LIGHT-INDUCED VOLUME CHANGES IN SPINACH CHLOROPLASTS

A light-dependent mechanism that results in a slow, high-amplitude swelling of spinach chloroplasts in vitro has been discovered. The swelling is readily observed by optical and gravimetric methods, and by the use of an electronic particle counter; all show a 100 per cent increase of chloroplast volume in the light with an approximately 10-minute half-time. The existence of an osmotic mechanism for chloroplast swelling in the dark is confirmed. The volume of illuminated chloroplasts versus NaCl concentration represents the addition of osmotic and light effects. The action of light is enhanced by electron flow cofactors, such as phenazine methosulfate (PMS). However, neither conditions for ATP hydrolysis or synthesis nor NH₄Cl influence the time course and extent of swelling. Hence, high-amplitude chloroplast swelling is light- (or electron flow), but not energy-dependent. A remarkable inhibitory effect of inorganic phosphate on chloroplast swelling is observed in the light, but not in the dark. Another action of light on chloroplasts is known to result in a shrinkage of chloroplasts which is rapid, reversible, energy-dependent, and requires phosphate. Thus phosphate determines the action of light on chloroplast volume. Since shrinkage is reversible, but swelling is not, it may be that they reflect physiological and deteriorative processes, respectively. Chloroplasts and mitochondria appear to control their volume by similar mechanisms.     

The Rate of Photosynthesis in Isolated Chloroplasts

Chloroplasts were isolated using aqueous and nonaqueous procedures.Aqueous chloroplasts lost approximately 50 per cent, of theirsoluble proteins during isolation. Nonaqueous chloroplasts retainedall their soluble enzymes, but lost their ability to performthe light reactions of photosynthesis. It was possible to reconstitutea chloroplast system of higher activity by adding soluble enzymesfrom nonaqueous chloroplasts to protein-deficient aqueous chloroplasts.The properties of the reconstituted chloroplast system wereas follows: 1. The CO₂ fixation rate of the reconstituted chloroplast system( 4 µM./. chlorophyll/hr.) was 3–4 times that ofthe aqueous chloroplasts ( I µM./. chlorophyll/hr.). Thefixation of aqueous chloroplasts isapparently limited in partby lack of soluble enzymes. 2. During light-fixation, the reconstituted chloroplast systemaccumulated PGA. This indicates that the reduction of PGA totriosephosphate is a rate-limiting step in this system. 3. It was possible to increase the CO₂ fixation to 12 µM.CO₂/mg. chlorophyll/ hr. by addition of ATP and TPNH to thesystem, but the reduction of PGA was still rate-limiting. 4. Further increase in the fixation rate was obtained by concentratingthe reaction mixture. Part of the striking differences of theCO₂-fixing capabilities of chloroplasts in vivo and in vitrois caused by dilution effects. Extrapolation of the dilutioneffect to the protein concentration which exists in chloroplastsyields a CO₂ fixation rate of approximately 30 µM./mg.chlorophyll/hr. 5. Inhibitors which are located in vivo outside the chloroplastsaffect the CO₂ fixation in vitro. 6. Under consideration of the examined factors which influencethe CO₂ fixation of isolated chloroplasts, it is possible toraise the fixation from approximately 1 per cent, to at least15 per cent, of the fixation in vivo.     

Swelling of spinach chloroplasts induced by HCHO and KMnO4, an effect partially hidden by the wide size range of chloroplasts

Both KMnO₄ and HCHO in concentrations used for fixation forelectron microscopy induce pronounced swelling of spinach chloroplasts.However, since electron microscopy samples small numbers, itis possible to overlook the swelling effect because the sizerange of the swollen chloroplasts can overlap the extremelywide range of chloroplasts in living mesophyll cells. HCHO fixesspinach chloroplasts only after 16 hr incubation, as verifiedby failure of the chloroplaststo swell when subsequently washedwith water. However, the HCHO treatment fails to prevent aninitial swelling and KMnO₄ further swells chloroplasts pre-fixedwith HCHO. Spinach chloroplasts in vivo measured in face area27.7 0.06 µ² mean value, 23.8 µ² mode value, range6.2 to 102.9 µ², and their distribution is skewed so thatthe coefficient of skewness is 0.15. Chloroplasts isolated directlyinto phosphate buffered 4% HCHO after 24 hrs measured in facearea 58.2 µ² mean value, 46.5 µ² mode value, range22 to 121 µ², and the coefficient of skewness increasedto 0.24. When such chloroplasts were additionally treated withphosphate buffered 2.8 % KMnO₄ the spinach chloroplasts measuredin facearea 96.4 1.40 µ² mean value, 86.1 µ² modevalue, range22 to 203 µ², and the coefficient of skewnessunchanged at 0.24. Volumes of spinach chloroplasts isolatedin NaCl as reported in the literature approach the volumes ofchloroplasts swollen by HCHO and KMnO₄. Some problems concerningsampling difficulties because of wide size ranges and skeweddistributions are discussed. ¹ Present address: Department of Agriculture, Bangkhen ExperimentStation, Bangkok, Thailand. ² Present address: Department of Biology, Wright State University,Dayton, Ohio 45431 U.S.A.     

