Effects of Isoprenoid Alcohols on Oxygen Exchange of Isolated Chloroplasts in Relation to their Possible Physiological Effects on Stomata

Farnesol accumulates in water-stressed plants of Sorghum, andhas been shown to induce closure of stomata in the Commelinabioassay. In the present study we have found that it inhibitsbicarbonate- and PGA-stimulated O₂ evolution of isolated Pisumchloroplasts. Farnesol apparently disrupts the membranes ofisolated plastids and most of the effects that we have observedcan probably be attributed to this. Apart from nerolidol, relatedisoprenoid alcohols had no comparable effects, but the fattyacid linoleic acid appeared to cause damage of a similar nature.These compounds all caused stomata of Commelina to close. It is suggested that, while farnesol causes irreversible damageto membranes when applied exogenously in in vitro systems, itmight act in vivo as a regulator of membrane structure and soaffect the solute movements on which the turgor changes of guardcells depend.     

Diurnal fluctuation of light and dark CO2 fixation in peeled and unpeeled leaves of Bryophyllum daigremontiana

Diurnal fluctuation of light and dark CO₂ fixation in peeledand unpeeled leaves of Bryophyllum daigremontiana was examined.A distinct difference in light CO₂ fixation was observed inunpeeled leaves but not in peeled ones. No measurable differencein dark CO₂ fixation was observed in either type. These resultsindicate that the leaves of CAM plants have a high capacityfor CO₂ fixation in the daytime, but it is suppressed by theclosing of the stomata. Also, the rapid depression of CO₂ uptakewhen the illumination was directed at on dark acidified leavescould be prevented by peeling off the epidermis. The net photosyntheticCO₂ uptake in peeled leaves was 77 µmoles/mg chllrophyll/hrin the 3rd leaf and 62 in the 4th leaf. (Received August 7, 1978; )     

Leaf Characteristics and Net Gas Exchange of Diploid and Autotetraploid Citrus

The effect of tetraploidy on leaf characteristics and net gasexchange was studied in diploid (2x ) and autotetraploid (4x) ‘Valencia’ sweet orange (Citrus sinensis (L.)Osb.) and ‘Femminello’ lemon (Citrus limon (L.)Burm. f.) leaves. Comparisons between ploidy levels were madeunder high irradiance (I) in a growth chamber or low total Iin a glasshouse. Tetraploids of both species had thicker leaves,larger mesophyll cell volume and lower light transmittance thandiploids regardless of growth I. Mesophyll surface area perunit leaf area of 2x leaves was 5–15% greater than on4x leaves. Leaf thickness and mesophyll cell volume were greaterin high I leaves than low I leaves. In high I, average leafarea was similar for 2x and 4x leaves, whereas in low I it was30% greater in 4x than in 2x leaves. Nitrogen and chlorophyllconcentration per cell increased with ploidy level in both growthconditions. The ratio of chlorophyll a:b was 25% greater in2x than in 4x leaves. When net CO₂assimilation rate (A_CO2) wasbased on leaf area, 4x orange leaves had 24–35% lowerA_CO2than their diploids. There were no significant differencesin A_CO2between 2x and 4x orange or lemon leaves when expressedon a per cell basis. Overall, lower A_CO2per unit leaf area oftetraploids was related to increase in leaf thickness, largermesophyll cell volume, the decrease in mesophyll area exposedto internal air spaces, and the lower ratio between cell surfaceto cell volume. Such changes probably increased the resistanceto CO₂diffusion to the site of carboyxlation in the chloroplasts. Cell volume; chlorophyll; irradiance; leaf thickness; nitrogen; photosynthesis; ploidy; Citrus limon ; C. sinensis ; ‘Valencia’ sweet orange; ‘Femminello’ lemon     

Separation of Direct and Indirect Responses of Stomata to Light: Results from a Leaf Inversion Experiment at Constant Intercellular CO2 Molar Fraction

