Stomatal uptake

Exposure route The means by which a contaminant enters an organism (e.g., inhalation, stomatal uptake, ingestion). 

Deposition of SO2 and NO2 to plants mainly occurs through stomatal uptake. The uptake by the surfaces of the receptors mentioned takes place by solution in water or water films. For highly soluble SO2, transfer to moist surfaces is limited only by atmospheric transfer, as long as no equilibrium exists between gas phase and solution phase. During rainfall the accumulated material will be leached out (leaves, soil) or washed off (twigs, trunks). 

The reported (14) mechanisms of action of allelochemlcals Include effects on root ultrastructure and subsequent Inhibition of Ion absorption and water uptake, effects on hormone-induced growth, alteration of membrane permeability, changes In lipid and organic acid metabolism, inhibition of protein synthesis and alteration of enzyme activity, and effects on stomatal opening and on photosynthesis. Reduced leaf water potential Is one result of treatment with ferulic and p-coumaric acids (15). Colton and Einhellig (16) found that aqueous extracts of velvetleaf (Abutllon theophrastl Medic.) Increased diffusive resistance In soybean fGlycine max. (L.) Merr.] leaves, probably as a result of stomatal closure. In addition, there was evidence of water stress and reduced quantities of chlorophyll In Inhibited plants. 

Two reports based on rather gross measuring techniques found a close correlation between ozone uptake and transpiration. These studies indicated that stomatal control is the prime factor in controlling pollutant uptake and that cuticular sorption is negligible in relation to stomatal absorption. These findings are generally supported by past work that indicates that stomata are the prime sites of pollutant entry... 

The vegetation zones along the slope and at the crest are subjected to oxidant exposure differently. Even though the lower chaparral receives longer exposure, the peak concentrations coincide more closely with the maximal evaporative stress for the day. There is some support for the hypothesis that stomates would be closed during this period and that pollutant uptake would be lower. There is in fact very little visible injury to the species in this zone. In contrast, the daily oxidant peak occurs well after the temperature and vapor-pressure gradient maximums in... 

Although stomata in plants treated with HF and SO2 showed some tendency to close as a result of exposures which depressed apparent photosynthetic rates, these phytotoxicants inhibited CO2 uptake rates more by affecting biochemical processes within the leaves than by impeding gas transfer by inducing stomatal closure. 

Figure 2.21 schematically depicts the dry deposition of a pollutant to a typical surface in the form of resistances (Lovett, 1994 Wesely and Hicks, 1999). In this case, the surface resistance rsurf has been broken down even further into a combination of parallel and series resistances (rs, rm, rct, rsoil, rwa(cl, etc.). Since leaves may absorb pollutants either through stomata or through the cuticles, the absorption into the leaf is represented by two parallel resistances, rcl for the cuticular resistance and rs for the stomatal resistance, which is in series with a mesophyllic resistance rm. Also shown are resistances for uptake into the lower part of the plant canopy and into water, soil, or other surfaces. 

During the late afternoon when the vapor pressure gradient declines, ponderosa pine stomata may open wider, resulting in greater oxidant uptake and simultaneous depression of carbon dioxide fixation. Some knowledge of stomatal function would be useful to see if there is any relationship between intraspecific oxidant tolerance and ability to close stomates in the presence of elevated ozone concentrations. This mechanism is an inherited characteristic of an ozone-resistant onion variety which closes its stomates when exposed to ozone (30). It is not known if this mechanism is involved in conditioning interspecific tolerance or sensitivity of the important conifer species. 

Air-side deposition velocities have been studied extensively for the uptake of inorganic gases in plants. However, the deposition of PCDD/Fs introduces a new dimension to this subject, since these compounds are deposited primarily to the surface of the cuticle,45 not to the stomatal openings as in the case of most inorganic trace gases. Hence the receptor surface is fundamentally different for... 

Phenolic acids are known to alter photosynthetic and respiration rates, cause stomatal closure, reduce chlorophyll content, modify the flow of carbon into various metabolic pools, and alter nutrient uptake in affected tissue (61-73). A common denominator for these multiple effects appears to be the action of phenolic compounds on membranes. They are soluble in membranes, and cause a reduction in ion accumulation in cells (71-73). Several phenolic acids cause membrane depolarization, especially at low pH, increasing membrane permeability to ions (72,73). This action undoubtedly impairs the proton gradient and ATP-driven ion transport. Logically, the effects phenolic acids have on membranes could disturb the water balance and mineral nutrition of seedlings, and research in my laboratory has established such a relationship. 

Patterson et al. (2001) examined the uptake of F by Wyoming big sagebrush and western wheatgrass in the area where the PL and TP transects were made, and reported very little or no translocation of F from roots to plant leaves. The solubility estimates, combined with the observations of plant uptake, indicate that precipitation of CaF2 in these soils may limit root uptake of F. Therefore, the soil geochemistry supports the Patterson (2002) findings of F plant uptake via stomatal entry. 

We next consider what would happen to C02 uptake if the stomata (stomates) provided no resistance to C02 entry. Instead of a gas-phase resistance for C02 of 280 s m-1, it might then be only 80 s m-1 for the... 

Stomatal opening leading to the C02 uptake that is necessary for photosynthesis results in an inevitable loss of water. A useful parameter relating the two fluxes and showing the total C02 fixed (benefit) per unit water lost... 

What is the optimal behavior of stomata over the course of a day WUE (Eq. 8.39) is maximized by minimal stomatal opening because transpiration is decreased more than photosynthesis by partial stomatal closure that is, changes proportionally more than does 7c02 as changes (Fig. 8-23 also Section 8.4E). However, minimal stomatal opening can lead to very little net CO2 uptake. Thus a more pertinent consideration might be the maximal amount of CO2 that can be taken up for a certain amount of water transpired. The amount of water lost depends on the plant condition and environmental factors and should be considered over the course of a whole day. 

Figure 8-23. Relation between net pho tosynthesis (Jco and transpiration (Jwv) as stomatal conductance is varied. The three curves depict various PPF levels, indicated as low, medium, and high. The circles indicate where the slope dJwv/dJco2 1000 H20/C02 (the slope is at 45° on each curve, as the ordinate is in mmol m-2 s l and the abscissa is in [xmol m-2 s which differ by a factor of 1000). Cu ticular transpiration is ignored. Hie numbers indicate changes in the fluxes, on the medium PPF curve for no net change in transpiration and at the same slope on the high PPF curve for no change in net C02 uptake. Curve shapes indicate that Jwv increases faster than does Jco2as stomata open.

Figure 8-24. Relation between stomatal C02 conductance ( co2 anc net COz uptake for various categories of C3 and C4 plants under optimal conditions and a jV of 360 [xmol C02 mol-1. Ignoring boundary layer effects as a first approximation, note that each line can be represented by Jco2= co2 co2 so tlie slope equals the drop in 0O2 mole fraction across the stomata, which differs for the two photosynthetic types.

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