Effects Of Low Water Potential Research Articles

Enzymology and Regulation of δ1-Pyrroline-5-Carboxylate Synthetase 2 From Rice.

Under several stress conditions, such as excess salt and drought, many plants accumulate proline inside the cell, which is believed to help counteracting the adverse effects of low water potential. This increase mainly relies upon transcriptional induction of δ1-pyrroline-5-carboxylate synthetase (P5CS), the enzyme that catalyzes the first two steps in proline biosynthesis from glutamate. P5CS mediates both the phosphorylation of glutamate and the reduction of γ-glutamylphosphate to glutamate-5-semialdehyde, which spontaneously cyclizes to δ1-pyrroline-5-carboxylate (P5C). In most higher plants, two isoforms of P5CS have been found, one constitutively expressed to satisfy proline demand for protein synthesis, the other stress-induced. Despite the number of papers to investigate the regulation of P5CS at the transcriptional level, to date, the properties of the enzyme have been only poorly studied. As a consequence, the descriptions of post-translational regulatory mechanisms have largely been limited to feedback-inhibition by proline. Here, we report cloning and heterologous expression of P5CS2 from Oryza sativa. The protein has been fully characterized from a functional point of view, using an assay method that allows following the physiological reaction of the enzyme. Kinetic analyses show that the activity is subjected to a wide array of regulatory mechanisms, ranging from product inhibition to feedback inhibition by proline and other amino acids. These findings confirm long-hypothesized influences of both, the redox status of the cell and nitrogen availability, on proline biosynthesis.

Low Water Potential and At14a-Like1 (AFL1) Effects on Endocytosis and Actin Filament Organization.

At14a-Like1 (AFL1) is a stress-induced protein of unknown function that promotes growth during low water potential stress and drought. Previous analysis indicated that AFL1 may have functions related to endocytosis and regulation of actin filament organization, processes for which the effects of low water potential are little known. We found that low water potential led to a decrease in endocytosis, as measured by uptake of the membrane-impermeable dye FM4-64. Ectopic expression of AFL1 reversed the decrease in FM4-64 uptake seen in wild type, while reduced AFL1 expression led to further inhibition of FM4-64 uptake. Increased AFL1 also made FM4-64 uptake less sensitive to the actin filament disruptor Latrunculin B (LatB). LatB decreased AFL1-Clathrin Light Chain colocalization, further indicating that effects of AFL1 on endocytosis may be related to actin filament organization or stability. Consistent with this hypothesis, ectopic AFL1 expression made actin filaments less sensitive to disruption by LatB or Cytochalasin D and led to increased actin filament skewness and decreased occupancy, indicative of more bundled actin filaments. This latter effect could be partially mimicked by the actin filament stabilizer Jasplakinolide (JASP). However, AFL1 did not substantially inhibit actin filament dynamics, indicating that AFL1 acts via a different mechanism than JASP-induced stabilization. AFL1 partially colocalized with actin filaments but not with microtubules, further indicating actin-filament-related function of AFL1. These data provide insight into endocytosis and actin filament responses to low water potential stress and demonstrate an involvement of AFL1 in these key cellular processes.

Screening of drought tolerant rice through morpho-physiological and biochemical approaches

Abstract Drought is one of the major abiotic stresses that delimit rice production and yield especially in rainfed ecosystems. To identify the promising drought tolerant rice cultivars, an effort was made in the present study to screen few traditional rice landraces of Assam, India, aimed at investigating effect of low water potential on growth. Seeds of twenty one traditional rice cultivars along with ‘Sahbhagi Dhan’ (tolerant) and ‘IR64’ (sensitive) were subjected to drought stress. Drought was imposed in one week old seedlings by 15% PEG-6000 in Yoshida medium for 7 days followed by 20% PEG-6000 for another 7 days until the drought symptoms appeared. All the experiments were conducted in randomized block design with three replicates. Germination percentage, root length, shoot length, root fresh and dry weight, shoot fresh and dry weight, chlorophyll content and relative water content (RWC) were evaluated after stress treatment. On the basis of Standard Evaluation Score, 8 rice varieties showing high drought tolerability were finally selected for further biochemical analyses. Proline content, lipid peroxidation level and hydrogen peroxide concentration in shoots and roots were investigated. Increase in proline content and decrease in hydrogen peroxide and lipid peroxidation implied its protective role in tolerant rice cultivars against drought stress. To identify the drought tolerant rice cultivars, a thorough understanding of the various mechanisms that govern the yield of rice under drought condition is a prerequisite.

