Striga hermonthica decreases photosynthesis in Zea mays through effects on leaf cell structure

Maize seedlings were grown in pots either with or without preconditionedseeds of the parasitic weed, Striga hermonthica. After between4 and 8 weeks, net photosynthesis in the leaves of maize plantsinfected with Striga decreased compared to leaves of uninfectedcontrol plants. The activities of four enzymes of photosyntheticmetabolism were, however, little affected by infection. A pulse-chaseexperiment using ¹⁴CO₂ showed that C₄ acids were the main earlyproducts of assimilation even when the rate of photosynthesiswas much decreased by infection, but more radio-activity appearedin glycine and serine than in leaves of healthy maize plants.Leaves of infected maize required longer to reach a steady rateof photosynthesis upon enclosure in a leaf chamber than leavesof uninfected plants after similar treatment. Electron microscopy of transverse sections of the leaves ofinfected maize indicated that the cell walls in the bundle sheathand vascular tissue were less robust than in leaves of healthyplants. The results suggest that infection with Striga causesan increase in the permeability of cell walls in the bundlesheath, leakage of CO₂ from the bundle sheath cells and decreasedeffectiveness of C₄ photosynthesis in host leaves. Key words: Zea mays, Striga hermonthica, photosynthesis, photorespiration, enzyme activity     

Changing Responses of Stomata to Abscisic Acid and CO2 as Leaves and Plants Age

Willmer, C. M., Wilson, A. B. and Jones, H. G. 1988. Changingresponses of stomata to abscisic acid and CO₂ as leaves andplants age.—J. exp. Bot. 39: 401–410. Stomatal conductances were measured in ageing leaves of Commelinacommunis L. as plants developed; stomatal responses to CO₂ andabscisic acid (ABA) in epidermal strips of C. communis takenfrom ageing leaves of developing plants and in epidermal stripsfrom the same-aged leaves (the first fully-expanded leaf) ofdeveloping plants were also monitored. Stomatal behaviour wascorrelated with parallel measurements of photosynthesis andleaf ABA concentrations. Stomatal conductance in intact leavesdecreased from a maximum of 0-9 cm s^{– 1} at full leaf expansionto zero about 30 d later when leaves were very senescent. Conductancesdeclined more slowly with age in unshaded leaves. Photosynthesisof leaf slices also declined with age from a maximum at fullleaf expansion until about 30 d later when no O₂ exchange wasdetectable. Exogenously applied ABA (0.1 mol m^{– 3}) didnot affect respiration or photosynthesis. In epidermal stripstaken from ageing leaves the widest stomatal apertures occurredabout 10 d after full leaf expansion (just before floweringbegan) and then decreased with age; this decrease was less dramaticin unshaded leaves. The inhibitory effects of ABA on stomatalopening in epidermal strips decreased as leaves aged and wasgreater in the presence of CO₂ than in its absence. When leaveswere almost fully-senescent stomata were still able to open.At this stage, guard cells remained healthy-looking with greenchloroplasts while mesophyll cells were senescing and theirchloroplasts were yellow. Similar data were obtained for stomatain epidermal strips taken from the same-aged leaves of ageingplants. The inhibitory effects of ABA on stomatal opening alsodecreased with plant age. In ageing leaves both free and conjugated ABA concentrationsremained low before increasing dramatically about 30 d afterfull leaf expansion when senescence was well advanced. Concentrationsof free and conjugated ABA remained similar to each other atall times. It is concluded that the restriction of stomatal movements inintact leaves as the leaves and plants age is due mainly toa fall in photosynthetic capacity of the leaves which affectsintracellular CO₂ levels rather than to an inherent inabilityof the stomata to function normally. Since stomatal aperturein epidermal strips declines with plant and leaf age and stomatabecome less responsive to ABA (while endogenous leaf ABA levelsremain fairly constant until leaf senescence) it is suggestedthat some signal, other than ABA, is transmitted from the leafor other parts of the plant to the stomata and influences theirbehaviour. Key words: Abscisic acid, CO₂, Commelina, leaf age, senescence, stomatal sensitivity     

