Onset of and recovery from nitrogen stress during reproductive growth of soybean

Photosynthetic rates and allocation of dry matter, nitrogen, and nonstructural carbohydrates were determined during onset of and recovery from a nitrogen stress for reproductive soybean (Glycine max [L.] Merrill cv Ransom) plants. Until the beginning of seed fill, non-nodulated plants were grown in flowing solution culture with 1.0 mM NO3- in a complete nutrient solution. One set of plants then was transferred to minus-nitrogen solution for 24 d of seed fill; a second set was transferred to a minus-nitrogen solution for 14 d followed by return to the complete solution with 1.0 mM NO3- for the remaining 10 d of seed fill; and a third set was continued on the complete solution. Net CO2 exchange rates of individual leaves, which remained nearly constant during seed fill for nonstressed plants, declined at an accelerated rate during onset of nitrogen stress as the specific content of reduced nitrogen in the leaves was decreased by remobilization of nitrogen to support pod growth. The rate of nitrogen remobilization out of leaves initially was relatively greater than the decrease in photosynthetic rate. While rate of pod growth declined in response to the developing nitrogen stress, photosynthetic assimilation of carbon exceeded reproductive demand and nonstructural carbohydrates accumulated within tissues. Following resupply of exogenous NO3-, specific rate of NO3- uptake by roots was enhanced relative to nonstressed plants. While there was little increase in content of reduced nitrogen in leaves, net remobilization of nitrogen out of leaves ceased, and the decline in photosynthetic rate stabilized at about 51% of that for nonstressed plants. This level of photosynthesis, combined with the availability of elevated pools of carbohydrates accumulated during stress, was sufficient to support the increases in both the specific rates of NO3- uptake and the rate of pod growth during recovery.     

Daytime soybean transcriptome fluctuations during water deficit stress

Background

Since drought can seriously affect plant growth and development and little is known about how the oscillations of gene expression during the drought stress-acclimation response in soybean is affected, we applied Illumina technology to sequence 36 cDNA libraries synthesized from control and drought-stressed soybean plants to verify the dynamic changes in gene expression during a 24-h time course. Cycling variables were measured from the expression data to determine the putative circadian rhythm regulation of gene expression.

Results

We identified 4866 genes differentially expressed in soybean plants in response to water deficit. Of these genes, 3715 were differentially expressed during the light period, from which approximately 9.55 % were observed in both light and darkness. We found 887 genes that were either up- or down-regulated in different periods of the day. Of 54,175 predicted soybean genes, 35.52 % exhibited expression oscillations in a 24 h period. This number increased to 39.23 % when plants were submitted to water deficit. Major differences in gene expression were observed in the control plants from late day (ZT16) until predawn (ZT20) periods, indicating that gene expression oscillates during the course of 24 h in normal development. Under water deficit, dissimilarity increased in all time-periods, indicating that the applied stress influenced gene expression. Such differences in plants under stress were primarily observed in ZT0 (early morning) to ZT8 (late day) and also from ZT4 to ZT12. Stress-related pathways were triggered in response to water deficit primarily during midday, when more genes were up-regulated compared to early morning. Additionally, genes known to be involved in secondary metabolism and hormone signaling were also expressed in the dark period.

Conclusions

Gene expression networks can be dynamically shaped to acclimate plant metabolism under environmental stressful conditions. We have identified putative cycling genes that are expressed in soybean leaves under normal developmental conditions and genes whose expression oscillates under conditions of water deficit. These results suggest that time of day, as well as light and temperature oscillations that occur considerably affect the regulation of water deficit stress response in soybean plants.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1731-x) contains supplementary material, which is available to authorized users.     

Ribonuclease activity in roots of soybean seedlings subjected to chilling stress and recovery

Many developmental and physiological changes, including alterations of enzyme activities, occur in plants under low temperature stress. In this study the total ribonuclease activity was determined in crude extracts from root tips of soybean seedlings germinated at 25 °C, subjected to chilling conditions (10°C) and recovered at optimal temperature (25°C). Measurements of RNase activity were performed every 24 hours starting from the third to the 10-th day of growth. We found that chilling caused a considerable increase in ribonuclease activity (in comparison with the control), with an activity peak on the fourth day of the cold treatment. The enzyme activity in root extracts of the plants recovered after cold stress decreased along with the time of recovery. No differences were found in approximate molecular weight (35 kDa) and pH optimum (6.0) for ribonucleases extracted from control and chilled soybean roots.     