Previous work has shown that stomata respond directly to light,but it was not clear whether the only additional response isthrough CO₂, or whether some other metabolite is involved inthis response. Gas exchange experiments were done with normallypositioned and inverted leaves of Hedera helix to investigatethis problem. The macroscopic optical properties of the leavesand their anatomical structure were also studied. These experimentssnowed that there is no need to postulate the existence of amessenger other than CO₂ to explain the indirect response ofstomata to light. The experiments also showed that leaf inversionaffects both stomatal conductance and photosynthesis, and highlightthe difficulties involved in the interpretation of the effectof leaf inversion on stomata when stomatal conductance measurementsare not done concurrently with measurements of CO₂ flux densityand intercellular CO₂ molar fraction Key words: Hedera helix, ivy, gas exchange, leaf inversion, stomatal conductance, light, CO₂ flux density, photosynthesis     

Patterns of Stomatal Behaviour, Transpiration, and CO2 Exchange in Pea Following Infection by Powdery Mildew (Erysiphe pisi)

A comparison was made of stomatal behaviour, and related phenomena,between leaves of garden pea (Pisum sativum cv. Feltham First)inoculated with powdery mildew fungus (Erysiphe pisi) and uninfectedleaves on healthy plants. Twenty four hours after inoculation,stomata opened more widely in the light in infected leaves thanin healthy leaves. Thereafter, stomatal opening was progressivelyreduced by infection and stomata failed to close completelyin the dark until, 7 d after inoculation, all movements ceasedand stomata remained partly open. Transpiration in the lightfollowed closely the pattem of stomatal opening and, after anearly increase compared with healthy controls, was progressivelyreduced by infection. Evidence is presented that transpirationfrom the fungus was less than the reduction in transpiraationfrom the leaf which was caused when development of the myceliumincreased the boundary layer resistance of the leaf. Seven daysafter inoculation, transpiration in the dark was greater frominfected leaves than from healthy leaves because of partly openstomata in the dark. Net photosynthesis in infected leaves was reduced within 24h of inoculation to a level below that found in healthy leavesand thereafter it declined progressively. The initial reductionwas due to a transient increase in photorespiration, for whenthe glycolate pathway was inhibited by a 2% O₂ concentrationthere was no difference between the (gross) photosynthetic ratesof healthy and infected leaves. Changes in photorespirationrate were confirmed from the interpretation of the CO₂ burston darkening. Reduced stomatal opening was a contributory causeof the reduction in net photosynthesis in the later stages ofinfection. Since the rate of gross photosynthesis, but not therate of photorespiration, of infected plants fell below thatof healthy plants, and infected plants had a higher rate ofrelease of CO₂ in the dark than healthy plants from the thirdday after inoculation onwards, infected plants consume an increasinglygreater proportion of their photosynthate in respiratory processesthan do healthy plants. The CO₂ compensation point of infectedplants increased at every time of sampling after inoculation.     

Stomatal Opening in Light of Different Wavelengths: Effects of Blue Light Independent of Carbon Dioxide Concentration

Stomatal opening in Xanthium pennsylvanicum was found to besignificantly greater in blue light than in red. Experimentsin which leaves were placed in a closed system and allowed toestablish their own steady-state carbon dioxide concentrationshowed that when the CO₂ concentration was about the same asthat in red, opening was much greater in blue light. Blue lightof low intensity could cause as great an opening as red of higherintensity, even though the CO₂ concentration was much higherin blue. Stomatal opening in light is considered as involvingat least two reactions: (1) a response to the removal of CO₂by photosynthesis; (2) a response to blue light not dependenton the removal of CO₂. Blue light became increasingly effective, relative to red, asthe length of night was increased over the range 2 to 14 hours.This might, in part, explain previously observed effects ofnight length on rate of opening in light. The initial very rapid phase of closure in darkness appearedto be independent of CO₂ accumulation, for it was not preventedby flushing the intercellular spaces with air free of CO₂. Itis suggested that closure in darkness, like opening in light,should be considered as involving components both dependentupon, and independent of, CO₂ concentration.     

