Effects of water stress on nitrogenase activity in Abuts incana

Tolerance to water stress was studied in plants of grey alder, Alnus incana (L.) Moench, grown in a climate chamber in pots of sand supplied with a nitrogen-free nutrient solution. The plants were subjected to a single drying and recovery cycle, during which acetylene reduction, transpiration and stomatal resistance were measured. At different stress levels the plants were placed in a closed system to equilibrate the water potential in the plant-soil system. The water potential of the plants was determined, after which they were watered and their recovery studied. Nitrogenase activity showed low tolerance to water deficit. At moderate stress (−0.6 to −0.8 MPa) acetylene reduction was reduced by half, and at more severe stress, (< −1 MPa) activity was near zero. There was a rapid decrease in nitrogenase activity coincident with stomatal closure, which indicates a continuous need for photoassimilates for nitrogenase activity. Nodules or nitrogenase activity seemed to be weak sinks for assimilates compared with root pressure bleeding. Measurements of nitrogenase activity in root nodule homogenates supplied with ATP and reductant suggested a loss of active nitrogenase in the nodules in response to water stress. The recovery from moderate stress or long dark treatment took several days, and recovery from severe stress took still longer. Shortage of assimilates and disturbances in oxygen and nitrogen balances in the nodules are discussed as reasons for the reduced nitrogenase activity in response to water stress.     

Threshhold for recovery in the marsh halophyte Spartina alterniflora grown under the combined effects of salinity and soil drying

A study quantifying the physiological threshhold at which Spartina alterniflora plants are able to tolerate the interactive effects of salinity and soil drying was conducted in a climate controlled greenhouse. The experiment consisted of two levels of salinity (3-5 ppt, L and 35-38 ppt, H) as well as four dynamic water levels: flooding (water level maintained 3-5 cm above the soil surface at high tide and 10 cm below the soil surface at low tide for entire study duration, F), 8-day drought (water level maintained at least 20 cm below the soil surface at high tide for 8 days then flooded, 8 days), 16-day drought (water level maintained at least 20 cm below the soil surface at high tide for 16 days then flooded, 16 days), and 24-day drought (water level maintained at least 20 cm below the soil surface at high tide for 24 days then flooded, 24 days). Plant gas exchange and growth responses were measured along with soil conditions of redox potential and water potential. Significant decreases were seen in plant gas exchange and growth in response to increases in salinity and soil drying. Survival was 100% for all flooded treatments while increased salinity combined with soil drying decreased survival to 86% in both low salt/24-day drought plants (LD24) and high salt/16-day drought plants (HD16). The lowest survival rate was seen in the high salt/24-day drought treatment (HD24) at 29%. Therefore, it appears that the critical time for recovery from the combined effects of increased salinity and soil drying may greatly diminish after two weeks from the onset of stress conditions. Consequently, if salinity continues to increase along the MRDP, marshes dominated by S. alterniflora may be more susceptible to short-term drought and likewise large-scale marsh browning.     

Nodule Activity and Allocation of Photosynthate of Soybean during Recovery from Water Stress

Nodulated soybean plants (Glycine max [L.] Merr. cv Ransom) in a growth-chamber study were subjected to a leaf water potential (Ψ_w) of −2.0 megapascal during vegetative growth. Changes in nonstructural carbohydrate contents of leaves, stems, roots, and nodules, allocation of dry matter among plant parts, in situ specific nodule activity, and in situ canopy apparent photosynthetic rate were measured in stressed and nonstressed plants during a 7-day period following rewatering. Leaf and nodule Ψ_w also were determined. At the time of maximum stress, concentration of nonstructural carbohydrates had declined in leaves of stressed, relative to nonstressed, plants, and the concentration of nonstructural carbohydrates had increased in stems, roots, and nodules. Sucrose concentrations in roots and nodules of stressed plants were 1.5 and 3 times greater, respectively, than those of nonstressed plants. Within 12 hours after rewatering, leaf and nodule Ψ_w of stressed plants had returned to values of nonstressed plants. Canopy apparent photosynthesis and specific nodule activity of stressed plants recovered to levels for nonstressed plants within 2 days after rewatering. The elevated sucrose concentrations in roots and nodules of stressed plants also declined rapidly upon rehydration. The increase in sucrose concentration in nodules, as well as the increase of carbohydrates in roots and stems, during water stress and the rapid disappearance upon rewatering indicates that inhibition of carbohydrate utilization within the nodule may be associated with loss of nodule activity. Availability of carbohydrates within the nodules and from photosynthetic activity following rehydration of nodules may mediate the rate of recovery of N₂-fixation activity.     

