics or growth and pH changes were recorded in maize coleoptiles using two independent measuring systems. The growth rate of the segments was measured simultaneously with medium pH changes. Membrane potential changes in parenchymal cells of the segments were also determined for chosen variants. The question of whether anion transport is involved in auxin-induced growth of maize coleoptile segments was primarily studied using anion channel blockers [anthracene-9-carboxylic acid (A-9-C) and 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS)]. In addition, experiments in which KCl was replaced by KNO3 were also performed. Both anion channel blockers, added at 0·1 mm, diminished indole-3-acetic acid (IAA)-induced elongation growth by ~30 %. Medium pH changes measured simultaneously with growth indicated that while DIDS stopped IAA-induced proton extrusion, A-9-C diminished it by only 50 %. Addition of A-9-C to medium containing 1 mm KCl did not affect the characteristic kinetics of IAA-induced membrane potential changes, while in the presence of 10 mm KCl the channel blocker stopped IAA-induced membrane hyperpolarization. Replacement of KCl with KNO3 significantly decreased IAA-induced growth and inhibited proton extrusion. In contrast to the KCl concentration, the concentration of KNO3 did not affect the growth-stimulatory effect of IAA. For comparison, the effects of the cation channel blocker tetraethylammonium chloride (TEA-Cl) on IAA-induced growth and proton extrusion were also determined. TEA-Cl, added 1 h before IAA, caused reduction of growth by 49·9 % and inhibition of proton extrusion. These results suggest that Cl(-) plays a role in the IAA-induced growth of maize coleoptile segments. A possible mechanism for Cl(-) uptake during IAA-induced growth is proposed in which uptake of K(+) and Cl(-) ions in concert with IAA-induced plasma membrane H(+)-ATPase activity changes the membrane potential to a value needed for turgor adjustment during the growth of maize coleoptile cells.

Changes in shoot water relations parameters of yellow-cedar (Chamaecyparis nootkatensis) in response to environmental conditions

Yellow-cedar (Chamaecyparis nootkatensis (D. Don) Spach) shoot water relations parameters were measured in response to (i) seasonal changes in photoperiod and temperature and (ii) controlled changes in root temperature and soil moisture. Pressure–volume curves determined measurements of shoot water relation parameters. Osmotic potential at saturation (Ψsat) and turgor loss point (Ψtlp) were lowest during February at −1.73 and −2.35 MPa, respectively. Both Ψsat and Ψtlp increased in the spring to a high in July of −1.05 and −1.28 MPa, respectively, when shoot growth was occurring. Total turgor (ΨPTotal) was 18.2 MPa in December and declined to its lowest level of 5.98 MPa in July. Increased ΨPTotal from late summer through winter could be attributed to osmotic and (or) elastic components, while decreased ΨPTotal in the spring was due to a decrease in both osmotic and elastic components. Both Ψsat and Ψtlp decreased as root temperatures were reduced from 22 to 1 °C. There was an increase in ΨPTotal as root temperatures declined from 15 to 8 °C because of both osmotic and elastic adjustment, with osmotic adjustment the primary factor. Turgor decreased at root temperatures below 8 °C, even though osmotic adjustment was occurring, and this was due to a reduction in the elastic component. Both Ψsat and Ψtlp decreased in response to drought, though Ψtlp decreased at a greater rate. Increased turgor during the initial stages of long-term drought results from an increase in both osmotic and elastic components, while increased turgor was solely due to the osmotic component as drought became more severe. Under rapid drought, turgor adjustment was minimal because the increased osmotic component was offset by a decrease in the elastic component. Results from these experiments indicate that turgor maintenance of yellow-cedar occurs through the use of both osmotic and elastic components in varying degrees at different times of the year and under differing environmental conditions. Keywords: Chamaecyparis nootkatensis, osmotic potential at saturation and turgor loss point, total turgor with osmotic and elastic components, seasonal patterns, root temperature, drought.

