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.     

Nitrogen Fixation (C(2)H(2) Reduction) by Broad Bean (Vicia faba L.) Nodules and Bacteroids under Water-Restricted Conditions

Water potentials of leaves and nodules of broad bean (Vicia faba L.) cultivated on a sandy mixture were linearly and highly (r² = 0.99) correlated throughout a water deprivation of plants. A decrease of 0.2 megapascal of the nodule water potential (Ψ_nod) induced an immediate 25% inhibition of the highest level of acetylene reduction of broad bean nodules attached to roots. This activity continued to be depressed when water stress increased, but the effect was less pronounced. Partial recovery of optimal C₂H₂ reduction capacity of mildly water stressed nodules (Ψ_nod = −1.2 megapascals) was possible by increasing the external O₂ partial pressure up to 60 kilopascals. The dense packing of the cortical cells of nodules may be responsible for the limitation of O₂ diffusion to the central tissue. Bacteroids isolated from broad bean nodules exhibited higher N₂ fixation activity with glucose than with succinate as an energy-yielding substrate. Bacteroids from stressed nodules appeared more sensitive to O₂, and their optimal activity declined with increasing nodule water deprivation. This effect could be partly due to decreased bacteroid respiration capacity with water stress. Water stress was also responsible for a decrease of the cytosolic protein content of the nodule and more specifically of leghemoglobin. The alteration of the bacteroid environment appears to contribute to the decline in N₂ fixation under water restricted conditions.     

Effect of RotyIenchulus reniformis on Reflectance of Cotton Plant Leaves

Differences between light reflectance from leaves of cotton (Gossypiurn hirsutum) plants grown with a low- or no-nematode (Rotylenchulus reniformis) population (nonstressed), and from leaves grown with a high nematode population (stressed) were measured in field and greenhouse experiments. Reflectance was measured spectrophotometrically in the laboratory on single leaves and spectroradiometrically in the field on plant canopies. Nematode-stressed cotton plants were stunted with fewer, smaller, and darker-green leaves than nonstressed plants. Over the 0.5- to 2.5-/μm waveband, stressed leaves had lower reflectance than nonstressed leaves of the same chronological age for both field- and greenhouse-grown plants. Reflectance differences between stressed and nonstressed leaves in the visible (0.5 to 0.75 μm), near-infrared (0.75 to 1.35 μm) and infrared water absorption (1.35 to 2.5 μm) regions were primarily caused by differences in leaf chlorophyll concentration, mesophyll structure, and water content, respectively. Results indicate the potential for remotely sensing nematode-infested plants to distinguish them from normal plants.     

Osmotic adjustment, symplast volume, and nonstomatally mediated water stress inhibition of photosynthesis in wheat

At low water potential (ψ_w), dehydration reduces the symplast volume of leaf tissue. The effect of this reduction on photosynthetic capacity was investigated. The influence of osmotic adjustment on this relationship was also examined. To examine these relationships, comparative studies were undertaken on two wheat cultivars, one that osmotically adjusts in response to water deficits (`Condor'), and one that lacks this capacity (`Capelle Desprez'). During a 9-day stress cycle, when water was withheld from plants grown in a growth chamber, the relative water content of leaves declined by 30% in both cultivars. Leaf osmotic potential (ψ_s) declined to a greater degree in Condor plants. Measuring ψ_s at full turgor indicated that osmotic adjustment occurred in stressed Condor, but not in Capelle plants. Two methods were used to examine the degree of symplast (i.e. protoplast) volume reduction in tissue rapidly equilibrated to increasingly low ψ_w. Both techniques gave similar results. With well-watered plants, symplast volume reduction from the maximum (found at high ψ_w for each cultivar) was the same for Condor and Capelle. After a stress cycle, volume was maintained to a greater degree at low ψ_w in Condor leaf tissue than in Capelle. Nonstomatally controlled photosynthesis was inhibited to the same degree at low ψ_w in leaf tissue prepared from well-watered Condor and Capelle plants. However, photosynthetic capacity was maintained to a greater degree at low ψ_w in tissue prepared from stressed Condor plants than in tissue from stressed Capelle plants. Net CO₂ uptake in attached leaves was monitored using an infrared gas analyzer. These studies indicated that in water stressed plants, photosynthesis was 106.5% higher in Condor than Capelle at ambient [CO₂] and 21.8% higher at elevated external [CO₂]. The results presented in this report were interpreted as consistent with the hypothesis that there is a causal association between protoplast (and presumably chloroplast) volume reduction at low ψ_w and low ψ_w inhibition of photosynthesis. Also, the data indicate that osmotic adjustment allows for maintenance of relatively greater volume at low ψ_w, thus reducing low ψ_w inhibition of chloroplast photosynthetic potential.     