STIMULATORY EFFECT OF CARBON DIOXIDE UPON THE HILL REACTION AS OBSERVED WTTH THE ADDITION OF CARBONIC ANHYDRASE TO REACTION MIXTURE

The stimulatory effect of CO₂ upon the HILL reaction by isolatedchloroplasts was observed with erythrocyte carbonic anhydraseas a supplementary agent for CO₂ deprivation. Addition of thisenzyme to the reaction media remarkably shortened the time requiredto obtain the maximal effect of CO₂ The degree of stimulationwas rather small (below 50 per cent) and varied depending onthe preparation of chloroplasts. In general, the effect wasgreater with broken chioroplasts than with whole chloroplasts.The lowering of light intensity diminished the CO₂-effect. ¹ Present address: Laboratory of Biological Chemistry, TokyoInstitute of Technology, Meguro-ku, Tokyo. (Received April 6, 1962; )     

The Response of Phaseolus vulgaris L. to Root-zone Anaerobiosis, Waterlogging and High Sodium Chloride

Phaseolus vulgaris L. grown at a range of external concentrationsof NaCl (0 to 80 mM) responded differently to gaseous anaerobiosis(N₂ gas) in nutrient solution or stagnant waterlogging of theroot-zone. With similar patterns of distribution of Na⁺ andCl^- occurring in the plants with comparable NaCl treatments,and similar final concentrations of Na⁺ and Cl^- in plants grownunder both root-zone conditions, rates of uptake of Na⁺ andCl^- were much higher in plants with the stagnant waterloggedrootzones. After 72 h stagnant waterlogging, plant tops fromplants grown at 40 mM NaCl contained 1.42 per cent Na⁺ and 3.44per cent Cl^- (d. wt basis) while after 9 days exposure to NaClwith gaseous anaerobiosis, leaf tissue contained 1.49 per centNa⁺ and 4.28 per cen Cl^- (d. wt basis). Plants exposed to 40mM external NaCl were severely damaged within 72 h when grownwith stagnant waterlogged root-zones; those grown with N₂ anaerobiosiscontinued growth and development over the 9 d period. Plantsgrown in nutrient solution showed changes in distribution andconcentration of Na⁺ and Cl^- when oxygen concentration was reducedbelow 21 per cent O₂ (full aeration). Phaseolus vulgaris. L., bean, mineral salt distribution, anaerobiosis, salinity, waterlogging     

Carbon Assimilation at Low Carbon Dioxide Levels: I. APPARATUS AND TECHNIQUE

The construction and operation of a versatile apparatus forthe measurement of CO₂ exchange of detached plant parts is described. CO₂ concenteration was measured with an accuracy of about ±3per cent using a commercial infra-red gas analyser; measurementswere made at ambient CO₂ levels between 10 and 10,000 p.p.m.(0.001 per cent. and 1.0 per cent. by volume), at leaf temperaturesbetween 5°C. and 40°C. (±0.1°C.) and at lightintensities up to 2,000 foot candles. The measurements were made on either a fixed volume of gas repeatedlypassed over the leaf, or on a stream of gas passing over theleaf once only, or with any desired combination of these two. Rates of gas flow (up to 801./hr.) could be controlled to finelimits independent of changes in flow resistance and measuredwith an accuracy of at least ±1 per cent., if required.     