Incomplete turgor adjustment in Cladophora rupestrisunder fluctuating salinity regimes

Turgor pressure fluctuates strongly in Cladophora rupestris subjected to low salinities and shows only a small tendency to readjust to the normal value in full seawater (incomplete turgor adjustment). This was revealed by direct turgor pressure measurements and by chemical analyses of osmotic solutes after exposure of upper and lower shore Cladophora to the different salinity regimes occurring in the intertidal zone or representing steady state osmotic acclimation. The main internal osmotic solutes were K + , Cl − , amino acids, NO 3 − and glycine betaine. Na + , SO 4 2− and PO 4 3− were of less importance. The sum of the charges on the cations was similar to that for the anions. K + , Cl − and, to a lesser extent, amino acids were responsible for limited turgor pressure adjustment which did occur. The concentrations of the major osmotic solutes were influenced not only by salinity but also by light: those of amino acids and NO 3 − were increased while those of K + and Cl − were decreased under illumination. Cladophora populations from the upper and lower shore differed in their ability to restore internal K + and Cl − levels on transfer to full seawater after long term exposure to low salinity. This may indicate ecotypic variation.

Osmotic and Turgor Adjustment in Rosa Foliage Drought-stressed under Varying Irradiance

The influence of irradiance and drought on osmotic and turgor adjustment was examined in leaves of rose (Rosa hybrida L. `Samantha'). Plants cultured under full ambient light in the greenhouse were placed in shade chambers and, after 2 weeks of acclimation, exposed to drought for 21 days. Treatments consisted of a water stress factor (well-watered and drought-stressed) and an irradiance factor (100%, 70%, and 30% of ambient irradiance). Pressure-volume analyses of leaves indicated that osmotic potentials at full turgor were decreased 0.42, 0.36, and 0.23 MPa by drought in the 100%, 70%, and 30% irradiance treatments, respectively. Plants stressed under 100% and 70% irradiance exhibited similar osmotic adjustments. Plants under 30% irradiance had higher osmotic potentials at full turgor under well-watered conditions than plants in the other two irradiance treatments and showed only 55% as much adjustment to drought. In each irradiance treatment, drought induced an increase in elastic modulus and a decrease in relative water content at zero turgor. Turgor pressures were higher across a range of relative water contents in plants in the two higher irradiance treatments under both soil moisture treatments. Turgor also was higher at any particular water potential at 100% and 70% irradiance than 30% irradiance, within each soil moisture treatment. Heavy, but not mild, shading inhibited osmotic and turgor adjustments in leaves during drought.

Using Pressure-Volume Analysis to Characterize NaCl-induced Osmotic and Turgor Adjustment in in vitro Cultures

The applicability of pressure-volume (PV) analysis for characterizing osmotic and turgor adjustments in in vitro cultures was tested using salinized orchardgrass callus. Thermocouple psychrometry was used to derive PV curves in three experiments: (a) adjustment to varying concentrations of NaCl, (b) recovery from NaCl stress and (c) influence of NaCl acclimation to an additional, increased NaCl challenge. Callus growth and water relations were unaffected by 50 mM NaCl. Harvest and zero turgor osmotic potentials were decreased about 0.7 and 0.9 MPa, respectively, in callus grown for three weeks on 100 and 150 mM NaCl. Turgor was 0.2 to 0.4 MPa higher in 100 and 150 mM treatments across a range of callus water potential, compared to 0 or 50 mM NaCl. Salinization resulted in decreased estimates of callus elasticity and increased values of relative water and osmotic water content at zero turgor. The effects of NaCl stress on elasticity and osmotic potential were still evident three weeks after transfer to fresh medium containing 0 mM NaCl. A three-week acclimation of callus to 50 mM NaCl increased turgor and decreased osmotic potential and elasticity compared with unacclimated callus when cultures were subsequently grown on 100 mM NaCl. Apoplastic water percentages ranged from 21 to 35 % and showed no response to salinization. Psychrometric PV analysis appears to offer a promising alternative for the assay of component water potentials and water contents, elasticity and apoplastic/symplastic water partitioning in callus tissue.