The combined effect of water stress and temperature on seed germination of Chihuahuan Desert species

Interactions between temperature and moisture on seed germination are not clearly understood. We hypothesized that high temperatures and low moisture would inhibit seed germination. We analyzed germination under four temperatures (18–36 °C) and under five water limiting conditions, using Polyethylene glycol (PEG) 6000 (0.0–1.0 MPa) for eight dominant species from Cactaceae (Isolatocereus dumortieri, Echinocactus platyacanthus, and Ferocactus histrix), Asparagaceae (Yucca decipiens, Agave salmiana, and Agave lechuguilla) and Fabaceae (Acacia schaffneri and Prosopis laevigata) from the Chihuahuan Desert. We used the September (when seed germination is more likely to occur) mean (18 °C), the historic highest mean temperature for this month (25 °C), the maximum temperature registered for the month (32 °C), and 36 °C, considering a 4 °C increase. Species had fewer seeds germinating with decreasing moisture and high temperature. Prosopis laevigata however, showed high germination across temperatures and on almost all water potentials. Isolatocereus dumortieri and E. platyacanthus had few seeds germinating at 18 °C but with a negative effect of low water potential for 18 °C and 36 °C. This is the first study including the combined effect of water potential and temperatures on germination of a set of Chihuahuan Desert species.

Influence of PEG generated osmotic stress on shoot regeneration and some biochemical parameters in Lathyrus culture

Several Lathyrus species and in particular Lathyrus sativus (grass pea) display much tolerance to drought and have great agronomic potential as grain and forage legumes. Studies on these crop species could improve our understanding of the mechanisms of plant resistance to water stress. This investigation was aimed at examining the effect of low water potential generated by polyethylene glycol (PEG) 6000 on seed germination, shoot regeneration and some biochemical parameters related to drought stress. Seeds of eight Lathyrus accessions (six of L. sativus, one of L. cicera, one of L. tingitanus) were cultured on MB (MS macro and microelements with B5 vitamins) medium with addition of 5 mg/l BAP (6-benzyloaminopurine). Different levels of water-stress were induced by using three concentrations of PEG 6000 (50, 100 and 150 g/l) in MB medium. Osmotic stress had no influence on seed germination of any of the accessions tested. However, an increased concentration of PEG in the media caused a decrease in the multiplication rate and vigour of newly regenerated shoots. The percentage content of dry matter and proline content in Lathyrus shoots increased significantly under osmotic stress. In turn, water stress had a little effect on photosynthetic pigments.

Effect of low water potential on phytochrome-induced germination, endosperm softening and cell-wall mannan degradation in Datura ferox seeds

Phytochrome-induced softening of the micropylar endosperm of Datura ferox seeds is followed by radicle protrusion and preceded by a significant increase in the extractable activity of β-mannanase and β-mannosidase (6 and 3 times over the far-red light controls, respectively) and a decrease in the mannan content of the micropylar endosperm cell walls. This relationship between phytochrome-induced germination, endosperm softening and mannan degradation was studied in Datura ferox seeds subjected to a range of water potentials during the presence of Pfr. Low water potential inhibited the decrease in mannose-rich cell-wall insoluble polysaccharides, the increase in activity of β-mannosidase and endosperm softening. A good correlation was found between activity of β-mannosidase measured at 45 h after exposure to a red light pulse and germination counted 24 h later when, with different external water potentials, germination varied between 0 and 90%. In contrast, no inhibitory effect of low water potential on in vitro β-mannanase activity was detected. We suggest that the in vivo action of β-mannanase might be limited by accumulation of manno-oligosaccharides caused by inhibition of β-mannosidase. These data are consistent with the concept of a link between mannan mobilization and endosperm softening. Part of the effect of low water potential on Pfr-induced germination may be mediated by inhibition of β-mannosidase activity, with consequences for mannan hydrolysis and endosperm softening.

