Stomatal Responses to Pressure Changes and Interruptions in the Water Supply of Detached Leaves of Zea mays L.

It is supposed that oscillations in stomatal conductance are associated with the dynamic properties of the loop in which rate of evaporation affects, through physiological processes, Self-sustained oscillations, having a period of about 40 min, in stomatal aperture, and plant functioning associated with stomatal aperture, are known to occur in many plant species (1,3). Several models of the mechanism responsible for these oscillations have been described (3, 7-9, 1 1). They have in common one basic assumption: that the oscillations are due to the properties of the loop in which rate of evaporation affects, through physiological processes, the aperture of stomata and stomatal aperture in turn affects rate of evaporation. The models differ in the way they describe the physiological processes. The model described by Cowan (3) suggests itself: can we manipulate the total gain of the loop in an unambiguous way and show that the stability of the stomatal mechanism is affected in the manner expected? Any attempt to alter the characteristics of the physiological component of the loop would fall short of what is required because it would presume a detailed understanding of the processes involved. On the other hand, the environmental component of loop gain, which is the sensitivity of rate of evaporation to change in stomatal aperture, can be readily manipulated in a way which is unequivocal because the physical process involved is well understood. We report here on two experiments in which environmental gain has been altered and the effects on the stability of the stomatal mechanism have been observed. MATERIALS AND METHODSThe plants used were cotton (Gossypiumn hirsutum L. cv.Deltapine Smooth Leaf), 1 month old, having five or six leaves with a total area of 0.05 to 0.06 M2. The apparatus is described in detail elsewhere (6). In brief, it consists of a small closedcircuit wind tunnel. The working section, which contains the upper parts of the plant, is of Plexiglas and is illuminated from above by a xenon arc lamp. The flux of radiation is 80 w m-2 in the wavelength range 370 to 820 nm. Wind speed is about 1 m s-'. The stem of the plant is sealed in the floor of the working section, the roots below being immersed in aerated Hoagland solution.Ambient temperature is regulated, through the combination of a water-cooled heat exchanger and an electrical heater in the air stream approaching the plant, in such a way as to maintain the temperature of the leaves, rather than that of the air stream, as constant as possible. Thermojunctions (44 s.w.g. copper-constantan) are inserted into the main veins of three of the leaves, with the reference junctions in ice. The signals are continuously recorded on strip-chart; one is amplified and used to provide the error signal for proportional control of the electrical heating of the air stream. The bandwidth varies with thermal load, but is always less than 1 C. The humidity of the air stream is sensed by a dew-point hygrometer and is proportionally controlled by regulating the f...

Section: Methodsmentioning

confidence: 63%

“…This indicates either that the gain of the internal transfer function was extremely high, or that there was an element of open loop control of leaf conductance. i.e., the stomata responded directly to changes of the external environment (10)(11)(12).…”

Section: Methodsmentioning

confidence: 99%

Oscillations in Stomatal Conductance

Farquhar

Cowan²

1974

“…Though the hypothesis is usually applied to effects on mesophyll-related function, it is possible that stomatal closure may also involve a photoassimilate accumulation mechanism. For example, increased soluble carbohydrate in epidermal tissue or increased apoplast sucrose concentration may prevent full stomatal opening (23). If this was the case, the translocation-obstructing treatments would not be expected to affect stomatal opening unless the leaves were maintaining net CO2 fixation.…”

mentioning

confidence: 99%

Effect of Obstructed Translocation on Leaf Abscisic Acid, and Associated Stomatal Closure and Photosynthesis Decline

