The Influence of Nitrogen Supply on the Use of Nitrate and Ribulose 1,5-bisphosphate Carboxylase/Oxygenase as Leaf Nitrogen Stores for Growth of Potato Tubers (Solanum tuberosum L.)

Millard, P. and Catt, J. W. 1988. The influence of nitrogensupply on the use of nitrate and ribulose 1,5-bisphosphate carboxylase/oxygenaseas leaf nitrogen stores for growth of potato tubers (Solanumtuberosum L.).—J. exp. Bot. 39: 1–11. The capacity of field-grown potato plants to store N in theirleaves for re-use during tuber growth was studied in two experiments.Increasing the N application from 0 g to 25 g N m^–2 providedplants with more N than they needed for growth and so allowedaccumulation of N in their leaves, principally as nitrate andprotein. Ribulose 1,5-bisphosphate carboxylase/oxygenase (RUBISCO)concentrations increased by approximately 120% in response toN application. During tuber growth there was an export of nitrate-Nfrom the leaves of N-replete plants and of RUBISCO-N from bothhigh and low N plants. RUBISCO-N was mobilized more rapidlyfrom leaves than N from other proteins and, together with nitrate,in one experiment accounted for over 90% of the N lost fromthe leaves irrespective of the N treatment. The potential contributionof mobilization of N stored in RUBISCO to the N content of tubersat final harvest was calculated as being between 11–15%,and appeared to be unaffected by the N supply to the plants. The distribution of N accumulating within the canopy, in responseto N application was studied. Nitrate accumulated predominantlyin the lowermost (shaded) leaves, while reduced N (includingRUBISCO) was found mainly in the younger leaves at the top ofthe canopy. This is discussed in relation to the growth of theplant and the supply of N. Key words: Solanum tuberosum, nitrogen, nitrate, ribulose, 1,5-bisphosphate carboxylase/oxygenase, storage     

Cellular Levels of Ribulose 1,5 Bisphosphate Carboxylase and Chloroplast Compartment Size in Wheat Mesophyll Cells

Pyke, K. A. and Leech, R. M. 1987. Cellular levels of ribulose1,5 bisphosphate carboxylase and chloroplast compartment sizein wheat mesophyll cells.—J. exp. Bot. 38: 1949–1956. The amount of the photosynthetic enzyme ribulose 1,5 bisphosphatecarboxylase (RUBISCO),as determined in mesophyll cells in primarywheat leaves was related to the size of the chloroplast compartmentwithin the cell for wheat species of three ploidy levels. Asimilar comparison was made for several genotypes of the hexaploidbreadwheat Triticum aestivum. Estimation of total chloroplastvolume per mesophyll cell was made assuming chloroplasts tobe oblate spheroid in shape. A significant correlation was found between the amount of RUBISCOper cell and the total chloroplast volume per cell for diploid,tetraploid and hexaploid wheat species. A significant correlationbetween cellular RUBISCO level and total chloroplast volumeper cell was also observed for a range of genotypes of the hexaploidT. aestivum but these genotypes of T. aestivutn accumulate agreater amount of RUBISCO per unit chloroplast volume than doany other wheat species. For these genotypes of T. aestivumthe stromal concentration of RUBISCO was estimated at 0·5mol m^–3 with a ribulose Msphosphate binding site concentrationof 4·0 mol m^–3. These results are discussed with respect to a gene dosage hypothesisto explain the accumulation of RUBISCO in leaf mesophyll cells. Key words: Ribulose, bisphosphate carboxylase, wheat chloroplasts, mesophyll cells     

The Measurement of Ribulose 1, 5-bisphosphate Carboxylase/Oxygenase Concentration in the Leaves of Potato Plants by Enzyme Linked Immunosorbtion Assays

Catt, J. W. and Millard, P. 1988. The measurement of ribulose1, 5-bisyphosphate carboxylase/ oxygenase concentration in theleaves of potato plants by enzyme linked immunosorbtion assays.—J.exp. Bot. 39: 157-164. An enzyme linked immunosorbtion assay (ELISA) has been developedfor the measurement of ribulose l, 5-bistfphosphate carboxylase/oxygenase(RUBISCO) concentrations in potato leaves. Extracts of leaveswere made using a buffer containing detergent and used to coatELISA plates. The amount of RUBISCO binding was estimated usinga double antibody technique. Various methods of extraction wereused and assessed and the efficiency of the assay was testedusing purified RUBISCO. The method is applicable to the determinationof RUBISCO from diverse sources. Key words: Ribulose 1, 5-bisphosphate carboxylase/oxygenase, immunosorbtion, Solanum tuberosum     

