The Effect of Sulphur Dioxide on the Growth of Lolium perenne L., Lolium multiflorum Lam., Dactylis glomerata L., and Phleum pratense L.

Lockyer, D. R. 1985. The effect of sulphur dioxide on the growthof Lolium perenne L., Lolium multiflorum Lam., Dactylis glomerataL., and Phleum pratense L.?J. exp. Bot. 36: 1851-1859. Fouragriculturally important grasses, Lolium perenne L., Loliummultiflorum Lam., Dactylis glomerata L. and Phleum pratenseL. were exposed to sulphur dioxide (SO₂) in a system of exposurechambers. The plants were exposed for a total of 43 d to meanconcentrations of SO₂ in the air of 0,87 or 448 (µg m^–3and herbage was harvested twice. All four grasses showed chloroticlesions after exposure to the highest concentration of SO₂.The effect of SO₂ on the yield of herbage was statisticallysignificant only at the second harvest and at the highest concentration;the dry weights of shoots of D. glomerata and L. perenne werereduced by 33% and 16% respectively. Significant effects ofSO₂ were also found on the 'transpiration coefficients' measuredfor D. glomerata and P. pratense. The grasses differed in theiruptake of sulphur from the atmosphere but this was not relatedto their sensitivity to SO₂. Total–S concentration inthe shoots of L. perenne, L. multiflorum and D. glomerata increasedalmost linearly in response to increasing SO₂ concentration;with P. pratense only the highest SO₂ concentration raised total-Sabove the level in control plants. These increases were almostentirely due to the accumulation of sulphate–S. Key words: Sulphur diozide, Lolium perenne, Lolium multiflorum, Dactylis glomerata, Phleum pratense     

Selection and symbiotic properties ofRhizobium leguminosarum biovarphaseoli strains harboring pRtr5a

TheRhizobium leguminosarum biovartrifolii symbiotic plasmid pRtr5a has been transferred toR. leguminosarum biovarphaseoli RCR 3644-S1. The transconjugant selection had been done byTrifolium pratense plants. All transconjugants lacked the resident pSym, but had complete pRtr5a, and were Fix⁺ onT. repens andT. alexandrinum, Fix^– onT. subterraneum, and formed a few small white and Fix^– nodules onPhaseolus vulgaris. It is shown that this nodulation onP. vulgaris is due to pRtr5a. The presence of pRtr5a and/or the passage throughTrifolium pratense nodules provoke(s) the recipient strain symbiotic plasmid loss.     

A Comparative Study on the Effects of H2S and SO2 Fumigation on the Growth and Accumulation of Sulphate and Sulphydryl Compounds in Trifolium pratense L., Glycine max Merr. and Phaseolus vulgaris L.

Maas, F. M., De Kok, L. J., Peters, J. L. and Kuiper, P. J.C. 1987. A comparative study on the effects of H₂S and SO₂ fumigationon the growth and accumulation of sulphate and sulphydryl compoundsin Trifolium pratense L., Glycine max Merr. and Phaseolus vulgarisL.—J. exp. Bot. 38: 1459-1469. The effects of 0—25 mm³ dm³ H₂S and SO₂ on growth andsulphur content of shoots of Trifolium pratense, Glycine maxand Phaseolus vulgaris were studied. After 2 weeks of fumigationthe yield of T. pralense was reduced by 32% by H₂S, but notaffected by SO₂. Yield of G. max was not affected by H₂S, butreduced by 20% by SO₂, whereas that of P. vulgaris was increasedby 11% by H₂S and not affected by SO₂. Increases in sulphydrylcontent were already observed after 24 h of exposure to H₂Sand SO₂ in all plants. The increase was greatest in T. pratenseand smallest in P. vulgaris and, except for T. pratense, alwaysgreater in the H₂S-exposed than the SO₂-exposed plants. Oneday of exposure resulted in an increase in sulphate contentonly in the SO₂-fumigated plants, with the highest accumulationin T. pratense and the lowest in P. vulgaris. After 2 weeksan increase in sulphate content was also observed in the H₂S-exposedplants. This increase was also highest in T. pratense and lowestin P. vulgaris. Transpiration rate was not affected by a 24 h exposure to H₂Sor SO₂ and was highest in T. pratense, intermediate in G. maxand lowest in P. vulgaris. The order of theoretical rates of deposition of H₂S and SO₂correlated with the observed increases in sulphydryl contentduring the first 24 h of exposure in both H₂S and SO₂-fumigatedplants and with the increase in sulphate content in the SO₂-exposedplants. The increases in sulphydryl content were only 8% ofthe theoretical H₂S and SO₂-deposition fluxes, whereas sulphateaccumulation accounted for at least 57% of the theoretical SO₂-depositionflux. Key words: Air pollution, clover, French bean, Glutathione, Soybean, sulphur metabolism.     