The wheat LEA protein Em functions as an osmoprotective molecule in Saccharomyces cerevisiae.

The biased amino acid composition and aperiodic (random coil) configuration of Group 1 late embryogenesis-abundant (LEA) proteins imply that these proteins are capable of binding large amounts of water. While Group 1 LEAs have been predicted to contribute to osmotic stress protection in both embryonic and vegetative tissues, biochemical support has been lacking. We have used Saccharomyces cerevisiae as a model system to test the putative osmoprotective function of a wheat Group 1 LEA protein, Em. We demonstrate that expression of Em protein in yeast cells is not deleterious to growth in media of normal osmolarity and attenuates the growth inhibition normally observed in media of high osmolarity. Enhanced growth is observed in the presence of a variety of osmotically active compounds indicating that Em protein is capable of mitigating the detrimental effect of low water potential in a relatively non-specific manner. These results are the first biochemical demonstration of an osmoprotective function for a Group 1 LEA and suggest that the yeast expression system will be useful in dissecting the mechanism of protection through structure-function studies.

Canola seed germination and seedling emergence from pre‐hydrated and re‐dried seeds subjected to salt and water stresses at low temperatures

SummaryLow soil temperatures and low water potentials reduce and delay the seed germination of canola (Brassica rapa L., B. napus L.) in western Canada. Germination is also very sensitive to the salinity effects of nitrogen fertiliser placed with the seed, especially when the seed bed is relatively dry. The effects of pre‐hydration and re‐drying treatment on canola (Brassica rapa L. cv. Tobin) seed germination and seedling emergence at 10°C subjected to either a water or salt stress were determined. Low water potentials, induced by polyethylene glycol (PEG 8000), low soil moisture, or high concentrations of salts, reduced both germination and seedling emergence, and increased the time to 50% germination and emergence of seeds at 10°C. At equal osmotic potentials, Na2SO4 was less inhibitory on low temperature germination than either NaCl or PEG, suggesting that the sulphate ion partially alleviated the inhibitory effects of low water potential. Solutions of NaCI produced more abnormal seedlings compared to Na2SO4, suggesting that NaCl was more toxic than Na2SO4 during seedling development. Pre‐hydration and re‐drying partially overcame the inhibitory effects of both low water potential and salts on seed germination and seedling emergence at 10°C. The seed treatment increased the germination rate in Petri dishes and seedling emergence from a sandy loam soil. Water potentials or soil water contents required to inhibit 50% germination or emergence at 10°C were lower for treated seeds compared to control seeds. Salt concentrations inhibiting 50% emergence were higher for treated seeds than control seeds. Neither treated nor control seeds produced seedlings which emerged if the soil water content was lower than 9% or when the soil was continuously irrigated with salt solutions of 100 mmol kg‐1 of NaCl or 50 mmol kg‐1 of Na2SO4. These results suggest that the pre‐hydration and re‐drying treatment did not lower the base water potentials at which seedling emergence could occur. Abnormal seedlings were observed in both treated and control seeds, particularly if the soil was watered with NaCl solutions; however, the seed treatment reduced the number of abnormal seedlings.

Effect of Water Potnetial of Culture Medium on Geotropic Response of Primary Seminal Root in Japanese Wheat Cultivars.