Responses of transpiration and hydraulic conductance to root temperature in nitrogen- and phosphorus-deficient cotton seedlings

Suboptimal N or P availability and cool temperatures all decrease apparent hydraulic conductance (L) of cotton (Gossypium hirsutum L.) roots. The interaction between nutrient status and root temperature was tested in seedlings grown in nutrient solutions. The depression of L (calculated as the ratio of transpiration rate to absolute value of leaf water potential [Ψ_w]) by nutrient stress depended strongly on root temperature, and was minimized at high temperatures. In fully nourished plants, L was high at all temperatures ≥20°C, but it decreased greatly as root temperature approached the chilling threshold of 15°C. Decreasing temperature lowered Ψ_w first, followed by transpiration rate. In N- or P-deficient plants, L approached the value for fully nourished plants at root temperatures ≥30°C, but it decreased almost linearly with temperature as roots were cooled. Nutrient effects on L were mediated only by differences in transpiration, and Ψ_w was unaffected. The responses of Ψ_w and transpiration to root cooling and nutrient stress imply that if a messenger is transmitted from cooled roots to stomata, the messenger is effective only in nutrient-stressed plants.     

Effect of form and level of applied nitrogen on nitrogenase and nitrate reductase activities in faba beans

The effects of nitrogen applied at increasing levels of 0, 4, 8, 16 and 32 mM N (KNO₃ or NH₄Cl) were studied in faba bean (Vicia faba) nodulated byRhizobium leguminosarum bv.viceae RCR lool. Nitrogenase activity was higher at 4 and 8 mM N than the zero N treatment (control), but 16 and 32 mM N significantly reduced the efficiency of nodule functions. Nitrate reductase activities (NRA) of leaves, stems, roots, nodules and nodule fractions (bacteroid and cytosol) were increased with rising the NO₃ ^? or NH₄ ⁺ levels. NRA decreased in the order of nodules>leaves>stems>roots. Cytosolic NR was markedly higher than that recorded in the bacteroid fractions. Nitrate levels were linearly correlated to NRA of nodules. Accumulation of NO₂ ^? within nodules suggests that NO₂ ^? inhibits nodule’s activity after feeding plants with NO₃ ^? or NH₄ ⁺.     

Effect of osmotically induced leaf moisture stress on nodulation and nitrogenase activity ofGlycine max

