Responses of Commelina communis L. Guard Cell Protoplasts to Abscisic Acid

Fitzsimons, P. J. and Weyers, J. D. B. 1987. Responses of Commelinacommunis L. guard cell protoplasts to abscisic acid.—J.exp. Bot. 38: 992–1001. Guard cell protoplasts (GCPs) isolated from the leaf epidermisof Commelina communis L. responded to abscisic acid (ABA) ina manner which was qualitatively and quantitatively similarto that of intact stomata. ABA inhibited swelling of GCPs underlow-CO₂ conditions and swollen GCPs responded to the hormoneby shrinking. Both the absolute volume decrease and the initialrate of shrinking were commensurate with the extent and ratesof solute loss computed for ABA-treated intact, open stomata.This indicates that GCPs represent a suitable experimental systemfor studies of ABA-mediated solute fluxes. A radiotracer equilibrationmethod was developed for the rapid estimation of GCP osmoticvolume changes. Using this technique it was found that, on average,82% of the reduction in solute content caused by ABA treatmentwas due to the loss of K⁺. It is envisaged that electroneutralitymight be maintained during ABA-induced shrinkage of GCPs bynet inward proton movement leading to acidification of the vacuole. Key words: Abscisic acid, Commelina communis L., guard cells, protoplasts     

The Effects of Cytokinins and ABA on Stomatal Behaviour of Maize and Commelina

Epidermal strips and leaf fragments of Commelina and leaf fragmentsof maize were incubated on solutions containing naturally-occurringor synthetic cytokinins and/or ABA. The effects of these treatmentson stomatal behaviour were assessed. Cytokinins alone did notpromote stomatal opening in either species but concentrationsof both zeatin and kinetin from 10^–3 to 10^–1 molm^–3 caused some reversal of ABA-stimulated closure ofmaize stomata. The reversal of the ABA effect increased withincreasing cytokinin concentration. Cytokinins had no effecton ABA-stimulated closure of Commelina stomata. When appliedalone, at high concentration (10^–1 mol m^–3), toCommelina epidermis or leaf pieces both zeatin and kinetin restrictedstomatal opening. Key words: ABA, Cytokinins, Stomata, Maize, Commelina     

Calcium Inhibits Ion-Stimulated Stomatal Opening in Epidermal Strips of Commelina communis L.

Inoue, H. and Katoh, Y. 1987. Calcium inhibitsion-stimulatedstomatal opening in epidermal strips of Commelina communis L.—J.exp. Bot. 38: 142–149. Ca²⁺ suppressed both the ion-stimulated stomatal opening andH⁺ extrusion of pre-illuminated epidermal strips isolated fromCommelina communis L. In the absence of Ca²⁺, the rate of H⁺release was 18 nmol H⁺ cm^–2 h^–1 per epidermal stripunit area in 150 mol m^–3 KCL at pH 7?4. Half-maximum inhibitionof stomatal opening was observed with 220 mmol m^–3 ofCa²⁺. The hexavalent dye, ruthenium red, showed concentration-dependentprevention of the inhibition by Ca²⁺ of the ion-stimulated stomatalopening. The effect of ruthenium red was non-competitive, andthe K₁ for the calcium inhibition was found to be 3?6 mmol m^–3.The calcium inhibition of H⁺ extrusion was also prevented byruthenium red. These results suggest that Ca²⁺ inhibits theactivity of electrogenic H⁺ translocating ATPase of the guardcell plasma membrane and leads to the suppression of stomatalopening. Key words: Calcium, Commelina communis, ruthenium red, stomata     

Direct Organogenesis from Petiole and Thin Cell Layer Explants in Sugar Beet Cultured In Vitro