Antioxidants in sun and shade leaves of sour orange trees (Citrus aurantium) after long-term acclimation to elevated CO2

Antioxidative systems and the contents of pigments, malondialdehyde,soluble protein, and carbohydrate were investigated in sun-and shade-acclimated leaves of sour orange (Citrus aurantium)trees that had been grown for 7.5 years under ambient and elevated(+300 µmol mol^–1) atmospheric CO₂ concentrations.Sunacclimated leaves contained higher ascorbate, glutathioneand soluble carbohydrate contents and higher catalase activitiesthan shade-acclimated leaves. The activities of superoxide dismutases,which belonged to the family of Cu/Zn-isozymes, were similarin sunand shade-acclimated leaves and decreased in responseto enhanced CO₂. In shade-acclimated leaves, none of the otherparameters studied was affected by elevated CO₂. In sun-acclimatedleaves elevated CO₂ caused increases in carbohydrate and ascorbatecontents. There was no evidence for enhanced lipid peroxidationas assessed from the determination of the malondialdehyde contentsunder either conditions. Key words: Ascorbate, catalase, CO₂ enrichment, global change, glutathione, superoxide dismutase     

Delay in the Response of Stomata to Abscisic Acid in CO2-free Air

Abscisic acid (10^–5 M) was fed via their petioles to leavesdetached from well watered plants of Xanthium strumartum, whilethe intercellular spaces were flushed with air of known CO₂content. A closing response to ABA occurred in the presenceor absence of CO₂, and the stomata responded to CO₂ whetheror not ABA was supplied to the leaves. A factorial experimentrevealed no interaction between CO₂ and ABA, and suggested thattheir effect on the rate of closure was purely additive. Theonly evidence of interdependence between the two corn poundswas a delay in the response to ABA in C0 air, which was moremarked in a high light intensity. A hypothesis which is consistentwith the data is that ABA induces stomatal closure by interferingwith the energy supply required for the active transport processeson which guard cell turgor depends. The inhibitory action ofABA takes longer in CO₂-free air because, in the absence ofCO₂ fixation, energy is available from chioroplasts as wellas mitochondria.     

Comparison of Palaeobotanical Observations with Experimental Data on the Leaf Anatomy of Durmast Oak [Quercus petraea(Fagaceae)] in Response to Environmental Change

To test whether stomatal density measurements on oak leaf remainsare reliable tools for assessing palaeoatmospheric carbon dioxideconcentration [CO₂], under changing Late Miocene palaeoenvironmentalconditions, young seedings of oak (Quercus petraea,Liebl.) weregrown at elevatedvs.ambient atmospheric [CO₂] and at high humiditycombined with an increased air temperature. The leaf anatomyof the young oaks was compared with that of fossil leaves ofthe same species. In the experiments, stomatal density and stomatalindex were significantly decreased at elevated [CO₂] in comparisonto ambient [CO₂]. Elevated [CO₂] induced leaf cell expansionand reduced the intercellular air space by 35%. Leaf cell sizeor length were also stimulated at high air humidity and temperature.Regardless of a temperate or subtropical palaeoclimate, leafcell size in fossil oak was not enhanced, since neither epidermalcell density nor length of the stomatal apparatus changed. Theabsence of these effects may be attributed to the phenologicalresponse of trees to climatic changes that balanced temporalchanges in environmental variables to maintain leaf growth underoptimal and stable conditions.Quercus petraea,which evolvedunder recurring depletions in the palaeoatmospheric [CO₂], maypossess sufficient phenotypic plasticity to alter stomatal frequencyin hypostomatous leaves allowing high maximum stomatal conductanceand high assimilation rates during these phases of low [CO₂].Copyright1998 Annals of Botany Company Atmospheric CO₂, high humidity, elevated temperature,Quercus petraea,durmast oak, Late Miocene, palaeoclimates, leaf anatomy, stomatal density, stomatal index     