Summary The effect of 2-day cycles of osmotically induced leaf moisture stress followed by partial recovery on the nodulation and nitrogenase activity of 2 soya cultivars was studied. Fourteen days after plant inoculation (mid-growth stage) the total leaf electrochemical water potential (w^leaf) of control plants ranged from –0.8 to –1.9 bars, whereas the concentrations of osmoticum (polyethylene glycol 4000) induced w^leaf values ranging from –1.4 (recovery value) to –3.1 bars (low stress), –1.8 to –4.4 bars (mild stress), and –2.2 to –6.2 bars (medium stress). The low stress treatment reduced nodule numbers and their specific activity in both cultivars, without affecting nodule size or the time required for nodule initiation. Nodule initiation was delayed in both cultivars by the mild and medium stress treatments, the former treatment reducing the number and size of the nodules and such nodules exhibited very low specific activity. The medium stress treatment prevented the further development of nodule initials, which remained inactive throughout the experiment. Such results imply an effect of water stress on the infection process and on nodule morphogenesis. The reduction in nodule numbers observed in water stressed plants was not associated with a reduced number of rhizobia in the rhizoplane nor due to an effect on root growth or root hair formation.At a stage prior to the formation of macroscopic nodule initials, the roots of plants under medium stress (w^leaf=–5.5 bar)s) had a higher content of abscisic acid (ABA) (4-fold increase) and a lower content of gibberellin (GA)-like substances (21.4% reduction) as compared to control plants (w^leaf=–1.0 bar). Although the medium stress treatment slightly increased the stomatal resistance of leaves, photosynthetic and transpiration rates were unaffected. Similar alterations of the hormononal balance occurred in the nodulated roots of plants subjected to naturally induced leaf moisture stress.Since the foliar application of ABA (1.92×10^–5 M) to unstressed plants inhibited nodulation (45% reduction in nodule numbers), the increased endogenous content of thishormone in the roots of plants under leaf moisture stress may provide some physiological insight into the inhibitory effect of water stress on the nodulation process.     

Characterization of delta-Aminolevulinic Acid Formation in Soybean Root Nodules

Formation of the heme precursor δ-aminolevulinic acid (ALA) was studied in soybean root nodules elicited by Bradyrhizobium japonicum. Glutamate-dependent ALA formation activity by soybean (Glycine max) in nodules was maximal at pH 6.5 to 7.0 and at 55 to 60°C. A low level of the plant activity was detected in uninfected roots and was 50-fold greater in nodules from 17-day-old plants; this apparent stimulation correlated with increases in both plant and bacterial hemes in nodules compared with the respective asymbiotic cells. The glutamate-dependent ALA formation activity was greatest in nodules from 17-day-old plants and decreased by about one-half in those from 38-day-old plants. Unlike the eukaryotic ALA formation activity, B. japonicum ALA synthase activity was not significantly different in nodules than in cultured cells, and the symbiotic activity was independent of nodule age. The lack of symbiotic induction of B. japonicum ALA synthase indicates either that ALA formation is not rate-limiting, or that ALA synthase is not the only source of ALA for bacterial heme synthesis in nodules. Plant cytosol from nodules catalyzed the formation of radiolabeled ALA from U-[¹⁴C]glutamate and 3,4-[³H]glutamate but not from 1-[¹⁴C]glutamate, and thus, operation of the C₅ pathway could not be confirmed.     

Transpiration- and growth-induced water potentials in maize

Recent evidence from leaves and stems indicates that gradients in water potential (ψ_w) necessary for water movement through growing tissues are larger than previously assumed. Because growth is sensitive to tissue ψ_w and the behavior of these gradients has not been investigated in transpiring plants, we examined the water status of all the growing and mature vegetative tissues of maize (Zea mays L.) during high and low rates of transpiration. The ψ_w measured in the mature regions of the plant responded primarily to transpiration, while the ψ_w in the growing regions was affected both by transpiration and growth. The transpiration-induced potentials of the mature tissue formed a gradient of decreasing ψ_w along the transpiration stream while the growth-induced potentials formed a gradient of decreasing ψ_w from the transpiration stream to the expanding cells in the growing tissue. The growth-induced gradient in ψ_w within the leaf remained fairly constant as the xylem ψ_w decreased during the day and was associated with a decreased osmotic potential (ψ_s) of the growing region (osmotic adjustment). The growth-induced gradient in ψ_w was not caused by excision of the tissue because intact maize stems exhibited a similar ψ_w. These observations support the concept that large gradients in ψ_w are required to maintain water flow to expanding cells within all the vegetative tissues and suggest that the maintenance of a favorable gradient in ψ_w for cell enlargement may be an important role for osmotic adjustment.     

