Relative control potential of abscisic acid, carbon dioxide and light in responses of Phaseolus vulgaris stomata

We outline a theoretical framework for comparing the relative effectiveness of hormonal and other factors in the control of physiological responses. This involves determining a 'baseline' set of conditions, then adjusting each factor in turn with the others held constant. The initial rate of response to each change in condition is estimated and compared among factors to arrive at a quantitative indication of control potential under the specified baseline conditions – the control coefficient. The suitability of the stomatal response as a model system for such studies is discussed. A portable infra-red gas analyser was used to investigate the control potential of abscisic acid (ABA), CO₂ and light – measured as photosynthetic photon flux density (PPFD) – over responses of Phaseolus vulgaris L. stomata at baseline conditions appropriate to an unstressed plant under moderate light conditions. The ranking of control coefficients observed under these conditions was light>CO₂>ABA. The control coefficients of CO₂ and ABA were 0.28 and 0.11 times that of light, respectively. These results are discussed in relation to the sensitivity of the plant material and the experimental conditions. Implications for a hormonal role for ABA in the control of stomatal movements are considered. We conclude that this method can provide valuable information concerning the relative control potential of hormonal and other influences in the context of differences in baseline conditions and in relation to changes in sensitivity of plant material.     

The roles of carbon dioxide and abscisic acid in the production of ethylene

Since CO₂ is liberated in the conversion of ACC to ethylene, the evidence that ethylene production by plant tissues is actually promoted by CO₂ calls for an explanation. Accordingly, the formation of ethylene by oat (Avena sativa L. cv. Victory) leaves and by apple (Golden Delicious) and pear (Pyrus communis L. cv. Anjou) tissue in very low levels of CO₂ has been studied. The gas chromatograph was modified to measure CO₂ and ethylene at the same time, by reducing both to methane. (Response of the gas chromatograph to CO₂ concentrations is linear.) The work establishes a clear difference between the endogenous production of ethylene and its production from applied ACC, a difference which holds about equally for leaves and for fruit tissue. The difference is in the CO₂ requirement, namely, lowering the CO₂ level by 99% or more decreases the production of ethylene from applied ACC by 50–60%, but it does not decrease, or even slightly increases, its production from endogenous precursors. Thus, while the need for CO₂ has not been explained, it has at least been delimited. The responses to abscisic acid (ABA) also differ, but in the reverse direction, the endogenous production of ethylene being decreased up to 70% by ABA. while the liberation from ACC is promoted about 20%. ABA also promoted the respiratory CO₂ production by 30% or, in presence of 1-aminocyclopropane-1-carboxylic acid (ACC), by over 50%. Inhibition of ethylene production by cobalt or EDTA shows no such differentiation, while inhibition by Na catechol-4,6-disulfonate (Tiron) shows a small difference. It is concluded either that endogenous ethylene is formed by an enzyme system different from that reacting with ACC, or (more likely) that when ethylene arises from endogenous precursors the reaction does not proceed by way of free ACC, but by some activated form of it.     

The effect of pH on stomatal sensitivity to abscisic acid

Abstract. The sensitivity of stomata of Commelina communis L. to abscisic acid (ABA) was evaluated by analysing the initial rates of response to the compound at different hormone concentrations. This was carried out at pH 6.8 and pH 5.5. The data were modelled and statistically analyzed by means of a computer program employing non-linear regression techniques and step-down analysis of variance. The response kinetics as quantified in terms of three sensitivity parameters were found to differ significantly between the two pH values. This finding is discussed in relation to previous research on purified ABA-binding proteins.     