Nodule activity and allocation of photosynthate of soybean during recovery from water stress

Nodulated soybean plants (Glycine max [L.] Merr. cv Ransom) in a growth-chamber study were subjected to a leaf water potential (psi w) of -2.0 megapascal during vegetative growth. Changes in nonstructural carbohydrate contents of leaves, stems, roots, and nodules, allocation of dry matter among plant parts, in situ specific nodule activity, and in situ canopy apparent photosynthetic rate were measured in stressed and nonstressed plants during a 7-day period following rewatering. Leaf and nodule psi w also were determined. At the time of maximum stress, concentration of nonstructural carbohydrates had declined in leaves of stressed, relative to nonstressed, plants, and the concentration of nonstructural carbohydrates had increased in stems, roots, and nodules. Sucrose concentrations in roots and nodules of stressed plants were 1.5 and 3 times greater, respectively, than those of nonstressed plants. Within 12 hours after rewatering, leaf and nodule psi w of stressed plants had returned to values of nonstressed plants. Canopy apparent photosynthesis and specific nodule activity of stressed plants recovered to levels for nonstressed plants within 2 days after rewatering. The elevated sucrose concentrations in roots and nodules of stressed plants also declined rapidly upon rehydration. The increase in sucrose concentration in nodules, as well as the increase of carbohydrates in roots and stems, during water stress and the rapid disappearance upon rewatering indicates that inhibition of carbohydrate utilization within the nodule may be associated with loss of nodule activity. Availability of carbohydrates within the nodules and from photosynthetic activity following rehydration of nodules may mediate the rate of recovery of N2-fixation activity.     

Distribution of 14CO2 during ontogeny of chickpea (Cicer arietinum) grown at two moisture levels

The distribution of assimilates of ¹⁴CO₂ in ethanol soluble and insoluble fractions was measured at 20-day intervals from 45–135 days after sowing (DAS) in chickpea (Cicer arietinum) grown at two moisture levels. The contribution of pre-flowering assimilates to pods, although very low, was higher under the stress conditions. At the time of harvest, the recovery of ¹⁴C in pods was 0.4 and 0.9% of the total ¹⁴C fed 45 DAS in soluble and 2.5 and 5.1% in insoluble fractions in control and stressed plants, respectively. The %¹⁴C received by nodules continuously decreased with increasing age of plants. Stressed plants diverted more ¹⁴C to nodules, compared to control, during vegetative and flowering stages. During active seed filling, stressed plants diverted more ¹⁴C to reproductive parts and less to nodules, compared to control. Significant amounts of ¹⁴C were retgined by the stem and leaves during the seed-filling period and it appears that there is scope for the remobilisation of pre-flowering, as well as post-flowering assimilates for seed-filling of chickpea.     

Water relations of pine seedlings in relation to root and shoot growth

The effects of water stress on growth and water relations of loblolly and white pine seedlings were studied during series of drying cycles. As mean soil water potential decreased, growth of roots, needles, and buds decreased. Growth of roots during successive severe drying cycles was not uniform, however. A study of needle and root extension showed that of the total growth of roots for 3 7-day drying cycles, only 6% occurred during the third cycle, while needle extension was uniform for the 3 cycles. The difference in response of needles and roots to drying cycles may be attributed primarily to the effect of water stress on the growing region. When subjected to a severe stress, roots matured toward the tip and became dormant, resulting in less growth during subsequent drying cycles. The intercalary growing region of needles, however, was not altered seriously enough by the stress to cause a difference in amount of growth during each drying cycle.