Photosynthesis by Flag Leaves of Wheat in Relation to Protein, Ribulose Bisphosphate Carboxylase Activity and Nitrogen Supply

The effects of nitrate supply on the composition (cell numbers,protein and chlorophyll contents) of flag leaves of winter wheatgrown with two amounts of N fertilizer and of spring wheat grownin the glasshouse under controlled nitrate supply are describedand related to photosynthesis. Nitrogen deficiency decreasedthe size of leaves, mainly by reducing cell number and, to asmaller extent, by decreasing cell volume. Protein content perunit leaf area, per cell and per unit cell volume was largerwith abundant N. Total soluble protein, ribulose bisphosphatecarboxylase-oxygenase (RuBPc-o) protein and chlorophyll changedin proportion irrespective of nitrogen supply and leaf age.Photosynthesis per unit area of flag leaf and carboxylationefficiency in both winter and spring wheat were proportionalto the amount of total soluble protein up to 7.0 g m^–2and to the amount of RuBPc-o protein up to 4.0 g m^–2.However, photosynthesis did not increase in proportion to theamount of total soluble or RuBPc-o protein above these amounts.In young leaves with a high protein content the measured ratesof photosynthesis were lower than expected from the amount andactivity of RuBPc-o. Carboxylation per unit of RuBPc-o protein,measured in vitro, was slightly greater in N-deficient leavesof winter wheat but not of spring wheat. RuBPc-o activity perunit of RuBPc-o protein was similar in winter and spring wheatleaves and remained approximately constant with age, but increasedin leaves showing advanced senescence. RuBPc-o protein fromN-deficient leaves migrated faster on polyacrylamide gels thanprotein from leaves with high N content. Regulation of the rateof photosynthesis in leaves and chloroplasts with a high proteincontent is discussed. The conductance of the cell to the fluxof CO₂ from intercellular spaces to RuBPc-o active sites iscalculated, from cell surface areas and CO₂ fluxes, to decreasethe CO₂ partial pressure at the active site by less than 0.8Pa at an internal CO₂ partial pressure of 34 Pa. Thus the decreasein partial pressure of CO₂ is insufficient to account for theinefficiency of RuBPc-o in vivo at high protein contents. Otherlimitations to the rate of photosynthesis are considered. Key words: Wheat, photosynthesis, nitrogen, ribulose, bisphosphate carboxylase     

The Influence of Nitrogen Supply on the Uptake and Remobilization of Stored N for the Seasonal Growth of Apple Trees

M26 apple rootstocks were grown in sand culture and suppliedwith three rates of nitrogen (N) with the irrigation: none,0·8 mol N m^–2 or 8·0 mol N m^–2. Allthe N supplied to the trees was labelled with ¹⁵N at 5·0atom percent enrichment. The effect of N supply on tree growth,N uptake and the remobilization of N from stems for the annualgrowth of the trees was measured. Increasing the N supply increasedleaf growth, but had no effect upon root mass and so alteredthe root/leaf dry matter ratio Plants receiving no fertilizer N had to rely entirely upon storedreserves of N for their seasonal growth. Initially this N wasused for leaf growth, which stopped after a few weeks. Thereafterthe N-deficient plants retranslocated some of the N from theirleaves to support root growth. Increasing the N supply had littleeffect upon the amount of N remobilized for growth, althoughwell-fertilized plants accumulated N in their leaves and didnot retranslocate any to support root growth. The partitioningof N between roots and shoots was, therefore, altered by increasingthe N supply. Amino acid analysis of stems showed that the majorforms of N remobilized during growth were protein rich in asparagineand arginine The results show the importance of internal N cycling for thegrowth of young apple trees, and are discussed in relation toother studies of N cycling in deciduous trees Malus domestica Borkh., nitrogen, remobilization, growth, partitioning, storage     