Effects of humidity on short-term responses of stomatal conductance to an increase in carbon dioxide concentration

The magnitude of the response of stomatal conductance to a change in the concentration of carbon dioxide external to the leaf from 350 to 700 cm³ m^–3 was found to be extremely variable from day to day in the field in Glycine max , Hordeum vulgare and Triticum aestivum . It was found that the leaf-to-air water vapour pressure difference (LAVPD) during the midday measurements of the stomatal response to carbon dioxide affected the magnitude of the response. On days when LAVPD was low, no significant change in conductance occurred with the increase in carbon dioxide concentration. When LAVPD was higher, conductance decreased by 24–52% with the increase in carbon dioxide within a few minutes. The sensitivity of conductance was approximately linearly related to LAVPD in wheat and barley. Experiments with G. max in the field indicated that, on days with low LAVPD, increasing the LAVPD just around the measured portion of a leaflet made stomatal conductance responsive to increased carbon dioxide. This result was also obtained under laboratory conditions with G. max , Helianthus annuus and Amaranthus retroflexus . In G. max , it was determined that leaves in which conductance was not responsive to the increase in carbon dioxide could be made responsive even at low LAVPD by the injection of abscisic acid into their petioles. Because it is known that abscisic acid sensitizes stomata to carbon dioxide, these results are consistent with the idea that abscisic acid may be involved in the response of stomatal conductance to changes in LAVPD.     

Direct and acclimatory responses of stomatal conductance to elevated carbon dioxide in four herbaceous crop species in the field

In order to separate the net effect of growth at elevated [CO₂] on stomatal conductance (g_s) into direct and acclimatory responses, mid‐day values of g_s were measured for plants grown in field plots in open‐topped chambers at the current ambient [CO₂], which averaged 350 μmol mol^?1 in the daytime, and at ambient + 350 μmol mol^?1[CO₂] for winter wheat, winter barley, potato and sorghum. The acclimatory response was determined by comparing g_s measured at 700 μmol mol^?1[CO₂] for plants grown at the two [CO₂]. The direct effect of increasing [CO₂] from 350 to 700 μmol mol^?1 was determined for plants grown at the lower concentration. Photosynthetic rates were measured concurrently with g_s. For all species, growth at the higher [CO₂] significantly reduced g_s measured at 700 μmol mol^?1[CO₂]. The reduction in g_s caused by growth at the higher [CO₂] was larger for all species on days with low leaf to air water vapour pressure difference for a given temperature, which coincided with highest conductances and also the smallest direct effects of increased [CO₂] on conductance. For barley, there was no other evidence for stomatal acclimation, despite consistent down‐regulation of photosynthetic rate in plants grown at the higher [CO₂]. In wheat and potato, in addition to the vapour pressure difference interaction, the magnitude of stomatal acclimation varied directly in proportion to the magnitude of down‐regulation of photosynthetic rate through the season. In sorghum, g_s consistently exhibited acclimation, but there was no down‐regulation of photosynthetic rate. In none of the species except barley was the direct effect the larger component of the net reduction in g_s when averaged over measurement dates. The net effect of growth at elevated [CO₂] on mid‐day g_s resulted from unique combinations of direct and acclimatory responses in the various species.     

Reduced respiratory responses to carbon dioxide after propranolol: a central action.

In a double-blind study in six subjects propranolol significantly reduced the respiratory sensitivity to carbon dioxide rebreathing. This effect seems to have been due to beta-adrenergic blockade, since it was not seen with D-propranolol. In two subjects increasing doses of propranolol caused progressive reductions in respiratory sensitivity to values below normal and similar to those of patients with ventilatory failure. These changes are probably due to a central action of propranolol.     

Guard cell-specific inhibition of Arabidopsis MPK3 expression causes abnormal stomatal responses to abscisic acid and hydrogen peroxide