Assimilation of carbon dioxide in mesophyll chloroplasts and bundle sheath strands mechanically isolated from corn leaves

Mesophyll chloroplasts capable of assimilating 1.2 µmolesCO₂ per milligram chlorophyll per hour were isolated from 7-day-oldcorn (Zea mays, Nagano No. 1) leaves. Addition of phosphoenolpyruvateincreased the rate of CO₂ fixation in light up to 22 µmolesper milligram chlorophyll per hour, whole exogenously addedribose 5-phosphate and adenosine triphosphate brought aboutonly small increases. The CO₂ fixation products were mostlymalate and aspartate. Bundle sheath strands isolated from the same plants were capableof assimilating 3–26 µmoles CO₂ per milligram chlorophyllper hour. The fixation rate increased 3- to 5-fold on additionof ribose 5-phosphate and adenosine triphosphate, while exogenousphosphoenolpyruvate had no effect. The bulk of early productsof light-induced CO₂ fixation were phosphate esters. These results indicate that corn mesophyll chloroplasts initiallyfix CO₂ by phoenolpyruvate carboxylase and that reductive pentosephosphate cycle occurs in corn bundle sheath cells, but notin the mesophyll chloroplasts. (Received January 25, 1974; )     

Vieillissement de l'appareil photosynth?tique II. Effet synergique de la lumi?re et du vieillissement in vitro sur les activit?es photochimiques de chloroplastes isol?s d'?pinard

The consequences of chloroplast ageing in vitro were furtherinvestigated, especially on the photochemical activities ofthese organelles. Ageing of chloroplasts in dark was accompanied by decreasesin activities for photohydrolysis and cyclic and non-cyclicsyntheses of ATP, photoreduction of NADP⁺ and O₂ evolution;but there was no decrease in ferricyanide photoreduction. Therates of decrease in these activities were comparable to therate of increase in chloroplast volume. Complete inhibitionswere reached when maximum chloroplast swelling had occurred,i.e. after 5 to 6 hr of incubation at 20?C in a Tris-NaCl (pH8) medium. Ageing in the light resulted in much accelerateddecreases in activities tested; the loss of capacity for light-inducedshrinkage was also accelerated by the light during ageing. Thus,light acts synergetically towards the ageing process. Moreover,light and, to a less extent, dark ageing, resulted in an uncouplingof chloroplast photophosphorylation and associated electronflow measured by ferricyanide photoreduction. The part of theelectron flow which is induced by coupling (+ ADP, Pi, MgCl₂,pH 8) or by uncoupling (+ NH₄C1, pH 7) was found to be verysensitive to light ageing. The NADP⁺ photoreduction loss wasrestored by addition of the ascorbate-DCPIP electron donor system,suggesting that ageing interferes with the integrity of photosystemII. In many respects, these effects of ageing are comparable withthe action of detergents and fatty acids on the structure andphotochemical activities of chloroplasts, especially in thatthey attack the energy transducing mechanism in chloroplasts. (Received May 24, 1969; )     

Some Factors Concerning the Centripetal Disposition of Bundle Sheath Chloroplasts during the Leaf Development of Eleusine coracana

Factors concerning the chloroplast disposition in bundle sheathcells were investigated in finger millet (Eleusine coracanaGaertn.), and NAD malic enzyme type C₄ plant with the centripetalarrangement of bundle sheath chloroplasts. Segments were cutfrom immature regions of emerging leaves in which the centripetalarrangement of bundle sheath chloroplasts had not yet been established.The leaf segments were floated on solutions with or withoutreagents. Sections were made of the segments at time intervalsand the distribution of bundle sheath chloroplasts was observedby light microscopy. The bundle sheath chloroplasts migratedto the vascular bundle and established a centripetal arrangementby 12-16 h in control solutions. Auxins, cycloheximide and cytochalasinB inhibited the disposition of bundle sheath chloroplasts whilechloramphenicol and colchicine had no effect. The inhibitoryeffect of auxins appeared only at early stages of chloroplastmigration while cycloheximide and cytochalasin B were effectiveeven at later stages. Cessation of elongation growth, cytoplasmicprotein synthesis and microfilaments seemed to be associatedwith the centripetal disposition of bundle sheath chloroplasts.Copyright1993, 1999 Academic Press Bundle sheath chloroplast, C₄ plant, chloroplast orientation, Eleusine coracana, finger millet     

Volume changes of isolated spinach chloroplasts on illumination in the presence of phenazine methosulfate