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     

Stomatal Functioning of In Vitro and Greenhouse Apple Leaves in Darkness, Mannitol, ABA, and CO2

Leaves from in vitro and greenhouse cultured plants of Malusdomestica (Borkh.) cv. Mark were subjected to 4 h of darkness;4 h of 1 M mannitol induced water stress; 1 h of 10^–4M to 10^–7 M cis-trans abscisic acid (ABA) treatment; 1h of 0.12% atmospheric CO₂. Stomatal closure was determinedby microscopic examination of leaf imprints. In all treatments,less than 5% of the stomata from leaves of in vitro culturedplants were closed. The diameter of open stomata on leaves fromin vitro culture remained at 8 µm. In contrast, an averageof 96% of the stomata on leaves of greenhouse grown plants wereclosed after 4 h in darkness; 56% after 4 h of mannitol inducedwater stress; 90% after 1 h of 10^–4 M ABA treatment; 61%after 1 h in an atmosphere of 0.12% CO₂. Stomata of in vitroapple leaves did not seem to have a closure mechanism, but acquiredone during acclimatization to the greenhouse environment. Thelack of stomatal closure in in vitro plants was the main causeof rapid water loss during transfer to low relative humidity.     

Effect of the Mesophyll on Stomatal Opening in Commelina communis

The effect of a number of factors on the opening of stomatain the intact leaf and in the isolated leaf epidermis of Commelinacommunishas been investigated. Stomata in the intact leaf opened widein the light and closed rapidly on transfer to the dark. Theywere also sensitive to CO₂. In contrast, stomata in isolatedepidermis floated on an incubation solution containing 100 molm^–3KCl responded neither to light nor CO₂. They opened as widelyas those in the intact leaf when treated with fusicoccin. Stomata in isolated epidermis opened almost as wide as thosein the intact leaf when they were incubated with isolatedmesophyllcells in the light. The solution in which the mesophyll cellswere incubated was separated by centrifugation. Themedium fromcells previously incubated in the light caused the stomata inisolated epidermis to open but that from cells kept inthe darkhad no effect. A similar effect was observed when isolated chloroplastswere incubated with the isolated epidermis.However, the supernatantfrom the chloroplast suspension had no significant effect onstomatal opening. These results indicate that the mesophyll plays an importantrole in stomatal opening in the light. The mesophyll appearstoproduce in the light, but not in the dark, a soluble compoundwhich moves to the guard cells to bring about stomatal opening.Theexperiments with isolated chloroplasts suggest that this substanceis a product of photosynthesis. Key words: Commelina communis, stomata, light, mesophyll     

Elevated carbon dioxide affects the patterning of subsidiary cells in Tradescantia stomatal complexes

The influence of elevated CO₂ concentration (670 ppm) on thestructure, distribution, and patterning of stomata in Tradescantialeaves was studied by making comparisons with plants grown atambient CO₂. Extra subsidiary cells, beyond the normal complementof four per stoma, were associated with nearly half the stomatalcomplexes on leaves grown in elevated CO₂. The extra cells sharedcharacteristics, such as pigmentation and expansion, with thetypical subsidiary cells. The position and shape of the extrasubsidiary cells in face view differed in the green and purplevarieties of Tradescantia. Substomatal cavities of complexeswith extra subsidiary cells appeared larger than those foundin control leaves. Stomatal frequency expressed on the basisof leaf area did not differ from the control. Stomatal frequencybased on cell counts (stomatal index) was greater in leavesgrown in CO₂-enriched air when all subsidiary cells were countedas part of the stomatal complex. This difference was eliminatedwhen subsidiary cells were included in the count of epidermalcells, thereby evaluating the frequency of guard cell pairs.The extra subsidiary cells were, therefore, recruited from theepidermal cell population during development. Stomatal frequencyin plants grown at elevated temperature (29 C) was not significantlydifferent from that of the control (24 C). The linear aggregationsof stomata were similar in plants grown in ambient and elevatedCO₂. Since enriched CO₂ had no effect on the structure or patterningof guard cells, but resulted in the formation of additionalsubsidiary cells, it is likely that separate and independentevents pattern the two cell types. Plants grown at enrichedCO₂ levels had significantly greater internode lengths, butleaf area and the time interval between the appearance of successiveleaves were similar to that of control plants. Porometric measurementsrevealed that stomatal conductance of plants grown under elevatedCO₂ was lower than that of control leaves and those grown atelevated temperature. Tradescantia was capable of regulatingstomatal conductance in response to elevated CO₂ without changingthe relative number of stomata present on the leaf. Key words: Elevated CO₂, stomata, subsidiary cells, patterning     