MAP kinases have been linked to guard cell signalling. Arabidopsis thaliana MAP Kinase 3 (MPK3) is known to be activated by abscisic acid (ABA) and hydrogen peroxide (H(2)O(2)), which also control stomatal movements. We therefore studied the possible role of MPK3 in guard cell signalling through guard cell-specific antisense inhibition of MPK3 expression. Such transgenic plants contained reduced levels of MPK3 mRNA in the guard cells and displayed partial insensitivity to ABA in inhibition of stomatal opening, but responded normally to this hormone in stomatal closure. However, ABA-induced stomatal closure was reduced compared with controls when cytoplasmic alkalinization was prevented with sodium butyrate. MPK3 antisense plants were less sensitive to exogenous H(2)O(2), both in inhibition of stomatal opening and in promotion of stomatal closure, thus MPK3 is required for the signalling of this compound. ABA-induced H(2)O(2) synthesis was normal in these plants, indicating that MPK3 probably acts in signalling downstream of H(2)O(2). These results provide clear evidence for the important role of MPK3 in the perception of ABA and H(2)O(2) in guard cells.     

Species-dependent changes in stomatal sensitivity to abscisic acid mediated by external pH

The direct effects of pH changes and/or abscisic acid (ABA) on stomatal aperture were examined in epidermal strips of Commelina communis L. and Arabidopsis thaliana. Stomata were initially opened at pH 7 or pH 5. The stomatal closure induced by changes in external pH and/or ABA (10 microM or 10 nM) was monitored using video microscopy and quantified in terms of changes in stomatal area using image analysis software. Measurements of aperture area enabled stomatal responses and, in particular, small changes in stomatal area to be quantified reliably. Both plant species exhibited a biphasic closure response to ABA: an initial phase of rapid stomatal closure, followed by a second, more prolonged, phase during which stomata closure proceeded at a slower rate. Changes in stomatal sensitivity to ABA were also observed. Comparison of these effects between C. communis and A. thaliana demonstrate that this differential sensitivity of stomata to ABA is species-dependent, as well as being dependent on the pH of the extracellular environment.     

Differential growth responses to sodium salts involve different abscisic acid metabolism and transport in Prosopis strombulifera

In this work, the response of the halophytic shrub Prosopis strombulifera to lowering an osmotic potential (Ψ_o) to ?1.0, ?1.9, and ?2.6 MPa generated by NaCl, Na₂SO₄, and the iso-osmotic combination of them was studied at 6, 12, and 24 h after reaching such values in the growing media. By analyzing the content of abscisic acid (ABA) and related metabolites and transpiration rates, we observed that ABA content varied depending on type of salt, salt concentration, organ analyzed, and age of a plant. ABA content in leaves was much higher than in roots, presumably because of rapid biosynthesis and transport from roots. Leaves of Na₂SO₄-treated plants had the highest ABA content at Ψ_o ?2.6 MPa (24 h) associated with sulfate toxicity symptoms. Significant content of ABA-glucose ester (ABA-GE) was found in both the roots and leaves, whereas only low content of phaseic acid (PA) and dihydrophaseic acid (DPA). The roots showed high ABA-GE accumulation in all treatments. The highest content of free ABA was correlated with ABA-GE glucosidase activity. The results show that ABA-GE and free ABA work together to create a specific stress signal.     

The role of the mesophyll in stomatal responses to light and CO2

Stomatal responses to light and CO₂ were investigated using isolated epidermes of Tradescantia pallida , Vicia faba and Pisum sativum . Stomata in leaves of T. pallida and P. sativum responded to light and CO₂, but those from V. faba did not. Stomata in isolated epidermes of all three species could be opened on KCl solutions, but they showed no response to light or CO₂. However, when isolated epidermes of T. pallida and P. sativum were placed on an exposed mesophyll from a leaf of the same species or a different species, they regained responsiveness to light and CO₂. Stomatal responses in these epidermes were similar to those in leaves in that they responded rapidly and reversibly to changes in light and CO₂. Epidermes from V. faba did not respond to light or CO₂ when placed on mesophyll from any of the three species. Experiments with single optic fibres suggest that stomata were being regulated via signals from the mesophyll produced in response to light and CO₂ rather than being sensitized to light and CO₂ by the mesophyll. The data suggest that most of the stomatal response to CO₂ and light occurs in response to a signal generated by the mesophyll.     