Chloroplasts suspended in phosphate buffer with PMS swelledrapidly on illumination, their volumes reaching a maximum levelwithin 5 min. Subsequently they shrank noticeably. Both swellingand subsequent shrinkage were reversible; in the former, chloroplastsswelled on illumination and shrank on turning off the light.In the latter,olume change occurred in the opposite direction.Light-induced swelling in the presence of PMS disappeared onalternating light and dark several times. Thereafter, only shrinkagewas observed. Only shrinkage took place on illumination when PMS was addedto Tris-washed, or heated chloroplasts, or when chloroplastswere suspended in phosphate buffer with other electron transportcofactors such as FMN and vitamin K₃ or in acetate buffer insteadof phosphate in the presence of PMS. PMA and CCCP (low concentration)inhibited swelling with PMS. Quinacrine had no effect on volumechanges with PMS, while antimycin A and CCCP (high concentration)completely prevented both swelling and shrinkage. These suggestthat volume changes are a result of competition between swellingand shrinking activities which depend on utilization of highenergy intermediates formed by PMS-induced photochemical reactions. (Received February 18, 1970; )     

Metabolism of Radioactive Sugars by Tobacco leaf Disks

Destarched tobacco-leaf disks were floated on per cent. (w/v)solutions of sucrose uniformly labelled with ¹⁴C in either theglucose or fructose moiety, and on invert sugar in which onehexose only was so labelled. The experiments were carried outin an atmosphere of oxygen at 25° C. Seventy-five per cent,of the sugar lost from the external solutions was recoveredas starch, sucrose, fructose, glucose, and CO₂. With sucroseas the substrate, 30 per cent, of the material was recoveredas CO₂ and 17 per cent. each as starch, sucrose, fructose, andglucose. With invert sugar as the substrate, 30 per cent, wasagain recovered as CO₂ only 20 per cent. as the three sugarstogether, and 50 per cent. as starch. Whichever hexose was initiallylabelled and whether the sugar was supplied as sucrose or hexose,the relative specific activities of starch and sucrose in theleaf disks and of the CO₂ evolved were equal or nearly equalto that of the sugar supplied. With sucrose as the substratethe sucrose in the disks retained its asymmetry of label, andfree hexoses produced were similarly asymmetrically labelled.When invert sugar was the substrate the sucrose synthesizedwas strongly labelled in both moieties, as also were the freehexosea. It is concluded that fructose and glucose free or combinedin sucrose were equally available for starch synthesis and CO₂,formation, and that there can be no question of preferentialutilization of one or other hexose. Starch and CO₂ must arisefrom a common source in which readily formed derivatives ofthe hexoses are rapidly equilibrated. Free hexose cannot participatedirectly in either sucrose or starch synthesis. Accumulationof sugar not immediately metabolized and inversion of sucrosetake place at a site remote from the common pool. A scheme toaccommodate the results is discussed.     

Overcoming Incompatibility in Brassica campestris L. by Carbon Dioxide, and Dark Fixation of the Gas by Self- and Cross-pollinated Pistils

Supplementing pollen suspension cultures with CO₂ (3–5per cent) caused a marked increase in germination and tube growthin vitro in Brassica campestris L. cv. toria. A weakening ofself-incompatibility by increased CO₂ levels from 3–5per cent was observed. The percentage of pollen tubes whichpenetrated the cuticle layer of stigmatic papilla cells in self-pollinatedpistils was high when CO₂ level was 5 per cent. Phosphoenolpyruvate (PEP) carboxylase activity was greater in the pollengerminated in 4 per cent CO₂ as compared to air (0.03 per cent).A possible role of CO₂ for self-recognition and control of pollentube growth is proposed, proposed. Brassica campestris L., carbon dioxide, self-incompatibility, phosphoenol pyruvate carboxylase     

Increased CO2 fixation by Pisum sativum chloroplasts in vitro reflecting a change in coupling caused by illuminating the plants

Illumination of pea plants caused a doubling in the rate ofCO₂ fixation by the subsequently isolated chloroplasts comparedwith the rate obtained for chloroplasts from plants in the dark.This enhancement in the CO₂ fixation rate was half-maximal for800 lux incident on the plants and was 90% light saturated at2000 lux. The half-time for the enhancement of the CO₂ fixationrate following illumination of the plants was about 4 min andthe half-time for its reversal when the plants were placed backin the dark was 5 min. Illuminating the plants had relativelylittle effect on the O₂ evolution rate of the subsequently isolatedchloroplasts. Moreover, the ferricyanide reduction rate by theisolated chloroplasts was also essentially unaffected by theillumination condition of the plants from which the chloroplastswere isolated. Consequently, light on the plant apparently causesa doubling in the CO₂ fixed per electron moving in the photosyntheticelectron transport pathway. This enhanced coupling is discussedin terms of a concomitant increase in endogenous photophosphorylationand flattening of the chloroplasts in vivo, other changes causedby light incident on the plant. (Received January 16, 1970; )     

