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     

Sap Exudation via the Epidermis of Leaves

Reports of stomatal guttation in Gladiolus and of active waterexudation in Vidafaba could not be confirmed by an experimentaland anatomical investigation. Exudation of water from detachedpine needles is considered to be due to processes distinct fromthose causing guttation. Water soaking of leaves, or the flooding of leaf air spaces,and the wettability of cell walls under root pressure as distinctfrom their wettability by water externally applied are discussedin relation to the experimental results.     

Direct Measurements of Turgor Pressure Potentials of Guard Cells: II. THE MECHANICAL ADVANTAGE OF SUBSIDIARY CELLS, THE SPANNUNQSPHASE, AND THE OPTIMUM LEAF WATER DEFICIT

Evidence of the mechanical advantage of subsidiary cells wasobtained by simultaneous measurements of turgor pressure potentialsin adjacent subsidiary and guard cells using injection circuitswith two separate needles. In Tradescantia virginiana the mechanicaladvantage approaches two. Using the same technique evidencewas obtained that the Spannungsphase is, in the first place,a turgor relations phenomenon due to the mechanical advantageof epidermal or subsidiary cells. In addition, the evidenceindicated that the elastic properties of guard cell walls mayundergo changes during the Spannungsphase when potassium iontransport commences. During these measurements it was confirmedthat the optimum leaf water deficit for maximum stomatal openingoccurs when the epidermal turgor is near zero. Under these conditionsthe width of the stomatal pore is a function of the turgor pressureof the guard cells, since at zero turgor of the subsidiary cellstheir mechanical advantage has disappeared.     

Studies in Stomatal Behaviour: VIII. STOMATAL RESPONSES TO TEMPERATURE AND CARBON DIOXIDE CONCENTRATION IN ALLIUM CEPA L., AND THEIR RELEVANCE TO MID-DAY CLOSURE

It has been reported in the literature that onion leaf stomatatend to close at noon (Loftfield, 1921) and that the minimumcarbon dioxide concentration () in onion leaf tissue rises from0.012 per cent to 0.024 per cent. When the leaf temperatureincreases from 30°C. to 35°C (light intensity 900.f.c.)(Heath and Orchard, 1957). Experiments were therefore carried out of test the hypothesisthat raising the left temperature above about 30°C. causesstomatal closing movements in onion leaves (e.g. mid-day closure)by increasing the carbon dioxide concentration in the leaf tissue.This hypothesis has been supported by the results of these experiments.Another and diametrically opposed effect of temperature on stomatalmovements in onion leaves has, however, been discovered, Namelythat, provided the final width of stomatal pores.     

The Minimum Intercellular-space CO2-Concentration (T) of Maize Leaves and its Influence on Stomatal Movements

Unlike other leaves investigated, maize leaves were found tobe able to exhaust atmospheric CO₂-content to zero concentration.This occurred at 20? C. with a light intensity of 100 f.c. andat 30? C. with a light intensity of 500 f.c. The influences of temperature, light intensity, and waterstrainon this property of maize leaves were investigated systematicallyand a permanent aftereffect of waterstrain on the leaves wasfound. Stomatal conductance measurements showed that maize stomataare sensitive to CO₂-concentrations between zero and 100 p.p.m.,a circumstance not yet reported for other leaves.     

The Measurement of Stomatal Responses to Stimuli in Leaves and Leaf Discs

A comparison has been made of stomatal responses in intact leaves,leaf discs supplied with water via their cut edges and leafdiscs floating on water. Xanthium pennsylvanicum leaf discswatered via their cut edges appeared to be more turgid thanintact leaves; this considerably slowed down the rate of stomatalopening but it slightly increased the final steady-state stomatalopening. When the water potential of such leaf discs was loweredby pre-treatment with mannitol solutions rates of stomatal openingincreased whereas maximum steady-state openings decreased. In tobacco leaf discs floating on water the stomata in contactwith water were wider open than those in contact with normalair and they did not respond to treatment with carbon dioxide-freeair. The rate of photosynthesis was severely reduced in tobaccoleaf discs floating with the lower epidermis on water, mostprobably owing to the slow rate of diffusion of carbon dioxidein water. By floating such discs on osmotica the degree of stomatalopening was increased, however, a response to treatment withcarbon dioxide-free air was still not measurable. It is postulatedthat, on account of the relative unavailability of carbon dioxidefrom the water, the carbon dioxide concentration in the substomatalcavities of the lower surface is abnormally low, irrespectiveof whether ordinary air or carbon dioxide-free air is availableto the upper surface. A comparison between porometer readings and measurements ofsiliconerubber impressions of stomatal pores taken from insidethe porometer cup confirmed that the silicone-rubber impressionmethod of assessing stomatal responses to stimuli has severelimitations, especially at small stomatal apertures.     

Stomatal Opening in Light of Different Wavelengths: Effects of Blue Light Independent of Carbon Dioxide Concentration

Stomatal opening in Xanthium pennsylvanicum was found to besignificantly greater in blue light than in red. Experimentsin which leaves were placed in a closed system and allowed toestablish their own steady-state carbon dioxide concentrationshowed that when the CO₂ concentration was about the same asthat in red, opening was much greater in blue light. Blue lightof low intensity could cause as great an opening as red of higherintensity, even though the CO₂ concentration was much higherin blue. Stomatal opening in light is considered as involvingat least two reactions: (1) a response to the removal of CO₂by photosynthesis; (2) a response to blue light not dependenton the removal of CO₂. Blue light became increasingly effective, relative to red, asthe length of night was increased over the range 2 to 14 hours.This might, in part, explain previously observed effects ofnight length on rate of opening in light. The initial very rapid phase of closure in darkness appearedto be independent of CO₂ accumulation, for it was not preventedby flushing the intercellular spaces with air free of CO₂. Itis suggested that closure in darkness, like opening in light,should be considered as involving components both dependentupon, and independent of, CO₂ concentration.     

Further Observations on the Minimum Intercellular Space Carbon-Dioxide Concentration ({Gamma}) of Maize Leaves and the Postulated Roles of 'Photo-respiration' and Glycollate Metabolism

The elevated -values of maize leaves under water strain havebeen found to be due to a reduction in the rate of carbon-dioxideintake during photosynthesis; these -values do not respond tochanges in the concentration of oxygen. No evidence has beenfound of an increased rate of carbon-dioxide evolution fromilluminated maize leaves under water strain (photo-respiration)although the presence of glycollate in these leaves could bedemonstrated.