CO2 fixation in leaves of a CAM plant without lower epidermis and the effect of CO2 on their deacidification

The effects of peeling the epidermis off Bryophyllum daigremontianumleaves on CO₂ uptake in light and darkness were investigated.Light-induced CO₂ uptake in the daytime was markedly enhancedin the peeled leaves, but dark fixation of CO₂ carried out atmidnight was not. The difference in promotion of CO₂ uptakein light and darkness was due to stomatal closing in the dayand opening at night. Also, deacidification was strikingly inhibitedby CO₂ in peeled leaves. (Received February 3, 1977; )     

Photosynthetic Carbon Reduction and Carbon Oxidation Cycles are the Main Electron Sinks for Photosystem II Activity During a Mild Drought

Stomatal closure can explain the inhibition of net CO₂ uptakeby a leaf subjected to a mild drought: the photosynthetic apparatusappears resistant to lack of water. Changes in both the watercontent of leaves maintained in a constant environment and theambient CO₂ molar fraction during measurements on well-hydratedleaves lead to similar effects on net CO₂ uptake and whole chainelectron transport as estimated by leaf chlorophyll fluorescencemeasurements. In particular, it is shown that photosystem II(PSII) functioning and its regulation are not qualitativelychanged during desiccation and that the variations in PSII photochemistrycan simply be understood by changes in substrate availabilityin this condition. Moreover, an analysis of the literature showsthat when inhibition of net CO₂ uptake by C₃ leaves under drought(Phaseolus vulgaris L., Helianthus annus L. and Solanum tuberosumL.) was lower than 80 %, elevated CO₂ completely restored thephotosynthetic capacity. The CO₂ molar fraction in the chloroplastsdeclines as stomata close in drying leaves. As a consequence,in C₃ plants, ribulose-1,5-bisphosphate oxygenation increasesand becomes the main sink for photosynthetic electrons. Dependingon the prevailing photon flux density, the O₂ uptake throughphotorespiratory activity can entirely replace carbon dioxideas an electron acceptor, or not. The rate of the Mehler reactionremains low and unchanged during desiccation. However, droughtcould also involve CO₂-sensitive modification of the photosyntheticmetabolism depending on plant growth conditions and possiblyalso on plant species.     

The Greenhouse Effect: Acclimation of Tomato Plants Growing in High CO2, Photosynthesis and Ribulose-1, 5-Bisphosphate Carboxylase Protein

Tomato plants were grown in solution culture in a controlledenvironment at 20 ?C with a 12 h photoperiod of 400 µmolquanta m^–2 s^–1 PAR with either normal ambient CO₂,approximately 340 vpm, or with 1000 vpm CO₂. The short- andlong-term effects of CO₂ enrichment on photosynthesis were determinedtogether with the levels of ribulose-1, 5-bisphosphate carboxylase(RuBPco) E.C. 4.1.1.39 [EC] protein and activity throughout leafdevelopment of the unshaded 5th leaf above the cotyledons. Thehigh CO₂ concentration during growth did not appreciably affectthe rate of leaf expansion or final leaf area but did increasethe fresh weight per unit area of leaf. With short-term CO₂enrichment, i.e. only during the photosynthesis measurements,the light-saturated photosynthetic rate (P_max) of young leavesdid not increase while those reaching full expansion more thandoubled their net rate of CO₂ fixation. However, with longerterm CO₂ enrichment, i.e. growing the crop in high CO₂, theplants did not maintain this photosynthetic gain. While theCO₂ concentration during growth did not affect the peak in P_maxmeasured in 300 vpm CO₂ or P_max in 1000 vpm CO₂, RuBPco proteinor its activity, the subsequent ontogenetic decline in theseparameters was greatly accelerated by the high CO₂ treatment.Compared with plants grown in normal ambient CO₂ the high CO₂grown leaves, when almost fully expanded, contained only approximatelyhalf as much RuBPco protein and P_max in 300 vpm CO₂ and P_maxin1000 vpm CO₂ were similarly reduced. The loss of RuBPco proteinmay be a major factor associated with the accelerated fall inP_max since it was close to that predicted from the amount andkinetics of RuBPco assuming RuBP saturation. In the oldest leavesexamined grown in high CO₂ additional factors may be limitingphotosynthesis since RuBPco kinetics marginally overestimatedP_max in 300 vpm CO₂ and the initial slope of photosynthesisin response to intercellular CO₂ was also less than expectedfrom the extractable RuBPco. Key words: Lycopersicon esculentum (Mill.) cv. Findon Cross, CO₂ enrichment, acclimation to high CO₂, photosynthesis, RuBPco protein and activity     