Transpiration of loblolly pine was lower in the second drying cycle than in the first. Needle water potential after rewatering was as high as that of control plants watered daily; root resistance was apparently not important in restricting transpiration during a second drying cycle. Needle diffusion resistance of loblolly pine, measured with a low-resistance diffusion porometer, was slightly higher during the second drying cycle than during the first. In addition, many primary needles were killed during the first period of stress. These factors contributed to the reduction of transpiration during the second drying cycle. Diffusion resistance of Coleus increased and transpiration ceased during the first drying cycle while water potential remained relatively high. After rewatering, both leaf resistance and transpiration returned to the control level, presumably because the stress during the first period of drying was not severe. The diffusion resistances observed for well-watered plants were 30 to 50 sec·cm⁻¹ for loblolly pine, 3 to 5 sec·cm⁻¹ for Coleus, and 4 to 6 sec·cm⁻¹ for tomato. These values agree closely with those reported by other workers.

     

Effects of repeated application of water stress on water status and growth of wheat

The effects on water status and growth of controlled cycles of water stress applied at various stages of development were studied on a semi-dwarf spring wheat (Triticum aestivum L.). The plants were grown in controlled environment chambers of the Duke University Phytetron at 24/18°C with a 12-h photo-period at about 600 μE m^?2 s^?1. Groups of plants were subjected to severe water stress by withholding irrigation, beginning at the 7th leaf, early anthesis, or early dough stages of development. A second cycle started 9 to 13 days after termination of the first cycle and maintained until the flag leaf water potential reached –25 bars at each of the growth stages. The lower leaves showed sign of wilting as indicated by curling in the first drying cycle at –7 bars and in the second cycle at –9 bars of leaf water potential during all stages of growth. Although these leaves recovered completely upon rewatering, onset of senescence was accelerated by three days in stressed plants. A preliminary drying cycle did not increase the ability of the plants to withstand subsequent stress because of severity of stress. Water stress of –25 bars at all three stages of growth reduced seed yield. The reduction was greater when a second stress cycle was also applied. Stress applied during early anthesis stage produced the smallest and the least number of seeds. The lack of osmotic adjustment probably was due to very rapid and severe development of water stress.     

Effects of high temperature on nodulation and nitrogen fixation by Phaseolus vulgaris L.

Screening of Rhizobium leguminosarum bv. phaseoli strains showed some that were able to nodulate common beans (Phaseolus vulgaris L.) at high temperatures (35 and 38°C/8 h/day). The nodulation ability was not related to the capability to grow or produce melanin-like pigment in culture media at high temperatures. However, nodules formed at high temperatures were ineffective and plants did not accumulate N in shoots. Two thermal shocks of 40°C/8 h/day at flowering time drastically decreased nitrogenase activity and nodule relative efficiency of plants otherwise grown at 28°C. Recovery of nitrogenase activity began only after seven days, when new nodules formed; total incorporation of N in tops did not recover for 2 weeks. Non-inoculated beans receiving mineral N were not affected by the thermal shock, and when growing continuously at 35 or 38°C had total N accumulated in shoots reduced by only 18%.     

Leaf expansion of four sunflower (Helianthus annuus L.) cultivars in relation to water deficits. II. Diurnal patterns during stress and recovery

Abstract. Leaf expansion of four sunflower cultivars ( Helianthus annuus L. cvs. Hysun 31, Havasupai, Hopi and Seneca) was monitored continuously in a growth cabinet through the final stages of a drying cycle and then throughout the first 2 days after rewatering in order to study the responses of leaf expansion to water deficits. Comparable plants were also measured throughout a diurnal cycle in a glasshouse.
In the cabinet, leaf extension was faster in the dark than in the light, but an extended dark period suppressed leaf extension. At similar leaf water potentials, the rate of leaf extension was greater in the light than in the dark, but as the osmotic potential was lower in the light than in the dark, the relationship between turgor pressure and leaf extension rate was similar in both environments. Throughout the drying and recovery cycles turgor and leaf extension rate was positively correlated: no significant differences among cultivars were observed.
In the plants grown and measured in the glasshouse, leaf expansion occurred at lower leaf water potentials in stressed than in unstressed plants, but the relationship between leaf expansion and turgor was similar in both stressed and unstressed plants as a result of a lowering of the osmotic potential in the former. Diurnal turgor maintenance resulting from osmotic adjustment was almost half that occurring during a complete drying cycle. During the day, the leaf expansion rate increased linearly with turgor pressure in all cultivars: the expansion rate per unit turgor pressure was greater in the glasshouse than in the growth cabinet. Nocturnal leaf expansion in the stressed and unstressed plants was not, however, correlated with turgor pressure.     