Effects of Temporary Droughts on Photosynthesis of Alfalfa Plants

The effect of temporary droughts on photosynthesis, total conductanceto water vapour, intercellular CO² concentration, CO² compensationpoint, light-response curves, photorespiration, dark respiration,chlorophyll content, and ribulose 1,5-bisphosphate (RuBP) carboxylase(EC 4.1.1.39 [EC] ) activity has been examined in nodulated alfalfaplants (Medicago saliva cv. Aragón). Plants were subjectedto moderate (S1/RS1) or severe (S2/RS2) cycles of drought (drought/recovery).Photosynthetic light-response curves showed decreased light-saturatedphotosynthetic capacity and decreased apparent quantum yield.Upon rewatering, photosynthesis did not recover whereas conductancedid in moderately stressed plants. Calculated electron transportrate also declined in drought-stressed plants, but upon rewatering,moderately stressed plants exhibited a total recovery. Comparison of photosynthetic intercellular CO² response curvesin well-watered and stressed leaves led to the assertion thateffects in chloroplast metabolism contribute significantly tophotosynthetic inhibition. Although the validity of this entireline of research has been questioned by some recent studiesbecause the occurrence of heterogeneous stomatal closure wouldaffect these curves, in our case, the effect of water stresswas investigated in experimental systems where stomata had beenremoved. Measurements of in vitro RuBP carboxylase activityand protein content showed a strong decline during drought treatmentsand upon rewatering no recovery was observed. Therefore, ourresults suggest the major implication of non-stomatal factorsin the decline of photosynthesis in alfalfa plants under cyclicdrought conditions. Key words: Alfalfa, water deficits, photosynthesis, ribulose, 1,5-bisphosphate carboxylase activity, stomatal limitation     

Abnormal Behaviour of Stomata in Barley Leaves Infected with Rhynchosporium secalis (Oudem.) J. J. Davis

Infection of barley leaves by the fungus Rhynchosporium secalisincreases the degree to which stomata open in the light. Openingis enhanced at CO₂, levels between 0 and 4000 parts/10⁶. Theability of stomata to close in the dark, as normal, is retaineduntil an advanced stage of tissue necrosis is reached. Increased stomatal opening is confined to those areas of theleaf which have been colonized by the fungus. Abnormal stomatalbehaviour results from the loss of osmotically active substancesfrom the epidermis of diseased leaves with a consequent alterationof the turgor relations between guard cells and their surroundingepidermal cells.     

