The Effect of Defoliation on Carbohydrate, Protein and Leghaemoglobin Content of White Clover Nodules

Single white clover plants grown in pots of Perlite in a controlledenvironment and completely dependent on N₂ fixation were defoliatedto various degrees (46–85 per cent of shoot weight removed).The soluble protein content of nodules declined by about 20per cent and leghaemoglobin content by 50 per cent in the first4–7 d after defoliation but increased again to controllevels as new leaf tissue appeared. In the short term (2–3h) carbohydrate content of nodules declined to different extentsdepending on the severity of defoliation. The initial declinein N₂ fixation and the respiration associated with it, appearednot to be related to the instantaneous carbohydrate contentof nodules but rather to the supply of current photosynthatefrom the shoot. After 24–48 h, however, the carbohydratecontent of nodules had declined to low levels, regardless ofthe severity (46 or 71 per cent shoot removed) of defoliation.As new leaf tissue appeared carbohydrate levels in all partsof the plant gradually recovered towards control levels. Microscopic examination of nodule sections indicated that onlyafter very severe defoliation (80–85 per cent shoot removed)was nodule deterioration evident. Even here, as the plant establishednew leaves, the damage to nodules was repaired and no noduleloss was apparent. Trifolium repens, white clover, defoliation, carbohydrate, protein, leghaemoglobin     

Diurnal Production and Utilization of Photosynthate in Nodulated White Clover

Gordon, A. J., Mitchell, D. F., Ryle, G. J. A. and Powell, C.E. 1987. Diurnal production and utilization of photosynthatein nodulated white clover.—J. exp. Bot. 38: 84–98. A steady-state ¹⁴C-labelling technique was used to examine thediurnal carbon fixation, storage and export characteristicsof white clover leaves. Approximately 70% of fixed carbon wasexported to other organs during the photoperiod. The remainingcarbon was stored mainly as starch (80% at the end of the photoperiod)with smaller amounts of sucrose, hexoses and charged compounds.Carbon export from the leaf at night was provided by remobilizationof starch. During the photoperiod it was estimated that c.60% of carbonexported from the leaf was directed towards the nodulated root;45% to nodules and 15% to roots. The 40% directed towards theshoot was supplemented by a further 11% of carbon (in the formof amides) re-exported from the nodules. During the photoperiod, all organs of the plant accumulatedcarbohydrate which was available for use during darkness, inconjunction with a diminished supply of exported carbon fromleaves. Nodules exhibited a striking pattern of carbohydratestorage and depletion. The levels of sucrose and starch in thenodules at the end of the photoperiod were sufficient to maintainN₂ fixation for 8–9 h of the 12 h dark period. We proposethat continued import from leaves provided the additional sucrosenecessary to support undiminished nodule function throughoutthe entire dark period. Key words: White clover, photosynthate, starch, carbohydrate, nodules, N₂ fixation     

Synthetic and Maintenance Respiratory Losses of 14CO2 in Uniculm Barley and Maize

The respiratory losses of ¹⁴CO₂ from whole plants of uniculmbarley and maize were measured following exposure of the wholeplant or a single leaf to air containing a uniform specificactivity of ¹⁴CO₂ for 30–60 min during normal photosynthesis.The total respiratory efflux of ¹⁴CO₂ could be described interms of two main components: an intense efflux characterizedby a half-life of 4–8 h, which was identified with thebiosynthesis of new tissue in meristems; and a much less intenseefflux characterized by a half-life of 26–120 h, whichwas primarily identified with the maintenance of metabolic activity.The (bio)synthetic efflux of ¹⁴CO₂ totalled between 25 and 35per cent of the labelled assimilate (¹⁴C: ¹⁴C), and was generallyinsensitive to temperature and light intensity. The maintenanceefflux totalled between 12 and 27 per cent of the labelled assimilateor its derivatives: the total generally increased with hightemperature and low light intensity. The rate of the maintenanceefflux showed a normal temperature response (Q102). It is concludedthat the efficiency of conversion of assimilate into new growthis unlikely to exceed 65 per cent in the long term, and willgenerally be less.     

