Chloroplast Protein Synthesis During Barley Leaf Growth and Senescence: Effect of Leaf Excision

Chloroplast protein synthesis was measured during the expansion,maturity and senescence of the oldest leaf of barley, Hordeumvulgare L., var. Hassan. A maximum rate of protein synthesisoccurred near the end of the expansion stage 9 d after sowing.Protein synthesis increased again at the beginning of senescenceand reached a new maximum at day 14 after sowing. Detachmentand incubation of leaves in the dark stimulated chioroplastprotein synthesis by fully expanded or by senescent leaves butnot by expanding leaves. If the detached leaves were kept inthe light, chloroplast protein synthesis was stimulated in fullyexpanded but not in senescent leaves. Short treatments (18 h)of leaf segments with growth substances in either light or indarkness, significantly changed the rate of protein synthesisshown by chloroplasts. The relationship between chloroplastprotein synthesis and leaf senescence is discussed. Key words: Hormones, light, maturity     

Increase in DNA Content of Primary Leaves of Phaseolus vulgaris upon Decapitation

Changes in DNA content of bean (Phaseolus vulgaris) primaryleaves after decapitation were investigated. When apical budswere removed at 11 d, DNA content per leaf increased by about20% at 15 d and then decreased in parallel with the controls.The RNA and chlorophyll contents, fresh weight, and leaf areaexpressed on a single leaf basis changed in the same manneras the DNA content in response to decapitation. But when bothapical and lateral buds were removed, all these values continuedincreasing during the test period. Thus, growing lateral budsand apical buds have the same effect on the DNA change in primaryleaves as that due to ageing of the leaves. Cell number perleaf was not increased by any treatment, indicating that theobserved increase in the DNA content of primary leaves is ascribableto an increase in DNA per cell. Next, the whole shoots above the nodes of primary leaves wereremoved at various ages. The response of primary leaves to decapitationvaried according to their age. With age, they lost the abilityto increase their fresh weight, area, and chlorophyll contentbut not their DNA and RNA contents in response to decapitation.Decapitation stimulated chloroplast replication only withinthe period in which chloroplasts were replicating in controlleaves, but it induced chloroplast enlargement at any age. Therefore,the increase in DNA content after decapitation may be partiallydue to an increase in the amount of chloroplast DNA. When stems were heat-girdled above the nodes of the primaryleaves, these leaves showed responses similar to but smallerthan those to decapitation. The senescence of primary leavesseems to be controlled by the distribution of substances whichare transported from the roots.     

Changes of Anatomical Features, Photosynthesis and Ribulose Bisphosphate Carboxylase-Oxygenase Content of Mango Leaves

Changes in anatomical and physiological features, includingchanges in amount per unit area of anthocyanin and chlorophyll,in leaves of seedling mango (Mangifera indica L. cv. Irwin)trees were determined to understand what controls the rate ofphotosynthesis (Pn) at various stages of development. The youngleaves of seedling trees contained high concentrations of anthocyanin.During enlargement of leaves, the disappearance of anthocyaninand the accumulation of chlorophyll occurred concomitantly;the anthocyanin content began to decrease markedly once theleaf area had reached a maximum. During the early period ofleaf development, the thickness of mesophyll tissue decreasedtemporarily, but when the length of the leaf reached half thatof a mature leaf, the mesophyll began to thicken again. Smallstarch grains appeared in the chloroplasts of the young leavesand chloroplast nucleoids (ct-nuclei) were distributed throughoutthe chloroplasts. When leaves matured, ct-nuclei were displacedto the periphery of chloroplasts because of the accumulationof large starch grains. Compared with young leaves, green andmature leaves contained greater concentrations of ribulose bisphosphatecarboxylase-oxygenase (RuBisCO) protein. The results of immunocytochemicalexamination of RuBisCO under the light microscope reflectedthe results of electrophoresis measurements of RuBisCO. Pn waslow during the chocolate-coloured stage of early leaf development.In green and mature leaves Pn was higher; the average Pn was7·6 mg CO₂ dm^-2 h^-1 under light at intensities above500 µmol m^-2 s^-1.Copyright 1995, 1999 Academic Press Mangifera indica L., mango leaf, chloroplast nucleoids, chloroplast ultrastructure, starch accumulation, anthocyanin, chlorophyll, DAPI staining, SDS-PAGE, immunocytochemical technique     