Effects of abscisic acid and its derivatives on stomatal closing

Abscisic acid and its derivatives, formed with the terminalcarboxyl group replaced respectively by aldehyde, hydroxymethyland methyl groups, were examined for their effects on stomatalclosing. Only the derivative with the methyl group was inactive.The acid and the other two derivatives were very active forclosing stomata at low concentrations. (Received January 28, 1975; )     

Changes in apoplastic pH and membrane potential in leaves in relation to stomatal responses to CO2, malate, abscisic acid or interruption of water supply

Low CO2 concentrations open CO2-sensitive stomata whereas elevated CO2 levels close them. This CO2 response is maintained in the dark. To elucidate mechanisms underlying the dark CO2 response we introduced pH- and potential-sensitive dyes into the apoplast of leaves. After mounting excised leaves in a gas-exchange chamber, changes in extracellular proton concentration and transmembrane potential differences as well as transpiration and respiration were simultaneously monitored. Upon an increase in CO2 concentration transient changes in apoplastic pH (occasionally brief acidification, but always followed by alkalinization) and in membrane potential (brief hyperpolarization followed by depolarization) accompanied stomatal closure. Alkalinization and depolarization were also observed when leaves were challenged with abscisic acid or when water flow was interrupted. During stomatal opening in response to CO2-free air the apoplastic pH increased while the membrane potential initially depolarized before it transiently hyperpolarized. To examine whether changes in apoplastic malate concentrations represent a closing signal for stomata, malate was fed into the transpiration stream. Although malate caused apoplastic alkalinization and membrane depolarization reminiscent of the effects observed with CO2 and abscisic acid, this dicarboxylate closed the stomata only partially and less effectively than CO2. Apoplastic alkalinization was also observed and stomata closed partially when KCl was fed to the leaves. Respiration increased on feeding of malate or KCl, or while abscisic acid closed the stomate. From these results we conclude that CO2 signals modulate the activity of plasma-membrane ion channels and of plasmalemma H+-ATPases during changes in stomatal aperture. Responses to potassium malate and KCl are not restricted to guard cells and neighbouring cells.     

Responses of stomatal conductance to light, humidity and temperature in winter wheat and barley grown at three concentrations of carbon dioxide in the field

Responses of leaf stomatal conductance to light, humidity and temperature were characterized for winter wheat and barely grown at ambient (about 350 μmol mol^?1 in the daytime), ambient + 175 and ambient + 350 μmol mol^?1 concentrations of carbon dioxide in open‐topped chambers in field plots over a three year period. Stomatal responses to environment were determined by direct manipulation of single environmental factors, and those results were compared with responses derived from natural day to day variation in mid‐day stomatal conductance. The purpose of these experiments was to determine the magnitude of reduction in stomatal conductance at elevated [CO₂], and to assess whether the relative response of conductance to elevated [CO₂] was constant across light, humidity and temperature conditions. The results indicated that light, humidity and temperature all significantly affected the relative decrease in stomatal conductance at elevated [CO₂]. The relative decrease in conductance with elevated [CO₂] was greater at low light, low water vapour pressure difference, and high temperature in both species. For measurements made at saturating light near mid‐day, the ratio of mid‐day stomatal conductances at doubled [CO₂] to that at ambient [CO₂] ranged from 0.42 to 0.86, with a mean of 0.66 in barley, and from 0.33 to 0.80, with a mean of 0.56 in wheat. Day‐to‐day variation in the relative effect of elevated [CO₂] on conductance was correlated with the relative stimulation of [CO₂] assimilation rate and with temperature. Some limitations of multiple linear regression, multiplicative, and ‘Ball–Berry' models as summaries of the data are discussed. In barley, a better fit to the models occurred in individual years than for the combined data, and in wheat a better fit to the models occurred when data from near the end of the season were removed.     

