Metabolism of S-methylcysteine and its sulfoxide in Chinese cabbage, Brassica pekinensis Rupr.

The metabolism of sulfur of S-methyl-L-cysteine and ($)S-methyI-L-cysteinesulfoxide in Chinese cabbage was studied. Results showed thatthese sulfur-bearing amino acids are metabolized to cysteineby demethylation. This conclusion is based on; 1) the relativelyhigh recovery of radioactivity in the cysteine of the insolublefraction a short time after the administration of 35S-labeledS-methyl-L-cysteine or ($)S-methyl- L-cysteine sulfoxide; 2)the similarity in the ratios of ³H to ³⁵S in the cysteine ofthe insoluble fraction and in the double-labeled cysteine partof S-methyl-L-cysteine and its sulfoxide fed to detached leavesand 3) the obvious structural relationship between cysteineand its methyl derivative, S-methyl-L-cysteine. Moreover, we ascertained that the sulfur of S-methyl-L-cysteinewas also utilized when supplied to plants; i.e. tomato, tobaccoand cucumber, in which this sulfur amino acid has not been found. (Received June 30, 1971; )     

Salinity Stress and the Content of Proline in Roots of Pisum sativum and Tamarix tetragyna

Amino acid composition of the free amino acid pool and the TCA-insolubleprotein fraction were investigated in root tips of pea and Tamarixtetragyna plants grown at various levels of NaCl salinity. Salinitystress induced an increase of proline content, mainly in thefree amino acid pool in both plants, and of proline or hydroxyprolinecontent in the protein. Externally-supplied proline was absorbedand incorporated into protein, by pea roots, more effectivelythan by Tamarix roots. Salinity stress, apparently, stimulatedthe metabolism of externally-supplied labelled proline. Pearoots have a very large pool of free glutamic acid; however,70 per cent of the ¹⁴C from externally-supplied ¹⁴C-U-glutamicacid was released as CO₂. Very small amounts of it were incorporatedinto protein. No measurable amount of radioactivity could bedetected in any one of the individual amino acids, either ofprotein hydrolysate or the free amino acid pool. Proline very effectively counteracted the inhibitory effectof NaCl on pea seed germination and root growth. A similar effectbut to a lesser degree was achieved with phenylalanine and asparticacid. The feasibility of proline being a cytoplasmic osmoticumis discussed.     

Soil nitrogen form and plant nitrogen uptake along a boreal forest productivity gradient

We present results from a study of soil solution concentrations of ammonium (NH₄⁺), nitrate (NO₃^-), and amino acid N over one growing season along a local 90-m-long plant productivity gradient in a boreal forest. Three forest types are found along the gradient: an ericaceous dwarf-shrub type between 0 and 40 m, a low-herb type between 40 and 80 m, and a tall-herb type at 90 m. Soil sampling of the mor layer was performed in June, July, August and October in the three forest types. In addition, plant uptake of NH₄⁺, NO₃^- and the amino acid glycine was investigated. A mixture of the three N forms was injected into the soil; one N form at a time was labeled with ¹⁵N, and in the case of glycine also with ¹³C. In the dwarf-shrub forest, where plant productivity was low, the soil N pool was strongly dominated by amino acid N. There, plants took up more NH₄⁺ than NO₃^-. Glycine uptake did not differ significantly from either NH₄⁺ or NO₃^- uptake. Along the gradient, soil concentrations of NH₄⁺ and NO₃^- increased, as did plant productivity. In the low-herb forest NH₄⁺ comprised a major portion of the soil N pool, and plants took up more NH₄⁺ than NO₃^- or glycine. In the tall-herb forest, NO₃^- was as abundant as NH₄⁺, and together these two N forms dominated the soil N pool. Here, plants took up nearly equal amounts of NO₃^- and NH₄⁺, and this uptake exceeded that of glycine severalfold. Apart from the overall preference for NH₄⁺ that plants exhibited throughout the gradient, the results show a correlation between soil concentrations of amino acids and NO₃^- and plant preferences for these N forms.     