It has been observed that wheat has a shallow root system in wet soil and a deep root system in dry soil. However, the control mechanisms of root growth depending on soil water status remain unclear. Since root geotropic response seems to play an important role in the formation of the root system, the effect of water potential of culture medium on the geotropic response of the primary seminal root in Japanese wheat cultivars was examined. Agar medium with different concentrations of polyethylene glycol (PEG-6000) and vermiculite medium with different levels of water content were used. Growth angle of the primary seminal root in Minaminokomugi, which usually shows small root growth angle, became larger in low water potential media (-0.05 MPa) than in the control (O MPa). Effects of low water potential on root geotropic responses in the agar medium were also examined in 133 Japanese wheat cultivars. Minaminokomugi and 50 other cultivars showed large responses in the low water potential medium. It is considered that low water potential of the culture medium stimulates positive root geotropic responses in certain Japanese wheat cultivars.

Effect of low water potential on photosynthesis in intact lichens and their liberated algal components

Earlier experiments (T.D. Brock 1975, Planta124, 13-23) addressed the question whether the fungus of the lichen thallus might enable the algal component to function when moisture stress is such that the algal component would be unable to function under free-living conditions. It was concluded that the liberated phycobiont in ground lichen thalli could not photosynthesize at water potentials as low as those at which the same alga could when it was present within the thallus. However, our experience with lichen photosynthesis has not substantiated this finding. Using instrumentation developed since the mid-1970's to measure photosynthesis and control humidity, we repeated Brock's experiments. When applying "matric" water stress (equilibrium with air of constant relative humidity) we were unable to confirm the earlier results for three lichen species including one of the species,Letharia vulpina, had also been used by Brock. We found no difference between the effects of low water potential on intact lichens and their liberated algal components (ground thallus material and isolated algae) and no indication that the fungal component of the lichen symbiosis protects the phycobiont from the adverse effects of desiccation once equilibrium conditions are reached. The photosynthetic apparatus of the phycobiont alone proved to be highly adapted to water stress as it possesses not only the capability of functioning under extremely low degrees of hydration but also of becoming reactivated solely by water vapor uptake.

Growth of the Maize Primary Root at Low Water Potentials

Primary roots of maize (Zea mays L. cv WF9 x Mo17) seedlings growing in vermiculite at various water potentials exhibited substantial osmotic adjustment in the growing region. We have assessed quantitatively whether the osmotic adjustment was attributable to increased net solute deposition rates or to slower rates of water deposition associated with reduced volume expansion. Spatial distributions of total osmotica, soluble carbohydrates, potassium, and water were combined with published growth velocity distributions to calculate deposition rate profiles using the continuity equation. Low water potentials had no effect on the rate of total osmoticum deposition per unit length close to the apex, and caused decreased deposition rates in basal regions. However, rates of water deposition decreased more than osmoticum deposition. Consequently, osmoticum deposition rates per unit water volume were increased near the apex and osmotic potentials were lower throughout the growing region. Because the stressed roots were thinner, osmotic adjustment occurred without osmoticum accumulation per unit length. The effects of low water potential on hexose deposition were similar to those for total osmotica, and hexose made a major contribution to the osmotic adjustment in middle and basal regions. In contrast, potassium deposition decreased at low water potentials in close parallel with water deposition, and increases in potassium concentration were small. The results show that growth of the maize primary root at low water potentials involves a complex pattern of morphogenic and metabolic events. Although osmotic adjustment is largely the result of a greater inhibition of volume expansion and water deposition than solute deposition, the contrasting behavior of hexose and potassium deposition indicates that the adjustment is a highly regulated process.

Effects of Low Water Potential on Cortical Cell Length in Growing Regions of Maize Roots

Roots growing under low water potential commonly exhibit a marked decrease in growth rate and in diameter. Using median longitudinal sections of fixed maize (Zea mays L. cv WF9 x Mo 17) seedling roots, we investigated the cellular basis for these effects. Cortical cells in the shortened elongation zone of water stressed roots were longer than cortical cells in the comparable location of well-watered roots. Nearly twofold differences in cell length were seen in the region 2 to 4 millimeters behind the root apex. The shortened growth zone, however, leads to a final mean cortical cell length approximately 30% shorter in the stressed roots. These differences were present regardless of the age of the control roots. These data, and the slower growth rate seen in water-stressed roots, suggest that the water deficit causes a significant reduction in the rate of cell supply to the cortical cell files.