Regulation of Ribulose Bisphosphate Carboxylase Activity in Intact Wheat Leaves by Light, CO2, and Temperature

The activity of the enzyme ribulose bisphosphate carboxylase(RuBPCase) was estimated after rapidly extracting it from intactwheat leaves pretreated under different light and CO₂ levels.No HCO₃^– was added to the extraction buffer since it isshown to inhibit RuBPCase. The activity increased as light intensityor CO₂ concentration during pretreatment was increased. Enzymeactivity increased as temperature during pretreatment was decreased.Light activation did not affect the affinity of RuBPCase forCO₂. A K_m of 30 µM CO₂ under air level O₂ was determined.CO₂, light and temperature are three main limiting factors ofphotosynthesis. It seems that the activity of RuBPCase is regulatedby these factors according to the requirements for CO₂ fixation.     

Effects of elevated CO2 concentrations on three montane grass species: III. Source leaf metabolism and whole plant carbon partitioning

Agrostis capillaris L.⁵, Festuca vivipara L. and Poaalpina L.were grown in outdoor open-top chambers at either ambient (340 3µmol mol^–1) or elevated (6804µmol mol^–1)concentrations of atmospheric carbon dioxide (CO₂) for periodsfrom 79–189 d. Photosynthetic capacity of source leaves of plants grown atboth ambient and elevated CO₂ concentrations was measured atsaturating light and 5% CO₂. Dark respiration of leaves wasmeasured using a liquid phase oxygen electrode with the buffersolution in equilibrium with air (21% O₂, 0.034% CO₂). Photo-syntheticcapacity of P. alpina was reduced by growth at 680 µmolmol^–1 CO₂ by 105 d, and that of F. vivipara was reducedat 65 d and 189 d after CO₂ enrichment began, suggesting down-regulationor acclimation. Dark respiration of successive leaf blades ofall three species was unaltered by growth at 680 relative to340 µmol mol^–1 CO₂. In F. vivipara, leaf respirationrate was markedly lower at 189 d than at either 0 d or 65 d,irrespective of growth CO₂ concentration. There was a significantlylower total non-structural carbohydrate (TNC) concentrationin the leaf blades and leaf sheaths of A. capillaris grown at680µmol mol^–1 CO₂. TNC of roots of A. capillariswas unaltered by CO₂ treatment. TNC concentration was increasedin both leaves and sheaths of P. alpina and F. vivipara after105 d and 65 d growth, respectively. A 4-fold increase in thewater-soluble fraction (fructan) in P. alpina and in all carbohydratefractions in F. vivipara accounted for the increased TNC content. In F. vivipara the relationship between leaf photosyn-theticcapacity and leaf carbohydrate concentration was such that therewas a strong positive correlation between photosynthetic capacityand total leaf N concentration (expressed on a per unit structuraldry weight basis), and total nitrogen concentration of successivemature leaves reduced with time. Multiple regression of leafphotosynthetic capacity upon leaf nitrogen and carbohydrateconcentrations further confirmed that leaf photosynthetic capacitywas mainly determined by leaf N concentration. In P. alpina,leaf photosynthetic capacity was mainly determined by leaf CHOconcentration. Thus there is evidence for down-regulation ofphotosynthetic capacity in P. alpina resulting from increasedcarbohydrate accumulation in source leaves. Leaf dark respiration and total N concentration were positivelycorrelated in P. alpina and F. vivipara. Leaf dark respirationand soluble carbohydrate concentration of source leaves werepositively correlated in A. capillaris. Changes in source leafphotosynthetic capacity and carbohydrate concentration of plantsgrown at ambient or elevated CO₂ are discussed in relation toplant growth, nutrient relations and availability of sinks forcarbon. Key words: Elevated CO₂, Climate change, grasses, carbohydrate partitioning, photosynthesis, respiration     