Preliminary Studies on the Photosynthetic Structures of Trifolium alpinum L. as Related to Productivity

Trifolium alpinum L. is a high-quality alpine forage plant growingspontaneously from 1900 to 2800 m above sea level and is widelydistributed in Piedmont and the Valle d'Aosta (Italy), whereit can reach population frequencies of 90 per cent. Yields weredetermined on forage harvested in the Valle dell'Orco (Piedmont)and were comparable to cultivated clovers from higher latitudes;yields decreased progressively as the elevation increased. Thechemical and nutritional characteristics of the forage, thoughcomparable to clovers cultivated in the Po valley (Italy), were,however, more constant. The structure of the leaf lamina asrelated to elevation was investigated using light microscopy,TEM and SEM. This is complemented by data on chlorophyll concentration,succulence, specific leaf weight and area. At all elevationsT. alpinum lacks, apart from bundle sheath cell chloroplastsin a centrifugal arrangement, the structural characteristicsof C₄ plants. The chlorophyll a:b ratio (less than four) istypical of a C₂ plant. Succulence indices (S and S_m) were verylow, making CAM pathway photosynthesis unlikely. Unusual anddifficult to interpret structures included: small functionalchloroplasts in both the epidermises, stomata present almostexclusively in the upper epidermis and mitochondria enveloped(or enclosed) by chloroplasts. It was observed that, as theelevation increases, populations are selected which are well-adaptedfor gas exchange (increase in specific leaf area, stomatal densityand intercellular spaces) and characterized by a decrease inthe grana thylacoid:integrana thylacoid ratio (consistent withthe increase in the chlorophyll a:b ratio), the per cent water,S_m and the specific leaf weight. Trifolium alpinum L., alpine trefoil, leaf structures, photosynthesis, yield, elevation, C₂, C₄     

The Carbon Balance of a Legume and the Functional Economy of its Root Nodules

Budgets for carbon and nitrogen in shoot, root, and nodulesof garden pea (Pisum sativum L.) are drawn up for a 9-d intervalin the life cycle, from data on nitrogen fixation, carbon accumulationin dry matter, respiratory output of plant organs, and organicsolute exchange between shoot and nodulated root. Of the carbon gained photosynthetically by the shoot from theatmosphere 26 per cent is incorporated directly into its drymatter, 32 per cent translocated to the nodules, and 42 percent to the supporting root. Of the nodules’ share, 5per cent is consumed in growth, 12 per cent in respiration,and 15 per cent returned to the shoot via the xylem, as aminocompounds generated in nitrogen fixation. Growth and respirationof the root utilize, respectively, 7 and 35 per cent. The respiratory efficiency of a nodulated root in terms of nitrogenfixation (5.9mg C per mg N₂-N fixed) is found to be very similarto that of an uninoculated root assimilating nitrate (6.2 mgC per mg NO₃-N reduced). The nodules require in growth, respiration,and export 4.1 mg C ( 10.3 mg carbohydrate) for each mg N whichthey fix. The nodules consume 3 ml O₂ for every 1 ml N₂ utilized in fixation. In exporting a milligram of fixed nitrogen the nodules requireat least 0.35 ml of water. Almost half of this requirement mightbe met by mass flow into the nodules via the phloem.     

Carbon Dioxide Fixation and Ribulose-1, 5-bisphosphate Carboxylase Activity in Intact Leaves of Sugar Beet Plants Exposed to Salinity and Water Stress

The influence of salinity in the growing media on ribulose-1,5-bisphosphate (RuBP) carboxylase and on CO₂ fixation by intactsugar beet (Beta vulgaris) leaves was investigated. RuBP carboxylase activity was mostly stimulated in young leavesafter exposure of plants for 1 week to 180 mM NaCl in the nutrientsolution. This stimulation was more effective at the higherNaHCO₂ concentrations in the reaction medium. Salinity also enhanced CO₂ fixation in intact leaves mostlyat rate-limiting light intensities. A 60 per cent stimulationin CO₂ fixation rate was obtained by salinity under 450 µEm^–2 s^–1. At quantum flux densities of 150 µEm^–2 s^–1 (400–700 nm) this stimulation was280 per cent. Under high light intensities no stimulation bysalinity was found. In contrast, water stress achieved by directleaf desiccation or by polyethylene glycol inhibited enzymeactivity up to fourfold at –1.2 MPa. Beta vulgaris, sugar beet, ribulose-1, 5-bisphosphate carboxylase, salt stress, water stress, carbon dixoide fixation, salinity     

