Effect Of Osmoticum Research Articles

Respiration metabolism in mitochondria of pea seedlings of different age under water deficit and rewatering

The effects of an osmoticum (mannitol) on growth, water content, respiration, and proline content in epicotyls of etiolated two-day-old (S2) and three-day-old (S3) pea (Pisum sativum L., cv. Flora-2) seedlings were studied. Water deficit was produced by placing plant roots in 0.6 M mannitol for 2–4 days. Control S3 epicotyls contained more water than S2 epicotyls; they were also characterized by more intense mitochondrial activity (in state 3, V3) and by the higher value of the respiratory control (RC) due to a fourfold decrease of proportion of weakly phosphorylating cyanide-resistant respiration (CRR). After 2-day-long treatment with the osmoticum, S2 epicotyls lost more water than S3; water deficit in them attained 40% (28% in S3). In S3 treatment, epicotyls continued to grow at the almost unchanged rate, and the content of proline in them changed insignificantly, whereas in S2 seedlings epicotyl growth stopped, and proline accumulated in them. In the presence of the osmoticum, V3 and RC reduced, but in S3 treatment they retained higher than in treatment S2. In mitochondria of S3 subjected to dehydration, the proportion of the cytochromic pathway of oxidation (Vcyt) remained high, but the absolute values of the respiration rate reduced, whereas in S2 both these indices reduced and the proportion of CRR increased. After rewatering, indices of mitochondrial activity (the rate of substrate oxidation, RC value) were restored, but in S3 they were higher. The ratio of Vcyt to CRR in S3 shifted toward CRR, whereas in S2, in contrast, it was shifted toward Vcyt. When temperature during seedling growth and experiment performing was declined from 22°C (in control) to 18°C, growth of S3 epicotyls was retarded to the rate characteristic of S2 at 22°C. Under the effect of osmoticum, they, like S2, acquired a capacity to accumulate proline. Nevertheless, mitochondrial activity in such S3 turned out to be higher than in S2 grown at 22°C. The results obtained than, as distinct from traditional notes, 3-day-old pea seedlings manifested the higher tolerance to water deficit than 2-day-old seedlings.

Incubation under fluctuating temperatures reduces mean base water potential for seed germination in several non-cultivated species

Seeds ofCarduus acanthoides,Cynara cardunculus,Cirsium vulgare,Brassica campestris, andSisymbrium altissimumwere incubated at a range of decreasing osmotic potentials (Ψo) under fluctuating temperatures or the median temperature of the fluctuation cycle. Fluctuating temperatures promoted total seed germination in water and at reduced osmotic potential. Total germination was reduced as the Ψodecreased. However, this trend was smallest under fluctuating temperatures, signalling a higher tolerance of seeds to reduced osmotic potential. Effects of osmoticum and temperature were modelled with the hydrotime model. The parameters estimated from the model, the hydrotime constant (θH), the mean base water potential Ψb(50) and its standard deviation (σΨb) gave good descriptions of germination time courses. For all species, incubation under fluctuating temperatures shifted Ψb(50) values downwards without modifying their distribution substantially. This accounted for the greater tolerance of germination to reduced Ψounder fluctuating temperatures. To confirm that these effects were mediated by temperature fluctuationsper se, the behaviour ofC. acanthoidesandC. cardunculusincubated at the minimum, the mean and the maximum temperature of the fluctuation cycle was also analysed. Constant maximum and minimum temperatures of the cycle did not stimulate germination, nor did they shift Ψb(50) towards more negative values. The hydrotime model provides a physiologically based quantitative description for germination promotion due to fluctuating temperature.