Photosynthesis of White Clover Leaves as Influenced by Canopy Position, Leaf Age, and Temperature

Microswards of white clover (Trifolium repens L.) were grownin controlled environments at 10/7, 18/13 and 26/21 °C day/nighttemperatures. The vertical distribution of leaves of differentages and their rates of ¹⁴CO₂-uptake in situ were studied. Extending petioles carried the laminae of young leaves throughthe existing foliage. A final position was reached within 1/4to 1/3 of the time between unfolding and death. Newly unfoldedleaves had higher rates of ¹⁴CO₂-uptake per leaf area than olderones at the same height in the canopy. At higher temperatures,the decrease with age was faster. However, the light-photosynthesisresponse of leaves which were removed from different heightsin the canopy varied much less with leaf age than did the ratesof ¹⁴CO₂-uptake in situ. The comparison of the rates of ¹⁴CO₂-uptake in situ with thelight-photosynthesis response curves suggests that young leavesreceive more light than older ones at the same height in thecanopy. This would imply that young white clover leaves havethe ability to reach canopy positions having a favourable lightenvironment. This ability may improve the chances of survivalof white clover in competition with other species. Trifolium repens L., white clover, photosynthesis, canopy, leaf age, ¹⁴CO₂-uptake, ecotypes, temperature     

Reactions of birch leaves to changes in light during early ontogeny: comparison between in vivo and in vitro techniques to measure carbon uptake

Trends in several photosynthetic parameters and their responseto changed growth light were followed for 15 d in leaves ofyoung birch saplings using a rapid-response gas exchange measuringequipment. These in vivo measurements were compared to biochemicalassays that were made from the same leaves after the gas exchangestudies. The measurements were made on leaves that were selectedprior to the study and were at that time of similar age. Forthe first 7 d the photosynthetic parameters were followed fromthe growth conditions of moderate light (200 µmol m^–2s^–1; referred to as controls later in the text). On day7 some of the saplings were transferred to grow either underhigh (450 µmol m^–2 s^–1; referred to as highlight plants) or low (75 µmol m^–2 s^–1; referredto as low light plants) light and the capability of the preselectedleaves for acclimation was followed for 6 d. For comparison,at the end of the experiment the measurements were made on bothcontrols and on young leaves that had developed under high andlow light. Generally the in vivo measured rate of CO₂ uptake (gross photosynthesis)both at 310 ppm CO₂ and 2000 ppm CO₂ corresponded very wellto the biochemically determined CO₂ fixation capacity in vitroafter rapid extraction (measured as the initial and total activityof Rubisco, respectively). However, if the flux of CO₂ intothe chloroplasts was limited by the closure of the stomata,as was the case of the high light plants, then the in vitromeasured Rubisco activity was greater than the in vivo measuredCO₂ uptake. V_max, calculated from the mesophyll conductanceat 1% O₂, exceeded the initial activity of Rubisco (assayedat saturating RuBP and CO₂) constantly by 60%. The catalyticactivity of Rubisco in birch leaves was overall very low, evenwhen calculated from the total activity of Rubisco (K_cat 0.63–1.18 s^–1), when compared to herbaceous C₃ species. Signs of light acclimation were not observed in most of thephotosynthetic parameters and in chloroplast structure whenmature birch leaves were subjected to changes in growth lightfor 6 d. However, the change of the growth light either to highor low light caused day-to-day fluctuations in most of the measuredphotosynthetic parameters and in the case of the high lightplants signs of photoinhibition and photodestruction were alsoobserved (decrease in the amount of chlorophyll and increasein chlorophyll a/b ratio). As a result of these fluctuationsthese plants achieved a new and lower steady-state conditionbetween the light and dark reactions, as judged from the molarratio of RuBP to Rubisco binding site. Key words: Acclimation, photosynthesis, light, Rubisco, birch     