Soil temperature and intermittent frost modulate the rate of recovery of photosynthesis in Scots pine under simulated spring conditions

An earlier onset of photosynthesis in spring for boreal forest trees is predicted as the climate warms, yet the importance of soil vs air temperatures for spring recovery remains to be determined. Effects of various soil- and air-temperature conditions on spring recovery of photosynthesis in Scots pine (Pinus sylvestris) seedlings were assessed under controlled environmental conditions. Using winter-acclimated seedlings, photosynthetic responses were followed after transfer to different simulated spring conditions. Recovery rates for photosynthetic electron transport and net CO(2) uptake were slower in plants from cold or frozen soil compared with controls. In addition, a greater fraction of light absorbed was not used photochemically, but was dissipated thermally via xanthophyll cycle pigments. Intermittent frost events decreased photosynthetic capacity and increased thermal energy dissipation. Within a few days after frost events, photosynthetic capacity recovered to prefrost levels. After 18 d under spring conditions, no difference in the optimum quantum yield of photosynthesis was observed between seedlings that had been exposed to intermittent frost and control plants. These results show that, if air temperatures remain favourable and spells of subfreezing air temperatures are only of short duration, intermittent frost events delay but do not severely inhibit photosynthetic recovery in evergreen conifers during spring. Cold and/or frozen soils exert much stronger inhibitory effects on the recovery process, but they do not totally inhibit it.     

Increase in nitrate uptake by soybean plants during interruption of the dark period with low intensity light

Diurnal patterns of net NO⁻₃ uptake by nonnodulated soybean [ Glycine max (L.) Merr. cv. Ransom] plants growing in flowing hydroponic culture at 26 and 16°C root temperatures were measured at hourly intervals during alternate days of a 12-day growth period. Ion chromatography was used to determine removal of NO⁻₃ from the culture solution. Day and night periods of 9 and 15 h were used during growth. The night period included two 6-h dark periods and an intervening 3-h period of night interruption by incandescent lamps to effect a long-day photoperiod and repress floral initiation. At both root temperatures, the average specific rates of NO⁻₃ uptake were twice as great during the night interruption period as during the day period; they were greater during the day period than during the dark periods; and they were greater during the dark period immediately following the day period than during the later dark period that followed the night interruption. While these average patterns were repetitious among days, measured rates of uptake varied hourly and included intervals of net efflux scattered through the day period and more frequently through the 2 dark periods. Root temperature did not affect the average daily specific rates of uptake or the qualitative relationships among day, dark and night interruption periods of the diurnal cycle.     

呼吸活性的下降可解释水分亏缺对银合欢根瘤固氮酶活性的抑制(英)

在温室条件下研究了水分亏缺对银合欢根瘤的水分关系、固氮酶活性（乙炔还原活性）、呼吸活性以及蔗糖代谢有关酶活性的影响。随着土壤含水量的下降,根瘤水势也相继下降。土壤干旱不但显著地抑制了根瘤乙炔还原的活性,而且对根瘤的呼吸活性、ATP的产生以及催化蔗糖降解的碱性转化酶和蔗糖合成酶的活性也具强烈的抑制作用。然而,根瘤可溶性总糖含量则不受土壤干旱的影响。用呼吸抑制剂DNP处理根瘤后,其固氮酶活性、呼吸活性及ATPI含量都受到极显著的抑制。这都表明,水分胁迫对根瘤呼吸活性及ATP产生的抑制可解释干旱条件下固氮酶活性的下降。     

Matrix metalloproteinase induction in fibrosis and fibrotic nodule formation due to silica inhalation