Physiological and Ecological Studies in the Analysis of Plant Environment: XI. A Further Assessment of the Influence of Shading on the Growth of Different Species in the Vegetative Phase

In a further analysis of the effects of varying light intensityon growth and development in the vegetative phase the reactionsof thirteen herbaceous species have been recorded. In some experimentsthe degree of shading has been extended to 0.055 daylight, alevel near or below the compensation point. For Lathyrus maritimus, Trifolium pratense, and Vicia faba,the net assimilation rate is directly related to the logarithmof the light intensity, but for Helianthus annuus, T. repent,T. hybridum, Medicago sativa, Phaseolus multiflorus, and Loliummultiflorum the relationship, though curvilinear, is not logarithmic.It is concluded that for all species the assimilation rate ofunshaded plants was limited by light even though in high summerthe recorded light energy between 4,000–7,000 A averaged1,900–2,200 foot-candles. For all these species between daylight and 0.12 daylight theleaf-area ratio rises as the intensity decreases and in generalthe trend is logarithmic. When the degree of shading is increasedto 0.055 daylight the logarithmic relationship still holds forL. maritimus and V. faba though this level is below the compensationpoint. For other species, such as P. multiflorus andH. annuusthe trend may be reversed below 0.12 daylight and the ratiothen falls. When the light intensity is reduced from daylight to 0.5 daylight,then for the species already cited and for Lolium perenne, Phleumpratcnse, and Festuca pratensis the relative growth-rate isinvariably depressed. At 0.055 daylight the relative growth-ratenever exceeded 1 per cent. per day. For L. perennet, P. pratense,and Dactylis glomerata the reactions to shading of ‘hay’and ‘grazing’ strains were different. The ecological and physiological implications of these findingsare discussed.     

The Influence of Foliar Applications of Urea on the Nodulation Pattern of Certain Leguminous Species

In greenhouse experiments with seven species of legumes, thespraying of urea on to the leaves was shown to affect nodulationadversely, without impairing the growth of the plants. In Phaseolusvulgaris, Vicia sativa, and Pisum sativum three-times-weeklysprays of 1 per cent. aqueous urea either prevented or markedlyreduced nodule development during the 8-week experimental period.In Medicago sativa and Trifolium pratense the urea treatmentresulted in a delay in nodulation so that numbers of nodulesat the first sampling (4 weeks) were reduced, while numbersat later samplings were higher since nodulation had been delayeduntil the root system was larger and provided a greater numberof potential nodule sites. In Trifolium hybridum and T. repensthe urea-treated plants showed reduced nodulation throughoutthe 6-week experimental period. In these experiments the advance effects on nodulation cannotbe due to high concentration of combined nitrogen in the rootingmedium, but it is suggested they derive from a high level ofnitrogen within the plant.     

Comparisons of the Respiratory Effluxes of Nodules and Roots in Six Temperate Legumes

The specific respiration rates of nodulated root systems, ofnodules and of roots were determined during active nitrogenfixation in soya bean, navy bean, pea, lucerne, red clover andwhite clover, by measurements on whole plants before and afterthe removal of nodule populations. Similar measurements weremade on comparable populations of the six legumes, lacking nodulesbut receiving abundant nitrate-nitrogen, to determine the specificrespiration of their roots. All plants were grown in a controlled-environmentclimate which fostered rapid growth. The specific respiration rates of nodulated root systems ofthe three grain and three forage legumes during a 7–14-dayperiod of vegetative growth varied between 10 and 17 mg CO₂g^–1 (dry weight) h^–1. This mean value consistedof two components: a specific root respiration rate of 6–9mg CO₂ g^–1 h^–1 and a specific nodule respirationrate of 22–46 mg CO₂ g^–1 h^–1. Nodule respirationaccounted for 42–70 per cent of nodulated root respiration;nodule weight accounted for 12–40 per cent of nodulatedroot weight. The specific respiration rates of roots lackingnodules and utilizing nitrate nitrogen were generally 20–30per cent greater than the equivalent rates of roots from nodulatedplants. The measured respiratory effluxes are discussed in thecontext of nitrogen nitrogen fixation, nitrate assimilation. Glycine max, Phaseolus vulgaris, Pisum sativum, Medicago sativa, Trifolium pratense, Trifolium repens, soya bean, navy bean, pea, lucerne, red clover, white clover, nodule respiration, root respiration, fixation, nitrate assimilation     

The Influence of Genes ar and n on Senescence in Pisum sativum L.