Correlation of Activity and Amount of Ribulose 1,5-bisphosphate Carboxylase with Chloroplast Stroma Crystals in Water-Stressed Willow Leaves

Changes in the activity and amount of ribulose 1,5-bisphosphate(RuBP)carboxylase (E.C. 4.1.1.39 [EC] ) were studied in well-watered plantsof Salix ‘aquatica gigantea’ and in similar plantsduring three different water stress treatments and after rewatering.The chloroplast ultrastructure of these plants was examinedby electron microscopy. The amounts of crystallized proteinin the chloroplast stroma were assessed according to the areaof crystal structure seen in the thin sections. RuBP carboxylase activity decreased with decreasing leaf waterpotentials but recovered upon rewatering, except when leaveshad been exposed to severe water stress. The percentage of totalchloroplast area made up of crystal inclusions decreased withdecreasing leaf water potentials. After rewatering, the crystalseither disappeared or the amount decreased markedly. Both RuBPcarboxylase activity and the area of crystal inclusions increasedinitially with increased extractable RuBP carboxylase proteinbut decreased with further increases above 6700–7000 µgRuBP carboxylase protein mg^–1 chlorophyll. In well-wateredand water-stressed plants the activity of RuBP carboxylase,based on amount of chlorophyll, increased with an increasingamount of crystal inclusions in the chloroplast stroma. In rewateredplants no such correlation was observed, and the low percentageof crystal inclusions in the chloroplast area was independentof RuBP carboxylase activity. Key words: Chloroplast stroma crystals, ribulose 1,5-bisphosphate carboxylase, Salix, water stress     

Nitrate Nutrition and Temperature Effects on Wheat: Enzyme Composition, Nitrate and Total Amino Acid Content of Leaves

Lawlor, D. W., Boyle, F. A., Kendall, A. C. and Keys, A. J.1987. Nitrate nutrition and temperature effects on wheat: Enzymecomposition, nitrate and total amino acid content of leaves.—J.exp. Bot. 38: 378–392. Wheat plants were grown in controlled environments in two temperatureregimes with two rates of nitrate fertilization. In some experimentstwo light intensities were combined with the nitrogen and temperaturetreatments. The composition of the third leaf was studied fromsoon after emergence until early senescence. The amounts ofchlorophyll, soluble protein, ribulose bisphosphate carboxylase-oxygenase(RuBPc-o) protein, nitrate, and total amino acids were measuredtogether with the activities of RuBPc-o, fructose- 1,6-bisphosphatase,glycolate oxidase, carbonic anhydrase, nitrate reductase, glutaminesynthetase and serine- and glutamate-glyoxylate aminotransferases.Additional nitrate supply increased the amounts, per unit leafarea, of chlorophyll, total soluble protein and RuBPc-o proteinand the activities of all the enzymes. The ratio of RuBP carboxylaseto RuBP oxygenase activity, when measured at constant CO²/O²ratio and temperature, was unaffected by growth conditions orleaf age. Leaves grown at the lower temperature, especiallywith more nitrate, contained much more soluble protein, nitratereductase, fructose bisphosphatase and free amino acids perunit area than the plants grown in the warmer conditions. However,young leaves grown in the warm contained more nitrate than thosegrown in the cool. Amounts of protein, amino acids and chlorophylland most enzyme activities reached maxima near full leaf expansionand decreased with age; additional nitrate slowed the decreaseand senescence was delayed. Nitrate content and nitrate reductaseactivities were highest in leaves before full expansion andthen fell rapidly after full expansion. Increased light intensityincreased the content of RuBPc-o protein at the higher rateof nitrate supply. Chloroplast components and, to a lesser extent,peroxisomal enzymes associated with photosynthetic nitrogenassimilation changed in proportion with different treatmentsbut nitrate reductase activity was not closely related to chloroplastenzymes. Control of tissue composition in relation to environmentalconditions is discussed. Key words: Nitrate nutrition, temperature, wheat, enzyme, amino acid, leaves, ribulose bisphosphate carboxylase oxygenase, nitrate reductase     

The Effects of Nitrogen Supply and Defoliation on the Seasonal Internal Cycling of Nitrogen in Molinia caerulea