Effect of Benzyladenine on Diurnal Changes in DNA Content per Chloroplast and Chloroplast Replication in Intact Bean Leaves

The effect of benzyladenine (BA) on the diurnal changes in DNAand Chl contents per chloroplast and chloroplast replicationin primary leaves of bean plants (Phaseolus vulgaris L.) grownunder a 16 h light/8 h dark cycle was studied. Experiments weremade on primary leaves in the early expansion phase, where celldivision had been completed but chloroplasts were replicating.In untreated controls, chloroplast number, Chl content and freshweight per leaf showed daily periodic changes. Chl content perchloroplast increased in the light period every day, and freshweight per leaf increased most rapidly in the early dark period.Chloroplast number per leaf increased rapidly in the early darkperiod on day 9, though the increase began a little earlierand was less sharp on days 8 and 10. During these periods, DNAcontent per chloroplast was decreasing due to chloroplast divisionas chloroplast DNA (ctDNA) per leaf remained unchanged throughoutthe experimental period. BA induced increases in Chi contentper chloroplast, ctDNA content and fresh weight per leaf within6 h of its application, regardless of whether it was appliedat or 10 h after the beginning of the light period. Applicationof BA at 10 h in the light period shifted the start of chloroplastreplication by 6 h compared to that in untreated controls. However,when BA was applied at the beginning of illumination, the startof chloroplast replication showed the same relative change intime as above. 5-Fluorodeoxyuridine (5-FdU) promptly preventedBA-induced increase in Chl content and chloroplast number perleaf as well as ctDNA content per leaf.     

Release of Nitrogen From Chloroplasts During Leaf Senescence in Rice (Oryza sativa L.)

In order to ascertain the possibility that nitrogen associatedwith chloroplasts serves as a major source of nitrogen redistributedfrom senescent leaves, chloroplasts were isolated from riceleaves and changes with leaf age in total leaf nitrogen andchloroplast nitrogen were examined. Results presented here showthat decrease in total leaf nitrogen during leaf senescencewas closely correlated with decrease of chloroplast nitrogenand roughly 85–95 per cent of leaf nitrogen released fromsenescent leaves during the experimental period could be accountedfor by a loss of chloroplast nitrogen. By dividing chloroplastnitrogen into two fractions, i.e. lamellar and stroma fractions,the question of which fraction was more deeply concerned withthe loss of leaf nitrogen was clarified. Results suggested thatin the vegetative stage of plant growth the stroma was mainlyresponsible for the loss of leaf nitrogen. On the other hand,nitrogen was released from lamellar and stromal fractions atalmost the same rate during the reproductive stage. Oryza sativa L., rice, chloroplasts, nitrogen, leaf senescence     

Changes in Chloroplast DNA Levels during Growth of Spinach Leaves

In young spinach leaves, 1–4 mm long, 7–10% of thetotal DNA of the leaf was chloroplast (pt) DNA. Growth in theseleaves was mainly by cell division with plastid division keepingpace with cell division and maintaining about 10 plastids percell. About 1% of the leaf cells were formed in 4.0 mm leaves.Both cell division and cell expansion contribute to the nextstage of leaf growth, which was quantitatively the major periodof new cell formation, nuclear DNA synthesis and ptDNA synthesis.Relative to the nuclear DNA level ptDNA levels rose to 21% ofthe total DNA and chloroplast.plastome copy numbers from 1500to 5000 per cell while chloroplast numbers rose from 10 to 30per cell. In the final period of leaf growth, cell expansionwas the main determinant of growth and chloroplast number percell rose to 180. In contrast to young leaves, newly emergedcotyledons contained 20% of their DNA as ptDNA and, during cellexpansion, cell number per cotyledon doubled. On average, thecells became octoploid, and chloroplast numbers and plastomecopy numbers rose to 500 and 22 000 per cell respectively. Similarlevels of nuclear ploidy, chloroplast number and plastome copynumber were induced in the first leaf pair of spinach followingdecapitation. When senescence was induced in mature leaves byshading, no loss of nuclear or ptDNA occurred. Following theonset of leaf yellowing and a form of senescence induced bynitrogen deficiency in leaves which had not fully expanded,there was preferential loss of ptDNA which fell from 8200 to3700 plastome copies per cell over an 11 d period. Key words: Spinach, Chloroplast, DNA, Ploidy     