The effect of genes ar (violet flowers, small hilum) and n (thick,fleshy pod wall) on whole plant in senescence peas was examinedby comparing Ar- with arar and N- with nn plants in segregatingprogenies. Homozygosity for ar or n significantly delayed the time whenthe plants were ready for harvest of their entire seed crop.These genes did not delay either the onset of reproduction orthe onset of apical arrest in the first instance. However, whereasAr- N- plants almost invariably senesced and died as the firstseed crop matured, the majority of arar and/or nn plants entereda period of secondary growth and a further fruiting cycle. Comparedwith Ar- plants, arar plants had over twice as many pods andseeds but individual seeds were 58 per cent lighter and totalseed yield (wt) was 19 per cent less. Pod length was unaffected.Compared with N-plants, nn plants had shorter pods (16 per cent),fewer seeds per pod (21 per cent), smaller seeds (20 per cent)and a lower total seed yield (wt 14 per cent less). It appearsthat ar and n impose a lower metabolic drain per reproductivenode as a consequence of their effects on hilum anatomy andpod morphology, respectively. These mutants disrupt the normalpattern of monocarpic senescence by breaking the coordinationbetween apical arrest and subsequent events. The developingseed crop delimited by the first arrest fails to cause plantdeath, possibly because sink size is less than in normal counterparts. Pisum sativum L, garden pea, senescence, hilum, pod, seed size, genetics     

Response of Self-thinning to Artificially Reduced Levels of Leaf Area in Monocultures of Trifolium pratense

Greenhouse grown monocultures of Trifolium pratense L, werepartially defoliated to test the hypothesis that the constantof proportionality (K) in the self-thinning rule is a functionof leaf area. This constant equates mean weight (w) and density(p) in self-thinning populations by the equation Pruning leaflets did not affect the pattern or occurrence ofdensity-related mortality but, as predicted, did affect K, whichwas reduced by 25 per cent as leaf area was decreased from anaverage of 7.3 to 3.9 m²m^–2. For both self-thinning andnon-self-thinning populations, leaf area was substituted forK in eqn (1) to give Multiple linear regression showed that this expression was significantfor all three defoliation treatments. Regressions with treedata grouped by genera were also significant and indicate thateqn (2) may be a more general expression of the relation betweenmean weight and density in even-aged monocultures. The self-thinningrule may be a special case of eqn (2) which expresses itselfwhen leaf area attains some upper limit. Trifolium pratense, red clover, leaf area, self-thinning, defoliation     

Cell Cycle Responses to Red or Far-red Light, or Darkness, in the Shoot Apex of Silene coeli-rosa L. During Floral Induction

Twenty-eight-day-old plants of Silene coeli-rosa L. were maintainedin short days (SD) for 9 d (0–8) or exposed to 7 longdays (LD), or 7 SD with a 5 min exposure at 1700 h of each dayto far-red (FR), red (R) or 5 min FR/5 min R, or 7 dark-interrupted(di = 1700–1720 h) LD. Treatments were followed by twofurther SD. The mitotic index and G1 and G2 proportions weremeasured in the shoot apices of plants sampled at 2000 h ofeach day of each replicated treatment. Exposure to 7 LD (= 100per cent flowering) resulted in significant increases, relativeto the SD controls, in both the G2 proportion and the mitoticindex on d 0 to 3, 7 and 8. Five minute FR (= 0 per cent flowering)resulted in cell cycle responses similar to those in LD onlyfrom d 0 to 2. R and FR/R (both = 0 per cent flowering) didnot result in any increases in the G2 proportion in the apexapart from d 3 of FR/R. However 5 min FR/5 min R, and to a lesserextent 5 min R, did result in significant increases in the mitoticindex on d 0, 1, 7, and 8. diLD (= 8–10 per cent flowering)also prevented any significant increases in the G2 proportionon d 0 to 3, and 5 to 8 but the mitotic index was again higheron these days compared with control data. Thus the transitionto floral growth for 90 per cent of the plants is associatedwith changes in the cell cycle in the shoot apex measured asincreases in the G2 proportion at 2000 h of LD 0 to 3 and 7to 8. Silene coeli-rosa L., cell cycle, flowering, phytochrome, shoot apex     