Setter¹,

Brun

Brenner³

1980

111

Pod removal or petiole girdling, which causes obstruction of translocation, was found in our previous study to cause reduced rates of photosynthesis in soybean leaves due to stomatal closure. The purpose of this research was to determine the involvement ofphotoassimilate accumulation and leaf abscisic acid (ABA) levels in the mechanism of stomatal closure induced by such treatments.Leaf glucose and sucrose levels increased during the initial 12-hour period after depodding or petiole girdling. Starch, which represents a much larger pool of leaf carbohydrate, was not perceptibly increased above control levels during the 12-hour posttreatment period. When leaflets were exposed to nonphotosynthetic enviromments (shading or COrfree air) for a 24-hour period after the translocation-obstructing treatments were applied and then returned to normal light or CO2 concentration, stomatal diffusive conductivity was reduced 65% and 85% with depodding and girdling, respectively. These reductions were comparable to those previously observed without an intervening nonphotosynthetic exposure, thus indicating that photosynthate accumulations were not necessary for the observed response.Free and bound ABA (released on alkaline hydrolysis) were determined by gas liquid chromatography with electron capture detection following preparative high performance liquid chromatography. Free ABA in monitored leaves increased almost 10-fold 48 hours after complete depodding and 25-fold 24 hours after petiole girdling of such leaves. By 3 hours after treatment, in a time course study, free ABA had increased 2-fold above control values in depodded and 5-fold in girdled leaves. Leafconcentrations of bound ABA did not appear to be related to the treatment effects on stomata.Thus, the translocation-obstructing treatments cause an increased level of ABA by a mechanism not involving accumulation of photoassimilate.Increased leaf ABA levels, which were independent of water stress or leaf water potential, appear to be involved in the stomatal closure response. It is suggested that the mechanism of increased leaf ABA levels following translocation-obstruction may be due to an interference with normal translocation of ABA out of leaves. in photosynthetic CO2 fixation rates, in part or entirely, by inducing stomatal closure (5,9,15,16,18,26). In soybeans we found, within 48 h after depodding or petiole girdling treatment, reductions of 70 to 90%o in CER3 due to partial stomatal closure (26). Such treatments can also cause reduced rates of CER by decreasing the gm to CO2 diffusion (nonstomatal factors). Some investigators have suggested that as a result of these decreases in gm, leaf intercellular space CO2 concentrations increase, thus causing stomatal closure (4, 9). This explanation cannot hold, however, in short-term (c 24 h) studies where no effect was found on CER apart from stomatal closure (16,26).Data from source/sink manipulation studies, where experimentally reduced translocation fluxes are associated with reduced rates of photosynthesis, ...

“…Interactions between these parameters are poorly understood and the control mechanisms for stomatal response are subject to controversy. Negative feedback relationships between leaf water status and stomatal aperture (4,21), and between internal…”

mentioning

confidence: 99%

Stomatal Response to Environment with Sesamum indicum. L.

Hall

Kaufmann²

1975

Leaf resistance of Sesamum indicurn L. increased when the humidity gradient between leaf and air was increased, at moderate temperatures, even though calculated carbon dioxide concentrations within the leaf decreased slightly. Mesophyll resistance remained relatively constant when humidity gradients were changed, indicating that the increases in leaf resistance were mainly caused by reductions in stomatal aperture and that nonstomatal aspects of photosynthesis and respiration were not affected. Low carbon dioxide concentrations inside the leaf decreased but did not eliminate resistance response to the humidity gradient. Internal carbon dioxide concentrations had little effect on resistance in humid air but had moderate effects on resistance with large humidity gradients between leaf and air. Stomatal response to humidity was not present at high leaf temperatures. Effects of humidity gradients on photosynthetic and stomatal responses to temperature suggested that large humidity gradients may contribute to mid-day closure of stomata and depressions in photosynthesis.The adaptation of plants to arid environments should be influenced by stomatal response to environment since optimal stomatal apertures would vary with changes in evaporative demand and with the changes in climate that influence net photosynthesis and the thermal balance of the plant. It is also probable that optimal stomatal apertures would depend upon phenotypic attributes such as heat tolerance and intrinsic photosynthetic capabilities, and upon the adaptive strategy of the plant (e.g., whether rapid growth or water use efficiency is more important).It has been suggested that stomata respond to several environmental parameters including radiation, internal CO2 concentrations, humidity gradients, and temperature. Interactions between these parameters are poorly understood and the control mechanisms for stomatal response are subject to controversy. Negative feedback relationships between leaf water status and stomatal aperture (4,21) (1,2,6,9,15,20). It has also been proposed that stomata may respond to the humidity gradient between leaf and air independently of effects 'This work was supported by National Science Foundation Grant GB 39856 to M. R. K.caused by changes in bulk leaf water status (3,12,16,23). This response system would provide an adaptive advantage in arid environments because it could prevent the development of water stress in the metabolically active cells within the leaf (23), and because it would promote the efficient use of water by plants (12). It has been suggested that stomata do not respond directly to humidity (18,22,28). Stomatal response to temperature is also a controversial subject with reports of stomata opening, closing, or not being affected by increases in temperature (1 8, 24). Stomatal response to temperature is important in relation to the maintenance of evaporative cooling and net photosynthesis, and to the avoidance of desiccation in hot, dry climates.The extent to which internal [CO,], humidity gradients,...