Incorporation of 2H from 2H2O into ABA in Tomato Shoots: Evidence for a Large Pool of Precursors

Tomato shoots were supplied simultaneously with S-[¹⁴C]abscisicacid and ²H₂O for 4 h. After a further 6 h incubation no deuteriumhad been incorporated into abscisic acid (ABA) extracted fromwilted or turgid shoots although up to 93% of the ABA had beensynthesized in the presence of ²H₂O as shown by the dilutionof [¹⁴C]ABA. This provides substantial evidence for a largepool of ABA precursors. When tomato shoots were incubated for6d in 80% ²H₂O between 20% and 32% of ABA molecules became labelledwith from 1 to 14 deuterium atoms. From this data the minimumsize of the precursor pool was calculated to be approximately35 times that of ABA. In addition, ABA from wilted plants containedsignificantly less deuterium than that from turgid plants suggestinga second or an enlarged pool of precursor in wilted plants. Key words: Abscisic acid, precursor pool, ²H incorporation     

The Partitioning of Photosynthetically Assimilated 14C into Amino Acids in Wheat: 2. Distribution in Amino Acids of Wheat Grain Fractions

Changes in the distribution of ¹⁴C between free and bound aminoacids in wheat grains (Triticum aestivum L. cv. Arkas) at 10and 20 d post-anthesis are described. After ¹⁴CO₂, labellingof the flag leaf, ¹⁴C was initially more rapidly transferredto the grains of 20 d post-anthesis plants than for 10 d post-anthesisplants. However, after a 460 min chase period in the light theamount of ¹⁴C in the grains of the younger and older plantswere similar. In the younger, more rapidly growing grains, agreater proportion of the ¹⁴C was incorporated into structuraltissue and starch. ¹⁴C accumulation in the grains continuedduring the dark in the younger grains but not in the older grains. Although the overall ¹⁴C distribution between the free aminoacid and protein pools of the grain was similar for both treatments,the distribution within the albumin, prolamin and globulin fractionsand between the individual non-bound amino acids differed. Ofthe protein fractions, the albumins were initially the mostheavily labelled but after 460 min chase the prolamins containedmore ¹⁴C. The majority of the ¹⁴C in the albumin and globulinfractions after 280 min chase was in hydrolysable, non-aminoacid compounds. In both tissues, the free amino acid pools lostradioactivity in the dark but the solid residues and proteinscontinued to function as ¹⁴C sinks. Daily fluctuations in the radioactivity in free and bound alanineare consistent with the role of free alanine as a diurnal metabolicnitrogen pool. Wheat, Triticum aestivum¹⁴CO₂, amino acids, proteins, carbon metabolism     

ORIGIN OF AMINO ACIDS CONSTITUTING CELLULAR PROTEIN IN GERMINATING CONIDIOSPORES OF ASPERGILLUS NIGER

Formation of pool amino acids in germinating spores of Aspergillusniger strain 1617 was investigated. The pool amino acids comprisedmainly glutamic acid and alanine. Small amounts of pyruvateand -ketoglutarate were found to increase almost in parallelwith the course of increase in the amount of free amino acidsup to the stage of onset of active protein synthesis. Asparticglutamictransaminase activity was exhibited even in dormant spores andit developed in response to the increase in cellular protein.Alanine-glutamic transaminase activity, on the other hand, waslacking in dormant spores and appeared at the stage of accumulationof amino acids preceding protein synthesis. It was revealed from the experiments with ³⁵S-labeled sporesthat the dormant spores of this fungus contain two unidentifiedsulfur substances, and the sulfur of these substances is incorporatedinto the sulfur amino acids of the protein synthesized in germinatingspores. ¹Present address: Institute of Applied Microbiology, Universityof Tokyo, Tokyo (Received September 11, 1959; )     

Dynamics of Carbon Photosynthetically Assimilated in Nodulated Soya Bean Plants under Steady-state Conditions 2. The Incorporation of 13C into Carbohydrates, Organic Acids, Amino Acids and some Storage Compounds