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     

Photosynthesis by developing embryos of oilseed rape (Brassica napus L.)

The aim of this study was to assess the photosynthetic potentialof developing seeds of oilseed rape (Brassica napus L.) andto compare photosynthetic properties of embryo plastids withthose of leaf chloroplasts from the same species. Measurementsof CO₂-dependent O₂ evolution show that developing seeds ofB. napus are photosynthetically active in vitro. Essentially,all of the photosynthetic activity of the developing seed isaccounted for by the embryo. The rate of photosynthesis by developingembryos increased until the onset of desiccation, after whichit declined, so that by maturity embryos were no longer photosyntheticallyactive. Photosynthetic activity was positively correlated withchlorophyll content throughout development. Comparison of thephotosynthetic characteristics of leaf and embryo chloroplastsrevealed that rates of uncoupled electron transport were 2.5-foldgreater in those from the embryo. Light-saturated rates of CO₂-dependentO₂ evolution, per unit chlorophyll, and CO₂ saturation pointswere similar for chloroplasts from both tissues. However, light-saturationpoints and chlorophyll a/b ratios were lower for embryo thanfor leaf choroplasts. Embryos and embryo chloroplasts also containedconsiderably less ribulose 1,5-bisphosphate carboxylase/oxygenaseprotein per unit total protein, than leaves. Although excisedembryos were capable of high rates of CO₂-dependent O₂ evolution(90–100 mol mg^–1 chlorophyll h^–1) under asaturating photosynthetic photon flux density (PPFD), low transmittanceof light through the silique wall (30%), together with the highPPFD required to achieve light compensation points in developingseeds (500 mol m^–2 s^–1), suggests that photosynthesisin vivo is unlikely to make a net contribution to carbon economyunder normal environmental conditions. Key words: Embryo, development, photosynthesis, chloroplast, Brassica napus L.     

A Vertical Gradient of the Chloroplast Abundance among Leaves of Chenopodium album

The abundances of chloroplasts in leaves on the main stems ofChenopodium album at different height levels were investigatedin relation to the photosynthetic capacity and light environmentof the leaves. (1) The number of chloroplasts per mesophyllcell decreased with descending position of leaves, except foryoung developing leaves at the top of plants that had smallerchloroplast numbers per cell than matured leaves beneath them.Contents of chlorophyll and ribulose-1,5-bisphosphate carboxylase/oxygenaseper leaf area that were highest in the topmost young leavesand decreased with decreasing height level indicate that thereis a vertical gradient of chloroplast abundance per leaf areadecreasing from the top of the leaf canopy with depth. (2) Light-saturatingrate of photosynthetic oxygen evolution per leaf area of maturedleaves decreased more steeply with decreasing leaf positionthan the chloroplast number per cell. Gradients of chlorophylland the enzyme protein contents were also steeper than thatof the chloroplast number. Loss of photosynthesis in lower leavesis, therefore, ascribed partly to loss of whole chloroplastsand partly to reduced photosynthetic capacities of the remainingchloroplasts. (3) The chloroplast number per cell in newly expandedsecond leaves was comparable to those in leaves that have developedat later stages of the plant growth but decreased graduallyduring leaf senescence both in the dark and light. The formationof the vertical gradient of chloroplast abundance is, therefore,ascribed to loss of whole chloroplasts during senescence ofleaves. (4) Irradiance a leaf receives decreased sharply fromthe top of the canopy with depth. The physiological or ecophysiologicalsignificance of the vertical distribution of chloroplasts amongleaves was discussed taking light environments of leaves intoconsideration. (Received July 31, 1995; Accepted October 20, 1995)     