Plants of Molinia caerulea were grown in pots for two seasonsat two levels of nitrogen (N) supply and two levels of defoliation.All N supplied was enriched with ¹⁵N in the first season andwas at natural abundance in the second season. This allowedthe contribution of remobilization from overwintering storesto be discriminated from current root uptake in supplying Nfor new shoot growth in the second season. The effects of Nsupply and defoliation upon the internal cycling of N in M.caerulea were quantified. N was remobilized from both roots and basal internodes to supportnew shoot, especially leaf, growth in spring. Roots suppliedmore N than basal internodes. Since the remobilization mainlyoccurred before the onset of root N uptake, internal cyclingwas important for the earliest period of shoot growth. An increasedN supply increased the amount of N remobilized to new shootgrowth, however, the proportion of N remobilized from overwinteringstores was independent of N supply. Defoliation increased theamount of N remobilized from the roots, and had no effect onthe ¹⁵N content of basal internodes of plants receiving a lowsupply of N. Remobilization of N from leaves of undefoliatedplants occurred later in the season. Remobilization from leavessupplied flowers in plants receiving a low N supply and bothflowers and new basal internodes in plants receiving a higherN supply. Key words: Molinia caerulea, internal cycling, nitrogen, defoliation     

Photoinhibition under Atmospheric O2, the Activation State of RuBP Carboxylase and the Content of Photosynthetic Intermediates in Soybean and Wheat

Ward, D. A. and Drake, B. G. 1987. Photoinhibition under atmosphericO₂, the activation state of RuBP carboxylase and the contentof photosynthetic intermediates in soybean and wheat.—J.exp. Bot. 38: 1937–1948. Associations between photosynthesis, the activation state ofRuBP carboxylase and the contents of photosynthetic intermediateswere compared in soybean and wheat leaves before and after exposureto photoinhibitory treatments in the presence of atmosphericO₂. Exposing attached leaves to a supra-saturating irradiance(3 800 µmol quanta m^{– 2} s^–1) for 2 h in CO_2-freeair decreased carboxylation efficiency and the light-saturatedphotosynthetic rate in air by approximately 50%. Exposure tothe photoinhibitory treatment for periods in excess of 2 h didnot cause a further decrease of photosynthesis in soybean. Althoughphotosynthesis was reduced, the initial and total (fully-activated)activities of ribulose 1,5-bisphosphate carboxylase (RuBPCase)in leaf extracts were unaltered in each species by the photoinhibitorytreatment. This was true for leaves sampled under both air andat a rate-limiting intercellular CO₂ partial pressure (C_i) of75 µPa Pa^–1. The contents of ribulose l,5-bisphosphate(RuBP) and 3-phosphoglyceric acid (3-PGA) were reduced by thephotoinhibitory treatment in soybean leaves sampled in air andat a rate-limiting C_i, although the RuBP/3-PGA ratio was unaffected.The relative reduction of RuBP content in soybean leaves atrate-limiting C₁ was similar to the corresponding reductionof carboxylation efficiency. For wheat,the relative reductionof RuBP content at rate-limiting C_i (–19%) caused by thephotoinhibitory treatment was considerably less than the correspondingdecrease of carboxylation efficiency (–49%).The RuBP/3-PGAratio of wheat was also increased significantly by the photoinhibitorytreatment The significance of these observations to the regulationof CO₂-limited photosynthesis in leaves experiencing photoinhibitionunder atmospheric oxygen is discussed. Consideration is alsogiven to the previous contention that contemporary measurementsof initial activity in crude extracts may provide a spuriousindication of the amount of the enzyme-CO₂-Mg_{2 +} form of RuBPcarboxylase present in the leaf. Key words: Carboxylation efficiency, RuBP carboxylase, photoinhibition, RuBP, 3-PGA     

Photosynthetic and Photorespiratory Enzymes in Widely Divergent Plant Species with Special Reference to the Moss Ceratodon purpureus: Properties of Ribulose Bisphosphate Carboxylase/ Oxygenase, Phosphoenolpyruvate Carboxylase and Glycolate Oxidase