Pools of non-structural carbohydrates in soybean root nodules during water stress

The aim of this study was to examine how the pools of non-structural carbohydrates in soybean nodules are affected under water stress conditions depending on the nature of the symbiont strains with particular emphasis on the plant-borne carbohydrates sucrose and pinitol, and on trehalose, a compatible solute synthesized by the bacteroids. Soybean ( Glycine max [L.] Merr. cv. Maple Arrow) plants were inoculated with the nitrogen-fixing strains Bradyrhizobium japonicum 61-A-101 or USDA 110 spc4 and cultivated axenically under conditions in which nodules formed in an upper soil compartment while roots for water supply grew into a compartment filled with nutrient solution. When the nodules were well established (1 month post inoculation), 10% (w/v) PEG 6000 was added to the nutrient solution. This led to a slowly progressing, moderate water stress, as determined by measuring the decrease of transpiration, and to a decrease in nitrogen fixation. The pool sizes of the major non-structural nodule carbohydrates changed during progression of water stress. Sucrose, the major soluble carbohydrate in nodules of unstressed plants (2 and 4%, respectively of nodule dry weight depending on symbiont strain), strongly increased in nodules of stressed plants, reaching nearly 10% of dry weight. The activities of two major sucrose-consuming enzymes, sucrose synthase and alkaline invertase, decreased markedly in nodules of stressed plants. Starch decreased only transiently upon water stress. Pinitol, a cyclitol serving as compatible solute in many plants, increased more than 4 times, reaching about 1% of nodule dry weight during the stress. Trehalose, the major soluble carbohydrate synthesized by the bacteroids, increased in nodules colonized by USDA 110 spc4 from about 0.2 to 0.8% of nodule dry weight, while in nodules colonized by 61-A-101 it amounted to more than 1.5% of dry weight both with and without stress.     

Effects of ultraviolet-B radiation on plants during mild water stress. III. Effects on photosynthetic recovery and growth in soybean

Soybean { Glycine max (L.) Merr. ev. Essex} was grown from seed in a greenhouse under ultraviolet-B (UV-B, 280–320 nm) radiation supplied by filtered FS-40 sunlamps. On a weighted, total daily dose basis these plants received either 0 (control) or 2875 effective J m⁻² day⁻¹ UV-B_BE. When weighted with the generalized plant action spectrum (Caldwell 1971), this simulated the solar ultraviolet-B irradiance expected to occur at College Park, Maryland, USA (39°N) in the event the global stratospheric ozone column is reduced by 23%. The effects of ultraviolet radiation on the photosynthetic recovery from water stress were measured with an infrared gas analyzer. These effects were examined in plants which were either well-watered or previously preconditioned to water stress, during two distinct phenological stages of development. During the early stages of soybean growth, enhanced levels of UV-B reduced net photosynthesis by 25%, and water stress also reduced photosynthesis to nearly the same extent (by 20%). The combination of these two stresses resulted in smaller biomass than that produced by plants exposed to either stress independently. Photosynthesis in older, larger plants was much more sensitive to water stress and was reduced by as much as 50–60% in non-preconditioned plants. Although non-irradiated, non-preconditioned (control) plants recovered to only within 60% of their prestressed value, preconditioned plants recovered to within 70–80% during the 3 day recovery period. Both water stress and UV-B radiation affected non-stomatal conductance, while stomatal conductance was primarily affected by water stress.     