RNA Synthesis in Phaseolus Chloroplasts: I. RIBONUCLEIC ACID SYNTHESIS IN CHLOROPLAST PREPARATIONS FROM PHASEOLUS VULGARIS L. LEAVES AND SOLUBILIZATION OF THE RNA POLYMERASE

Chloroplast preparations from the young primary leaves of Phaseolusvulgaris L. cv. Canadian Wonder carry out the DNA-dependentincorporation of UTP into RNA at rates between 8 and 14 pmolUTP µg^–1 chlorophyll h^–1. It is estimatedthat 90% of the activity was localized in the chloroplasts.The incorporation proceeded for between 20 and 30 min at 35°C. The maximum rates of RNA synthesis were attained atpH 8.3, in the presence of 15 mM MgCl₂. Chloroplasts were alsoactive, to a lesser extent, with 1.5 mM MnCl₂. The simultaneouspresence of MnCl₂ and MgCl₂ resulted in inhibition of activity.Nuclear material prepared from young P. vulgaris leaves incorporatedUTP at a rate of about 12 pmol UTP µg^–1 DNA h^–1.On a chloroplast (Tritonsoluble) DNA basis chloroplast activitywas over 40-fold that of nuclei. Methods of solubilizing chloroplastRNA polymerase were explored. Yields of over 75% were achieved,but methods suitable for one species were not always successfulwhen applied to another. The highest yields of the P. vulgarisenzyme were obtained using EDTA and KCl. All methods resultedin solubilization of DNA. RNA synthesis by the soluble P. vulgarisenzyme proceeded for more than 40 min at 35 °C.     

Photosynthesis and Other Traits in Relation to Chloroplast Number during Soybean Leaf Senescence

Soon after attaining full expansion, soybean (Glycine max [L.] Merr.) leaves enter a senescence phase marked by decline in photosynthetic rate and the progressive loss of chloroplast activity and composition. Our primary goal was to determine if this loss could be accounted for by sequential degradation of whole chloroplasts or by simultaneous degeneration of all chloroplasts. Total photosynthesis (TPs) measured as ¹⁴CO₂ uptake, chloroplast number, ribulose 1,5-bisphosphate carboxylase activity, uncoupled photosynthetic electron transport activity, soluble protein content, and chlorophyll content declined progressively during the 37 days after full leaf expansion. During this period, chloroplast number per unit leaf area was constant for all genotypes studied. We conclude that leaf senescence may be a two-stage process wherein the first stage chloroplast activity and composition declines, but chloroplast numbers do not change. During a brief terminal stage (11 days in our experiment), whole chloroplasts may be lost as well. As a second objective we wished to determine if variation in single-leaf total photosynthetic rate among soybean cultivars is related to corresponding variation in chloroplast number and/or chloroplast activity/composition. By comparing the means for three cultivars known to have rapid leaf TPs and for the three known to have slow TPs, we found the former group to be superior to the latter for all the previously mentioned leaf physiological traits. This superiority was related primarily to differences in chloroplast number and only secondarily to differences in activity and composition per chloroplast.     

Quantitative Determination of Changes in the Number and Size of Chloroplasts in Naturally Senescing Leaves of Rice Seedlings