White Clover N2-Fixation in Response to Root Temperature and Nitrate: II. N2-FIXATION, RESPIRATION, AND NITRATE REDUCTASE ACTIVITY

Established, nodulated white clover plants were transferredto eight tanks of a flowing culture apparatus with solutiontemperatures of 5, 11, 17, and 25 ?C (two tanks per temperature).Shoot temperature and light environment were common to all plants.After 7 d, (10 mmol m^–3) was continuouslysupplied to one tank at each temperature while in the remainingfour tanks (one at each temperature) the plants were completelydependent on nodule N₂-fixation. Plants were randomly selected at intervals during the following14 d period in order to measure root and nodule respirationand acetylene reduction activity (ARA) in a flow-through systemset at the adapted root temperature. Additional plants wereassayed for in vitro nitrate reductase activity in leaves, roots,and nodules. Apparent nitrogenase activity (ARA) and respiration associatedwith it were each markedly affected by temperature in two ways;(1) Activity per unit weight of nodule was reduced at lowertemperatures; (2) Development of the plant, and thus also nodulemass, was restricted at lower temperatures which, in turn, restrictedtotal nodule activity per plant. The presence of nitrate significantly reduced ARA of nodules,particularly at higher temperatures. However, significant discrepancieswere found when N₂-fixation rates, estimated from the acetylenereduction assay, were compared with N₂-fixation rates calculatedfrom curves fitted to N accumulation data (minus the rate of uptake in the case of nitrate-treated plants). Carbon use efficiency (CO² respired per C₂H₄ produced) was notsignificantly affected by temperature or the presence of nitrate. Nitrate reductase activity (NRA) developed in all plant partsat the three highest temperatures, but not at 5 ?C. We calculatethat leaf NRA may account for 82, 75, and 68% of total nitratereduction at 11, 17, and 25 ?C respectively. Key words: Trifolium repens, white clover, N₂ fixation, root temperature, acetylene reduction assay, nitrate, nitrate reductase     

Studies in the Physiology of Flowering of Timothy (Phleum pratense L.): III. Effects of Shoot Age and Nitrogen Level on the Timing of Inflorescence Production

The influence of shoot age on flowering in S.48 timothy wasinvestigated in partially controlled conditions and outdoors.In artificial long days sensitivity to photo-induction increasedwith age of the shoot from soon after germination until aboutfive leaves were produced; no further increases in responseto photoperiod were evident in older plants. The minimum leafnumber to ear emergence in plants exposed to continuous lightfrom germination was nine; of these, five had been producedbefore spikelet initiation, while the remainder developed duringspike differentiation. In natural conditions, only main shoots on plants sown in Marchor earlier were sufficiently advanced to respond to the increasingnatural daylength and to produce ears in late June. In latersowings ear emergence became increasingly delayed until, inplants sown in June, some main shoots failed to develop inflorescences.Nitrogen deficiency further delayed or inhibited flowering inthese later-formed shoots. The minimum period of 10 weeks forinflorescence production in main shoots was attained in plantssown some weeks before midsummer. The equivalent period in subsidiarytillers arising in midsummer on the same plants decreased toless than 5 weeks. The implications of the results are discussedin relation to previous investigations in this species.     

The Respiratory Costs of Nitrogen Fixation in Soyabean, Cowpea, and White Clover: II. COMPARISONS OF THE COST OF NITROGEN FIXATION AND THE UTILIZATION OF COMBINED NITROGEN

Plants of soyabean, cowpea, and white clover were grown singlyin pots in Saxcil growth cabinets at 23/18 °C, 30/24 °C,and 20/15 °C, respectively, until seed maturation or for85 d (white clover). Two populations were produced within eachspecies: one population effectively nodulated and wholly dependentfor nitrogen on fixation in the root nodules, and a second populationcompletely lacking nodules but receiving abundant nitrate nitrogen.In each species, the two populations were compared in termsof rate of gross photosynthesis, rate of shoot respiration,and rate of root respiration. Source of nitrogen had littleor no effect on rate of photosynthesis or shoot respiration.In contrast, the rate of respiration of the nodulated rootsof plants fixing their own nitrogen was greater, sometimes two-foldgreater, than that of equivalent plants lacking nodules andutilizing nitrate nitrogen. This superiority in terms of rateof root respiration was generally confined to the period ofintense nitrogen fixation. An analysis of the magnitude of thisrespiratory burden in terms of daily photosynthesis indicatesthat, in all three legumes, plants fixing their own nitrogenrespire 11–13% more of their fixed carbon each day thanequivalent plants lacking nodules and utilizing nitrate nitrogen.     

