Stomatal apertures

Akita, S. and Moss, D. N. (1973). Photosynthetic responses to carbon dioxide and light by maize and wheat leaves adjusted for constant stomatal apertures. Crop Sci. 13,234-237. 

In this chapter we review recent developments, concentrating mostly on the effects of water stress because of its great importance, but trying where possible to identify general problems. As noted above, accumulation of ABA in leaves is common under stress, and we review its effects on gas exchange. If the sole direct effect of ABA were to reduce stomatal aperture, then assimilation rate would also decrease because of a lower intercellular partial pressure of C02,pi. Some experiments have suggested that pi is little affected by ABA and that the capacity for CO2 fixation has actually decreased. The situation has become complicated with the observation (Terashima et al., 1988) that application of ABA can cause non-uniform... 

Ishihara, K., Nishihara, T. Ogura, T. (1971). The relationship between environmental factors and behaviour of stomata in the rice plant. I. On the measurement of the stomatal aperture. Proceedings of the Japanese Society for Crop Science, 40, 491-6. 

Laisk, A. (1983). Calculation of photosynthetic parameters considering the statistical distribution of stomatal apertures. Journal of Experimental Botany, 34, 1627-35. 

Guard cells Specialized epidermal cells, crescent-shaped, contain chloroplasts, form defines stomatal pore Regulate stomatal aperture/pore for gas exchange... 

Genetic or other factors that induce stomatal closure will reduce plant sensitivity to oxidant pollutants. Generally, the sensitivity of plants at the time of exposure is controlled primarily by factors that affect the stomatal aperture. The internal resistance to gas flow may also influence leaf sensitivity. Factors that affect sensitivity during growth usually cause physiologic changes in the plant that tend to make it more resistant to the added stress of oxidant. Many of these stresses may alter membrane physiology and make the membranes either more or less sensitive to oxidant stress. 

MacKni t, M. L. The Effect of Ozone on Stomatal Aperture and Transpirati< i. M.S. Thesis. Salt Lake City University of Utah, 1968. 80 pp. 

Otto, H. W., and R. H. Daines. Plant injury by air pollutants Influence of humidity on stomatal apertures and plant response to ozone. Science 163 1209-1210. 1%9. 

Einhellig, F. A. 1971. Effects of tannic acid on growth and stomatal aperture in tobacco. Proc. South Dakota Acad. Sci. 50, 205-209... 

Einhellig, F. A. and Kuan, L.-Y. 1971. Effects of scopoletin and chlorogenic acid on stomatal aperture in tobacco and sunflower. Bull. Torrey Bot. Club 98, 155-162... 

Stomata close under dry conditions, and when this happens, the levels of CO2 in the mesophyll air spaces fall towards the compensation point. Since this is lower in C4 plants, they can photosynthesise with narrower stomatal apertures, so only lose half as much water per CO2 fixed as C3 plants. So although we may regard the C4 syndrome as an adaptation that reduces photorespiration, as far as plants are concerned, it is an adaptation to reducing water loss in hot climates. 

Stomatal resistance is a critical factor affecting pollutant uptake. The resistance is determined by stomatal number, size, anatomical characteristics, and the size of the stomatal aperture. Little or no uptake occurs when the stoma is closed. Stomatal opening is regulated by internal C02 content, temperature, humidity, light, water availability, and nutrient status, particularly potassium. Research shows that K+ ions in the guard cells regulate the guard... 

Figure 5 (A) Evidence that paraquat transiently affects chloroplasts of resistant Conyza and (B) that there are constitutively elevated levels of the Halliwell-Asada active oxygen detoxification pathway in the chloroplasts. A. Resistant and susceptible plants of Conyza bonariensis were sprayed to runoff with 0.1 mM paraquat and whole leaves were removed for measurement of photosynthesis at times thereafter as an estimation of paraquat arriving at, and affecting chloroplasts. Simultaneous measurements of stomatal aperture were made to ascertain that the stomates remained open. Source Data redrawn from (60). B. Normal enzyme levels (without paraquat treatment) in resistant and susceptible Conyza bonariensis. Source Collated and drawn from (57, 61).

Abscisic acid (ABA) 3-1 was originally detected because of its growth inhibitory properties. It is now known to play an important role in the control of a-amylase synthesis, and regulation of stomatal aperture during water stress. Phaseic acid (PA) 3-3 is an important metabolite of ABA. Over a hundred derivatives of ABA are known, activity correlations have been reviewed, and the difficulty of drawing firm conclusions due to differences in uptake, metabolism and sequestration between the different molecules assayed has been discussed [16-20]. In many correlations, racemates have been used, and it is possible that each enantiomer may be active, have a different type of activity, and/or interfere with the action of the other enantiomer. 

Much of our understanding of transporters in guard cells has come from efforts to dissect the effects of hormones on stomatal function. Arguably the largest contribution has come from the study of ABA-induced stomatal closure. Classically, ABA has also received the most attention as a phytohormone regulator of stomatal apertures (see review by Raschke... 

These benefits, thought to be the result of direct influences on physiological processes of the treated plants, are referred as to physiological effects [48a], and have been most extensively studied with kresoxim-methyl and pyradostrobin. These include effects like delayed senescence, altered CO2 compensation point, reduced stomatal aperture and water consumption, and better tolerance of oxidative stress. Significantly altered levels of enzyme activities (ACC synthase, nitrate reductase, peroxidases, alternative oxidase AOX) could be observed or inferred indirectly in vivo, but in no attempted cases directly with isolated enzymes in vitro. The simplest and therefore most convindng hypothesis to explain all these many... 

Recently it has been argued that some of the non-stomatal effects of salinity could be only apparent, being actually caused by spatial heterogeneity of stomatal aperture over the leaf. Terashima et al.(8) have demonstrated in ABA treated leaves that stomatal heterogeneity causes a systematic overestimation of intercellular CO2 partial pressure (p ) calculated from gas exchange. Similar effects have also been observed in water stressed plants (9> 10, ll). 

This possibility was evaluated using an oxygen electrode at saturating CO2 (COp partial pressure of 100 mbar). It has been demonstrated that such very high CO2 concentration can overcome the effect of heterogeneity of stomatal aperture (8, lO). Our results indicate that the apparent quantum yield of oxygen evolution was fairly insensitive to salinity (Table l), being about O.O8 (mol O2 evolved/mol incident quanta) in all salinity treatments. This value is very close to that reported previously for cotton (l ). We could not measure the quantum yield on the basis of absorbed photons. 

CO2 flux and H2O flux pass through stomata. Stomatal aperture depends on both internal and external factors. As Wong et a/., 1979, pointed out, the capacity of the mesophyll tissue to fix carbon allows the regulation of stomatal conductance (gc) For instance, DCMU, an inhibitor of photosynthetic electron transport, modifies both mesophyll CO2 assimilation and stomatal conductance in such a way that the intercellular concentration of CO2 (Ci) remains proportional to the air CO2 concentration (Ca) ... 

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