Rhizoplane colonization of pea seedlings by Rhizobium leguminosarum and a deleterious root colonizing Pseudomonas sp. and effects on plant growth

Some pseudomonads produce a toxin that specifically inhibits winter wheat (Triticum aestivum L.) root growth and the growth of several microorganisms. The toxin does not inhibit pea (Pisum sativum) root growth, but the organisms are aggressive root colonizers and their effect on Rhizobium leguminosarum growth, colonization, and nodulation of peas was not known. Peas were grown in Leonard jars in the greenhouse. Pea roots were inoculated with R. leguminosarum, a toxin-producing Pseudomonas sp., both, or neither (control). The Pseudomonas sp. colonized pea roots more rapidly and in greater number than R. leguminosarum after ten days. In the presence of the Pseudomonas sp., the R. leguminosarum population on the rhizoplane was less at ten days. When the roots were inoculated with both R. leguminosarum and Pseudomonas sp., the number of nodules were greater than when R. leguminosarum was inoculated alone, but nodule dry weight and pea shoot biomass were similar to plants inoculated with only R. leguminosarum. Although these results need confirmation with non-sterile soil and field studies, these preliminary results indicate that peas will not be affected by wheat root-inhibitory rhizobacteria.     

Vegetable Plant Part Relationships. III. Modification of Carrot (Daucus carota L.) Root and Shoot Weights by Gibberellic Acid and Daminozide

Aqueous foliar sprays of N-dimethylaminosuccinamic acid (daminozide)at 2000 p.p.m. and gibberellic acid (GA) at 100 p.p.m. wereapplied 45, 59, 82 and 100 days after sowing to Chantenay carrotswith population densities of 244, 495 and 883 plants m^–2.The plants were harvested on ten approximately weekly occasions;fresh weights were determined and d. wt estimates were obtainedfor the separated shoots (s) and roots (r). Allometric linearregressions of the logarithm of s on that of r at each harvestseparately, clearly showed that GA always increased shoot: rootratio and reduced root yield (by approximately 35 per cent)but could sometimes also increase whole-plant weight. Daminozideincreased root yield (by approximately 7 per cent from 80 tonnesha^–1) and tended to have effects opposite to those ofGA. Daucus carota L., carrot, root weight, shoot weight, N-dimethylaminosuccinamic acid (daminozide), gibberellic acid     

Population level processes in Rhizobium leguminosarum bv. trifolii: the role of founder effects

The importance of genotype-specific selection between host and symbiont, founder effect, and clonal reproduction in Rhizobia leguminosarum biovar trifolii populations is relatively unknown. A field experiment was conducted to sample 1268 isolates of R. l. bv. trifolii from four genotypically distinct Trifolium pratense plants for allozyme variation at nine loci. Genetic and genotypic variation, population genetic substructure, and linkage disequilibrium were estimated. Of the 1268 isolates 188 genotypically distinct strains (electrophoretic types or ETs) were identified with an average of 11.04 different ETs per plant. Total genetic diversity in the plot was 0.346 and most of the variation was found within plants (= 80%). Our data suggests that genotype-specific selection between the rhizobia and the four host-plant genotypes tested does not influence local population structure, but evidence of founder effect was present. Significant linkage disequilibrium was observed and is most likely due to the clonal reproduction of R. l. bv. trifolii.     

Silica and Ash in Tissues of Some Coastal Plants

Ash and silica contents and their depositional patterns in differenttissues of 44 Mississippi coastal plants were determined. Silicacontent of dried plants varied from no more than a trace inChenopodium album L. leaves to 7.37 per cent in Zizanopsis miliacea(Michx) Doell & Aschers leaves. Ash content varied from2.50 per cent in Lythrum lineare L. stems to 28.24 per centin Borrichia frutescens (L.) DC leaves. Plants in the same familytend to be alike in their ability to absorb or not absorb silica.Poaceae and Cyperaceae had consistently high concentrationsof silica. In contrast, the Asteraceae studied had very lowsilica contents but often had high contents of other minerals.Dicotyledonous plants studied had consistently lower silicacontents than the monocotyledons. Plants growing in salt watercontained considerable sodium chloride. Spectra were obtainedfor major elements in four different plants. Energy-dispersiveX-ray analysis shows that distribution of the element siliconis clearly related to certain epidermal structures such as guardcells, ridges, dumb-bells and balls that appear in electronmicrographs. Silica was deposited differently in each type ofplant studied. In many of the plants silica was deposited inrows of irregular-shaped particles running lengthwise of theleaf and in guard cells. In others, like Zizanopsis miliacea(Michx) Doell & Aschers, the deposit was sheet-like. Zizaniaaquatica L. not only had a sheet-like deposit, but the depositwas ridged and there were rows of dumb-bell-shaped silica cells.Related plants had similar structures. Euchlaena mexicana Schrad.,Tripsacum dactyloides (L.) L and Manisuris rugosa (Nutt.) Kuntzeall had irregular phytoliths similar to those in Zea mays L. coastal plants, marsh plants, ash content, silica deposition, scanning electron microscopy, energy-dispersive X-ray analysis, silicon distribution, X-ray diffraction patterns, spectra of elements in plants     