Matrix metalloproteinases (MMPs) are the principle enzymes that initiate degradation of collagen. We examined the role of MMPs during alveolar wall fibrosis and fibrotic nodule formation from silica exposure. Rats were exposed to filtered air or 15 mg/m(3) silica by inhalation for 5 days/wk, 6 h/day. Lungs were preserved by intratracheal instillation of fixative at 20, 40, 60, 79, and 116 days of exposure. Additional groups were fixed after 20, 40, and 60 days of exposure followed by 36 days of recovery. The number of nodules, defined by a collagenous core and a bounding cell layer detached from the alveolar wall, was determined by morphometry. Lungs showed increased alveolar wall collagen and fibrotic nodules at 79 and 116 days of exposure with increased collagenase and gelatinase activity. The number of nodules per lung in exposed groups increased from 619 +/- 447 at 40 days to 13,221 +/- 1,096 at 116 days (means +/- SE, n = 5). No nodules were seen in control lungs. Silica-exposed rats with a 36-day recovery in filtered air showed enhanced MMP activity over exposure to silica for the same duration with no recovery. MMP-2 and MMP-9 were significantly elevated in alveolar macrophages after 40-day exposure. Stromelysin expression was demonstrated in alveolar macrophages and cells within fibrotic nodules. TIMP-1 expression was not significantly altered. In summary, MMP activity was upregulated at 40 days of silica exposure and progressively increased during ensuing fibrotic responses. Early expression of stromelysin was found in fibrosing alveolar walls and fibrotic nodules.     

Nitrate Inhibition of Legume Nodule Growth and Activity : II. Short Term Studies with High Nitrate Supply

Soybean plants (Glycine max [L.] Merr) were grown in sand culture with 2 millimolar nitrate for 37 days and then supplied with 15 millimolar nitrate for 7 days. Control plants received 2 millimolar nitrate and 13 millimolar chloride and, after the 7-day treatment period, all plants were supplied with nil nitrate. The temporary treatment with high nitrate inhibited nitrogenase (acetylene reduction) activity by 80% whether or not Rhizobium japonicum bacteroids had nitrate reductase (NR) activity. The pattern of nitrite accumulation in nodules formed by NR⁺ rhizobia was inversely related to the decrease and recovery of nitrogenase activity. However, nitrite concentration in nodules formed by NR⁻ rhizobia appeared to be too low to explain the inhibition of nitrogenase. Carbohydrate composition was similar in control nodules and nodules receiving 15 millimolar nitrate suggesting that the inhibition of nitrogenase by nitrate was not related to the availability of carbohydrate.

Nodules on plants treated with 15 millimolar nitrate contained higher concentrations of amino N and, especially, ureide N than control nodules and, after withdrawal of nitrate, reduced N content of treated and control nodules returned to similar levels. The accumulation of N₂ fixation products in nodules in response to high nitrate treatment was observed with three R. japonicum strains, two NR⁺ and one NR⁻. The high nitrate treatment did not affect the allantoate/allantoin ratio or the proportion of amino N or ureide N in bacteroids (4%) and cytosol (96%).

     

Reversible dark-induced senescence of soybean root nodules

Nodule senescence was induced in intact soybean [Glycine max. (L.) Merr., cv Woodworth] plants by an 8-day dark treatment. Dark-induced senescence resulted in the complete loss of acetylene reduction activity, a 67% loss of total soluble protein, and an almost complete loss in total leghemoglobin of nodule extracts. Isoelectric focusing gels demonstrated a preferential loss of certain proteins, which was correlated with an increase in endoprotease specific activity toward azocasein. Nodules were completely green after the 8-day dark treatment. If plants were returned to a normal photoperiod after 8 days in the dark, nodules recovered from the dark treatment in 12 to 16 days. Acetylene reduction activity returned to normal, and both total soluble protein and leghemoglobin were resynthesized while protease activity against azocasein decreased to the level of control nodules. The nodule population that had turned green after 8 days in the dark exhibited a progressive increase in red color starting nearest the exterior of the nodule, and after 16 days of recovery nodules were indistinguishable from control nodules maintained under a normal photoperiod.     