INFLUENCE OF WATER AND SALT STRESS ON WATER RELATIONSHIPS AND CARBON DIOXIDE EXCHANGE OF TOP AND ROOTS IN BEANS

SUMMARYWhen low‐salt plants, grown in nutrient solution, were exposed to osmotic potentials in the root medium (πs)* of minus 2.7 and minus 4.7 bar due to the addition of KNO3, osmotic adjustment occurred. The leaf osmotic potential (π1,) decreased, leaf turgor potential (Ψp) and leaf water content (W) were maintained, no wilting took place, stomatal conductance (g) was partially regained and net photosynthesis was only moderately depressed.When low‐salt plants were exposed to πs of minus 2.7 and minus 4.7 bar due to the addition of PEG 1500, no adjustment to water stress was observed. Ψp decreased, the leaves became dehydrated and a high percentage wilted. Stomatal conductance, net photosynthesis, top dark respiration and root respiration were depressed. NO3‐uptake ceased completely at minus 4.7 bar.When low‐salt plants were exposed to πs of minus 4.7 bar, due to both PEG (minus 2 bar) and KNO3 (minus 2 bar), osmotic adjustment took place; π1 decreased, Ψp, W, g, net photosynthesis, top dark respiration and root respiration recovered to some extent, but the proportion of wilted leaves was high.When medium‐ and high‐salt plants, osmotically adjusted to πs of minus 2.7 and minus 4.7 bar due to KNO3 were exposed to water stress due to PEG (minus 2 and minus 4 bar), (Ψp and W were maintained, none or few leaves wilted and g was depressed less than in low‐salt plants. These responses are attributed to influence of pre‐osmotic adjustment on turgor maintenance and hydration of the plant.It is concluded that osmotic adjustment by inorganic solute uptake, before or during exposure to water and salt stress, can reduce the severe effects of low water potentials in the root medium.

Effect of low water potential of the culture medium on mesocotyl elongation of rice seedlings.

At a soil water content of water potential of ca. -3.1 to -6.2 bars, mesocotyl elongation of both indica and japonica type rice was extremely stimulated. It was also stimulated in an aseptic culture medium (2.5-4.0% mannitol, water potential ca. -3.5 to -5.5 bars). The stimulation of the mesocotyl may be caused by cell multiplication.

The influence of nitrate, water potential and oxygen tension on nitrogen fixation in detached pea roots

Nodulated pea roots were detached from the plant shoots and exposed to different nitrate concentrations, water potentials, and oxygen tensions. The rates of nitrogen fixation in the detached pea roots following these treatments were determined by the acetylene reduction assay. It was found that increasing the nitrate concentration up to 5 mM brought about reduced nitrogen fixing activity, and this occurred independently of the depressing effects of low water potential and low oxygen tension.

Effects of Low Water Potentials on Transpiration and Photosynthesis in Mitchell Grass (Astrebla lappacea)

Gas-exchange studies were carried out on potted Mitchell grass plants raised in the glasshouse and in growth cabinets. Provided that nutrition was adequate, the low irradiance of the growth cabinets did not impair the photosynthetic capacity at levels approaching full sunlight. The optimum temperature for net photosynthesis was in excess of 40�C. Close linear relationships were established between leaf water potential and both transpiration and net photosynthesis. Most of the variation in gas exchange could be attributed to changes in stomatal diffusive resistance. Although there was an increase in mesophyll resistance as leaf water potential decreased, the proportion of the total resistance attributable to the mesophyll became substantially smaller. Gas exchange exhibited a remarkable recovery after brief but severe drought, during which leaf water potentials fell to about -48 bars. There was some inhibition of both transpiration and photosynthesis for about one week following rewatering. It is concluded that A. lappacea demonstrates a high degree of true drought tolerance.