Increases in the Diffusion Resistances of Leaves in a Carbon Dioxide-Enriched Atmosphere

The stomatal closing reaction to CO₂, which has been observedin laboratory studies by many workers, was investigated in plant-growthcabinets under conditions similar to those in glass-houses withCO₂-enriched atmospheres. Lettuce and Xanthium plants grownin a normal atmosphere showed the expected stomatal closurein response to increasing CO₂ concentrations, and lettuce plantsgrown in 1000 ppm CO₂ showed the same response, even after 4weeks. Thus there was no evidence of acclimatization of thestomata and it is concluded that they must remain partiallyclosed during CO₂-enrichment. Estimates of diffusion resistancesto CO₂ intake in lettuce leaves showed that in light of 14.4J m^–2 S^–1, 880 ppm CO₂ in the atmosphere resultedin a concentration of 367 ppm near the mesophyll cell walls.If the stomata remained open this same internal concentrationcould be achieved with an external concentration of only 640ppm. There could, therefore, be some economic advantage if stomatalclosure were prevented during CO₂-enrichment.     

The Influence of Prior Water Stress and Abscisic Acid Foliar Spraying on Stomatal Responses to CO2, IAA, ABA, and Calcium in Leaves of Solanum melongena

The influence of a water stress or foliar ABA spraying pretreatmenton stomatal responses to water loss, exogenous ABA, IAA, Ca²⁺,and CO₂ were studied using excised leaves of Solanum melongena.Both pretreatments increased stomatal sensitivity of water loss,in the presence and absence of CO₂, but decreased stomatal sensitivityto exogenous ABA. CO₂ greatly reduced the effect of exogenouslyapplied ABA. IAA decreased leaf diffusion resistance for controland ABA sprayed leaves, but did not influence the LDR of previouslywater-stressed leaves. CA²⁺ did not influence LDR of any leavesof any treatments. Key words: Water stress, stomatal response, pretreatments     

The Damping and Reinitiation of the Circadian Rhythm of CO2 Output in Bryophyllum Leaves in Relation to their Malate Content

The circadian rhythm of CO₂ output in leaves of Bryophyllumfedtschenkoi damps out after 3–4 d in continuous darknessand a CO₂-free air stream at 15°C. The rhythm is reinitiatedafter a single exposure to white light of 2, 4, 6 or 8 h duration,damps out again after a further 3–4 d and can be reinitiatedfor a second time by a further exposure to light. During the exposure to light there is a burst of CO₂ outputconsistent with the decarboxylation of malate, and the rhythmbegins afterwards with an initial high rate of CO₂ fixation.Malate gradually accumulates in the leaves in continuous darknessto attain a maximum value (35 mol m^–3) at the time whenthe circadian rhythm disappears, and decreases to a low value(19 mol m^–3) after a 4 h exposure to light which reinitiatesrhythmicity. These results support the hypothesis that damping of the rhythmof CO₂ output in continuous darkness is due to the accumulationof malate in the leaf cells, eventually reaching such a levelthat its removal from the cytoplasm into the vacuole cannottake place, with the result that PEPc activity, upon which therhythm of CO₂ output depends, remains allosterically inhibited. Key words: CAM, circadian rhythm, Bryophyllum, CO₂-fixation, malate metabolism