The Role of Farnesol as a Regulator of Stomatal Opening in Sorghum

Fine, very dilute aqueous emulsions of all-trans farnesol appliedto intact leaves of Sorghum bicolor caused appreciable inhibitionof stomatal opening which persisted for 2 d, after which timethe stomata regained their capacity to open. The inhibitoryeffect of farnesol was not overcome by flushing the leaves withCO₂-free air, indicating that it was not the result of an accumulationof CO₂. This conclusion was supported by measurements of CO₂compensation, which increased only slightly after farnesol treatment. All-trans farnesol has previously been reported to be formedin water-stressed plants of Sorghum. The data presented heresuggest that it could be acting as an endogenous antitranspirant,in a comparable role to that already established for abscisicacid in several species. It would appear, however, to have aless prolonged inhibitory effect than abscisic acid, and itcould be responsible for the rapid responses of Sorghum stomatato water stress and their quick recovery after the plant hasregained turgor, a characteristic which distinguishes Sorghumfrom many other genera so far investigated.     

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; )     

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; )     

Stomata of Micropropagated DelphiniumPlants Respond to ABA, CO2, Light and Water Potential, but Fail to Close Fully

Morphological and physiological characteristics of micropropagatedplants of Delphinium cv. Princess Caroline were studied. Leavesproduced in vitro showed poor control of water loss which appearsto result from restricted responses by stomata and not frompoor cuticular development. Stomata of leaves produced in vitrowere larger and more frequent than those produced during acclimatization.Despite the fact that stomata from isolated epidermis of leavesproduced in vitro reduced their apertures when exposed to turgor-reducingtreatments, they did not close fully. This, together with highstomatal frequencies might explain the poor control of waterloss shown by intact leaves produced in culture when exposedto dry air. While leaves from acclimatized plants showed almostcomplete closure with ABA, low water potentials, darkness andCO₂, stomata from leaves produced in vitro reduced their apertureswhen exposed to those factors, but only to a limit. Therefore,stomata from leaves cultured in vitro seem to be partially functional,but some physiological or anatomical alteration prevents themfrom closing fully. Stomata from leaves produced in vitro wereparticularly insensitive to ABA which appears to be partly associatedwith the high cytokinin concentration in the culture medium.In the long-term, this stomatal insensitivity to ABA might contributeto plant losses when micropropagated plantlets are transferredto soil. Key words: Micropropagation, stomatal physiology, dehydration, PEG, ABA, BAP, darkness, CO₂, Delphinium     

Changes in apparent photosynthesis, CO2 compensation point and dark respiration of leaves of some Poaceae and Cyperaceae species with senescence

CO₂ exchange characteristics of detached mature and senescentflag leaves and of bracts in some Poaceae and Cyperaceae species,respectively, were studied using a closed IR system. Senescentleaves, 30 to 45 days after flowering, showed lower rates ofapparent photosynthesis and dark respiration, and higher CO₂compensation points (CCP) than those measured at the floweringstage. In senescent C₄-Poaceae, the increase of CCP was small(from 4.8 to 10.1 ppm on the average) with little influenceof temperature, and the photorespiration level, 0.4 mg CO₂/dm²/hr,was as low as that in mature leaf, indicating the presence ofnormal C₄-characteristics. On the other hand, a C₄-Cyperaceae,Cyperus microria Steud., showed extensive increases of CCP (from9 to 41 ppm) and photorespiration (from 0.8 to 2.1 mg CO₂/dm²/hr)with senescence. (Received August 25, 1979; )     

Stomatal Responses of Variegated Leaves to CO2 Enrichment

The responses of stomatal density and stomatal index of fivespecies of ornamental plants with variegated leaves grown attwo mole fractions of atmospheric CO₂ (350 and 700 µmolmol^-1) were measured. The use of variegated leaves allowed anypotential effects of mesophyll photosynthetic capacity to beuncoupled from the responses of stomatal density to changesin atmospheric CO₂ concentration. There was a decrease in stomataldensity and stomatal index with CO₂ enrichment on both white(unpigmented) and green (pigmented) leaf areas. A similar responseof stomatal density and index was also observed on areas ofleaves with pigmentation other than green indicating that anydifferences in metabolic processes associated with colouredleaves are not influencing the responses of stomatal densityto CO₂ concentrations. Therefore the carboxylation capacityof mesophyll tissue has no direct influence on stomatal densityand index responses as suggested previously (Friend and Woodward1990 Advances in Ecological Research 20: 59-124), instead theresponses were related to leaf structure. The stomatal characteristics(density and index) of homobaric variegated leaves showed agreater sensitivity to CO₂ on green portions, whereas heterobaricleaves showed a greater sensitivity on white areas. These resultsprovide evidence that leaf structure may play an important rolein determining the magnitude of stomatal density and index responsesto CO₂ concentrations.Copyright 1995, 1999 Academic Press Leaf structure, photosynthesis, stomatal conductance, CO₂, stomatal density, stomatal index     