Changes in the number and size of chloroplasts in senescingleaves of rice seedlings were determined. The method employedinvolves electron microscopic examination of large numbers ofcells and chloroplasts in the mesophyll of leaves at differentstages of senescence with the aid of a microcomputer. Analysisshowed that, once leaves had been fully expanded, the numberand size of the mesophyll cells remained unaltered throughoutthe course of senescence. By contrast, the quantity of chloroplastspresent in leaves decreased with advancing senescence. Whencompared with the newly expanded 6th leaves, the chloroplastnumber per unit area of mesophyll section was reduced by 40%and the mean cross section area of chloroplasts by 23% in theoldest leaves (3rd leaves) of seedlings. Chloroplasts occupied33% of the mesophyll section area in the 6th leaves and thepercentage decreased slightly in the 5th leaves and markedlyin lower leaves to reach 17% in the 3rd leaves. The rate ofoxygen evolution decreased approximately in parallel to thedecline in the chloroplast content. Thus, sequential decreasein the amount of chloroplasts is a main cause of loss of photosynthesisduring foliar senescence of rice seedlings. (Received May 31, 1989; Accepted October 17, 1989)     

Relationship between Leaf Structure and Gas Exchange in Wheat Leaves at Different Insertion Levels

Net photosynthesis rate (P_n), stomatal conductance to CO₂ andresidual conductance to CO₂ were measured in the last six leaves(the sixth or flag leaf and the preceding five leaves) of Triticumaestivum L. cv. Kolibri plants grown in Mediterranean conditions.Recently fully expanded leaves of well-watered plants were alwaysused. Measurements were made at saturating photosynthetic photonflux density, and at ambient CO₂ and O₂ levels. The specificleaf area, total organic nitrogen content, some anatomical characteristics,and other parameters, were measured on the same leaves usedfor gas exchange experiments. A progressive xeromorphic adaptation in the leaf structure wasobserved with increasing leaf insertion levels. Furthermore,mesophyll cell volume per unit leaf area (V_mes/A) decreasedby 52·6% from the first leaf to the flag leaf. Mesophyllcell area per unit leaf area also decreased, but only by 24·5%.However, nitrogen content per unit mesophyll cell volume increasedby 50·6% from the first leaf to the flag leaf. This increasecould be associated to an observed higher number of chloroplastcross-sections per mm² of mesophyll cell cross-sectional areain the flag leaf: values of 23000 in the first leaf and 48000in the flag leaf were obtained. P_n per unit leaf area remainedfairly constant at the different insertion levels: values of33·83±0·93 mg dm^–2 h^–1 and32·32±1·61 mg dm^–2 h^–1 wereobtained for the first leaf and the flag leaf, respectively.Residual conductance, however, decreased by 18·2% fromthe first leaf to the flag leaf. Stomatal conductance increasedby 41·7%. The steadiness in P_n per unit leaf area across the leaf insertionlevels could be mainly accounted for by an opposing effect betweena decrease in V_mes/A and a more closely packed arrangement ofphotosynthetic apparatus. Adaptative significance of structuralchanges with increasing leaf insertion levels and the steadinessin P_n per unit leaf area was studied. Key words: Photosynthesis, structure, wheat     

Developmental changes in ribosomal ribonucleic Acid and fraction I protein in wheat leaves

In light-grown wheat (Triticum aestivum L.) seedlings, the amount of chloroplast and cytoplasmic ribosomal RNA increased to a maximum in the first leaf near the end of its growth and declined by about 60% in the following 3 days. While total ribosomal RNA was declining, labeled uracil was still incorporated into cytoplasmic ribosomal RNA, but the rate of incorporation into chloroplast ribosomal RNA fell by more than 80%, as did the incorporation of labeled leucine into fraction I protein. Either there is greater replacement of cytoplasmic ribosomal RNA than chloroplast ribosomal RNA in mature leaves, or chloroplasts are able to repress the incorporation of exogenous precursor when there is no net synthesis of RNA.     

Starch Synthesizing Enzymes in Chloroplasts of Developing Leaves of Spinach (Spinacea oleracea L.)

In experiments in which RuDP carboxylase activity was used asa marker for the integrity of isolated chloroplasts, more than90% of the starch synthase activity and more than 80% of theADP-glucose pyrophosphorylase activity of spinach leaves wasfound to be located in chloroplasts. Less than 2% of the UDP-glucosepyrophosphorylase was associated with chloroplasts. The activityof starch synthase per chloroplast remained almost constantduring plastid replication in developing leaves on whole plantsand in leaf discs cultured for 7 d on agar under different lightconditions. The ADP-glucose pyrophosphorylase activity of chloroplastsincreased during leaf development and was much lower in dark-growntissues. The results suggest that the synthesis of starch iscontrolled by the synthesis of ADP-glucose pyrophosphorylaseas well as by the previously known control of activity by metabolitessuch as 3-phosphoglyceric acid and inorganic phosphate.     