Ammonium and nitrate uptake by soybean during recovery from nitrogen deprivation

Soybean [Glycine max (L.) Merrill] plants that had been subjected to 15 d of nitrogen deprivation were resupplied for 10 d with 1.0 mol m-3 nitrogen provided as NO3-, NH4+, or NH4(+) + NO3- in flowing hydroponic culture. Plants in a fourth hydroponic system received 1.0 mol m-3 NO3- during both stress and resupply periods. Concentrations of soluble carbohydrates and organic acids in roots increased 210 and 370%, respectively, during stress. For the first day of resupply, however, specific uptake rates of nitrogen, determined by ion chromatography as depletion from solution, were lower for stressed than for non-stressed plants by 43% for NO3- resupply, by 32% for NH4(+) + NO3- resupply, and 86% for NH4+ resupply. When specific uptake of nitrogen for stressed plants recovered to rates for non-stressed plants at 6 to 8 d after nitrogen resupply, carbohydrates and organic acids in their roots had declined to concentrations lower than those of non-stressed plants. Recovery of nitrogen uptake capacity of roots thus does not appear to be regulated simply by the content of soluble carbon compounds within roots. Solution concentrations of NH4+ and NO3- were monitored at 62.5 min intervals during the first 3 d of resupply. Intermittent 'hourly' intervals of net influx and net efflux occurred. Rates of uptake during influx intervals were greater for the NH4(+)-resupplied than for the NO3(-)-resupplied plants. For NH4(+)-resupplied plants, however, the hourly intervals of efflux were more numerous than for NO3(-)-resupplied plants. It thus is possible that, instead of repressing NH4+ influx, increased accumulation of amino acids and NH4+ in NH4(+)-resupplied plants inhibited net uptake by stimulation of efflux on NH4+ absorbed in excess of availability of carbon skeletons for assimilation. Entry of NH4+ into root cytoplasm appeared to be less restricted than translocation of amino acids from the cytoplasm into the xylem.     

Ammonium and nitrate uptake by soybean during recovery from nitrogen deprivation

Soybean [Glycine max (L.) Merrill] plants that had been subjectedto 15 d of nitrogen deprivation were resupplied for 10 d with1.0 mol m^–3 nitrogen provided as NO₃^–, NH₄⁺, orNH₄⁺+NO₃^– in flowing hydroponic culture. Plants in a fourthhydroponic system received 1.0 mol m^–3 NO₃^– duringboth stress and resupply periods. Concentrations of solublecarbohydrates and organic acids in roots increased 210 and 370%,respectively, during stress. For the first day of resupply,however, specific uptake rates of nitrogen, determined by ionchromatography as depletion from solution, were lower for stressedthan for non-stressed plants by 43% for NO₃^- resupply, by 32%for NH₄⁺ + NO₃^– resupply, and 86% for NH₄⁺ resupply. Whenspecific uptake of nitrogen for stressed plants recovered torates for non-stressed plants at 6 to 8 d after nitrogen resupply,carbohydrates and organic acids in their roots had declinedto concentrations lower than those of non-stressed plants. Recoveryof nitrogen uptake capacity of roots thus does not appear tobe regulated simply by the content of soluble carbon compoundswithin roots. Solution concentrations of NH₄⁺ and NO₃^– were monitoredat 62.5 min intervals during the first 3 d of resupply. Intermittent‘hourly’ intervals of net influx and net effluxoccurred. Rates of uptake during influx intervals were greaterfor the NH₄⁺ -resupplied than for the NO₃^– -resuppliedplants. For NH₄⁺ -resupplied plants, however, the hourly intervalsof efflux were more numerous than for NO₃^– -resuppliedplants. It thus is possible that, instead of repressing NH₄⁺influx, increased accumulation of amino acids and NH₄⁺ in NH₄⁺-resupplled plants inhibited net uptake by stimulation of effluxof NH₄⁺ absorbed in excess of availability of carbon skeletonsfor assimilation. Entry of NH₄⁺ into root cytoplasm appearedto be less restricted than translocation of amino acids fromthe cytoplasm into the xylem. Key words: Ammonium, nitrate, nitrogen-nutrition, nitrogen-stress, soybean     