Studies in the Physiology of Flowering of Timothy (Phleum pratense L.): IV. Effect of Shoot Age and Nitrogen Level on the Size of the Inflorescence

The main-stem ear lengths of S. 48 timothy plants increasedwith shoot age at the time of flowering, reaching a peak afterthe expansion of 20–25 leaves and then declining again:similar responses were also apparent in subsidiary tillers onthe same plants. In natural conditions, the maximum ear lengthswere attained on shoots originating in autumn and early winterand flowering the following summer; shoots arising later developedprogressively shorter ears. It is suggested that the potentialear length attainable on a shoot is linked with the size ofthe shoot apex built up during vegetative development. In early formed tillers, mainly primary and secondary, a reducednitrogen supply decreased the size of ears by limiting spikedifferentiation. The intensity of the nitrogen effect increasedin later generations of tillers, with the result that the influenceof shoot age on ear length became obscured. The results emphasizedthe importance of the internal relations between tillers inperennial plants, especially with regard to mineral nutrition.     

The Measurement and Prediction of Organ Growth in a Uniculm Barley

Measurements of leaf areas, net rates of photosynthesis, patternsof assimilate translocation, and of some aspects of respirationwere made at leaf-increment intervals during the expansion ofleaves 5, 6, 7, 8, and 9 on the single axis of a uniculm barley(Kindred Uniculm 97) grown in controlled environments. Thesedata were used as the primary inputs in a computer programmedeveloped to simulate the carbon metabolism and consequent weightchanges of the organs in the single-axis barley plant. The totalweight of plant tissue increased threefold between the expansionof the fifth and ninth leaves; during this period the simulationmodel generally predicted the daily growth increments to within10 per cent of the observed values. The predictions of dailygrowth increments in new leaf, stem, and root were less accurate.The simulation indicated that the proportion of photosyntheticproducts incorporated in new growth at the meristems declinedfrom some 54 per cent of the assimilate at the fifth leaf stageto 42–3 per cent at the ninth leaf stage. This declinein the efficiency of conversion of photosynthetic products appearedto be the result of an increase in maintenance respiration,which in turn stemmed from an approximately linear increasein total tissue weight; the proportion of photosynthetic productslost in the respiration associated with synthetic processesremained approximately constant throughout the growth period.     

Carbon and Nitrogen Yield, and N2 Fixation in White Clover Plants Receiving Simulated Continuous Defoliation in Controlled Environments

Single plants of white clover grown in controlled environments,and dependent for nitrogen on N, fixation, were defoliated at1 or 2 d intervals to 3, 2 and 1 expanded leaves per stolon(Expt 1), and to 1,0.5 (1 leaf on every alternate stolon) and0 expanded leaves per stolon (Expt 2), for 43–50 days Plants adapted to severe defoliation by developing much smallerleaves with a slightly reduced specific leaf area, more stolons,a smaller proportion of weight in leaf, root and nodules anda greater proportion of weight in stolons. The daily yield (materialremoved by defoliation) of d. wt and nitrogen generally decreasedwith severity of defoliation, as did the residual plant weight.However, the ‘efficiency’ of yield (daily yield/residualweight x 100) of dry matter and nitrogen was greater in themost severely defoliated treatments, attaining a maximum of5–6 % All plants adapted to the imposed defoliation regimes, howeversevere, with the result that even plants maintained withoutany fully expanded leaves invested a similar fraction of theirmetabolic resources in shoot and root as less severely defoliatedplants, and continued to grow and fix N₂, albeit at a very reducedrate of 1–2 mg Nd^–11. The energetic cost of N₂ fixation(acetylene reduction) remained constant in all treatments at31 mole CO₂ mole C₂H₄^–1, but there was some evidence thatrate of N₂ fixation per unit of nodule weight declined in themost harshly defoliated treatment. Trifolium repens, white clover, continous defolation, growth, N₂ fixation