Fluorescence and thermal imaging were used to examine the dynamics of stomatal patches for a single surface of Xanthium strumarium L. leaves following a decrease in ambient humidity.
Patches were not observed in all experiments, and in many experiments the patches were short‐lived. In some experiments, however, patches persisted for many hours and showed complex temporal and spatial patterns.
Rapidly sampled fluorescence images showed that the measurable variations of these patches were sufficiently slow to be captured by fluorescence images taken at 3‐min intervals using a saturating flash of light.
Stomatal patchiness with saturating flashes of light was not demonstrably different from that without saturating flashes of light, suggesting that the regular flashes of light did not directly cause the phenomenon.
Comparison of simultaneous fluorescence and thermal images showed that the fluorescence patterns were largely the result of stomatal conductance patterns, and both thermal and fluorescence images showed patches of stomatal conductance that propagated coherently across the leaf surface.
These nondispersing patches often crossed a given region of the leaf repeatedly at regular intervals, resulting in oscillations in stomatal conductance for that region. The existence of these coherently propagating structures has implications for the mechanisms that cause patchy stomatal behaviour as well as for the physiological ramifications of this phenomenon.
Previous studies have suggested that the red light and CO2 responses of stomata are caused by a signal from the mesophyll to the guard cells.
Experiments were conducted to test the idea that this signal is a vapour‐phase ion. Stomata in isolated epidermes of Tradescantia pallida were found to respond to air ions created by an electrode that was positioned under the epidermes. Anthocyanins in the epidermes of this species were observed to change colour in response to these air ions, and this change in colour was attributed to changes in pH.
A similar change in lower epidermal colour was observed in intact leaves upon illumination and with changes in CO2concentration. Based on the change in epidermal colour, the pH of the epidermis was estimated to be approximately 7.0 in darkness and 6.5 in the light.
Stomata in isolated epidermes responded to pH when suspended over (but not in contact with) solutions of different pH.
We speculate that stomatal responses to CO2 and light are caused by vapour‐phase ions, possibly hydronium ions that change the pH of the epidermis.
The response of stomata to red and blue light was investigated using small fibre optics (66 µm diameter) to control light levels on a single pair of guard cells without affecting the surrounding tissue.
Low intensity red light (50 µmol m–2 s–1) applied to the entire leaf caused stomata to oscillate continuously for several hours with no apparent decrease in amplitude with time. Adding low intensity blue light (50 µmol m–2 s–1) caused stomata to stop oscillating, but oscillations resumed when the blue light was removed.
Adding the same intensity of red light to an oscillating leaf changed the amplitude of the oscillations but did not stop them. When blue light was added to a single guard cell pair (using a fibre optic) in a red-light-illuminated leaf, the stoma formed by that pair stopped oscillating, but adjacent stomata did not.
Red light added to a single guard cell pair did not stop oscillations. Finally, blue light applied through a fibre optic to areas of leaf without stomata caused proximal stomata to stop oscillating, but distal stomata continued to oscillate.
The data suggest that blue light affects stomata via direct effects on guard cells as well as by indirect effects on other cells in the leaf.
Stomatal responses to leaf temperature (Tl) and to the mole fractions of water vapour in the ambient air (wa) and the leaf intercellular air spaces (wi) were determined in darkness to remove the potential effects of changes in photosynthesis and intercellular CO2 concentration.
Both the steady‐state and kinetic responses of stomatal conductance (gs) to wa in darkness were found to be indistinguishable from those in the light. gs showed a steep response to the difference (Δw) between wa and wi when wa was varied.
The response was much less steep when wi was varied. Although stomatal apertures responded steeply to Tl when Δw was held constant at 17 mmol mol−1, the response was much less steep when Δw was held constant at about zero.
Similar results were obtained in the light for Δw = 15 mmol mol−1 and Δw ≈ 0 mmol mol−1.
These results are discussed in the context of mechanisms for the stomatal response to humidity.
The role of the mesophyll in stomatal functioning in thin amphistomatous leaves was investigated by altering gas‐exchange for one surface and observing the effects on stomatal conductance for the other surface.
Three methods of perturbing gas exchange on the adaxial surface were used. First, gas exchange for the adaxial surface was completely blocked with plastic wrap or vacuum grease. Second, leaves were inverted to induce closure of the adaxial stomata. And third, ambient humidity for the adaxial surface was reduced to induce stomatal closure on that surface.
Experiments were performed at low light intensity and three different CO2 concentrations to test the idea that stomatal responses in thin amphistomatous leaves are partially controlled by a signal from the mesophyll that varies with light and CO2.
In general, stomata on the abaxial surface opened when gas‐exchange on the adaxial surface was reduced, with the largest increases in conductance occurring at high CO2 concentration.
The data are discussed with respect to role of a purported signal from the mesophyll and the partitioning of that signal between the two surfaces of the leaf.
A new mechanism for stomatal responses to humidity and temperature is proposed. Unlike previously-proposed mechanisms, which rely on liquid water transport to create water potential gradients within the leaf, the new mechanism assumes that water transport to the guard cells is primarily through the vapour phase. Under steady-state conditions, guard cells are assumed to be in near-equilibrium with the water vapour in the air near the bottom of the stomatal pore. As the water potential of this air varies with changing air humidity and leaf temperature, the resultant changes in guard cell water potential produce stomatal movements. A simple, closed-form, mathematical model based on this idea is derived. The new model is parameterized for a previously published set of data and is shown to fit the data as well as or better than existing models. The model contains mathematical elements that are consistent with previously-proposed mechanistic models based on liquid flow as well as empirical models based on relative humidity. As such, it provides a mechanistic explanation for the realm of validity for each of these approaches.
The ability of guard cells to hydrate and dehydrate from the surrounding air was investigated using isolated epidermes of Tradescantia pallida and Vicia faba. Stomata were found to respond to the water vapour pressure on the outside and inside of the epidermis, but the response was more sensitive to the inside vapour pressure, and occurred in the presence or absence of living, turgid epidermal cells.
Experiments using helium–oxygen air showed that guard cells hydrated and dehydrated entirely from water vapour, suggesting that there was no significant transfer of water from the epidermal tissue to the guard cells.
The stomatal aperture achieved at any given vapour pressure was shown to be consistent with water potential equilibrium between the guard cells and the air near the bottom of the stomatal pore, and water vapour exchange through the external cuticle appeared to be unimportant for the responses.
Although stomatal responses to humidity in isolated epidermes are the result of water potential equilibrium between the guard cells and the air near the bottom of the stomatal pore, stomatal responses to humidity in leaves are unlikely to be the result of a similar equilibrium.