The effect of osmoticum on ascorbate and glutathione metabolism during white spruce ( Picea glauca) somatic embryo development

Water stress is an important factor which regulates organized development of both zygotic and somatic embryos. Somatic embryos of white spruce were cultured in the presence of polyethylene glycol (PEG), a non-plasmolyzing agent which increases embryo quality and number, and mannitol, a plasmolyzing agent. The effects of these two compounds on both ascorbate and glutathione metabolism were investigated at different stages of embryo development. Compared to control and mannitol-treated embryos, embryos treated with PEG accumulated higher levels of endogenous ascorbate (ASC) in its reduced form, especially during the first half of the maturation period. This increase, also observed in immature seeds, was mainly the result of two different processes: activation of the de novo ASC machinery, and recycling of ASC from ascorbate free radicals (AFR) which was modulated by the activity of ascorbate free radical reductase (AFRR, EC. 1.6.5.4). The activity of this enzyme increased during the early phases of development in both PEG-treated somatic embryos and seeds. Compared to control somatic embryos, mannitol and PEG were shown to change the levels of reduced (GSH) and oxidized glutathione (GSSG). In particular, a constant decline in the GSH/GSSG ratio was observed in the presence of PEG. This pattern was also observed in maturing white spruce seeds. Overall, these data indicate that applications of non-plasmolyzing agents in the culture medium of spruce somatic embryos result in seed-like fluctuations of the ascorbate–glutathione metabolism, which may have a positive effect on embryo yield.

Early changes of Cl- efflux and H+ extrusion induced by osmotic stress in Arabidopsis thaliana cells.

In various plant materials changes in turgor pressure, following hyper- or hypo-osmotic stress, were associated with the activation or inactivation of the plasma membrane H+ -ATPase, respectively. To see if the turgor changes might indirectly influence H+ -ATPase activity by regulating ion fluxes through plasma membrane, we investigated, in cultured cells of Arabidopsis thaliana (L.) Heynh., the early effects of hyper- and hypo-osmotic stress on Cl- fluxes in comparison, in the case of hyper-osmotic treatment, with its effect on net H+ extrusion. The results obtained showed that hyper-osmotic stress (200 mM mannitol) quickly reduced Cl- efflux (-70%) from cells preloaded with 36 C1- for 18 h. This inhibiting effect was independent of the simultaneous mannitol-induced stimulation of Cl- influx and rapidly reversible after removal of the hyper-osmotic treatment. The inhibition of Cl- efflux was associated with a stimulation of net H+ extrusion, and these two effects showed the same dependence on the external mannitol concentration. Fusicoccin (FC, 20 µM), which stimulated H+ extrusion to about the same extent as 200 mM mannitol, did not affect Cl- efflux. When cells preloaded with 36 C1- for 18 h in the presence of mannitol (from 25 up to 200 mM) were eluted in a mannitol-free medium an early and strong increase in Cl- efflux was found. The increase of Cl- efflux was already detectable for a small hypo-osmotic jump (25 mM), and was reduced (-50%) by the anion channel inhibitor A9C (300 µM). These results lead to exclude a direct causal relationship mediated by Em changes between the effects of osmoticum on Cl- efflux and net H+ extrusion, and favour the view that the changes in turgor pressure induced by hyper/hypo-osmotic stress may respectively induce an early inactivation/activation of stretch-sensitive anion channels.

Sugar Efflux from Attached Seed Coats of Glycine max (L.) Men.

Ellis, E. C. and Spanswick, R. M. 1987. Sugar efflux from attached seed coats of Glycine max (L.) Merr.-J. exp. Bot. 38: 1470-1483. Sugar efflux (sucrose + glucose) from attached seed coats of Glycine max (L.) Merr. was measured at high sampling rates to improve the kinetic characterization of seed coat exudation. This study confirms that sugar efflux in seed coats has at least two components, and demonstrates that the concentration of mannitol osmoticum bathing the seed coat may influence one or both of these components. High leaf irradiance increased sugar efflux relative to a low leaf irradiance at the same mannitol concentration. A high concentration of mannitol (500 mol m~3) enhanced sugar efflux relative to a medium concentration (100 mol m~3) under both high and low leaf irradiance. A low mannitol concentration (10 mol m~3) stimulated sugar efflux (relative to 100 mol m~3) to a greater extent when leaf irradiance was high. Rapid changes in mannitol concentration produced immediate stimulations of sugar efflux. Effects of osmoticum on sugar efflux are explained by simultaneous turgor-mediated effects on import of sucrose by the phloem and retrieval of apoplastic sucrose, presumably by seed coat parenchyma. Key words—Glycine max, seed coat, turgor-sensitive transport, assimilate transport.