Rintamäki, E. and Aro, E.-M. 1985. Photosynthetic and photorespiratoryenzymes in widely divergent plant species with special referenceto the moss Ceratodon purpureus: Properties of ribulose bisphosphatecarboxylase/oxygenase, phosphoenolpyruvate carboxylase and glycolateoxidase.—J. exp. Bot. 36: 1677–1684. Km(CO₂) values and maximal velocities of ribulose bisphosphatecarboxylase/oxygenase (E.C. 4.1.1.39 [EC] ) were determined for sixplant species growing in the wild, consisting of a moss, a fernand four angiosperms. The maximum velocities of the RuBP carboxylasesvaried from 0.13 to 0.;62 µmol CO₂ fixed min^–1 mg^–1soluble protein and the Km(CO₂) values from 15 to 22 mmol m^–3CO₂. The highest Km(CO₂) values found were for the moss, Ceratodonpurpureus, and the grass, Deschampsia flexuosa. These plantsalso had the highest ratios of the activities of RuBP carboxylaseto RuBP oxygenase. Glycolate oxidase (E.C. 1.1.3.1 [EC] ) activitieswere slightly lower in D.flexuosa, but not in C. purpureus,than for typical C₃ species. Phosphoenolpyruvate carboxylase(E.C. 4.1.1.31 [EC] ) was not involved in the photosynthetic carboxylationby these two plants. However, another grass, Phragmites australis,was intermediate in PEP carboxylase activity between C₃ andC₄ plants The properties of RuBP carboxylase/oxygenase are discussedin relation to the activities of PEP carboxylase and glycolateoxidase and to the internal CO₂ concentration. Key words: RuBP carboxylase, oxygenase, Km(CO₂), moss     

Export, Mobilization, and Respiration of Assimilates in Uniculm Barley during Light and Darkness

The respiratory losses and the pattern of carbon supply froma leaf of unicuim barley were examined during a complete diurnalperiod using a steady state ¹⁴C-labelling technique. After a delay of c. 1 h a portion of the ¹⁴C exported from acontinuously assimilating leaf was lost in respiration in thelight. This respiratory loss amounted to c. 20% of the total¹⁴C fixed. A further 28% of the total ¹⁴C fixed was respiredduring the dark period. In the light, carbon was fixed at a rate of c. 8·9 mgC dm^{–2 h–1} and exported from the leaf at ^c. 5·3mg C dm^{–2 h–1}. Dark export averaged ^c. 31% of theday-time rate. Carbohydrate was stored in the leaf during the day and was almostcompletely remobilized during the dark. Sucrose, the major reservecarbohydrate, was exported first whilst the starch level remainedconstant. After some 9 h of darkness, sucrose declined to alow level and starch remobilization began.     

Effect of Nitrogen on Growth, Yield and Photorespiratory Activity in Spring Wheat

Spring wheat plants growing in pots in controlled environmentrooms were given extra nitrogen after flag leaf emergence. Theeffect of nitrogen on growth, yield, the activity of ribulose1,5–bisphosphate carboxy–lase/oxygenase and thedistribution of¹⁴C in photorespiratory intermediates and indifferent parts of the plants was determined. Extra nitrogenincreased the movement of ¹⁴C to the ear and increased grainyield by 29 per cent, mainly because of an increase in grainnumber. Though extra nitrogen delayed senescence of the leaves,the growth of the ear in the later stages was not increasedin proportion to the extra green area. The relative inefficiencyof leaf area with extra nitrogen, which has also been foundin the field, was not due to a reduction in photosynthesis perunit leaf area. Nor was there evidence of an increase in photorespirationas reflected by a greater flow of carbon into the photorcspiratorymetabolites glycine and serine, or an increase in the activityof ribulose 1,5–bisphosphate oxygenase relative to thecarboxylase. We suggest that there may be an increase in theloss of carbon in dark respiration. Triticum aesttvum, nitrogen, growth, yield, photorespiration     

A CO2 Concentrating System in Leaves of Higher C3-Plants Predicted by a Model Based on RuBP Carboxylase/Oxygenase Kinetics and 14CO2/12CO2 Exchange