Nitrogen Fixation by Subterranean Clover at Varying Stages of Nodule Dehydration: I. CARBOHYDRATE STATUS AND SHORT-TERM RECOVERY OF NODULATED ROOT RESPIRATION

The nodule water potential (_nod) of subterranean clover (Trifoliumsubterraneum L.) cv. Seaton Park incubated in a flow-throughgas-exchange system was induced to decline independently ofleaf water potential (₁) by passing a continuous dry airstreamover the nodulated roots of intact well-watered plants. Reducedtranspiration by plants whose nodules had become dehydratedwas hypothesized to be related to the decline in nitrogen fixation.Whole-plant and nodule soluble carbohydrates increased as _noddeclined. Throughout an 8 d period of continual nodule dehydration,the gaseous diffusion resistance of nodules increased and theoptimum pO₂ for nitrogenase activity declined from 52 to 28kPa. Following rehydration of the nodulated roots between days4 and 5 and between days 7 and 8, nodulated root respirationincreased to or above pre-stress levels whereas nitrogenaseactivity did not recover. Re-establishment of initial ratesof nodulated root respiration was due to the stimulation ofgrowth and maintenance respiration, not to the respiration coupledto nitrogenase activity. Although no recovery of nitrogenaseactivity occurred, the elapsed time from the introduction ofacetylene into the gas stream flowing past the nodules untilmeasurement of the acetylene-induced decline in nitrogenaseactivity, decreased substantially. This was characteristic ofan increase in the permeability of the nodules to gaseous diffusionupon rehydration. However, calculated values of nodule diffusionresistance after the 24 h periods of rehydration did not indicateany recovery of gaseous diffusion resistance based on measurementsof the respiration coupled to nitrogenase activity. Hence, useof a diffusion analogue (i.e. Fick's Law) in conjunction withnodule respiratory CO₂ efflux was unable to predict changesin permeability of the variable barrier of legume nodules followingnodule dehydration and recovery. Key words: Subterranean clover, gaseous diffusion, respiration, carbohydrates, drought     

Nitrogen distribution index ofCajanus cajan L. during drought and rehydration

Relative competition among various plant parts for N during water stress,i.e. nitrogen distribution index (NDI) was determined in relation to specific nitrogenase activity (SNA) and nodule and soil nitrogen in both indeterminate (H-77-216) and determinate (ICPL-151) types of pigeonpea (Cajanus cajan L.) under greenhouse conditions. Two levels of water stress,i.e. moderate (soil Ψ_w) -0.77 MPa) and severe (soilΨ_w -1.34 MPa) were created by witholding the irrigation at vegetative (40 DAS) and flowering (70 DAS) stages. At vegetative stage under moderate stress the highest NDI was in nodules of cv. H-77-216 and in leaf of cv. ICPL-151. Under severe stress both the cultivars showed negative values of NDI, with maximum loss of N from root and nodules. Cultivar ICPL-151 behaved differently at flowering and vegetative stages. Very high loss of N from different plant parts was seen at flowering under severe stress. All the plant parts showed gain in N during rehydration. Loss and gain in N at both the stages under stress and rehydration respectively, correlated with available N in soil. Specific nitrogenase activity (SNA) and nodule N were maximum at moderate stress and related with NDI values of leaf and nodules.     

Nitrogen metabolism of soybeans: I. Effect of tungstate on nitrate utilization, nodulation, and growth

The effects of N source (6 mm nitrogen as NO₃⁻ or urea) and tungstate (0, 100, 200, 300, and 400 μm Na₂ WO₄) on nitrate metabolism, nodulation, and growth of soybean (Glycine max [L.] Merr.) plants were evaluated. Nitrate reductase activity and, to a lesser extent, NO₃⁻ content of leaf tissue decreased with the addition of tungstate to the nutrient growth medium. Concomitantly, nodule mass and acetylene reduction activity of NO₃⁻-grown plants increased with addition of tungstate to the nutrient solution. In contrast, nodule mass and acetylene reduction activity of urea-grown plants decreased with increased nutrient tungstate levels. The acetylene reduction activity of nodulated roots of NO₃⁻-grown plants was less than 10% of the activity of nodulated roots of urea-grown plants when no tungstate was added. At 300 and 400 μm tungstate levels, acetylene reduction activity of nodulated roots of NO₃⁻-grown plants exceeded the activity of comparable urea-grown plants.     