Detrez, C., Tetu, T., Sangwan, R. S. and Sangwan-Norreel, B.S., 1988. Direct organogenesis from petiole and thin cell layerexplants in sugar beet cultured in vitro.—J. exp. Bot.39: 917–926. Plant regeneration was obtained by direct bud formation frompetiole as well as from thin cell layer explants taken fromsugar beet (Beta vulgaris L.) plants grown in vitro. The budswere mainly induced in the blade-petiole transition zone ofthe explants. High frequency bud regeneration was observed inpetiole and thin layer explants of 10 different breeding linesof sugar beet tested. Organogenesis resulted when petiole explantsexcised from 8-d-old seedlings grown on half-strength Murashigeand Skoog medium (MS) containing 3.0 mg dm^–3 naphthaleneacetic acid (NAA), 3.0 mg dm^–3 6-benzylaminopurine (BAP)and 1.0 mg dm^–3 2, 3, 5, triiodobenzoic acid (TIBA) werecultured on MS with 3.0 mg dm^–3 NAA and 3.0 mg dm^–3BAP. Thin cell layer strips isolated from shoot apices culturedon MS medium supplemented with 0–9 mg dm^–3 BAP or1.0 mg dm^–3 indolebutyric acid (IBA) formed adventitiousbuds on MS medium containing 0–5 mg dm^–3 NAA + 5.0mg dm^–3 BAP. Histological studies confirmed the sub-epidermalorigin of shoots. Key words: Beta vulgaris, direct organogenesis, in vitro culture, petiole, regeneration, thin cell layer     

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.     

Effects of Light/Dark on Anion Fluxes in Isolated Guard Cells of Commelina communis L.

The effects of light/dark on anion fluxes in isolated guardcells of Commelina communis L. have been studied, using ⁸²Brand ³⁶Cl. Transfer of open guard cells from light to dark hasno effect on the ⁸²Br influx, but produces a marked transientstimulation of ⁸²Br or ³⁶Cl efflux, similar to the effect ofsuch transfer on the ⁸⁶Rb fluxes, and to the effects on both⁸⁶Rb and ⁸²Br fluxes of adding ABA. On return of guard cellsto light, after the transient, there is a further reductionin Cl/Br efflux. It is argued that control of a specific processof ion extrusion is important in regulating the ability of guardcells to stay open. In three out of four batches of steady-statetissue labelled with ⁸²Br, the plasmalemma fluxes were highenough, relative to the tonoplast fluxes, for the efflux kineticsto be separable into two exponential components, allowing estimationof bromide contents in cytoplasm and vacuole (Q_c and Q_v), andfluxes at plasmalemma and tonoplast. With opening in light,Q_c increased by 3.9 ± 0.4 pmol mm^–2 µm^–1and Qy by 5.2 ± 0.6 pmol mm^–2 µm^–1(change in content per mm² of epidermis perµm change inaperture). Using rough estimates for the volumes of cytoplasmand vacuole these figures suggest that at 6.1 µm in thedark the concentrations were about 63 mol m^–3 in the cytoplasmand 35 mol m^–3 in the vacuole, rising to about 185 molm^–3 in the cytoplasm and 125 mol m^–3 in the vacuole,at 16.7 µm aperture in light. Neither increase can providean adequate increase in salt concentration to account for theosmotic change required, and some solute other than potassiumsalt must also be involved. In one experiment with 82Br andin the only experiment with 36Cl the plasmalemma flux was lower,and not high enough relative to the tonoplast flux to allowseparation of two phases in the efflux curves, and calculationof cytoplasmic and vacuolar contents and fluxes. The effectsof transfer from light to dark were, nevertheless, similar inboth types of tissue. Key words: Commelina communis L., Light/dark effects, Anion fluxes, Guard cells     

A Critical Assessment of the Role of Potassium and Osmolarity in Stomatal Opening