The Effects of Nitrogen Supply on the Absorption and Distribution of Copper in Red Clover (Trifolium pratense L.) Grown in Flowing Solution Culture with a Low, Maintained Concentration of Copper

The absorption and distribution of Cu in red clover (Trifoliumpratense L.) were measured in plants grown in flowing solutionculture with Cu maintained throughout at 0.5 µg 1^–1and N supplied either as nitrate or through symbiotic fixation.Although there was a decrease in Cu absorption, both with time,and with a depleted nitrate supply, it increased to its formerrate when nitrate was adjusted to 10 mg N 1^–1 after aperiod of depletion. Differences in absorption between plantsdependent upon fixation and those supplied with nitrate wererelated to the slower initial growth of the plants fixing N.Considerable proportions (> 30 per cent) of the absorbedCu were retained by the roots. At the final harvest, and withthe exception of plants grown with nitrate adjusted to 0.1 mgN 1^–1 after a period of depletion, the proportion of theCu retained was related to the concentration of N in the roots.The different N treatments produced differences in Cu concentrationin the shoots, and the effects were greater in the youngestfully expanded leaves than in older leaves. Trifolium pratense L., red clover, absorption, copper, flowing solution culture, nitrogen     

Accumulation and Distribution of Nutrients in Fruits of Castor Bean (Ricinus communis L.)

The nutrition of developing fruits of Ricinus communis was studiednear Perth, Western Australia, where the species grows as aweed on poor sandy soil. Fruits required 60 days to mature anddehydration of the capsule began 20 days before the seeds ripened.Mature seeds accumulated 49 per cent of the fruit dry matterand over 80 per cent of its P, Zn and Cu, 50–80 per centof its Mg, N, Fe and Mn, 41–46 per cent of its S and Caand 11–21 per cent of its K and Na. Losses of nutrientsfrom capsules during fruit ripening were: Zn, 73 per cent, P,42 per cent, Cu, 23 per cent and Mn, 8 per cent. Dry matter,N, K, S, Ca, Mg, Na and Fe were not withdrawn from capsules.Apparent retranslocation from capsules could have provided from6–28 per cent of the Zn, Mn, P and Cu in mature seeds.Seeds from plants on poor sandy soil were small but had adequatelevels of nutrients when compared with those from plants growingon a fertile loam. Concentrations of all nutrients except P were higher in youngcapsules than in young seeds, but levels of N, P, Mg, Fe, Znand Cu were higher in mature seeds than in mature capsules.The intake of most nutrients by fruits was out of phase withdry matter accumulation, especially in capsules, and the elementsappeared to accumulate in fruit parts independently of eachother. Glutamine accounted for over 85 per cent of the amino-Nin phloem sap destined for fruits. Potassium made up over 90per cent of the inorganic cations in phloem exudate. Of theminor elements in the exudate, Fe was present at highest concentrationand Cu at the lowest. The results showed that retranslocation from the capsule madea very small contribution to the nutrition of seeds. It is suggestedthat R. communis would require a sustained supply of soil nutrientsto ensure maximum seed yield, partly due to the restricted retranslocationof most nutrients from capsules. Ricinus communis L., castor bean, mineral nutrition, translocation, retranslocation     

The Effect of Gibberellic Acid on Fibre-cell Length

Measurements were made of fibre-cells from plants of Corchorusolitorius L., Hibiscus cannabinus L., and Cannabis sativa L.Which had been sprayed with gibberellic-acid solution. Fibre-cellsfrom treated plants showed a highly significant increase inlength, 20–130 per cent, for the whole stem and as muchas 400 per cent. for a single intermode. Gibberellic acid increased the variation in cell-length andthe positive skewness of the distribution of the variate. Differences in cell-length can be related to the developmentalsequence of the shoot and the variation in internode-length.     