Comparative Response of Nodulated and Nitrogen-Supplied Cowpea (Vigna unguiculata (L.) Walp. var. Tuy) Plants to Infection by Cowpea Mosaic Virus

The effect of infection by the Cowpea Mosaic Virus (CpMV) onseveral parameters relevant to symbiotic nitrogen fixation wasdetermined in cowpea (Vigna unguiculata (L.) Walp. var. Tuy)plants nodulated with two strains of Rhizobium cowpea: IVIC–124and IVIC–38. Plants were virus-infected at the seedlingstage before Rhizobium inoculation. The effect of CpMV infectionon plant growth was analysed in nodulated and nitrogen-suppliedplants at 18, 25 and 35 d after germination. At all developmentalstages of nodulated plants CpMV infection caused a reductionof leaf chlorophyll content, leaf area, dry weight of shootsand roots, total nodule weight and nodule number. Most of thenodules from 18- and 25-d-old CpMV-infected plants did not exhibitleghaemoglobin pigmentation. CpMV infection delayed the onsetof nitrogenase activity in nodules of the rhizobial strain IVIC–124and the enzyme activity measured on a per plant basis was reducedin both strains at the first and second harvests. Significantnitrogenase activity was detected in 35-d-old infected plants.Some of the nodules of the rhizobial strain IVIC-124 and mostof the nodules from plants nodulated with the strain IVIC-38developed leghaemoglobin; however, the nodule-specific nitrogenaseactivity, estimated on a milligram nodule dry weight basis,was always higher in virus-infected plants, particularly in18-d-old CpMV-infected plants harbouring the IVIC–124strain. CpMV-infected nodules had a larger peribacteroidal space,a reduced number of peribacteroid units, a greater number ofbacteroids per unit, a lower number of vesicles and 88% lowertotal reducing sugar content. Starch accumulation was detectedin infected leaves of nodulated plants during the first harvest,while high levels of leaf reducing sugars and protein were presentat the second harvest. In healthy nodulated plants the rhizobialstrain IVIC–124 was shown to be more efficient than IVIC–38in promoting plant growth. However, the results indicate thatnodulation by rhizobial strain IVIC–124 and growth ofplants harbouring this strain were affected to a greater extentby virus infection. The effect of CpMV infection on leaf chlorophyllcontent, leaf area, carbohydrate level, leaf proteins and growthof nitrogen-supplied plants, as well as the symptoms inducedin the leaves, were less conspicuous than in nodulated plants. Key words: Cowpea, Rhizobium, virus infection, nodule untrastructure     

Plasma cortisol and 17β-oestradiol levels in roach exposed to acute and chronic stress

Plasma cortisol levels were measured as an indicator of physiological stress in roach subjected to brief handling, or to a 14-day period of confinement, and in undisturbed control fish, during winter (water temperature 5° C) and summer (16° C), at which time plasma 17 β-oestradiol levels were also determined. Cortisol levels in undisturbed roach were low (mean 8·1 ng ml⁻¹ at 5° C; 1·4 ng ml⁻¹ at 16° C) and both handling and handling+confinement elevated blood cortisol levels significantly to 400 and 140 ng ml⁻¹, respectively (at 5° C) and 700 and 600 ng ml⁻¹, respectively (at 16) C). Blood cortisol levels had almost returned to baseline within 4 h following handling alone but in fish subjected to handling and prolonged confinement cortisol levels remained elevated for up to 168 h. Differences in baseline and poststress levels of cortisol, and in the rate of recovery from acute stress, were observed at the two different temperatures and the possible factors underlying these differences are discussed. Circulating levels of 17 β-oestradiol were reduced significantly within 24 h of exposure to either acute handling or chronic confinement indicating that the reproductive endocrine system in roach is sensitive to disruption by stressors.     

Effect of Temperature on the Nitrogenase Activity of Intact and Detached Nodules in Lotus and Stylosanthes

Temperature and plant age influenced the nitrogenase activityof Lotus and Stylosanthes nodules. Time course studies usingnodulated plants in closed vials showed a decline in activityafter 48 h; regassing with 10% (v/v) acetylene in air partiallyrestored the activity. Transfer of plants from 15, 20, 25, and30 ?C to 40 ?C immediately stopped activity; this was completelyrestored within 1 h after return to original temperatures. Detached nodules cultured on nitrogen-free agar medium exhibiteda sucrose concentration (2–8%, w/v)-dependent nitrogenaseactivity at each temperature. With 6% sucrose prolonged activity,up to 96 h, was obtained. Decline in nitrogenase activity indetached nodules was due partly to exposure to air during excisionand transfer to the medium.