An attempt has been made to separate the osmotic effect fromthe ionic effect of KCI in stomatal responses. For this purposeisolated illuminated epidermis from species with and withoutsubsidiary cells were treated with KCI (0-250 mOs kg^–1)and with mannitol (0-250 mOs kg^–1). Since osmolarity wasmade the basis of comparison, the effect of mannitol had tobe observed immediately, before guard cell contents could haveleached into the incubation medium. When plotting aperturesagainst osmolarity sigmoid curves were obtained with KCI, butwith mannitol straight lines resulted provided that prior tostripping and incubation leaves were briefly illuminated. Whilst in lower concentrations (60 mOs kg^–1 for Viciafaba; 90 mOs kg^–1 for Zantedeschia aethiopica; 190 mOskg^–1 for Commelina communis) pores were wider in mannitolthan in KCI, in concentrations above these values the situationwas reversed. It appeared therefore that KCI had either an inhibitoryor a promoting effect. Inhibition was most pronounced when atthe beginning of incubation stomata were closed; the inhibitoryeffect on stomata without subsidiary cells occurred at low concentrations(0-60 mOs kg^–1) whereas when subsidiary cells were presentinhibition occurred at up to 190mOs kg^–1. Other experiments started with KCI solutions of 50 mOs kg^–1for Vicia faba, 85 mOs kg^–1 for Zantedeschia aethiopicaand 115 mOs kg^–1 for Commelina communism; mannitol wasadditionally used to give the progressive increases in osmolarity.Degrees of opening were then reached which with KCI alone couldonly be attained at the very highest concentrations. Starch disappearance was followed using the periodic-acid-silvertest; by using either ⁸⁶Rb or ⁴³K it was shown that ion uptakewas restricted to guard cells alone only at osmolarities exceeding200 mOs kg^–1. On the basis of these observations it was concluded that K transportdoes not represent the major mechanism of stomatal regulation. Key words: Stomata, Potassium, Osmolarity     

The Effects of Fusicoccin on Anion Fluxes in Isolated Guard Cells of Commelina communis L.

Clint, G. M. 1987. The effects of fusicoccin on anion fluxesin isolated guard cells of Commelina communis L.—J. exp.BoL 38: 863–876. The effects of 3?10^–2 mol m^–3 fusicoccin (FC) onbromide fluxes and contents in isolated guard cells of Commelinacommunis L. have been studied using K⁸²Br at pH 3?9 and pH 6?7.At pH 3?9 FC caused a reduction in both the influx and the effluxof ⁸²Br, whereas at pH 6?7 FC had no effect on the influx butcaused a transient increase in the efflux of ⁸²Br. There wasno obvious change in bromide content with FC treatment at eitherpH. The behaviour of the anion fluxes in response to FC suggeststhat FC does not act solely via a hyperpolarization at the plasmalemma.A redistribution of bromide between the intracellular compartmentssuggests that anion flux from the cytoplasm to the vacuole maybe stimulated by FC at pH 3?9. The failure of guard cells toincrease their anion content on treatment with FC despite anincrease in stomatal aperture and in cation content suggeststhat in FC-induced stomatal opening excess cation is balancedby organic acid synthesis within the guard cell. Key words: Fusicoccin, guard cells, ion fluxes, Commelina communis     

Ion Fluxes in 'Isolated' Guard Cells of Commelina communis L.

Ion fluxes have been measured in ‘isolated’ guardcells of Commelina communis L. using ⁸⁶RbCl and K⁸²Br, in epidermalstrips in which all cells other than guard cells have been killedby treatment at low pH. To avoid problems of slow free spaceexchange most fluxes have been measured at pH 3.9, at whichstomata open well in K(Rb) Cl(Br) and are stable for many hours.At pH 3.9 the intracellular ⁸⁶Rb exchanged as a single compartmentwith a half-time of 2–3 h, independent of external concentration(C_o). The influx of ⁸⁶Rb rose with concentration, to a V_maxof about 23 pmol mm^–2 h^–1. The efflux curve of ⁸²Brcould be well fitted by two exponential terms, with half-timesof 38 min (independent of C_o), and 5–35 h (falling withincreasing C_o). Bromide contents of cytoplasm and vacuole (Q_cand Q_v), and fluxes at plasmalemma and tonoplast, were calculatedfrom the efflux kinetics. Over C_o 20–60 mM, as the apertureincreased from 7 µm to 17 µm, the tonoplast flux(0.5–11.5 pmol mm^–2h^–1) was always much lessthan the plasmalemma flux (7–77 pmol mm^–2 h^–1).Q_c and Q_v both increased with aperture. The increase in Q_c of10.3 pmol mm^–2 µm^–1 is adequate to accountfor the osmotic changes required to change the aperture, aspreviously estimated. However, the change in vacuolar contentof only 5.9 pmol mm^–2 µm^–1 is much too smallto account for the osmotic changes required, or to balance thecytoplasmic changes. It appears therefore that increasing KBroutside not only increases the cytoplasmic salt content, andthe Br flux at the tonoplast, but also stimulates the vacuolaraccumulation of some other solute.     