In situ Observations of Stomatal Movements

Kappen, L., Andresen, G. and L?sch, R. 1987. In situ observationsof stomatal movements.—J. exp. Bot. 38: 126–141. A device is described by which stomatal movements in situ canbe observed and recorded continuously in light and in darkness.It is mounted in a conditioned CO₂ exchange measuring chamberso that stomatal movements can be observed whilst CO₂ exchange(photosynthesis and respiration) of the same leaf is measured.Advantages and limitations are discussed. By this method itwas shown that stomata of Vicia faba although responding inthe same direction to environmental stimuli exhibited a widerange of pore widths. Responses to changes of air humidity andof CO₂ content were clearly evident when the leaves were exposedto light. Before stomata closed due to decreasing water vapourpressure differences between leaf and air they showed a markedwidening of the pore. An inverse response occurred when watervapour pressure deficit decreased. In darkness stomata did notrespond to such changes. Key words: Stomata, leaf gas exchange, microscopic observation     

Growth at elevated CO2 leads to down-regulation of photosynthesis and altered response to high temperature in Quercus suber L. seedlings

The effects of growth at elevated CO₂ on the response to hightemperatures in terms of carbon assimilation (net photosynthesis,stomatal conductance, amount and activity of Rubisco, and concentrationsof total soluble sugars and starch) and of photochemistry (forexample, the efficiency of excitation energy captured by openphotosystem II reaction centres) were studied in cork oak (Quercussuber L.). Plants grown in elevated CO₂ (700 ppm) showed a down-regulationof photosynthesis and had lower amounts and activity of Rubiscothan plants grown at ambient CO₂ (350 ppm), after 14 monthsin the greenhouse. At that time plants were subjected to a heat-shocktreatment (4 h at 45C in a chamber with 80% relative humidityand 800–1000 mol m^–2 s^–1 photon flux density).Growth in a CO₂-enriched atmosphere seems to protect cork oakleaves from the short-term effects of high temperature. ElevatedCO₂ plants had positive net carbon uptake rates during the heatshock treatment whereas plants grown at ambient CO₂ showed negativerates. Moreover, recovery was faster in high CO₂-grown plantswhich, after 30 min at 25C, exhibited higher net carbon uptakerates and lower decreases in photosynthetic capacity (A_max aswell as in the efficiency of excitation energy captured by openphotosystem II reaction centres (F_vJF_m than plants grown atambient CO₂. The stomata of elevated CO₂ plants were also lessresponsive when exposed to high temperature. Key words: Elevated CO₂, temperature, acclimation, photosynthesis, Quercus suber L.     

Extreme Phosphate Deficiency Decreases the In Vivo CO2/O2 Specificity Factor of Ribulose 1,5-Bisphosphate Carboxylase-Oxygenase in Intact Leaves of Sunflower