Relation between Anion Transport and Water Flow in Tomato Plants

AbstractLowering the water potential of culture solutions from −0.4 to −5.4 atm reduced both phosphorus and bromide transport to the shoot, hut the content in the roots was not affected. Reductions in phosphorus transport to the shoot were measured during the first four hours of treatment and were related to concurrent decreases in water flow and not to an impairment of active phosphorus transport. The effect of low water potential on phosphorus transport to shoots was similar at external phosphorus concentrations between 0.6 and 15 mg/l.Phosphorus transport was greater in the dark at −0.4 atm than in the light at −5.4 atm even when these treatments gave the same overall rates of water flow; this is attributed to a different pattern of water flow through the various root zones. The results suggest that the main effect of water flow on anion transport to shoots occurred after the ions had been actively adsorbed by the roots and was not due to mass flow increasing ion delivery to sites of active uptake.

Effects of low water potential on ion uptake and loss for excised roots

In excised roots of barley and tomato plants, lowering the water potential of nutrient solutions to-10.4 and-20.4 atm decreased the uptake of bromide and phosphorus while increasing the loss of these ions to the external solutions.

Effects of low water potentials on respiration and on glucose and acetate uptake, by Chlorella pyrenoidosa

Chlorella pyrenoidosa was subjected to a range of water potentials and the effects of these treatments on endogenous respiration and on the uptake and respiration of glucose and acetate were measured.For a given water potential the reductions were greatest for glucose, less for acetate, and least for endogenous respiration. At intermediate water potentials of about-10 atm, glucose respiration was depressed strongly at first, but this respiration approached control levels after two to three hours at low water potentials.The reduced respiration of substrates was caused by inhibition of glucose and acetate uptake, as demonstrated by (14)C uptake experiments over short periods. These effects on uptake are attributed to low water potentials, rather than to any possible competition between the molecules of the osmotica and the substrates. Evidence for this view includes the equal inhibitions of "glucose-induced" respiration by osmotica with such diverse molecular structure as mannitol, KCl, and polyethylene glycol 1540. More conclusively, glucose itself was used as an osmotic agent and its inhibition of "glucose-induced" respiration was very similar to that by mannitol solutions of equal water potentials.Respiratory activity was much less reduced than uptake. This was demonstrated by lowering the water potential of cells which had already absorbed glucose from a control medium. The subsequent respiration was much higher than that for cells continuously exposed to low water potential.The findings are discussed in relation to the reduced transport of ions and sucrose, which is known to occur in vascular plants subjected to a water stress.The results demonstrate the advantages of using a unicellular organism in the study of metabolic effects of water deficits in plants.

Effects of low water potentials on some aspects of carbohydrate metabolism in Chlorella pyrenoidosa

Chlorella pyrenoidosa was subjected to low water potentials and the resulting changes in carbohydrate metabolism were measured.Water deficit reduced the incorporation of (14)C-glucose into methanol insoluble compounds, principally starch and increased that into sucrose. Even moderate water deficit, for example potentials of -2.5 and -5 atm, greatly reduced the incorporation of (14)C-glucose into uridine diphosphate glucose, while (14)C levels of the hexose monophosphates changed little, indicating a direct stimulus of sucrose synthesis. This increased sucrose synthesis was one of the earliest effect of water deficit, because potentials of -2.5 and -5 atm did not reduce respiration and glucose uptake.At lower water potentials (-10 atm or less) there was reduced (14)C incorporation into all sugar phosphates. This resulted from a combination of reduced (14)C-glucose uptake and increased sucrose synthesis.Water potentials as low as -20 atm had little effect on acetate uptake, or on the (14)C levels in the intermediates of the TCA cycle. This confirmed that low water potentials do not directly inhibit respiratory pathways in Chlorella.The results are discussed in relation to the effect of water deficit on levels of various metabolites in vascular plants, which have been reported by other workers.