植物光合产物分配及其影响因子研究进展

植物光合产物分配受环境因子和生物因子的共同影响。为增进对植物对全球变化响应的理解, 从植物个体水平与群落/生态系统水平综述了植物光合产物分配的影响因子与影响机理的最新研究进展。植物个体在光照增强及受水分和养分胁迫时, 会将光合产物更多地分配到根系; CO₂浓度升高对植物光合产物分配的影响受土壤氮素的制约, 植物受氮素胁迫时, CO₂浓度升高会促进光合产物更多地分配到根系; 反之, 对植物光合产物分配没有影响。植物群落/生态系统的光合产物分配对环境因子的响应不敏感; 光合产物向根系的分配比例随其生长阶段逐渐降低。功能平衡假说、源汇关系假说和相关生长关系假说分别从环境因子、个体发育和环境因子与个体发育协同作用方面阐述了植物光合产物分配的影响机理。在此基础上,指出了未来拟重点加强的研究方向: 1)生态系统尺度的光合产物向呼吸部分的分配研究; 2)地下净初级生产力(belowground net primary productivity, BNPP)研究; 3)温室和野外条件下及幼苗和成熟林光合产物分配对环境因子响应的比较研究; 4)生态系统尺度的多因子控制试验; 5)整合环境因子和个体发育对植物光合产物分配格局的影响研究。     

Hepatocyte water volume and potassium activity during hypotonic stress

Hepatocytes exhibit a regulatory volume decrease (RVD) during hypotonic shock, which comprises loss of intracellular K⁺ and Cl^– accompanied by hyperpolarization of transmembrane potential (V _m ) due to an increase in membrane K⁺ conductance, (G _K). To examine hepatocyte K⁺ homeostasis during RVD, double-barrel, K⁺-selective microelectrodes were used to measure changes in steady-state intracellular K⁺ activity (a _K ⁱ ) and V _m during hyposmotic stress. Cell water volume change was evaluated by measuring changes in intracellular tetramethylammonium (TMA⁺). Liver slices were superfused with modified Krebs physiological salt solution. Hyposmolality (0.8×300 mosm) was created by a 50 mm step-decrease of external sucrose concentration. Hepatocyte V _m hyperpolarized by 19 mV from –27 ± 1 to –46 ± 1 mV and a _K ⁱ decreased by 14% from 91 ± 4 to 78 ± 4 mm when slices were exposed to hyposmotic stress for 4–5 min. Both V _m and a _K ⁱ returned to control level after restoring isosmotic solution. In paired measurements, hypotonic stress induced similar changes in V _m and a _K ⁱ both control and added ouabain (1 mm) conditions, and these values returned to their control level after the osmotic stress. In another paired measurement, hypotonic shock first induced an 18-mV increase in V _m and a 15% decrease in a _K ⁱ in control condition. After loading hepatocytes with TMA⁺, the same hypotonic shock induced a 14-mV increase in V _m and a 14% decrease in a _TMA ⁱ . This accounted for a 17% increase of intracellular water volume, which was identical to the cell water volume change obtained when a _K ⁱ was used as the marker. Nonetheless, hyposmotic stress-induced changes in V _m and a _K ⁱ were blocked partly by Ba²⁺ (2 mm). We conclude that (i) hepatocyte V _m increases and a _K ⁱ decreases during hypotonic shock; (ii) the changes in hepatocyte V _m and a _K ⁱ during and after hypotonic shock are independent of the Na⁺-K⁺ pump; (iii) the decrease in a _K ⁱ during hypotonic stress results principally from hepatocyte swelling.This work was supported by grant AA-08867 from the Alcohol, Drug Abuse, and Mental Health Association.     

A global analysis of plant recovery performance from water stress

Plant post‐drought recovery performance is essential to predict shifts in ecosystem dynamics and production during frequent climate change‐driven drought events. Yet, it is not clear how post‐drought recovery is related to evolutionary and geographic variations in plants. In this study, we generated a global data set of post‐drought recovery performance in 140 plant species from published studies. We quantified the plant post‐drought recovery performance by calculating a recovery index for multiple plant physiological and hydraulic parameters, including leaf water potential, net photosynthetic rate, leaf hydraulic conductance and shoot biomass. The magnitude of recovery among four plant functional types (deciduous angiosperms, evergreen angiosperms, gymnosperms, and crops), two plant growth forms (shrubs and trees), two water management strategies (isohydric and anisohydric), four xylem porosity types (diffuse, ring, semi‐ring and tracheid), and four major biomes (dry sclerophyll forest, boreal forest, temperate forest and tropical/subtropical forest) were compared. We found the inability to completely recover immediately after severe water stress is ubiquitous across all plant functional types and growth forms, while the rate and magnitude of post‐drought recovery varied greatly across different plant taxonomic categories and geographic ranges. In general, plant hydraulic architecture, leaf anatomy and physiology affect plants’ propensity towards recovery, and reflect evolutionary consequences of plant adaptation to their habitat. Due to the essential role of plant functional traits in regulating carbon storage in each biome, a better understanding plant post‐drought recovery performance could improve our predictions on ecosystem productivity in a rapidly changing climate.     