Mächler, F., Lehnherr, B., Schnyder, H. and Nösberger,J. 1985. A CO₂ concentrating system in leaves of higher C₃-plantspredicted by a model based on RuBP carboxylase/oxygenase kineticsand ¹⁴CO₂/¹²CO₂ exchange.–J. exp. Bot. 36: 1542–1550. A model is presented which compares the ratio of the two activitiesof the enzyme nbulose bisphosphate carboxylase/oxygenase asdetermined in vitro with the ratio of photosynthesis to photorespirationin leaves as determined from differential ¹⁴CO₂/¹²CO₂ uptakeor from CO₂ compensation concentration. Discrepancies betweenmeasurements made in vitro and in vivo are attributed to theeffect of a CO₂ concentrating system in the leaf cells. Interferencefrom dark respiration is discussed. A CO₂ concentrating systemis postulated which is efficient mainly at low temperature andlow CO₂ concentration. Key words: —Photosynthesis, photorespiration, ribulose bisphosphate carboxylase/oxygenase     

Modulated Degradation of Ribulose Bisphosphate Carboxylase in Leaves on Top-Pruned Shoots of the Mulberry Tree (Morus alba L.)

Yamashita, T. 1987. Modulated degradation of ribulose ftisphosphatecarboxylase in leaves on top-pruned shoots of the mulberry tree(Morus alba L.).—J. exp. Bot. 38: 1957–1964. The effects of pruning shoot tops on the synthesis and degradationof ribulose 1,5–Wsphosphate carboxylase (RuBPCase) inleaves on remaining shoots were investigated in mulberry trees.Leucine labelled with ¹⁴C was fed to leaf discs from field-grownmulberry trees and ¹⁴C incorporation into RuBPCase was examined.Proportion of ¹⁴C in RuBPCase to leucine–¹⁴C absorbedby leaf discs was remarkably lowered by top-pruning, thoughoccasionally a slight increase was observed soon after pruning.Yet RuBPCase content in leaves on top-pruned shoots became progressivelyhigher than that in leaves on intact shoots. Changes in ¹⁴Cin Ru1BPCase in leaves of mulberry saplings previously fed ¹⁴CO₂were followed. Following ¹⁴CO₂ feeding, the attainment of themaximal level of ₁₄C in RuBPCase was retarded by top-pruning.The highest level of ₁₄C in RuBPCase was maintained in leaveson top-pruned shoots but decreased in leaves on intact shoots.Specific radioactivity in RuBPCase continued to increase inleaves on top-pruned shoots even after attaining a maximum levelin the control leaves. These facts suggest that the increasein RuBPCase by top-pruning results from a cessation of its degradationfor the remobilization of nitrogen for newly developing leaveson shoot tops. Key words: RuBP carboxylase, shoot pruning, mulberry (Morus alba)     

Response of Soybean Canopy Photosynthesis to CO2 Concentration, Light, and Temperature

Photosynthetic rates of outdoor-grown soybean (Glycine max L.Merr. cv. Bragg) canopies increased with increasing CO₂ concentrationduring growth, before and after canopy closure (complete lightinterception), when measured over a wide range of solar irradiancevalues. Total canopy leaf area was greater as the CO₂ concentrationduring growth was increased from 160 to 990 mm³ dm^–3.Photosynthetic rates of canopies grown at 330 and 660 mm³ CO₂dm^–3 were similar when measured at the same CO₂ concentrationsand high irradiance. There was no difference in ribulose bisphosphatecarboxylase/oxygenase (rubisco) activity or ribulose 1,5-bisphosphate(RuBP) concentration between plants grown at the two CO₂ concentrations.However, photosynthetic rates averaged 87% greater for the canopiesgrown and measured at 660 mm³ CO₂ dm^–3. A 10°C differencein air temperature during growth resulted in only a 4°Cleaf temperature difference, which was insufficient to changethe photosynthetic rate or rubisco activity in canopies grownand measured at either 330 or 660 mm³ CO₂ dm^–3. RuBP concentrationsdecreased as air temperature during growth was increased atboth CO₂ concentrations. These data indicate that the increasedphotosynthetic rates of soybean canopies at elevated CO₂ aredue to several factors, including: more rapid development ofthe leaf area index; a reduction in substrate CO₂ limitation;and no downward acclimation in photosynthetic capacity, as occurin some other species. Key words: CO₂ concentration, soybean, canopy photosynthesis     

