Characterization of the Resistance to Oxygen Diffusion in Legume Nodules

A method for characterizing the resistance to oxygen diffusionin legume nodules has been developed. This is based on the assumptionsthat diffusion can be described using a simple resistance analogueand that the respiratory response of the bacteriod-containingcells to external oxygen concentrations can be analysed as adiffusion-limited process. Applying this analysis to experimentaldata from infact white clover plants allowed the total diffusionresistance to be separated into (a) a minimum resistance and(b) the extent to which this resistance can be increased. Whenthe carbohydrate status of the nodules was reduced by dark treatments,the minimum diffusion resistance increased, and after 24–28h darkness equalled the maximum resistance. At the same timethe ability to control this resistance was lost. White clover, nitrogen fixation, oxygen diffusion, nodule respiration     

Control of Nitrogen Fixation in a Legume Nodule: an Analysis of the Role of Oxygen Diffusion in Relation to Nodule Structure

A mathematical model of oxygen diffusion in a legume root noduleis constructed and validated. The required properties, functionsand possible locations of the major resistance to oxygen diffusionare investigated and support is given to the concept of a partwater-filled diffusion pathway. The model predicts that in theabsence of leghaemoglobin an infected cell would have oxygendamaged nitrogenase in bacteroids close to an air space andanaerobic conditions around those towards its centre. Leghaemoglobinis essential for maintaining a fairly uniform level of nitrogenaseactivity within the infected cells, but it cannot prevent damageat high oxygen concentrations. Nevertheless, the nitrogenaseactivity of a nodule can increase with increasing oxygen concentrationeven though some enzyme damage occurs. In the absence of a variablediffusion resistance, the oxygen tolerance of nitrogenase isrelated to carbohydrate supply and in a normal atmosphere nitrogenfixation is limited by oxygen diffusion. Oxygen, diffusion, nitrogen fixation     

Microclimate, Photosynthesis and Growth of Lucerne (Medicago sativa L.). II. Canopy Structure and Growth

The growth of lucerne var. Europe was examined in the fieldduring 1976. The annual dry matter production of unirrigatedlucerne during 1976, with no nitrogen fertilizer application,was 82.5 per cent greater than unirrigated S.24 perennial ryegrass,with a nitrogen application of 384 kg ha^–1. The mean aboveground growth rate of lucerne was 7.3 g DM m^–2 day^–1between March and early June, of which stem material contributeda maximum of 76.5 per cent. Significant losses of leaves andstems occurred from the end of April, indicating a loss of potentialforage material. Nitrogen analyses of the above ground crop suggested that in56 days lucerne yielded 10.7 per cent more nitrogen than didS.24 annually with a nitrogen fertilizer addition of 280 kgha^–1. Between 13 and 57 per cent of the daily photosynthate is translocatedbelow ground. Medicago sativaL, lucerne, dry matter production, canopy structure, nitrogen analyses     

Canopy Photosynthesis and Crop Growth Rate of Eight Temperate Forage Grasses

The rates of canopy and individual leaf photosynthesis, ratesof growth of shoots and roots, and the extinction coefficientfor light of eight temperate forage grasses were determinedin the field during early autumn. Canopy gross photosynthesiswas calculated as net photosynthesis plus dark respiration adjustedfor temperature using a Q₁₀ = 2. The relationships between canopygross photosynthesis and light intensity were hyperbolic, andthe initial slopes of these curves indicated that light wasbeing utilized efficiently at low light intensities. The initialslope depended on the distribution of light in the canopy andthe quantum efficiency of the individual leaves. The maximumrate of canopy gross photosynthesis reflected the maximum rateof individual leaf photosynthesis. Although the maximum rateof canopy gross photosynthesis was correlated with crop growthrate, there was no significant relationship between daily grossphotosynthesis and crop growth rate. Indeed, daily gross photosynthesisvaried by only 22 per cent, whereas the daily growth of shootsand roots varied by 120 per cent. This poor correlation is influencedby a negative correlation (P < 0.01) between the maximumrate of canopy gross photosynthesis and the initial slope ofthe curve relating canopy gross photosynthesis and light intensity.Difficulties in the interpretation of measurements of dark respirationappeared to confound attempts to relate daily net photosynthesisto crop growth rate, individual leaf photosynthesis, and theextinction coefficient for light.     

