Stomatal sensitivity to abscisic acid: can it be defined?

Abstract. Evidence is presented that the sensitivity of stomata to abscisic acid is greatly influenced by two factors which are known to vary in leaves, namely concentrations of potassium and of indol-3-ylacetic acid. It is suggested that the notion of 'quantitative tissue sensitivity', by which Trewavas explains the action of growth substances in developmental phenomena, may be applicable to stomatal responses to abscisic acid.     

Studies of the Inhibition of Stomatal Opening by Naphth-1-ylacetic Acid and Abscisic Acid

Evidence is presented which confirms the inhibitory action ofthe synthetic auxin, NAA, on stomatal opening. In contrast tothe natural auxin IAA, NAA exerts effects which, in some respects,resemble those of ABA. The mode of action of NAA on the guardcells is, however, thought to be different from that of ABA.When NAA and ABA are applied together, their combined actionresults in a greater reduction in stomatal aperture than wouldbe predicted from their separate effects. Key words: NAA, ABA, IAA Stomata, Commelina     

Responses of Stomata to IAA and Fusicoccin at the Opposite Phases of an Entrained Rhythm

The stomata of Commelma communis showed reduced opening responsesto light and low CO₂ concentrations during the night phase oftheir entrained circadian rhythm. Increased supplies of potassiumions, and treatments with indol-3-ylacetic acid and fusicoccin,failed to promote opening during the night phase to a levelequivalent to that in the day phase. The inability of fusiccocinto overcome the suppression of opening during the night phasecontrasts with its ability to counteract the closure inducedby agents such as CO₂, darkness and abscisic acid. It is concludedthat there are at least two basic mechanisms by which the turgorof guard cells can be regulated, one which is susceptible tooverriding control by fusicoccin and another which is unaffectedby fusicoccin. Several previous studies had shown a positive correlation betweenmalate in the epidermis (mainly located in guard cells) andstomatal opening. In the present experiments the aperture/malatecorrelation was broken in epidermis treated with fusicoccinduring the night phase of the rhythm. The amount of malate presentexceeded that associated with the same stomatal aperture inthe day phase. Possible explanations are (1) that fusicoccinstimulates similar proton fluxes out of the guard cells duringboth phases of the rhythm, but an unknown factor imposes a restrictionon stomatal opening during the night phase; (2) that there arelower proton fluxes in the night phase (limited, for example,by a reduced supply of ATP) but chloride availability or transportis reduced to an even greater extent so that a larger productionof malate in the guard cells is required. Key words: Stomata, IAA, Fusicoccin, Rhythms     

Effects of long‐term fox baiting on species composition and abundance in an Australian lizard community

Abstract We report on the effects of almost a decade of 1080‐fox baiting on a lizard community in a mosaic Australian habitat. Replicated comparisons of baited versus non‐baited control areas with near‐identical histories of bush fires, grazing and climate showed a higher density of red fox tracks (Canis vulpes) in the non‐baited areas. Furthermore, the fox‐baited areas showed a more than five times higher density of sand goannas (Varanus gouldii), a species that strongly overlaps the red fox in food niche breadth and is itself a direct target of fox predation, in particular its eggs and young. Exclusion of predators from a natural habitat led to significant increases in the density of small lizards, suggesting that predation can drive lizard population dynamics in this ecosystem. Replicated pitfall‐trapping in three habitats in the control areas (with high fox and low goanna density) versus the baited areas (with low fox and high goanna density) showed that fox baiting had positive effects on the density of diurnal scincid lizards in open grassland, whereas the control areas showed higher density of nocturnal gecko lizards. Our interpretation is that fox removal may result in a shift in the top predator towards the sand goanna. Historically, this indigenous, endemic species was the natural top predator. It has co‐evolved with its prey and that may have moulded it into a more efficient lizard predator per encounter than the introduced fox.     

The Circadian Rhythm of Stomatal Opening: Evidence for the Involvement of Potassium and Chloride Fluxes

Epidermal strips were taken from plants of Commelina communisat opposite phases of an entrained circadian rhythm, at timeswhich corresponded to the middle of the day phase and the middleof the night phase. Earlier observations of much reduced openingin response to external stimuli in the night phase were confirmed. Evidence is now presented that uptake or retention of specificions by the guard cells is affected by the phase of the rhythm.The overt expression of the rhythm was reduced if the epidermiswas presented with potassium iminodiacetate instead of potassiumchloride. Iminodiacetate is a non-absorbable anion, and thisresult suggested that chloride uptake is important for the fullexpression of the rhythm. The use of an anion channel inhibitor(DIDS) gave strong support for this conclusion. Reduced uptakeof chloride would explain an earlier observation that formationof malate is greater during the night phase. When the guard cells were presented with sodium chloride insteadof potassium chloride there was still an overt expression ofthe rhythm, but this was of reduced magnitude. This suggeststhat potassium movements might contribute to the rhythm, andstudies of ionic fluxes using ⁸⁶Rb as a tracer showed greatereffluxes during the night phase. It is concluded that the circadian rhythm in stomatal openingmay be the result of a varying ability of the guard cells toaccumulate or retain both chloride and potassium ions. Key words: Circadian rhythm, stomatal opening, Commelina communis, potassium and chloride fluxes     

Control of the Co2 Responses of Stomata by Indol-3ylacetic Acid and Abscisic Acid

Previous studies of stomatal behaviour on detached epidermisof Commelina communis L. have suggested that abscisic acid (ABA)and C02 act independently to cause stomatal closure. Evidenceis presented here that if indol-3ylacetic acid (IAA) is addedto the medium used for incubating the epidermis, an interactionbetween ABA and Co₂ becomes apparent. Increasing concentrationsof IAA reduce the ability of the stomata to respond to CO₂,and ABA appears to antagonize this effect. Recognition of therole of IAA enables us to reconcile earlier conflicting reportsconcerning the interdependence of effects of ABA and Co₂on stomata.