Elevated CO2 enhances stomatal responses to osmotic stress and abscisic acid in Arabidopsis thaliana

A , carbon assimilation rate
ABA, abscisic acid
Ci , intercellular space CO₂ concentration
g , leaf conductance
WUE, water use efficiency

Carbon dioxide and abscisic acid (ABA) are two major signals triggering stomatal closure. Their putative interaction in stomatal regulation was investigated in well-watered air-grown or double CO₂-grown Arabidopsis thaliana plants, using gas exchange and epidermal strip experiments. With plants grown in normal air, a doubling of the CO₂ concentration resulted in a rapid and transient drop in leaf conductance followed by recovery to the pre-treatment level after about two photoperiods. Despite the fact that plants placed in air or in double CO₂ for 2 d exhibited similar levels of leaf conductance, their stomatal responses to an osmotic stress (0·16–0·24 MPa) were different. The decrease in leaf conductance in response to the osmotic stress was strongly enhanced at elevated CO₂. Similarly, the drop in leaf conductance triggered by 1 μ M ABA applied at the root level was stronger at double CO₂. Identical experiments were performed with plants fully grown at double CO₂. Levels of leaf conductance and carbon assimilation rate measured at double CO₂ were similar for air-grown and elevated CO₂-grown plants. An enhanced response to ABA was still observed at high CO₂ in pre-conditioned plants. It is concluded that: (i) in the absence of stress, elevated CO₂ slightly affects leaf conductance in A. thaliana ; (ii) there is a strong interaction in stomatal responses to CO₂ and ABA which is not modified by growth at elevated CO₂.     

Effect of abscisic acid pretreatments on stomatal reopening in Vicia faba

After a pretreatment of 2 h exposure to a solution containing 2 × 10⁻⁴ M ABA, reopening of stomata occurred in epidermal strips of Vicia faba L. cv. Cavalier on an ABA-free incubation solution. After pretreatment with exogenous ABA stomatal apertures were greater when higher levels of KCl were incorporated into the solution used for reopening. Prolonged exposure to exogenous ABA (14 h) did not prevent stomatal reopening upon transfer to ABA-free solutions. However, for both ABA and ABA-free pretreatments, prolonged incubation (1 day after removal of epidermis) resulted in enhanced stomatal apertures when the epidermal strips were exposed to light. This effect was lost 2 days after removal of the epidermis and opening did not occur after 3 days. Epidermal strips containing endogenous ABA were obtained from wilted leaves. Reopening was greatly reduced by the endogenous ABA treatment, and variation of KCl concentration in the incubation solution had little effect on stomatal aperture. It is postulated that during wilting endogenous ABA becomes reversibly bound without loss of activity for a longer period than is obtained using exogenous ABA. The presence of other unidentified compounds may be involved in this process.     

General relations of stomatal responses to xylem sap abscisic acid under stress in the rooting zone – A global perspective

A sensitivity factor that quantifies the responsiveness of stomata to xylem sap abscisic acid concentration ([ABA]_xyl) is described, using the relation between [ABA]_xyl and maximum leaf conductance (g_max). Plotting g_max against this factor results in a common linear relationship for woody and herbaceous species from boreal to (semi-) arid climates. The global distribution of the sensitivity factor reveals an unexpected pattern which is inverse to rainfall, i.e., plants in humid climates respond more sensitively to ABA than plants in arid areas. The implications for the response of natural vegetation and consequences for agriculture are discussed.     

Stomatal responses to jasmonic acid, linolenic acid and abscisic acid in wild-type and ABA-deficient tomato plants

Wild-type and abscisic acid (ABA) -deficient (sitiens) tomato plants were used to analyse the effects of abscisic acid (ABA), butyric acid (BA), jasmonic acid (JA) and linolenic acid (LA) on assimilation and transpiration rates in detached leaves taking up those substances into the transpiration stream. BA did not affect assimilation and transpiration rates. ABA decreased assimilation and transpiration in both wild-type and ABA-deficient mutants. JA reduced the assimilation rate in both lines but induced a significant reduction of transpiration in the wild type only. The response to LA in both lines was slower than that to JA.