Measurement of the Apoplastic Activity of K+ and Cl- in the Leaf Epidermis of Commelina communis in Relation to Stomatal Activity

Bowling, D. J. F. 1987. Measurement of the apoplastic activityof K⁺ and Cl^– in the leaf epidermis of Commelina communisin relation to stomatal activity.–J. exp. Bot. 38: 1351–1355. Ionic activity of K⁺ and Cl^– in the apoplast of the lowerepidermis of the leaf of Commelina communis was measured usingion selective micro-electrodes. Large gradients across the stomatalcomplex were observed which were related to stomatal aperture.On stomatal closure the activity of K⁺ and Cl^– in theapoplast of the guard cell rose from 3·0 mol m^–3to 100 mol m^–3 and 33 mol m^–3 respectively. It wasconcluded that the apoplast is an important pathway for iontransport between the cells. Key words: Stomata, ionic activity, leaves, apoplast     

Stomatal responses of Vicia faba L. to indole acetic acid and abscisic acid

Evidence is presented that stomata in isolated epidermal peelsof Vicia faba L. open in darkness in response to the externalpresence of indole acetic acid (IAA) in the incubation medium.The effect of IAA is found to be overcome completely in thepresence of either TRIS or MES buffers. In the absence of buffer,V. faba stomata are shown to be influenced by IAA in a concentration-dependenttrend which reached a maximum at an [IAA] of 10^–3 molm^–3. Further investigations reveal that stomata in thisspecies can be shown to respond to the presence of IAA and anotherphytohormone, abscisic acid (ABA). IAA and ABA are demonstratedto be antagonistic in their effects provided the incubationconditions are suitable. The data are discussed in relationto stomatal responses of other species in different treatmentconditions. Recommendations are made with respect to standardizationof incubation media during epidermal peel experiments. Key words: Vicia faba, stomata, indole acetic acid, abscisic acid, buffers     

Electrical Currents at the Leaf Surface of Commelina communis and their Relationship to Stomatal Activity

A vibrating probe was used to detect and measure electricalcurrents at the surface of excised leaves and isolated leafepidermis from Commelina commnunis. Currents of up to 4.0 µAcm^–2 moving out from the leaf surface were observed whenthe stomata were open. When the stomata were almost closed nocurrent was detected and when they were fully closed a smallinwardly directed current was observed. There appeared to bea linear relationship between current and stomatal aperture.The current was stimulated by fusicoccin and eliminated by increasingthe external pH suggesting that it was brought about by a flowof H⁺ from the leaf surface. Key words: Electrical currents, leaves, stomata, vibrating probe     

Stomatal Responses to Applied ABA and CO2 in Epidermis Detached from Well-Watered and Water-Stressed Plants of Commelina communis L.

Epidermal strips from either well-watered or water-stressedplants of Commelina communis L. were subjected to a range ofABA concentrations (10^–6–10^–3 mol m^–3)in the presence (330 parts 10^–6 in air) or virtual absence(3 parts 10–6 in air) of CO₂. The stomatal response toCO₂ was greater in epidermis from water-stressed plants, althoughthere was a distinct CO₂ response in epidermis from well-wateredplants. Additions of ABA via the incubation medium had littleeffect on the relative CO₂ response. Stomata responded to ABAboth in the presence and virtual absence of CO₂, but the relativeresponse to ABA was greatest in the high CO₂ treatment. Whenwell-watered plants were sprayed with a 10^–1 mol m^–3ABA solution 1 d prior to use, the stomatal response of detachedepidermis to both CO₂ and ABA was very similar to that of epidermisdetached from water-stressed leaves. It is hypothesized thata prolonged exposure to ABA is necessary before there is anymodification of the CO₂ response of stomata.     