Nodulated soya bean (Glycine max L.) plants at the early floweringstage were allowed to assimilate ¹³CO₂ under steady-state conditions,with a constant ¹³C abundance, for 8 h in the light. The plantswere either harvested immediately or 2 d after the end of the¹³CO₂ feeding, divided into young leaves (including flower buds),mature leaves, stems+petioles, roots and nodules; the ¹³C abundancein soluble carbohydrates, organic acids, amino acids, starchand poly-ß-hydroxybutyric acid was determined witha gas chromatography-mass spectrometry. The rapid turnover of ¹³C in the sucrose pools observed in allorgans of the plants showed that sucrose was the principal materialin the translocation stream of primary products of photosynthesis.At the end of the ¹³CO₂ exposure, sucrose in the mature leavesas the major source organs and in the stems+petioles was labelledwith currently assimilated carbon to about 75 per cent, whereasa much higher labelling of sucrose was found in the roots andin the nodules. This suggests the existence of two or more compartmentedpools of sucrose in mature leaves and also in stems+petioles. The relative labelling patterns of individual organic acidsand amino acids were similar in various plant organs. However,the rapid turnover of succinate and glycine was characteristicof nodules. Treatment with a high concentration of nitrate inthe nutrient media increased the turnover rate of amino acidcarbon in shoot organs and roots, while it markedly decreasedthe labelling of amino acids in nodules. The cyclitols, exceptfor D-pinitol, were significantly labelled with assimilated¹³C in mature leaves, but in nodules, the labelling was verymuch less. In the nodules, which were actively fixing atmospheric nitrogen,a large proportion (80–90 per cent) of currently assimilatedcarbon was found as sucrose and starch at the end of the ¹³CO₂feeding. This was also true of the roots. On the other hand,in young growing leaves, the distribution of currently assimilatedcarbon into sucrose, starch and other soluble compounds wasmuch less. This suggests that a large amount of carbon assimilatedby and translocated to young leaves was used to make up structuralmaterials, mainly protein and cell wall polymers synthesis,during the light period. Glycine max L., soya bean, ¹³CO₂ assimilation, carbon metabolism in nodules     

Defoliation-Induced Stress in Nodules of White Clover: I. CHANGES IN PHYSIOLOGICAL PARAMETERS AND PROTEIN SYNTHESIS

Nodule function and protein synthesis were studied in defoliationstressed white clover plants. Uncut control plants (C) werecompared with plants from two defoliation treatments: (1) continuousdefoliation (CD) where all leaves and petioles were removedeach day; and (2) defoliated/recovered (DR) where, after removalof all leaves and petioles, new leaves were then allowed toregrow. After a single defoliation N₂ fixation (acetylene reductionactivity) and nitrogenase-linked respiration declined by morethan 80% within 3 h and by nearly 100% by 24 h. DR plants beganto fix nitrogen again at a very low level 3 d later and thereafterrose to control levels by 15 d. Continuously defoliated plantsnever recovered N₂ fixation capacity. Nodule protein complementwas assessed by polyacrylamide gel electrophoresis. Major changesoccurred in buffer soluble protein band patterns by 6 d in CDplants, but few changes were evident in SDS soluble proteins.By 9 and 14 d significant disruption of all proteins was evident.The prominent host plant protein, leghaemoglobin (Lb) had disappearedby 14 d. In DR plants the intensity of staining was reducedbut no major changes in band patterns were evident and by 21d nodules were rejuvenated. [³⁵S]-labelled methionine was incorporated into nodule proteinsfrom all treatments throughout the experiment. However, continuousdefoliation caused increasing variability between replicatesin the labelled band patterns. By 21 d CD, much of the labelledprotein was present as amorphous low M_r material which suggestseither disruption of the protein synthesizing machinery or rapidhydrolysis by proteolytic enzymes. Surprisingly [³⁵S]-methionine was never found in Lb from nodulesof any treatment. It is possible that white clover Lb does notcontain any methionine residues or that no synthesis of Lb occurred. Key words: Trifolium repens, white clover, defoliation, protein synthesis, nodules     

The Distribution of 14C-labelled Assimilates in Young Apple Trees as Influenced by Doses of Supplementary Nitrogen: II. Soluble Carbohydrates and Amino Acids

Methanol extracts of young MM.104 apple trees fed ¹⁴CO₂ viaa single leaf were fractionated to compare ¹⁴C activity in totalsoluble sugar and amino acid components. ¹⁴C activity in aminoacids increased after the supply of ammonium nitrate to thesoil in plants where ¹⁴C labelled carbohydrates were presentin the roots. Estimates of specific carbon activity gave lowervalues for the amino acid carbon than the general value fortotal soluble carbohydrates. The fractionation of subsequentmethanol extracts of MM.104 roots has shown that sucrose hadlower specific activity than other components. Although thelevels of activity would accord with sucrose being a substratefor amino acid synthesis, an alternative explanation for theobserved results involving a cyclical system for transferringnitrogen is postulated.     