Aerial root nodules in the tropical legume,Pentaclethra macroloba

Summary Symbiotic nitrogen fixation in angiosperms normally occurs in buried root nodules and is severely inhibited in flooded soils. A few plant species, however, respond to flooding by forming nodules on stems, or, in one case, submerged roots with aerenchyma. We report here the novel occurrence of aerial rhizobial nodules attached to adventitious roots of the legume,Pentaclethra macroloba, in a lowland tropical rainforest swamp in Costa Rica. Swamp sapdings (1–10 cm diameter) support an average 12 g nodules dry weight per plant on roots 2–300 cm above water, and nodules remain in aerial positions at least 6 months. Collections from four swamp plants maintained linear activity rates (3–14 moles C₂H₄/g nodule dry weight/hr) throughout incubations for 6 and 13 hrs; excised nodule activity in most legumes declines after 1–2 hrs. Preliminary study of the anatomy and physiology suggest aerial nodules possess unusual features associated with tolerance to swamp conditions. High host tree abundance and nodulation in the swamp compared to upland sites indicate the aerial root symbiosis may contribute more fixed nitrogen to the local ecosystem than the more typical buried root symbiosis.     

Hydraulic Failure Defines the Recovery and Point of Death in Water-Stressed Conifers

This study combines existing hydraulic principles with recently developed methods for probing leaf hydraulic function to determine whether xylem physiology can explain the dynamic response of gas exchange both during drought and in the recovery phase after rewatering. Four conifer species from wet and dry forests were exposed to a range of water stresses by withholding water and then rewatering to observe the recovery process. During both phases midday transpiration and leaf water potential (Ψ_leaf) were monitored. Stomatal responses to Ψ_leaf were established for each species and these relationships used to evaluate whether the recovery of gas exchange after drought was limited by postembolism hydraulic repair in leaves. Furthermore, the timing of gas-exchange recovery was used to determine the maximum survivable water stress for each species and this index compared with data for both leaf and stem vulnerability to water-stress-induced dysfunction measured for each species. Recovery of gas exchange after water stress took between 1 and >100 d and during this period all species showed strong 1:1 conformity to a combined hydraulic-stomatal limitation model (r² = 0.70 across all plants). Gas-exchange recovery time showed two distinct phases, a rapid overnight recovery in plants stressed to <50% loss of leaf hydraulic conductance (K_leaf) and a highly Ψ_leaf-dependent phase in plants stressed to >50% loss of K_leaf. Maximum recoverable water stress (Ψ_min) corresponded to a 95% loss of K_leaf. Thus, we conclude that xylem hydraulics represents a direct limit to the drought tolerance of these conifer species.     

Carbon Balance of Sorghum Plants during Osmotic Adjustment to Water Stress

The daily (24-hour) carbon balances of whole sorghum plants (Sorghum bicolor L. Moench cv BTX616) were continuously measured throughout 15 days of water stress, followed by rewatering and 4 more days of measurements. The plants were grown under controlled environment conditions typical of warm, humid, sunny days. During the first 12 days, osmotic potentials decreased in parallel with decreased water potentials to maintain pressure potentials near 0.5 kilojoules per kilogram (5 bars). Immediately before rewatering on day 15, the water potential was −3.0 kilojoules per kilogram. Osmotic adjustment at this point was 1.0 kilojoules per kilogram, as measured by the decrease in the water potential at zero turgor from its initial value of −1.4 kilojoules per kilogram.