Ion Fluxes Across the Transfusion Zone of Secreting Limonium Salt Glands Determined from Secretion Rates, Transfusion Zone Areas and Plasmodesmatal Frequencies

Faraday, C. D., Quinton, P. M. and Thomson, W. W. 1986. Ionfluxes across the transfusion zone of secreting Limonium saltglands determined from secretion rates, transfusion zone areasand plasmodesmatal frequencies.—J. exp. Bot. 37: 482–494. The epidermal salt-secreting glands of Limonium (Plumbaginaceae)are enclosed in a cuticular envelope. Ions and metabolites enterthe glands from the mesophyll through gaps in the cuticularenvelope, the transfusion zones. Net influxes of ions acrossthe transfusion zone were calculated from measurements of secretionrates and transfusion zone areas. When leaves of L. pereziiF. T. Hubb. were treated with 300 mol m^–3 NaCl, transfusionzone influxes of Na⁺ K⁺, Ca⁺⁺ and Cl^– as high as 7?0?10^–5,1.7?10^–5, 5?8?10^–7 and 8.5?10^–5 mol m^–2s^–1 respectively, were calculated. Assuming a transmembranepathway, these fluxes would be some of the highest reportedfor ions in plant cells. Key words: Salt glands, ion fluxes, ultrastructure     

In-Phase Cycling of Photosynthesis and Conductance at Saturating Carbon Dioxide Pressure Induced by Increases in Water Vapour Pressure Deficit

Bunce, J. A. 1987. In-phase cycling of photosynthesis and conductanceat saturating carbon dioxide pressure induced by increases inwater vapour pressure deficit.—J. exp. Bot. 38: 1413–1420. The leaf to air water vapour deficit was increased suddenlyfrom about 1·0 to 2·5 IcPa for single leaves ofsoybean (Glycine max L. Merr.) plants held at 30 °C, 2·0mmol m ^–2 s^–1 photosynthetic photon flux density(PPFD) and carbon dioxide pressures saturating to photosynthesis.After a lag of about 10 min, photosynthetic rate and stomatalconductance to water vapour began to decrease, and then cycledin phase with each other. The period of the cydes was about20 min. During these cycles the substomatal carbon dioxide pressurewas constant in the majority of leaves examined, and was alwaysabove saturation for photosynthesis. Epidermal impressions showedthat most stomata changed in aperture during the cycles, andthat very few were ever fully closed. Water potential measuredon excised discs changed by at most 0·1 MPa from theminima to the maxima in transpiration rate. In contrast, forleaves of sunflower (Helianthus animus L.) grown at low PPFD,the increase in VPD led to leaf wilting and decreased photosynthesis,followed by recovery of turgor and photosynthesis as stomatalconductance began to decrease. In these leaves photosynthesisand conductance then cycled approximately 180° out of phase.It is suggested that in soybeans decreased leaf conductanceinduced by high VPD provided a signal which decreased the rateof photosynthesis at carbon dioxide saturation by a mechanismthat was not related to a water deficit in the mesophyll. Key words: Photosynthesis, stomatal conductance, cycling, vapour pressure deficit     

Salt Tolerance in the Triticeae: Growth and Solute Accumulation in Leaves of Thinopyrum bessarabicum

Gorham, J., McDonnell, E., Budrewicz, E. and Wyn Jones, R. G.1985. Salt tolerance in the Triticeae: growth and solute accumulationin leaves of Thinopyrum bessarabicum.—J. exp. Bot. 36:1021–1031. The diploid wheatgrass Thinopyrum bessarabicum was found towithstand prolonged exposure to 350 mol m^–3 NaCl in hydroponicculture. During the gradual addition of salt to the externalmedium, osmotic adjustment was rapidly achieved by the accumulationof Na and Cl. Following osmotic adjustment constant leaf Naand Cl concentrations were maintained, and K was retained ata high level. Thinopyrum bessarabicum may be described as anosmoconformer, adjusting its internal osmotic pressure to 400–500mOsmol kg^–1 above that of the external medium in hydroponicculture. Both slower shoot initiation and reduced leaf lengthcontributed to the reduced growth rates at higher salinities.Leaf width was not affected. Increasing salinity resulted inincreases in leaf concentrations of phosphate, glycinebetaine,sucrose and proline, and in decreases in the concentrationsof nitrate, sulphate, magnesium, calcium, total amino acidsand organic acids. Thinopyrum bessarabicum exhibits salt tolerancecharacters which may be useful in wheat breeding. Key words: Salt stress, solute accumulation, osmotic adjustment, Thinopyrum     