Root Development and Source-Sink Relations in Carrot, Daucus carota L: II. EFFECTS OF ROOT PRUNING ON CARBON ASSIMILATION AND THE PARTITIONING OF ASSIMILATES

Physiological responses to root pruning were investigated bycomparing ¹⁴CO₂ fixation rates, the partitioning of ¹⁴C-labelledassimilate, and soluble and insoluble carbohydrate levels inthe leaves of carrot plants following the removal of some ofthe fibrous roots, or fibrous roots and part of the tap root.Root pruning reduced ¹⁴CO₂ fixation by 28–45% but leafspecific activity (¹⁴C assimilation g-¹ leaf fresh weight) wasunchanged. The proportion of total assimilate exported to theroot system increased following root pruning and this was atthe expense of the developing leaves. In younger plants (wherethe tap root received 10% of the assimilate) the supply of ¹⁴Cto the tap root was maintained in spite of root pruning. However,shortening the tap root to 3 cm in older plants (in which 30%of the fixed 14C was normally exported to the developing storageorgan), reduced its sink capacity and resulted in slightly greaterretention of ¹⁴C in the mature leaves. Greater concentrationsof insoluble carbohydrate were found in the mature leaves followingroot pruning but soluble sugar content was unaffected. Onlysmall differences were observed in the distribution of ¹⁴C betweensoluble and insoluble carbohydrate fractions when plants werefed ¹⁴CO₂ several days after the root pruning operations. Thesephysiological responses were mainly associated with the removalof fibrous roots and support the view that the fibrous rootsystem is more important than the developing storage organ inregulating growth in young carrot plants.     

The Redistribution of Labelled Sulphate Ions in Sunflower Plants subjected to Different Treatments

The effect of 0.5 and 5.0 mM sulphate concentrations and 15,25, and 35°C root temperatures on the redistribution of³⁵SO₄ in sunflower plants was studied at two rates of transpiration.It is concluded from the results that ³⁵SO₄ was lost rapidlyfrom the free space of the roots to the external solution andthat this losa was independent of the transpiration rates androot temperatures. The fact there was no marked decrease inthe ³⁵SO₄ content of the shoots during the ‘washing’process indicates that the free space is likely to be confinedto the cortical region of the roots, and there seems to be adiffusion barrier between the cortex and xylem preventing arapid movement of ³⁵SO₄ from the cortex to the xylem.     

The Effect of Different Levels of N Limitation on Sugars, Amino Acids, Growth and Biomass Partitioning in Broadbean (Vicia faba L.)

Broadbean plants (Vicia faba L.) were submitted to three differentlevels of steady state N limitation. Relative addition ratesof N were 0.06, 0.1 and 0.14d^-1. Plants were harvested at fiveevenly distributed times over a 2d period. Shoot growth correspondedwell with the imposed treatment. Root growth, relative to shootgrowth, was highest at the 0.06d^-1treatment. Diurnal patternsof soluble sugars, amino acids and starch were analysed. Onaverage, soluble sugar levels were highest in the plants ofthe 0.06d^-1treatment whereas average free amino acid levelswere highest in the 0.14d^-1treatment. Shoot growth increasedas the concentration of shoot amino acids increased. No suchcorrelation however could be found between root growth and freesugar levels in the root. Broadbean; Vicia faba L.; exponential addition; N limitation; free sugar; amino acids; diurnal cycle; functional equilibrium; starch     

The Influence of Nitrate and Ammonium Nutrition on the Growth of Wheat (Triticum aestivum) and Maize (Zea mays) Plants

The effects of NO^-₃ and NH⁺₄ nutrition on hydroponically grownwheat (Triticum aestivum L.) and maize (Zea mays L.) were assessedfrom measurements of growth, gas exchange and xylem sap nitrogencontents. Biomass accumulation and shoot moisture contents ofwheat and maize were lower with NH⁺₄ than with NO^-₃ nutrition.The shoot:root ratios of wheat plants were increased with NH⁺₄compared to NO^-₃ nutrition, while those of maize were unaffectedby the nitrogen source. Differences between NO^-₃ and NH⁺₄-fedplant biomasses were apparent soon after introduction of thenitrogen into the root medium of both wheat and maize, and thesedifferences were compounded during growth. Photosynthetic rates of 4 mM N-fed wheat were unaffected bythe form of nitrogen supplied whereas those of 12 mM NH⁺₄-fedwheat plants were reduced to 85% of those 12 mM NO^-₃-fed wheatplants. In maize supplied with 4 and 12 mM NH⁺₄ the photosyntheticrates were 87 and 82% respectively of those of NO^-₃-fed plants.Reduced photosynthetic rates of NH⁺₄ compared to NO^-₃-fed wheatand maize plants may thus partially explain reduced biomassaccumulation in plants supplied with NH⁺₄ compared to NO^-₃ nutrition.Differences in the partitioning of biomass between the shootsand roots of NO^-₃-and NH⁺₄-fed plants may also, however, arisefrom xylem translocation of carbon from the root to the shootin the form of amino compounds. The organic nitrogen contentof xylem sap was found to be considerably higher in NH⁺₄- thanin NO^-₃-fed plants. This may result in depletion of root carbohydrateresources through translocation of amino compounds to the shootin NH⁺₄-fed wheat plants. The concentration of carbon associatedwith organic nitrogen in the xylem sap of maize was considerablyhigher than that in wheat. This may indicate that the shootand root components of maize share a common carbon pool andthus differences induced by different forms of inorganic nitrogenare manifested as altered overall growth rather than changesin the shoot:root ratios.Copyright 1993, 1999 Academic Press Triticum aestivum, wheat, Zea mays, maize, nitrogen, growth, photosynthesis, amino acids, xylem     