Stomatal numbers of soybean and response to water stress

The relationship among stomatal density, photosynthetic rate, leaf conductance, plant growth, bean yield and kaempferol triglucoside (K9) in the leaves of soybean (Glycine max (L.) Merr.) was examined in two field tests. K9 in the leaves was associated with reduced stomatal density, reduced photosynthetic rate, reduced stomatal conductance, reduced plant weight and lower bean yield. Plants with high stomatal frequency (lacking K9) were better able to take advantage of increased water supply by increasing stomatal conductance (upper surface), transpiration and bean yield. Plants with low stomatal frequency (with K9) were unresponsive to irrigation and in this sense were more tolerant of water stress, but their overall yield was low.     

Light rather than iron controls photosynthate production and allocation in Southern Ocean phytoplankton populations during austral autumn

The role of iron and light in controlling photosynthate productionand allocation in phytoplankton populations of the Atlanticsector of the Southern Ocean was investigated in April–May1999. The ¹⁴C incorporation into five biochemical pools (glucan,amino acids, proteins, lipids and polysaccharides) was measuredduring iron/light perturbation experiments. The diurnal Chla-specific rates of carbon incorporation into these pools didnot change in response to iron addition, yet were decreasedat 20 µmol photons m^–2 s^–1, an irradiancecomparable with the one at 20–45 m in situ depth. Thissuggests that the low phytoplankton biomass encountered (0.1–0.6µg Chl a L^–1) was mainly caused by light limitationin the deep wind mixed layer (>40 m). Regional differencesin Chl a-specific carbon incorporation rates were not foundin spite of differences in phytoplankton species composition:at the Antarctic Polar Front, biomass was dominated by a diatompopulation of Fragilariopsis kerguelensis, whereas smaller cells,including chrysophytes, were relatively more abundant in theAntarctic Circumpolar Current beyond the influence of frontalsystems. Because mixing was often in excess of 100 m in thelatter region, diatom cells may have been unable to fulfil theircharacteristically high Fe demand at low average light conditions,and thus became co-limited by both resources. Using a modelthat describes the ¹⁴C incorporation, the consistency was shownbetween the dynamics in the glucan pool in the field experimentsand in laboratory experiments with an Antarctic diatom, Chaetocerosbrevis. The glucan respiration rate was almost twice as highduring the dark phase as during the light phase, which is consistentwith the role of glucan as a reserve supplying energy and carbonskeletons for continued protein synthesis during the night.     

Effect of potassium on growth and nitrate reductase during water stress and recovery in maize

Abstract The effect of potassium (0,50, 100 and 200 mg/pot) was studied on growth characteristics and nitrate reductase activity in maize (Zea mays) seedlings during water stress and subsequent recovery. In irrigated plants K⁺ increased the rate of leaf area expansion, leading to increased leaf area per plant. Increased leaf area was associated with decreased chlorophyll content. Water stress (–15 bars) enhanced the stomatal resistance of leaves which was further accentuated by K⁺ application. Nitrate reductase activity rose in irrigated plants 24 h after K⁺ application. Subsequently, as water stress developed, K⁺ helped to maintain higher NR activity for the first two days. However, K⁺ had no effect on half life of NR in light or darkness. During recovery from stress K⁺ aided to maintain the higher leaf expansion rate, the chlorophyll content and the stomatal resistance. The results above are discussed in relation to the ability of K⁺ to maintain better growth under water stress.     