Effects of moisture stress and senescence on the synthesis of abscisic acid in the primary leaves of bean

The capacity for ABA synthesis during moisture stress of primary leaves of bean ( Phaseolus vulgaris cv. Kinghorn) was defined in terms of leaf age and associated changes in several physiological parameters. The leaves were fully expanded within 9 days after emergence. Fresh and dry weights per unit of leaf area fell during all 5 weeks of the study, from leaf expansion through advanced senescence. The most significant losses in weight occurred during the third and fourth weeks and coincided with a sharp drop in protein content that began immediately after full-leaf. Chlorophyll concentrations declined rapidly during leaf expansion and then more slowly through the end of the fifth week when the leaves were ready to abscise. The ratio of chlorophyll a to b rose steeply over the first 4 weeks of the study.
Although a rapid loss of protein provided the most definitive indication of the early stages of leaf senescence, a marked decline in the ability to synthesize ABA was more closely associated with the termination of rapid leaf growth. This relationship between leaf expansion and the capacity for ABA synthesis during moisture stress remained unchanged when ABA content was expressed on a per unit chlorophyll, protein or dry weight basis.
A water deficit between 5 and 10% of fresh weight, representing a drop in water potential of less than 150 kPa, was sufficient to initiate accumulation of ABA in young leaves. Slightly more intensive levels of stress were required to stimulate ABA synthesis in senescent leaves, but total accumulation was less than one-tenth of the amount recorded in the younger tissue.     

Photoinhibition and Light Acclimation in Seedlings of Bischofia javanica, a Tropical Forest Tree from Asia

The aim of this study was to examine the potential for lightacclimation in shade grown seedlings of Bischofia javanica Blume.The seedlings were grown under simulated forest shade light(40 µmol m^–2 s^–1), and after transfer to ahigher light level (1200 µmol m^–2 s^–1), chlorophyllfluorescence induction kinetics, net photosynthesis, and changesin leaf chlorophylls and leaf anatomy were examined in leavesthat were fully developed prior to the transfer. The low-light (LL) leaf displayed photoinhibition immediatelyafter transfer to high-light (HL). This photo-inhibition wassubstantial, and continued for several days. Chlorophyll bleachingoccurred only after a certain degree of photoinhibition hadproceeded. Photosynthetic light acclimation commenced immediatelyafter severe photoinhibition. An increase in chlorophylls perunit leaf area was also immediate after severe bleaching. Thechanges in leaf chlorophylls over time were consistent withthe visual observations of bleaching and recovery. The leafweight per unit leaf area increased gradually on transfer toHL and finally it approached that of the newly formed HL leaf.Although fully expanded prior to transfer to HL, the leaf thicknesswas increased by about 45% and the leaf tissues became denserwithout changing the leaf area and the stomatal density. Finally,the net photosynthetic rate per unit leaf area was higher thanthat before exposure by 75% but less than that of newly formedHL leaf by more than 30%. Moreover, leaf movements were observedafter exposure to HL and also the formation of short epicormicshoots with a cluster of small leaves on the lower part of thestem during light acclimation. It is concluded that the fully expanded shade leaf has a wideacclimation plasticity. In addition to leaf acclimation, wholeplant responses such as leaf movements, the formation of epicormicshoots and the production of new ‘sun-type’ leavesunder HL may be of crucial importance to the success of thespecies following opening of the canopy. Photoinhibition, light acclimation, photosynthesis, fluorescence, tropical trees, shade, Bischofia javanica     

Photosynthetic Electron Transport During Senescence of the Primary Leaves of Phaseolus vulgaris L.: I. NON-CYCLIC ELECTRON TRANSPORT