Photosynthesis and Light Activation of Ribulose 1,5-bisphosphate Carboxylase in the Presence of Starch

Limitation of photosynthesis and light activation of ribulose,1,5-bisphosphate carboxylase (RuBPCO) were examined in the 5thleaf of seedlings of red clover (Trifolium pratense L. cv. Renova)for 5 d following an increase in photosynthetic photon fluxdensity (PPFD) from 200 to 550µmol quanta m^–2 s^–1.Net photosynthesis and its stimulation at 2.0 kPa O₂ initialactivity of rapidly extracted RuBPCO, standard activity of RuBPCOafter incubation of the extracts in the presence of CO₂, Mg²⁺,and inorganic phosphate and contents of soluble protein, starch,soluble sugars, and various photosynthetic metabolites weredetermined. Photosynthesis decreased and starch content increased.No decrease in photosynthesis was found if, when PPFD was increased,all leaves except the investigated 5th leaf were removed, suggestingthat the decrease in photosynthesis was due to accumulated carbohydrates.The stimulation of photosynthesis at 2.0 kPa O₂ did not decreaseand the ratio of the total foliar steady-state contents of triosephosphate to 3-phosphoglycerate increased suggesting that thedecrease in photosynthesis was not due to limiting inorganicphosphate in chloroplasts. Intercellular CO₂ partial pressureand RuBP content were not decreased. Nevertheless, the ratioof photosynthesis to initial RuBPCO activity decreased, suggestingthat the catalysis per active RuBPCO site was decreased. Theincrease in PPFD in the growth cabinet and the PPFD at whichleaves were preconditioned for 1 h, affected not only initialactivity but also the standard activity of RuBPCO. The resultssuggest that a varying proportion of RuBPCO was bound to membranesand was contained in the insoluble fraction of the extracts.A comparison of photosynthesis with extracted RuBPCO activitysuggested that membrane bound RuBPCO did not contribute to photosyntheticCO₂ fixation and that the binding and release to and from membranesmodulated actual RuBPCO activity in vivo. Key words: Photosynthesis, ribulose 1,5-bisphosphate carboxylase, starch     

Ribulose Bisphosphate Carboxylase Activity and Photosynthesis during Leaf Development in the Tomato

Besford, R. T., Withers, A. C. and Ludwig, L. J. 1985. Ribulosebisphosphate carboxylase activity and photosynthesis duringleaf development in the tomato.—J. exp Bot. 36: 1530–1541. The carboxylase activity of ribulose-1,5-bisphosphate carboxylase/oxygenaseand of phosphoenolpyruvate carboxylase, and the light saturatedrate of net photosynthesis were measured in the developing 5thleaf of tomato plants. Values for light saturated net photosynthesiswere also calculated from the measured carboxylase activitiesand estimates of internal CO₂ and oxygen concentrations. Thecalculated rate using the activity of ribulose bisphosphatecarboxylase alone for net CO₂ assimilation in 300 mm³ dm^–3CO₂ was greater than the measured rate at 80% and full expansionbut less than the measured rate in younger leaves. When theactivities of both the carboxylases were taken into accountbetter agreement was evident for young leaves but the rate wasfurther overestimated for older leaves The calculated rate forphotosynthesis in 1200 mm³ dm^–3 CO², assuming saturationof ribulose bisphosphate carboxylase with RuBP, was an overestimatefor young leaves but was close to the observed values for leavesnear full expansion. The results are discussed in terms of measuredconductances for CO₂ and the availability of RuBP in the leaf Key words: Tomato, leaf development, photosynthesis, RuBP carboxylase, oxygenase     

Combined Effects of Partial Defoliation and Nutrient Availability on Cloned Betula pendula Saplings: II. CHANGES IN NET PHOTOSYNTHESIS AND RELATED BIOCHEMICAL PROPERTIES