Cardiac triacylglycerol content and lipase activity during recovery from exercise

Female rats swam for 2-h to determine the temporal relationship between triglyceride (TG) repletion and TG lipase activity in the heart during recovery from exercise. Immediately after the exercise, plasma free fatty acids (FFA) had increased from a resting value of 0.44 +/- 0.04 to 0.84 +/- 0.04 mM. Heart TG concentration was reduced 75%, whereas the glycogen level was decreased 34% below control. TG lipase activity was elevated 33% above control activity. One hour after the end of the exercise, lipolytic activity was still 26% above control and did not return to the resting level until the 4th h of recovery. The cardiac TG concentration was back to control levels by the 2nd h after the swim. Plasma FFA concentrations remained elevated during the first 4 h of recovery and were back to the control level by h 8. Cardiac glycogen was "supercompensated" during recovery h 1 and 2 and returned to the preexercise level by h 4. These data indicate that TG is being synthesized in the heart while lipolytic enzyme activity is elevated above control levels. This points out that the rate of TG synthesis is in excess of the hydrolysis. Since plasma FFA concentrations are elevated during periods of augmented TG synthesis, substrate availability, namely plasma FFA, may play a key role in regulating the size of the intracellular TG pool.     

The dependence of soil microbial activity on recent photosynthate from trees

It remains a challenge to quantify and assess the importance of the direct plant below-ground flux of photosynthate carbon (C) to soil microorganisms, especially in forests because of the size of the plants and the great spatial heterogeneity of soils. We studied the importance of labile C inputs from trees on the respiratory activity of soil microorganisms by comparing the response of plots with and without girdled pine trees (Pinus sylvestris L.) to additions of C₄-sucrose, thus enabling us to differentiate between utilization of endogenous C₃-soil C sources and exogenous C₄-sucrose. In both girdled and non-girdled plots the respiration rate after sucrose application, i.e. substrate induced respiration measured in the field, was on average ca. double that of basal respiration rate measured in the field. However, the C₄-sucrose-induced increase in respiration of endogenous C₃-C was significantly higher in non-girdled plots. Expression of C₃-respiration as a percentage of induced respiration in the field showed that in girdled plots, C₃-respiration decreased after sucrose addition and, consequently, the induced respiration in the field was totally C₄-C based. A previous laboratory experiment found no increase in total respiration rate when C₄-sucrose was added to the soil substrate of non-mycorrhizal and ectomycorrhizal pine plants. Hence, we see no reason to attribute the increased respiration to (mycorrhizal) roots. Thus, our results indicate that despite the alleged C limitation of the soil microorganisms there is a fraction of SOM, or C within the microbial biomass that is available to microbial metabolism if their C limitation is relieved by the supply of labile C. This fraction corresponds to roughly 10–20% of biomass C of the heterotrophic organisms and seems to become exhausted in the long-term absence of supply of photosynthate to roots.     

The response and recovery of nitrogen fixation activity in soybean to water deficit at different reproductive developmental stages

Soybean (Glycine max [L.] Merr.) N₂ fixation is a primary plant mechanism responsible for meeting plant-N demand during seed development. Nitrogen fixation is recognized as a drought-sensitive mechanism; however, N₂ fixation response to water deficit and N₂ fixation recovery at different reproductive stages are not well documented. We tested the hypothesis that water deficit during late reproductive stages would inhibit N₂ fixation and lead to the breakdown of essential leaf proteins and an inability to recover N₂ fixation. Acetylene reduction activity (ARA) and N redistribution response to a 5-d drought period at flowering (R2), early seed fill (R5), and late seed fill (R6) were evaluated in one genotype (Hendricks, maturity group 0). Control plants maintained high rates of nodule activity until late seed fill. Plants drought stressed at R2 and R5 recovered ARA after rewatering and in some cases had higher nitrogenase activity than control plants during mid-seed fill. Recovery of ARA on plants stressed at R2 and R5 was associated with higher shoot N concentration than control plants at maturity. Drought stress at R6 reduced ARA, and the inability to recover ARA after stress alleviation at R6 resulted in decreased individual seed mass, which was likely caused by an acceleration of leaf N redistribution and a shorter seed-fill period. Results emphasized the importance of soybean N₂ fixation during late seed development on seed yield and that the ability to recover N₂ fixation following drought is dependent upon crop developmental stage.     