Sunflower plants were grown under controlled environmental conditionswith either 0 or 10 mol m^–3 phosphate (Pi). From steady-statemeasurements of gas exchange and chlorophyll fluorescence madeon intact leaves, the in vivo CO₂/O₂ specificity factor (invivo K_sp) of ribulose 1,5-Aisphosphate carboxylase-oxygenase(Rubisco) was determined following two methods based on modelsof C₃ photosynthesis by Brooks and Farquhar (1985) and Peterson(1989). The two methods gave in vivo K_sp values for controlsunflower leaves which were similar to published values forhigher plants. Extreme Pi deficiency decreased in vivo K_sp,in sunflower leaves compared to adequate Pi. This suggests thatPi deficiency affected photorespiration less than photosynthesis.The decrease in in vivo K_sp may be due to a real change in theenzyme kinetics favouring oxygenation more than carboxylationor due to an increase in the number of CO₂ molecules releasedper oxygenation; in which case the observed decrease in thein vivo K_sp determined on intact leaves will not agree numericallywith the true K_sp of Rubisco determined in vitro using purifiedenzyme from the same leaf. We discuss the implications of therelatively large photorespiration in Pi-deficient sunflowerleaves with respect to the increased dissipation of photosyntheticelectrons and photorespiratory recycling of Pi in thechloroplaststroma. Although our results on in vivo K_sp suggested a relativelylarger photorespiratory potential in Pi-deficient than controlsunflower leaves, photosynthesis was insensitive to O₂ in Pi-deficientleaves; the possible reasons for this phenomenon are discussed.Under extreme Pi deficiency, O₂ sensitivity of photosynthesisis not a reflection of the in vivo photorespiratory rates. Determinationof in vivo K_sp of Rubisco is a useful approach to study thephotorespiratory potential of intact leaves. Key words: Chlorophyll fluorescence, phosphate deficiency, photorespiration, photosynthesis, PSII quantum yield, Rubisco specificity factor     

Stomatal Responses and the Senescence of Leaves

Guard cell responses were examined in green and senescing leavesof Victa faba using detached epidermal strips to eliminate influencesfrom the mesophyll. Stomatal opening was greater in epidermalstrips from mature leaves than from senescing leaves althoughthe latter still retained the ability to respond to CO₂ andto kinetin. It was concluded that the decline in stomatal activityduring senescence is an independent but parallel process tochanges occurring in the mesophyll. Vicia faba, leaf senescence, stomata, kinetin     

Effect of Elevated CO2 on Stomatal Size and Distribution in Perennial Ryegrass

Plants of ryegrass (Lolium perenne L. cv. Melle) were grownfrom the early seedling stage in growth cabinets at a day/nighttemperature of 20/15 °C, with a 12-h photoperiod, and aCO₂ concentration of either 340 or 680 ± 15 µl1^–1 CO₂. Young, fully-expanded, acclimated leaves fromprimary branches were sampled for length of stomata, and ofepidermal cells between stomata, numbers of stomata and epidermalcells per unit length of stomatal row, numbers of stomatal rowsacross the leaf and numbers of stomatal rows between adjacentvein ridges. Elevated CO₂ had no significant effect on any ofthe measured parameters. Elevated CO₂, Lolium perenne, ryegrass, stomatal distribution, stomatal size     

Effects of Low Oxygen on Photosynthesis, Translocation and Growth in Green Pepper (Capsicum annuum)

Green pepper (Capsicum annuum cv. Bell Boy) plants were exposedin chambers to low (2%) oxygen and controlled carbon dioxideconcentrations. Vegetative and fruiting plants showed short-termincreases in net photosynthesis in low oxygen or elevated carbondioxide (up to 900 µl CO₂ l^–1). Photosynthesis ofyoung vegetative plants increased in low oxygen in the short-termbut there was no long-term benefit. Low oxygen enhancement ofphotosynthesis declined with time and after 10 d, leaf areaand root dry weight were less than in plants grown in normalair. Labelled assimilates were translocated from leaves to otherregions at similar rates in low oxygen and normal air. Low oxygenreduced respiratory losses from leaves and reduced the proportionof soluble carbohydrate converted to polysaccharide in all plantparts. Thus, low-oxygen environments decrease the utilisationof assimilates which then may lead to inhibition of photosynthesis. Capsicum annuum, photosynthesis, photorespiration, translocation, utilization of assimilates