Measurements of Nitrogen Fixation (C2H2), Photosynthesis and Respiration Using an Open System in the Natural Environment

A flow-through technique has been used successfully to measureacetylene reduction by an acetylene-sensitive legume in thefield. The results suggest that nodules in the field do notexhibit an acetylene-linked decline, implying that they havea much larger diffusion resistance in the field than a controlledenvironment. There was good agreement between the maximum ratesof nitrogen fixation, estimated using the acetylene reductiontechnique, and the maximum rates of nitrogen accumulation inthe biomass above ground. This suggests that the acetylene techniquecan give an accurate measurement of nitrogenase activity overtime periods of the order of hours and a useful daily measureof biological nitrogen fixation in the field; rates of nitrogenfixation below a depth of 0.2 m were negligible. When measurements of carbon dioxide were made by passing airupwards through the soil, steady state conditions were not achievedfor several hours because of the diffusive flux from the surroundingsoil. It is suggested that the unwanted inward diffusive fluxcan be prevented by ‘air-sealing’ the soil enclosure. Nitrogen fixation, acetylene reduction, carbon dioxide, diffusion resistance     

The Development and Use of a Flow-through Apparatus for Measuring Nitrogenase Activity and Photosynthesis in Field Crops

A flow-through apparatus for measuring nitrogenase activityin field-grown crops is described. The apparatus was used tomeasure nitrogenase activity in a sainfoin crop throughout agrowth season. A marked seasonal variation was observed whichwas partly the consequence of defoliation. The acetylene-basedestimate of the nitrogen fixed annually by sainfoin was less(146 kg N ha^–1) than the nitrogen accumulated in the shoots.Reasons for this difference are discussed with special emphasison the validity of the technique and alternative sources ofnitrogen. There were no treatment differences between the ratesof growth of either sainfoin or lucerne when grown with or withoutfertilizer nitrogen in the field. This result is in marked contrastwith results from a glasshouse experiment where sainfoin plantsutilizing nitrate-nitrogen exhibited faster growth rates thansimilar plants that were solely dependent on symbiotic nitrogenfixation. Key words: Nitrogenase, photosynthesis, field-technique, sainfoin, lucerne     

Microclimate, Photosynthesis and Growth of Lucerne (Medicago sativa L.). I. Microclimate and Photosynthesis

Measurements of microclimate and photosynthesis of lucerne var.Europe were made in the field during the spring of 1976. Themaximum rate of canopy gross photosynthesis (14.3 g CO₂ m^–2h^–1, I = ) was 2.5 times greater than that of S 24 perennialryegrass at the same LAI. This difference was due to differencesin individual leaf photosynthesis. The photosynthetic rate ofthe youngest fully expanded leaf of lucerne remained constantthroughout the experimental period at 3.6 g CO₂ m^–2 h^–1(300 W m^–2). Measurements of soil water potential profiles indicated thatlucerne extracted water from the soil to a depth of at least800 mm, with a region of maximum uptake between 400 and 600mm. This capability, with a moderate mean leaf resistance of460 s m^–1, conferred a high assimilation efficiency onlucerne, with a mean water use efficiency of 34 g H₂O lost pergram of carbohydrate assimilated, compared with 200 g H₂O pergram of carbohydrate for S 24. Medicago sativa L, lucerne, photosynthesis, assimilation efficiency     

The Influence of Water Stress on the Photosynthesis of a Simulated Sward of Perennial Ryegrass

The rate of canopy photosynthesis, single leaf photosynthesis,leaf resistance to gaseous exchange, and leaf water potentialof simulated swards of perennial ryegrass (Lolium perenne cv.S24) in a controlled environment, were determined during a periodof increasing water stress and recovery from that stress. Canopyphotosynthesis did not decline immediately water was withheldbut continued at an undiminished rate for several days; thereafterit fell rapidly, particularly at first. As water stress increasedsuccessive relationships between canopy photosynthesis and irradiancebecame more curved, indicating that the effect of water stressincreased with increasing irradiance. After the swards werere-watered canopy photosynthesis rose, most rapidly during thefirst 24 h. In general, the pattern of change of leaf waterpotential was similar to that of canopy photosynthesis, althougha more detailed examination of this relationship showed it tobe hysteresial; in particular, the fall in leaf water potentialpreceded that of canopy photosynthesis. Single leaf photosynthesisappeared to be the main agent through which water stress influencedcanopy photosynthesis although in the more severely stressedswards (leaf water potentials of about—15 bars) some leaftissue died and so limited the recovery of canopy photosynthesis.The leaf resistance to gaseous diffusion increased with increasingwater stress, as did the CO₂ compensation point, thereby influencingsingle-leaf photosynthesis and through it canopy photosynthesis.     

The Use of a Model to Investigate the Influence of some Environmental Factors on the Growth of Perennial Ryegrass

A modelof thegrasscrop isconstructed, inahichthe simulated valuesof crop morphology and physiology adapt automatically to changesin the environment. The model predicts photosynthesis, partitioningof assimilates and the growth or the cropin terms of leaves,stems and rmts from environmental data and the initial weightsof the crop components following defoliation. The model is usedto investigate the separate effects of light, temperature, canopystructure and maintenance respiration on the growth and denlopmcnt of the crop. It demonstrates how the crop attempts to optimizeaboveground growth, and the complexity of the interaction betweenthe physiological and environmental factors controlling growth. Lolium perenne, perennial ryegrass, photosynthesis, partition of assimilates, mathematical model