The effect of pot size on growth and transpiration of maize and soybean during water deficit stress

Many experiments are conducted in greenhouses or growth chambers in which plants are grown in pots. Considerable research has shown that pots can have a limiting effect on overall plant growth. This research was undertaken to examine the effects of pot size specifically on transpiration response of maize (Zea mays L.) and soybean (Glycine max L.) plants undergoing water-deficit stress. Maize and soybean experiments were conducted similarly, but as separate experiments. Maize plants were grown in 2.3, 4.1, 9.1, and 16.2 l pots sealed to prevent water loss except by transpiration. For each pot size, plants were divided into two watering regimes, a well-watered control and a water-deficit regime. Water deficits were imposed by simply not rewatering the pots. Soybean was examined in a similar manner, but only the three larger pot sizes were used in the experiment. For both maize and soybean, and in both watering regimes, there was a significant reduction of shoot dry weight and total transpiration with decreasing pot size. However, there were no significant differences among pot sizes in the fraction of transpirable soil water (FTSW) point at which transpiration began to decline (FTSW0.31 for maize and 0.35 for soybean) or in the overall relationship of transpiration rate to soil water content in response to water deficits. These results indicated that, regardless of pot size or plant size, the overriding factor determining transpirational response to drought stress was soil water content.     

Oxidative stress occurs during soybean nodule senescence

Several markers of oxidative stress were measured in 2- to 10-week-old soybean (Glycine max [L.] Merr.) nodules. There were increases in peroxides, protein carbonyls and modified DNA base concentrations with nodule age. The catalytic iron content also increased significantly during nodule ageing. Iron contained in the peribacteroid space was effective in promoting lipid peroxidation and this might contribute to the degradation of the peribacteroid membrane in senescing nodules. The concentration of the oxidized forms of glutathione and homoglutathione increased significantly during nodule development and the concentration of reduced glutathione and homoglutathione decreased during senescence. Taken together, these results are consistent with the development of oxidative stress in senescing nodules. Significant DNA and protein damage also occurred in the first days of nodule development, suggesting that an earlier period of oxidative stress might occur in the period over which the symbiosis becomes established. Received: 7 July 1998 / Accepted: 30 November 1998     

Effects of water stress on respiration in soybean leaves

The effect of water stress on respiration and mitochondrial electron transport has been studied in soybean (Glycine max) leaves, using the oxygen-isotope-fractionation technique. Treatments with three levels of water stress were applied by irrigation to replace 100%, 50%, and 0% of daily water use by transpiration. The levels of water stress were characterized in terms of light-saturated stomatal conductance (g(s)): well irrigated (g(s) > 0.2 mol H(2)O m(-2) s(-1)), mildly water stressed (g(s) between 0.1 and 0.2 mol H(2)O m(-2) s(-1)), and severely water stressed (g(s) < 0.1 mol H(2)O m(-2) s(-1)). Although net photosynthesis decreased by 40% and 70% under mild and severe water stress, respectively, the total respiratory oxygen uptake (V(t)) was not significantly different at any water-stress level. However, severe water stress caused a significant shift of electrons from the cytochrome to the alternative pathway. The electron partitioning through the alternative pathway increased from 10% to 12% under well-watered or mild water-stress conditions to near 40% under severe water stress. Consequently, the calculated rate of mitochondrial ATP synthesis decreased by 32% under severe water stress. Unlike many other stresses, water stress did not affect the levels of mitochondrial alternative oxidase protein. This suggests a biochemical regulation (other than protein synthesis) that causes this mitochondrial electron shift.     

Promoting effect and recovery activity from physical stress of the fruit of Morus alba

We examined the effects of the fruit of M. alba extracts on the changes of the monoamine oxidase (MAO) activities during and after the physical exercise in rat. Each activity was measured by used serotonin(5-HT) and benzylamine as substrate. Lactate dehydrogenase(LDH) activity and the concentrations of lactate in blood which were clinical indexes of physical exercise were also determined to compare with the relation of MAO activities. Those activities during and after the physical exercise have different tendency in each other enzyme. MAO-A activity was sharply decreased with stress by physical activities compared to the normal group, whereas MAO-B activity was increased for 60 minutes after exercise. All of these indexes were recovered to normal state by oral administration of extract of M. alba. These results of this study suggested M. alba may modulate the MAO activities during exercise and promote the capability of physical activities and show anti-stress effect. In general, MAO inhibitors have been used drugs for the purpose of treatment Parkinson's disease, dementia, depression. These results can apply to produce the health and functional foods that have modulating effects for these diseases.