Gross input of carbon was less but the fraction retained was greater because a smaller fraction was lost through respiration in stressed plants than in unstressed plants. This was attributed to a lower rate of biomass synthesis, and conversely a higher rate of storage of photosynthate, due to inhibition of leaf expansion. The reduction in the cost associated with biomass synthesis more than balanced any metabolic cost of osmotic adjustment. The net daily gain of carbon was always positive in the stressed plants.

There was a large burst of respiration on rewatering, due to renewed synthesis of biomass from stored photosynthate. Over the next 3 days, osmotic adjustment was lost and the daily carbon balance returned to that typical of nonstressed plants. Thus, osmotic adjustment allowed the stressed plants to accumulate biomass carbon throughout the cycle, with little additional metabolic cost. Carbon stored during stress was immediately available for biomass synthesis on rewatering.

     

Rhizobins, a Group of Peptides in the Free-Amino-Acid Pool of the Soybean-Rhizobium System

Free-living Rhizobium (according to Bergey's Manual of Systematic Bacteriology, [1984, The Williams & Wilkins Co., Baltimore], Bradyrhizobium) japonicum was found to release a peptide into the nutrient media. Soybean nodules contained this peptide and exuded it into the soil. The name “rhizobin A” is suggested for this peptide. Nodules also contained another peptide, rhizobin B, as well as an unidentified, ninhydrin-positive compound, rhizobin C. The three peptides were confined to the free-amino-acid pool of the soluble fraction and eluted consecutively from a cation-exchange column. Rhizobin A was isolated in a highly purified form; its molecular mass was approximately 1,600 daltons as determined by Sephadex gel filtration and mass spectrometry. The amino-acid composition could be determined only approximately, because a long time was necessary for acid hydrolysis, possibly due to unusual linkages. The rhizobin concentration in soybean nodules continually increased during 50 days of growth, from 2 to approximately 400 μg/g (fresh weight). When combined nitrogen was added to nodulated soybean and subsequently removed, nitrogenase activity, nodulation, and nodule growth first decreased and then recovered. The relative amount of rhizobin A followed a similar pattern. Rhizobins were not detected in the roots, stems, and leaves of nodulated soybean plants. They were present in Lupinus nodules, but absent in alder nodules.     

Correlation between the Maintenance of Photosynthesis and in Situ Protoplast Volume at Low Water Potentials in Droughted Wheat

Studies were undertaken to examine the relationship between water deficit effects on photosynthesis and the extent of protoplast volume reduction which occurs in leaves at low water potential (Ψ_w). This relationship was monitored in two cultivars (`Condor' and `Capelle Desprez') of cultivated wheat (Triticum aestivum) that differed in sensitivity to drought, and in a wild relative of cultivated wheat (Triticum kotschyi) that has been previously found to be `drought resistant.' When subjected to periods of water stress, Condor and T. kotschyi plants underwent osmotic adjustment; Capelle plants did not. Photosynthetic capacity was maintained to different extents in the three genotypes as leaf Ψ_w declined during stress; Capelle plants were most severely affected. Calculations of internal leaf [CO₂] and stomatal conductance from gas exchange measurements indicated that differences in photosynthetic inhibition at low Ψ_w among the genotypes were primarily due to nonstomatal effects. The extent of protoplast volume reduction that occurred in leaves at low Ψ_w was also found to be different in the three genotypes; maintenance of protoplast volume and photosynthetic capacity in stressed plants of the genotypes appeared to be correlated. When the extent of water stress-induced inhibition of photosynthesis was plotted as a function of declining protoplast volume, this relationship appeared identical for the three genotypes. It was concluded that there is a correlative association between protoplast volume and photosynthetic capacity in leaves of wheat plants subjected to periods of water stress.     