Water Relations of Red Clover Trifolium pratense L. as Affected by VA Mycorrhizal Infection and Phosphorus Supply Before and During Drought

Red clover Trifolium pratense L. plants were grown in a factorialdesign with four levels of added P and with and without a mycorrhizalinoculum, to test the separate effects of P nutrition and infectionon plant water relations. Under well-watered conditions, onlyuninfected plants on very low P soil showed reduced stomatalconductance and these had the lowest leaf P concentrations.During droughting, only plants with very high leaf P concentrationsmaintained high conductance. There was no evidence of increasedwater uptake by mycorrhizal plants. This and other evidencesuggests that mycorrhizal effects on water relations are secondaryconsequences of changes in P nutrition which are, in any case,inconsistent. Key words: Trifolium pratense, VAM, water relations, concentration, drought     

The Function of the Hilum in Some Papilionaceae in Relation to the Ripening of the Seed and the Permeability of the Testa

In seeds of Trifolium repens, T. pratense, and Lupinus arboreus,the hilum is a hygroscopically activated valve in the impermeableepidermis of the testa. When relative humidity was low the fissurein the hilum opened permitting the seed to dry out; when therelative humidity was high the fissure closed obstructing theabsorption of moisture. During seed-ripening the moisture contentfell readily to approximately 25 per cent., and thereafter moreslowly until the epidermis became impermeable at approximately14 per cent, moisture content. Further drying of the seed tookplace only by diffusion of water vapour through the hilum. ‘Hard’seeds tended to have a moisture content in equilibrium withthe lowest relative humidity to which they had been exposed.They absorbed moisture under conditions of gradually increasingrelative humidity such that the hilar fissure remained open.The duration of the impermeable condition increased with thedegree of desiccation brought about by loss of water throughthe hilum.     

Studies in the Growth of Young of Pinus sylvestris L.: I. The Annual Cycle of Assimilation and Growth

The annual cycle of growth in plants of P. sylvestris L. varyingin age from 2 to 5 years was studied mainly by following theirweight changes through periodic sampling. Immediately followingbud-break there was a period of about 1 month in which, althoughmeristematic and extension growth were rapid in both shoot androot, there was no gain in weight by the plant. There was evidencethat the needles remaining from previous years contained a considerableamount of storage material which was utilized at this time.During May and June the main increase in weight was in the newshoots, but in August and September assimilates were more evenlydivided between the new shoots on the one hand, and the oldstem and the root on the other. The average relative growth-ratesfor the months May to September were between 1·0 and0·4 per cent, per day in plants growing in favourableconditions, the lower rates being found in older plants. Duringthe months October to March slow increase in weight occurredin needles, stem, and root, and the R.G.R.s in two experimentswere 0·21 and 0·15 per cent, per day. The netassimilation rate fell with increasing age and in the summerof the fifth year was 0·06g./g./week or 0·10g./dm.²/week.This low value of N.A.R. is discussed in relation to the knownhigh rate of production of P. sylvestris stands and their evergreenhabit.     

Determination of Nitrate Reductase Activity in Barley Leaves and Roots

The inactivation of nitrate reductase in the leaves and rootsof barley (Hordeum vulgare L. cv. Mazurka) during and afterextracting was investigated. At 0 °C in the absence of casein,25 per cent of ‘total’. i.e. maximal in vitro, nitratereductase activity was lost during the 2 min extraction process,followed by a slower loss of activity while the extract wasstored in ice. Activity was maintained by adding a minimum of1 per cent casein to the extraction medium containing 0·1M phosphate (pH 7·5), 1 mM EDTA and 1 mM dithiothreitol.Nitrate reductase was stable for several hours in these extracts,but declined in a first order manner in the absence of dithiothreitol.Casein also prevented the initial loss while making root extracts,but had less effect during storage. Using casein and thiols, nitrate reductase activity in light,(as product of maximal in vitro rates and wt g^–1) in leaveswas 98 per cent of the total activity in 31-day-old plants grownwith full nutrient in water culture and 60-day-old field-grownplants receiving no fertilizer. Field-grown plants, however,exhibited only 17 per cent of the activity of culture-grownplants. Nitrate reductase in leaves of barley plants grown in waterculture had a diurnal rhythm. During the first 3 h of the lightperiod, activity increased to 1·3 x the ‘dark’value. This was followed by a temporary decrease and then byanother increase to a maximum of 1·7 x the ‘dark’value, occurring about 8 h after illumination. Activity thendecreased during the rest of the light period and in darkness. Hordeum vulgare L., barley, nitrate reductase