Psychrometric Pressure-Volume Analysis of Osmoregulation in Roots, Shoots, and Whole Sporophytes of Salinized Ceratopteris

Osmotic and turgor adjustment in roots, shoots, and whole sporophytes of a NaCl-tolerant mutant and a NaCl-sensitive wild-type strain of the fern Ceratopteris richardii Brongn. were characterized following exposure to 60 millimolar NaCl, using a psychrometric approach to pressure-volume analysis. Water potential components of whole plants and shoots at full or ambient hydration were similar between strains. Roots of the mutant, however, had osmotic potentials at full turgor and water potentials at zero turgor which were lower (0.32 and 0.46 megapascal, respectively) than those in roots of the wild type after salinization. Although compromised in the absence of NaCl, sporophytes of the mutant strain were larger and much less necrotic in 60 millimolar NaCl after 17 days, compared to the wild type. Root growth in the mutant strain was unaffected by salinization and far exceeded root growth in the wild type. Correlation coefficients of the linear regression and the general consistency and precision of the pressure-volume data demonstrated the feasibility of using PV curves for estimating water potential components of roots. The technique should also provide a means of studying osmoregulation in a variety of other rapidly equilibrating plant tissues.

Effects of manipulation of water and nitrogen regime on the water relations of the desert shrub Larrea tridentata.

Water and nitrogen regimes of Larrea tridentata shrubs growing in the field were manipulated during an annual cycle. Patterns of leaf water status, leaf water relations characteristics, and stomatal behavior were followed concurrently. Large variations in leaf water status in both irrigated and nonirrigated individuals were observed. Predawn and midday leaf water potentials of nonirrigated shrubs were lowest except when measurements had been preceded by significant rainfall. Despite the large seasonal variation in leaf water status, reasonably constant, high levels of turgor were maintained. Pressure-volume curve analysis suggested that changes in the bulk leaf osmotic potential at full turgor were small and that nearly all of the turgor adjustment was due to tissue elastic adjustment. The increase in tissue elasticity with increasing water deficit manifested itself as a decrease in the relative water content at zero turgor and as a decrease in the tissue bulk elastic modulus. Because of large hydration-induced displacement in the osmotic potential and relative water content at zero turgor, it was necessary to use shoots in their natural state of hydration for pressure-volume curve determinations. Large diurnal and seasonal differences in maximum stomatal conductance were observed, but could not easily be attributed to variations in leaf water potential or leaf water relations characteristics such as the turgor loss point. The single factor which seemed to account for most of the diurnal and seasonal differences in maximum stomatal conductance between individual shrubs was an index of soil/root/ shoot hydraulic resistance. Daily maximum stomatal conductance was found to decrease with increasing soil/root/ shoot hydraulic resistance. This pattern was most consistent if the hydraulic resistance calculation was based on an estimate of total canopy transpiration rather than the more commonly used transpiration per unit leaf area. The reasons for this are discussed. It is suggested that while stomatal aperture necessarily represents a major physical resistance controlling transpiration, plant hydraulic resistance may represent the functional resistance through its effects on stomatal aperture.

Recovery of Turgor by Wilted, Excised Cabbage Leaves in the Absence of Water Uptake

Cabbage leaves excised from a fully turgid plant wilt within 20 minutes to 2 hours (depending on plant age) with a loss of about 10% relative water content (RWC). If droughted for 2 to 4 days in a high relative humidity leaf chamber, they may acclimate, recovering their turgor without the absorption of water, in fact at a loss of 15 to 25% RWC. This turgor recovery in the absence of water uptake occurs only if (a) the rate of water loss is slow enough (about 1-5% RWC per day after the first 24 hours drought loss of about 15% RWC), (b) if the leaves are no longer growing actively. Osmotic adjustment accompanies the turgor adjustment, but cannot be the cause in the absence of water uptake. The recovery of turgor by wilted cabbage leaves in the absence of water uptake cannot be explained by (a) transfer of reserve water from apoplast to symplast either from the cell walls or from the vessel lumens by cavitation or (b) metabolic loss of dry matter and gain of water. It can be explained by a contraction of the cell walls around the partially dehydrated protoplasts, until they regain their elastic extensibility. These proposed cell wall changes during drought acclimation are therefore the opposite of those occurring during growth. This hypothesis therefore explains the long recognized inverse relation between growth and acclimation. Two predictions of this hypothesis were tested and substantiated.