Coupled, non-cyclic electron transport was measured for chloroplastsisolated from the primary leaves of Phaseolus vulgaris. Preparationsfrom young, fully expanded leaves gave good rates of electrontransport, but the rates obtained decreased by approximately80% during leaf senescence. Higher rates of electron transportwere recorded for chloroplasts isolated from primary leaveswhich had regreened following removal of the remainder of theshoot. With preparations from leaves of all ages, photophosphorylationwas coupled to electron transport with a mean P/2e ratio ofapproximately 1.3. No evidence was obtained for inactivationof chloroplasts from older leaves during isolation or assay,and it is suggested that the decrease in rate of electron transportover the period of senescence, and its increase during regreening,were consequences of changes in the composition and physicalproperties of the thylakoid membrane which occur in vivo. Thedecrease in rate of non-cyclic electron transport may be importantin limiting the rate of photosynthesis in the senescing leaves.     

Effect of Source-Sink Manipulations on the Crassulacean Acid Metabolism of Kalanchoe pinnata

The effect of manipulations of the sink-source at the above-groundlevel and girdling of source leaves was measured in 4-month-oldplants of the CAM species Kalancho pinnata (Lam.) Pers. At thisage plants developed five pairs of leaves. The upper fourthand fifth leaf pairs were not fully expanded and behaved ascarbohydrate sinks. Removal of the developing leaves induceda progressive accumulation of glucans and sugars in the matureleaves. The titratable acidity increased during the second weekbut accumulation was less than in the control plants three tofour weeks after sink removal. Similar, but more rapid, resultswere observed in mature leaves with girdled petioles. Up tothe second night after girdling dark CO₂ fixation increased,but decreased steadily afterwards. CAM Phase 4 (afternoon CO₂fixation) however, was more sensitive to girdling, being reducedby 38% on the first day, and disappearing completely 3 d aftergirdling. The glucan and sugar contents of girdled leaves increasedcontinuously after treatment, but day-night changes ceased completelyon the fifth day. Girdling also caused a considerable increasein chloroplast area, with up to 80% of their internal spaceoccupied by starch grains, leading to grana distortion. In girdledleaves, or in source leaves in plants lacking aerial carbohydratesinks, dawn-dusk changes in titratable acidity started to decreasewhen the leaf glucan content exceeded 1·0 mol equivalenthexoses kg^–1 dry weight. Increased sink strength throughshading of all leaves except one source leaf did not affectits CAM activity. The titratable acidity and non-structuralcarbohydrate content of the shaded mature leaves was reducedby around 55%. Removal of all the mature source leaves acceleratedthe maturation process of sink leaves, increasing titratableacidity at dawn and synthesis of glucans during the light period.The results support the hypothesis that CO₂ fixation in a CAMplant is controlled by accumulation of glucans in chloroplasts. Key words: CAM, glucan accumulation, sink-source ratio, CO₂ fixation     

Measuring the RGR of Individual Grass Plants

Vegetative growth of grasses was analysed by dry mass increaseof growing leaves.Holcus lanatuswas grown in a controlled environmentand leaf extension rates of leaf numbers 5–10 of the maintiller were monitored daily. Leaf appearance and leaf extensionrates (LER) of leaves 5–7 enabled the prediction of thefinal length and dry mass of leaf 8 during its growth. A linearincrease of leaf mass per unit leaf length (LML) of leaf 8 wasobserved during growth. After harvest the daily increase indry mass of growing leaves was calculated from the LER and correspondingincrease in LML. The relative growth rate (RGR) of the maintiller showed day-to-day fluctuations and was gradually reducedby 50% over a 16-d period. The RGR of the shoot was maintainedby tillering. The RGR of a single (grass) plant can be calculatedfrom four parameters only: LER, LML, leaf appearance and tillering.Variation of RGR over a period can be reconstructed after harvestand the impact of these four parameters on RGR can be established.Copyright1998 Annals of Botany Company. Relative growth rate, grass, leaf growth,Holcus lanatus.