Changes in water status and osmolyte contents in leaves and roots of olive plants (Olea europaea L.) subjected to water deficit

Two-year-old olive trees (Olea europaea L., cv. Coratina) were subjected to a 15-day period of water deficit, followed by 12 days of rewatering. Water deficit caused decreases in predawn leaf water potential (Ψ_w), relative water content and osmotic potential at full turgor (Ψ _π100) of leaves and roots, which were normally restored upon the subsequent rewatering. Extracts of leaves and roots of well-watered olive plants revealed that the most predominant sugars are mannitol and glucose, which account for more than 80% of non-structural carbohydrates and polyols. A marked increase in mannitol content occurred in tissues of water-stressed plants. During water deficit, the levels of glucose, sucrose and stachyose decreased in thin roots (with a diameter <1 mm), whereas medium roots (diameter of 1–5 mm) exhibited no differences. Inorganic cations largely contribute to Ψ _π100 and remained stable during the period of water deficit, except for the level of Ca²⁺, which increased of 25% in water-stressed plants. The amount of malate increased in both leaves and roots during the dry period, whereas citrate and oxalate decreased. Thin roots seem to be more sensitive to water deficit and its consequent effects, while medium roots present more reactivity and a higher osmotic adjustment. The results support the hypothesis that the observed decreases in Ψ_w and active osmotic adjustment in leaves and roots of water-stressed olive plants may be physiological responses to tolerate water deficit.     

Nonphotosynthetic CO(2) Fixation by Alfalfa (Medicago sativa L.) Roots and Nodules

The dependence of alfalfa (Medicago sativa L.) root and nodule nonphotosynthetic CO₂ fixation on the supply of currently produced photosynthate and nodule nitrogenase activity was examined at various times after phloem-girdling and exposure of nodules to Ar:O₂. Phloemgirdling was effected 20 hours and exposure to Ar:O₂ was effected 2 to 3 hours before initiation of experiments. Nodule and root CO₂ fixation rates of phloem-girdled plants were reduced to 38 and 50%, respectively, of those of control plants. Exposure to Ar:O₂ decreased nodule CO₂ fixation rates to 45%, respiration rates to 55%, and nitrogenase activities to 51% of those of the controls. The products of nodule CO₂ fixation were exported through the xylem to the shoot mainly as amino acids within 30 to 60 minutes after exposure to ¹⁴CO₂. In contrast to nodules, roots exported very little radioactivity, and most of the ¹⁴C was exported as organic acids. The nonphotosynthetic CO₂ fixation rate of roots and nodules averaged 26% of the gross respiration rate, i.e. the sum of net respiration and nonphotosynthetic CO₂ assimilation. Nodules fixed CO₂ at a rate 5.6 times that of roots, but since nodules comprised a small portion of root system mass, roots accounted for 76% of the nodulated root system CO₂ fixation. The results of this study showed that exposure of nodules to Ar:O₂ reduced nodule-specific respiration and nitrogenase activity by similar amounts, and that phloem-girdling significantly reduced nodule CO₂ fixation, nitrogenase activity, nodule-specific respiration, and transport of ¹⁴C photoassimilate to nodules. These results indicate that nodule CO₂ fixation in alfalfa is associated with N assimilation.     

Nitrate Reductase Activity in Maize (Zea mays L.) Leaves: II. Regulation by Nitrate Flux at Low Leaf Water Potential

Experiments were conducted to determine whether the nitrate flux to the leaves or the nitrate content of the leaves regulated the nitrate reductase activity (NRA) in leaves of intact maize (Zea mays L.) seedlings having low water potentials (ψ_w) when other environmental and endogenous factors were constant. In seedlings that were desiccated slowly, the nitrate flux, leaf nitrate content, and NRA decreased as ψ_w decreased. The decrease in nitrate flux was caused by a decrease in both the rate of transpiration and the rate of nitrate delivery to the transpiration stream. Upon rewatering, the recovery in NRA was correlated with the nitrate flux but not the leaf nitrate content.