Studies on the metabolism of a sulfur-oxidizing bacterium VI1. Fractionation and reconstitution of the elementary sulfur-oxidizing system of Thiobacillus thiooxidans

The sulfur-oxidizing system of a strain of Thiobacillus thiooxidanswas obtained in cell-free state. The system is resolved intothree fractions and can be reconstituted from these fractions.Both the soluble and particulate fractions are required forthe oxidation of elementary sulfur. The soluble fraction wasfurther separated into two fractions, the collodion membrane-permeable(S-P)and the impermeable(S-IP). S-P contains a low molecular weight,relatively heat stable substance(s) which is indispensable forthe reconstitution of the sulfur-oxidizing system and was identifiedas a pyridine nucleotide. The function of S-P can be replacedby NAD or NADP, but not by cysteine nor GSH. Oxidation of NADH₂ and NADPH₂ is catalyzed by the particulatefraction. Oxidation of the latter is much more rapid than thatof the former. Oxidation of NADPH₂ as well as sulfur oxidationis inhibited by cyanide, pCMB and CO, the CO-inhibition beingphoto-irreversible. However, strong inhibitors of sulfur oxidationsuch as DDC, 8-hydroxyquinoline and salicylaldoxime have noeffect on the oxidation of NADPH₂. The optimum pH values for sulfur and sulfite oxidations by thecell-free extract are shifted to the neutral side in comparisonwith pH values by intact cells. ¹V = References(I). ²Partly supported by a grant from the Ministry of Education. (Received April 3, 1969; )     

Relationship between photosynthetic carbon metabolism and essential oil biogenesis in peppermint under Mn-stress

Changes in growth and yield parameters, and ¹⁴CO₂ and (U-¹⁴C)sucrose incorporation into the primary metabolic pool, and essentialoil have been investigated under Mn-deficiency and subsequentrecovery in Mentha piperita, grown in solution culture. UnderMn-deficiency, CO₂ exchange rate, total chlorophyll, total assimilatoryarea, plant dry weight, and essential oil yield were significantlyreduced, whereas chlorophyll a/b ratio, leaf area ratio andleaf stem ratio significantly increased. In leaves of Mn-deficientplants, ¹⁴CO₂ incorporation into the primary metabolic pool(ethanol-soluble and -insoluble) and essential oil were significantlylower, whereas (U-¹⁴C) sucrose incorporation into these componentswas significantly higher as compared to the control. Among theprimary metabolites, the label was maximum in sugars, followedby organic acids and amino acids. A higher label in these metaboliteswas, in general, observed in stems of Mn-deficient plants ascompared to the control. Mn-deficient plants supplied with completenutrient medium for 3 weeks exhibited partial recovery in growthand yield parameters, and essential oil biogenesis. Thus, underMn-deficiency and subsequent recovery, the levels of primaryphotosynthetic metabolites and their partitioning between leafand stem significantly influence essential oil biogenesis. Key words: Mentha piperita, Mn-stress, ¹⁴CO₂ and [U-¹⁴C] sucrose incorporation, oil accumulation, primary photosynthetic metabolites     

Transformation of atmospheric NO2 absorbed in spinach leaves

NO₂ fumigation at 8 ppm of spinach plants resulted in nitriteaccumulation in the leaves in the dark but not in the light.When spinach plants were fumigated with ₁₅N-labeled NO₂ in thelight, amide nitrogen of glutamine, glutamic acid, -amino butyricacid and aspartic acid, in this order, were highly labeled with¹⁵N and nitrate was also labeled. These results suggest thatNO₂-nitrogen (at least some of it) is converted into nitriteand nitrate, and then actively assimilated into amino acidsthrough the glutamine synthetase/glutamate synthase pathwayin spinach leaves. ¹This work was conducted as a part of the special research project"Studies on evaluation and amelioration of air pollution byplants" (1976–1978) at the National Institute for EnvironmentalStudies. (Received July 24, 1978; )     

