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Apical dominance in Xanthium strumarium was influenced by the quality of illumination received at the end of the photoperiod. The involvement of the red/far-red regions of the spectrum was apparent. The persistence of the effects was partially dependent on the age of the individual buds concerned. Plants receiving 30 minutes of illumination from tungsten lamps after a 16-hour photoperiod from fluorescent tubes failed to branch, whereas plants given an identical photoperiod, both in terms of day-length and photosynthetically available light energy, but lacking the far-red from tungsten lamps, branched profusely.The influence of the spectral distribution of illumination on the levels of cytokinins and abseisic acid in the plant, and the correlation with the degree of branching, is presented and discussed. The cytokinin content was much higher in inhibited than released buds. The cytokinins present were probably not able to particinate in bud growth because of an accumulation of inhibitors resembling abscisic acid. The concentration of the inhibitors in inhibited buds was 50 to 250 times that occurring in all other plant parts examined.

Isolated epidermal strips of Commelina communis L. showed progressively smaller stomatal openings when incubated in abscisic acid solutions ranging in concentration from 10(-8) to 10(-4) M. The effects were reproducible and did not appear to be affected by the presence of auxin, gibberellic acid or kinetin. This specificity suggests that this method may prove valuable as a quick, sensitive bioassay for abscisic acid and other related compounds which might be used as antitranspirants on field crops. The fungal toxin fusicoccin, previously reported to cause increased stomatal opening on intact leaves, partially reversed the closure induced by abscisic acid.

Effects of fusicoccin alone and together with abscisic acid were observed on the stomatal complex of Commelina communis. The experimental material consisted of isolated epidermal strips incubated in a medium containing the ions required for stomatal opening. Fusicoccin stimulated opening and this was accompanied by potassium entry into the guard cells, and hydrolysis of the starch in their chloroplasts. Abscisic acid alone inhibited potassium entry and starch hydrolysis, but these effects could be almost entirely overcome by fusicoccin.Attempts were made to measure the solute potential of the guard cells under the various treatments. Abscisic acid clearly increased their solute potential, but no absolute measurements could be made in the presence of fusicoccin owing to a failure of plasmolysis even with mannitol solutions of solute potential as low as -35 bars. Experiments using isotopically labelled mannitol indicated a massive uptake into the epidermis in the presence of fusicoccin.The mechanism of stimulation of stomatal opening by fusicoccin probably depends in part on a stimulation of the normal processes associated with opening in the guard cells, but may also involve release of pressure due to destruction of the surrounding cells. The effectiveness of this toxin under natural conditions may depend on its ability to counteract effects of abscisic acid, the stress hormone that induces stomatal closure.

Detached shoots of Bryophyllum fedtschenkoi maintained in a closed system in the light exhibited an endogenous circadian rhythm in CO2 compensation. The rhythm was sensitive to changes in light intensity and temperature. At 15° C it damped rapidly in light of 78 J m(-2) s(-1), but at 10° C a rhythm of considerable amplitude was evident at this same light intensity. During the transient (i.e. the temporary state of the rhythm before it acquired its steady state) low compensation values between 0 and 5 ppm CO2 were achieved. When the plants were maintained at a higher light intensity prior to the measurements, the period of low compensation during the transient was extended, and zero values were obtained under some conditions.Studies of gas exchange at opposite phases of the rhythm revealed: (i) that the rate of uptake of (14)CO2 differed, both in light and darkness (the epidermis was removed during these observations to avoid interference from stomatal rhythms); (ii) 'photorespiration', estimated by extrapolation of the graph relating photosynthetic rate and CO2 concentration, was highest during the peaks of the rhythm in CO2 compensation; (iii) estimates of the capacity for 'photorespiration' by the glycine-1-(14)C assay indicated highest values during the troughs of the rhythm. These findings are discussed in relation to the C4-acid metabolism of this species. Low CO2 compensation is probably due to the activity of phosphoenolpyruvate carboxylase and not to the absence of processes involving CO2 evolution.

Abscisic acid (ABA) at a concentration of 100 μm reduced the mean stomatal aperture on isolated epidermis of Commelina communis from 9.5 to 3.1 μm. This closure resulted from a fall in osmotic pressure of the guard cells from 14.1 to 9.8 bars; the osmotic pressure of the subsidiary cells did not change significantly. Histochemical tests showed that the potassium concentration in guard cells was reduced by ABA-treatment, while the starch content of the chloroplasts increased. ABA was found to exert a significant effect on Rb(86) uptake into leaf discs, but this was relatively small in magnitude. It is concluded that ABA has a greater effect on ion uptake into guard cells than into the leaf tissues as a whole.Recent hypotheses of the stomatal mechanism are discussed in relation to these new observations, and the rejection by some writers of any major role for starch hydrolysis is challenged. Evidence from several sources suggests that starch disappearance occurs simultaneously with K(+) entry into guard cells. Breakdown of starch may lead to formation of organic anions, with which K(+) uptake may be associated. In this case starch breakdown would contribute as much to the increased osmotic pressure as does K(+) uptake.

Ten analogues of abscisic acid (ABA) have been tested for their activity in assays based on the induction of stomatal closing on isolated epidermis of Commelina communis L., and the inhibition of opening on intact leaves of Xanthium strumarium L. Three of the analogues showed activity comparable to that of ABA on the stomata of Commelina. The same analogues also showed similar activity and persistence as ABA when applied to the leaf surfaces of Xanthium. Molecular requirements for activity on stomata agree closely with those for activity in the Avena coleoptile assay. The possible value of the analogues as antitranspirants is considered.

Lipid extracts from Sorghum sudanese Stapf (cv Piper) that had been subjected to different degrees of water stress contained increasing amounts of a sesquiterpenoid, other than abscisic acid, as the amount of stress increased and subsequent bioassay showed that this substance was capable of inducing stomatal closure. Infra-red, nuclear magnetic resonance and mass spectral analysis after final purification by preparative gas-liquid chromatography showed conclusively that this substance was identical to all-trans-farnesol. Various derivatives, homologues and isoprenologues of all-trans-farnesol were tested for their ability to close stomata. Only trans-nerolidol approached all-trans-farnesol in activity when applied to isolated epidermis of Commelina communis L. Stomatal closure was induced slightly faster and more completely in the presence of all-trans-farnesol than in the presence of ABA at a similar molar concentration.