Circadian Rhythm of Stomatal Movements in Epidermal Strips

Measurements were made of changes in stomatal pore widths inepidermal strips of leaves ofVicia faba and Commelina communis.Strips were incubated in dilute KCI solutions (1 and 10 molm^–3) flowing through a perfusion chamber on the stageof a microscope and kept for 4 d in continuous light. Circadianrhythms of stomatal apertures were detected in both species.Although the amplitude was small it was statistically significant.It is concluded that at least partof the mechanism for the stomatalrhythm resides in the epidermis, probably in the guard cells. Key words: Cireadian rhythm, epidermal strips, stomata     

Acid-Induced Stomatal Opening in Commelina communis and Vicia faba

The effects of H^$ and fusicoccin (FC) on stomatal opening inthe dark were investigated using epidermal strips of Commelinacommunis and Vicia faba cv. Ryosai Issun. Citrate-phosphatebuffer induced maximal opening of stomata at pH 3.0 when testedover the range of 2.7 to 5.0. HCl at 1 mM also induced stomatalopening without appreciable accumulation of K^$ in the guardcells. After 4 hr treatment with 10 µM FC, stomata openedwith concomitant accumulation of K^$ in the guard cells, although1–2 hr treatment caused opening without concomitant K^$increase. These results suggest that stomatal opening can be caused bysalt accumulation and/or changes of the physicochemical conditionsin the cell wall of the guard cells due to high acidity. ¹ Present address: Biological Laboratory, Faculty of Education,Nagasaki University, Nagasski 852, Japan. (Received April 30, 1982; Accepted July 17, 1982)     

Statistical Distribution of Stomatal Apertures of Vicia faba and Hordeum vulgare and the Spannungsphase of Stomatal Opening

Plants of Vicia faba and Hordeum vulgare were grown in growthboxes with 7 mW cm^–2 PAR, 14 h day/10 h night, at 22/20°C. Stomata of attached leaves were measured under controlledconditions by means of an optical microscope and the distributionfunctions of the widths of pores were established. For Viciafaba they appeared to be symmetrical bell-shaped functions.In the process of stomatal opening or closure the shape of thedistribution remained constant, its maximum sliding left andright along the aperture axis. This result has been interpretedto mean that increments or decrements of apertures were equalfor all stomata independent of their individual apertures. Theconclusion has been drawn that the ‘driving force’is evenly distributed, equal for all stomata, and varies withinwider limits than is possible for stomatal apertures. Stomatalopening is limited by the closed state from below and by ananatomically possible maximum aperture from above.     

Modification of the CO2 Responses of Maize Stomata by Abscisic Acid and by Naturally-Occurring and Synthetic Cytokinins

Fragments of maize leaves were incubated at controlled temperatureand irradiance either on distilled water or on one of threeconcentrations of cytokinin (10^–1, 10^–2 and 10^–3mol m^–3). The effects of zeatin or kinetin on stomatalaperture were determined by stripping abaxial epidermis fromthe fragments after incubation and immediately measuring stomatalapertures under the microscope. At each cytokinin concentrationleaf pieces were incubated at 5 or 350 µmol mol^–1CO₂ with or without ABA (10^–1 mol m^–3). At 5.0 µmolmol^–1 CO₂ increasing the concentrations of zeatin hada negligible effect upon stomatal aperture. When air containing350 umol mol^–1 CO₂ was bubbled through the incubationsolutions, apertures of stomata incubated on water were morethan halved. Increasing cytokinin concentrations reduced theeffect of CO₂ on stomata and incubation on 10^–1 mol m^–3zeatin completely removed any CO₂ response. The addition ofABA restored the effect of CO₂, even at the highest cytokininconcentration. Key words: Maize, CO₂ response, ABA, Cytokinins