Responses of Noogoora Burr (Xanthium occidentale Bertol.) to Nitrogen Supply and Carbon Dioxide Enrichment

We studied the responses of Xanthium occidentale (Bertol.) (cockleburor Noogoora burr), a noxious weed, to atmospheric CO₂ enrichmentand nitrate-N concentrations in the root zone ranging from 0.5to 25 mM. CO₂ enrichment (1500 cm³ m^–3) increased dry-matterproduction to about the same extent (18 per cent) at all levelsof supplied N: most of the increment in dry matter was distributedequally between leaves and roots so that there was little effecton shoot-to-root dry-weight ratios. Growth was stimulated greatlyby N and plateaued at 12 mM supplied N. Shoot-to-root dry-weightand total N ratios increased with increasing N supply. CO₂ enrichmenthad no effect on the total amount of N accumulated by plants,but increased the N-use efficiency of leaves. Enriched plantshad lower concentrations and quantities of N in their leavesthan controls, and therefore lower shoot-to-root total N ratios.Little free NO₃^– accumulated in organs of control or enrichedplants. NO₃^– was the major form of N in xylem sap fromdetopped plants at low supplied NO₃-N, but amino N was equalin importance at high supplied NO₃-N in control and enrichedplants. Concentrations of NO₃^– were lower in the xylemsap of CO₂ enriched plants. It was concluded that the betterN-use efficiency of CO₂ enriched plants could result in increasedgrowth of X. occidentale in regions of marginal soil fertilityas atmospheric levels of CO₂ increase. CO₂ enrichment, nitrogen, Xanthium, Noogoora burr, cocklebur     

Phosphate Regulation of Nitrate Assimilation in Soybean

It is known that phosphorus deficiency results in alterationsin the assimilation of nitrogen. An experiment was conductedto investigate mechanisms involved in altered ¹⁵NO^–₃ uptake,endogenous ¹⁵N translocation, and amino acid accumulation insoybean (Glycine max L. Merrill, cv. Ransom) plants deprivedof an external phosphorus supply for 20 d in solution culture.Phosphorus deprivation led to decreased rates of ¹⁵NO^–₃uptake and increased accumulation of absorbed ¹⁵N in the root.Both effects became more pronounced with time. Asparagine, theprimary transport amino acid in soybean, accumulated in largeexcess in roots and stems. In roots of phosphorus-deprived plants,concentrations of ATP and inorganic phosphate declined rapidly,but dry weight accumulation was similar to or above that ofthe control even after 20 d of treatment. Arginine accumulationin leaves was greatly enhanced, even though ¹⁵N partitioninginto the insoluble reduced-N fraction of leaves was unaffected.The results suggest that decreases in NO^–₃ uptake in lowphosphorus plants could be caused by feedback control factorsand by limited ATP availability. The decline in endogenous Ntransport from the root to the shoot may be associated withchanges in membrane properties, which also result in paralleleffects on hydraulic conductance and the upward flow of waterthrough the plant. Key words: Phosphorus stress, nitrate uptake, nitrate translocation, arginine     

Pool Size of Electrons That Can Be Donated to P700+ As Determined in Intact Leaves: Donation to P700+ from Stromal Components Via the Intersystem Chain

A method for estimation of the functional pool size of electronsthat can be donated to P700⁺ after the illumination of intactleaves with actinic light is described. The complementary areasbetween the stationary level of P700⁺ attained by irradiationwith far-red light and the oxidation curves of P700 by far-redlight after a 50-ms multiple-turnover light (MT-area), and afterthe illumination of actinic light (AL-area) were determined.Since the MT-area represents the pool size of electrons in theintersystem chain, the ratio of the AL-area to the MT-area allowsus to estimate the pooi size of electrons stored during actinicillumination that can be donated to P700⁺ The ratio of the AL-areato the MT-area was determined for intact leaves of several C₃plants, and it was found to increase to a value of two or threewith increases in the intensity of actinic light, and it wasfurther increased by anaerobiosis. The pool size of electronsthat can be donated to P700⁺ from the stroma was estimated tobe in the range of 12 and 28 electrons per P700 under aerobicconditions. These results indicate that stromal components donateelectrons to P700⁺ through the intersystem chain in chloroplastsof C₃ plants. (Received July 3, 1992; Accepted September 29, 1992)