Ultrastructural Changes in Chloroplasts of Quercus rotundifolia Lam. in Response to Evernic Acid

The amount of total chlorophyll, chlorophylls a and b as wellas the ratio of a to b decreased in chloroplasts isolated fromQuercus rotundifolia leaves, kept for 17 d in a solution of35.5 µM evernic acid in 1 mM Na HCO₃, when compared withthe chloroplasts of control leaves (kept in NaHCO₃). The chloroplastsin the spongy parenchyma were smaller and the amount of starchand plastoglobuli lower. The number of grana per chloroplastsection, the number of thylakoids per grana and the height ofgrana stacks were also less in the chloroplasts of leaves treatedwith evernic acid. Quantitative ultrastructural differenceswere determined by means of electron microscopy and image analysistechniques. Quercus rotundifolia Lam., chloroplasts, ultrastructure, lichens, evernic acid     

Preparation of Radioactive Starch, Glucose and Fructose from C14O2

Radioactive starch, glucose and fructose have been preparedfrom tobacco leaves after assimilation of C¹⁴O₂. The apparatusused for photosynthesis consisted of a shallow Perspex leafchamber connected to a closed gas system, in which C¹⁴O₂ wasgenerated from BaC¹⁴O₂. Six leaves, area 14 to 18 sq. dm. whenexposed to bright sunlight with an initial CO₂ concentrationof 8 to 10 per cent., assimilated 3.35 g. of C¹⁴O₂ in 8 to 10hours. At least 80 per cent. of the C¹⁴O₂ supplied appearedin the leaves as starch and sugar and over 80 per cent. of theradioactivity was accounted for in these carbohydrates. Thespecific activity per m. atom of carbon of the isolated productswas 85 to 90 per cent. of that of the C¹⁴O₂. Small amounts ofradioactive carbon were also incorporated in the leaf proteinand in the celluose, hemicellulose and polyuronides.     

Induction of Light Dependent Pyruvate Transport into Chloroplasts of Mesembryanthemum crystallinum by Salt Stress

Mode of photosynthesis in Mesembryanthemum crystallinum changesfrom C₃ to Crassulacean acid metabolism (CAM) when the plantswere stressed with high salinity. [¹⁴C]Pyruvate uptake for 30s into intact chloroplasts isolated from leaves of the CAM modeof M. crystallinum was enhanced more than 5-fold in the lightcompared with that in the dark. The stromal concentration ofpyruvate in the light reached to more than 2.5 times of themedium. In contrast, little or no pyruvate uptake occurred inchloroplasts from C₃ leaves in either light or dark condition.The initial uptake rate (10 s incubation at 4°C) into theCAM chloroplasts in the light was about 3-fold higher than therate in the dark. K_m and V_max of the initial uptake in the lightwere 0.54 mM and 8.5 µmol (mg Chl)^–1 h^–1 respectively.These suggest that pyruvate was actively incorporated into theCAM chloroplasts against its concentration gradient across theenvelope in the light. When hydroponically grown M. crystallinumwere stressed by 350 mM NaCl, the capacity of chloroplasts forpyruvate uptake was induced in 6 d corresponding to the inductionof the activities of PEP-carboxylase and NAD(P)⁺-malic enzymesin response to salt stress. (Received October 12, 1995; Accepted January 19, 1996)     

The Photosynthetic Activity of the Wheat Ear

The contribution made by ear photosynthesis to grain yield wasfound to vary from 10 per cent to 44 per cent depending on thetechnique used, and on environmental conditions. A modifiedear-shading technique is described which overcomes some of theundesirable features of previous ear-shading methods. It wasalso found that ear photosynthesis comprises two processes,(a) the assimilation of atmospheric CO₂ and (b) the photosyntheticrefixation of the ear's respiratory CO₂. Dry-weight data andmeasurements of CO₂ exchange both indicated that this lattercomponent can make a significant contribution to grain yield.