The combined effects of partial defoliation and nutrient availabilityon net photosynthesis and related biochemical variables werestudied in cloned Betula pendula Roth saplings. The saplingswere randomly assigned to different nutrient levels (5, 1·5and 0·5 mol N m^–3) in aerated nutrient cultureand to the following defoliation treatments: (1) control (nodamage), (2) damage of the developing main stem leaves (halfof the leaf lamina removed), and (3)removal of the developingmain stem leaves (entire leaf lamina removed). The leaf immediatelybelow the damaged area in the treated plants, and the correspondingleaf in the control plants, were selected for study. Net photosynthesismeasurements and biochemical determinations were made 2, 8 and14 d after assigning the treatments. At intermediate and lownutrient levels the final net photosynthetic capacity was significantlyhigher in the saplings with the topmost leaves removed thanin the undamaged control saplings, indicating that the expressionof compensatory photosynthesis after partial defoliation isnot inhibited by nutrient deficiency. The photosynthetic enhancementwas closely associated with the increased initial activity ofribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco). However,the increased activity of Rubisco was not exclusively the resultof a higher amount of Rubisco. The expression of compensatoryphotosynthesis after partial defoliation in our study cannotunequivocally be attributed to an increased flow of nitrogento the remaining leaves. Key words: Partial defoliation, nutrient availability, net photosynthesis, nitrogen, Rubisco     

The Effects of Temperature and Form of Nitrogen Supply on the Relative Contribution of Root Uptake and Remobilization in Supplying Nitrogen for Laminae Regrowth of Lolium perenne L.

Plants of Lolium perenne L. cv. S23 were grown in sand culturesupplied with either ammonium (NH₄⁺) or nitrate (NO₃^–)in an otherwise complete nutrient solution at 12°C or 20°C.Three weeks after germination, plants were clipped weekly tosimulate grazing. After 10 weeks growth all nitrogen (N) wassupplied enriched with ¹⁵N to quantify the effects of form ofN supply and temperature on the relative ability of currentroot uptake and remobilization to supply N for laminae regrowth. The form of N supply had no effect on the dry matter partitioning,while at 20°C more dry weight was allocated to laminae regrowthand less to the remaining plant material. The current root uptakeof N, which subsequently appeared in the laminae regrowth, wassimilar for plants supplied with NH₄⁺ or NO₃^–, and bothwere equally reduced at the lower temperature of growth. Remobilizationof N to laminae regrowth was greater for plants receiving NH₄⁺than NO₃^–; remobilization with either form of N supplywas reduced at the lower temperature of growth. Remobilizationwas reduced to a lesser extent at 12°C than current rootuptake. It was concluded that remobilization became relativelymore important in supplying N for regrowth of laminae at lowertemperatures. Key words: Lolium perenne, ammonium, nitrate, temperature, remobilization     

Nitrogen Supply Affects the Remobilization of Nitrogen for the Regrowth of Defoliated Lolium perenne L.

Nitrogen remobilization from roots and pseudostems during regrowthof Lolium perenne L. was studied in miniswards grown with contrastinglevels of (NH₄)₂SO₄ in solution culture. Growth with a highN supply (5.0 mol m^–3) increased theweight of leaf laminae recovered at each of five weekly clippings,and decreased the proportion of photosynthate used for rootgrowth. Clipped plants growing in a steady-state were suppliedwith ¹⁵N for 48 h and the recovery of labelled N in laminaemeasured after five weekly cuts. Recovery of labelled N in thelaminae from the second clipping onwards was derived only fromremobilization of N from roots and pseudostem. Miniswards grownwith low N (0.5 mol m^–3) relied moreupon remobilization of N for lamina growth than did high N plants.Thus after 14 d 20% of lamina N was labelled in low N plantsbut only 3% was labelled in the high N treatment. Thereafter,N remobilization declined until at the final clipping after35 d, labelled N represented only 4% and 1 % of the lamina Nin the low and high N plants. When plants were not clipped beforethe labelling period, they took up more ¹⁵N if grown with highN than cut plants. Thereafter, the remobilization of N followeda similar pattern as in the cut plants. Exponential models were used to calculate the rate of N transferfrom roots and pseudostem to laminae. When grown with low N,the half-life of remobilization was 1.56 weeks. High N miniswardshad an initial rapid remobilization with a half-life of 0.66weeks, and a slower phase with a half-life of 2.98 weeks. Key words: Lolium perenne L., nitrogen supply, regrowth, remobilization, internal cycling