Báo cáo lâm nghiệp: "Ionic interactions between precipitation and leaf cuticles"

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precipitation and leaf cuticles Ionic interactions between T. Scherbatskoy University of Vermont, Botany Department, Burlington, VT 05405, U.S.A. 3.8 or 5.4 were applied to adaxial and ab- Introduction pH axial surfaces of mature, field-grown Acer sao- charum leaves which had been rinsed with deionized water. Ten experiments were con- The leaf cuticle is the most important rate- ducted at weekly intervals between July and limiting barrier controlling the movement of September in a high-humidity chamber to re- solutes between precipitation and the leaf duce drop evaporation. Drops were quantita- interior, but the mechanisms and rates by tively removed after precise times between 4 which ions diffuse through the cuticle and 128 min and analyzed for Cu Pb Zn2+, +, +, 22 K+, Ca and Mg2+. + 2 remain poorly defined. There is strong experimental evidence that cuticles contain hydratable pores up to 0.45 nm in radius Electrophysiology lined with polar groups (McFarlane and Adaxial cuticles were enzymatically isolated Berry, 1975; Sch6nherr, 1979; Seymour, (Orgell, 1955) from mature leaves of several 1980). These hydrophilic regions in cuticles tree species. Diffusion potentials across cuticles probably incorporate the polar substituents were measured in a flow-cell by pumping of waxes and cutin, as well as polyuronic unbuffered salt solutions across the two sides of the cuticle to create ion concentration gra- acids of the polysaccharides in the secon- dients. Electrical potentials across the cuticle dary cuticle. These polar groups and polar were measured for various concentration ratios, pores are likely to contribute greatly to the G,IC and were expressed as E!!= (y ), 0 i f -y , o = ion exchange capacity and transport prop- where y is the local voltage and the super- erties of cuticles. This research examined scripts refer to inside and outside the cuticle. the ionic interactions between leaf surfaces and precipitation, particularly the role of the leaf cuticle in mediating ion transport into and out of the leaves. Results Concentrations of Cu and Pb remain- + 2 + 2 Materials and Methods ing in applied solutions declined rapidly with time at pH 5.4 (Fig. 1); this was more pronounced on adaxial leaf surfaces. Ionic exchange Concentrations of Zn were unchanged + 2 pl drops of artificial precipitation Individual 50 under all treatments. K+, Ca and Mg + 2 + 2 representing regional ambient precipitation at
tended to increase with time, mostly responsible for the efflux of these at more so cations into solutions. Using published Ps pH 3.8. to predict exchange times, calculations Diffusion potentials are graphed in Fig. show that half-times for cation exchange 2 for A. saccharum in KCI under 2 ex- between the apoplast and precipitation by perimental regimes: 1) constant 10-fold cuticular permeation is about 10 days. concentration ratios at different ionic In the electrophysiology experiments, strengths, and 2) constant average ionic diffusion potentials approached ± 59 mV strength (10 mM) at different concentration at low ionic strength when the concentra- ratios. These graphs show 2 important tion ratio was 0.1 or 10 (Fig. 2), confirming typical for all species: diffusion responses the Nernstian behavior of this system. The potentials increased with decreasing ionic increase in potential with decreasing ionic strength, and potentials were asymmetric, strength indicates that the permeability being smaller when C IC, >1. i ratio, P increased as ionic on i an lP on ti ca strength decreased; this was due to ionic partition coefficients changing with ionic Discussion and Conclusion strength. The asymmetry of the potentials varied among cuticles and appeared to be caused by the asymmetric distribution of In the ion exchange experiments, adsorp- negative charge in the cuticle. tion of C and Pb was reduced at the + 2 U 2+ Potentials were inadequately modeled lower pH. This is consistent with a reduc- the Goldman equation, apparently due by tion in available exchange sites for cation to changing permeability ratios within the adsorption at the higher H concentra- + 0 3 cuticle. Pure cellulose membranes (pre- tion. Increased H concentration, on the + 0 3 sumed to be more structurally homo- other hand, would favor the release of K, + geneous than cuticles) did allow a good fit + 2 Ca and Mg from adsorption sites on + 2 between predicted and observed poten- the leaf surface. tials (Fig. 3). The kinetics in these experiments can The importance of electrical potential be used to predict cuticular permeability was evaluated by comparing the driving coefficients (P) that would be required if force for ionic flux due to: 1) only chemical this was the only exchange mechanism or 2) electrochemical potential. potential, operating. This leads to apparent cuticular Potentials were measured while mimicking 5 10- and 10- cm-s- for Cu and 4I + 2 P = natural conditions by applying artificial aci- +, 2 Pb respectively. These are much too dic precipitation to the outside of the large to represent cuticular permeation, cuticle and artificial apoplastic solution to suggesting instead that surface adsorption the inside. At pH 3.8, there was little dif- mechanisms dominate. For K+, Ca and + 2 ference in the driving force between the +, 2 Mg on the other hand, the calculated two models. At pH 5.4, however, ignoring apparent Ps are similar to those measured the electrical potential resulted in over- in isolated cuticles (McFarlane and Berry, estimating the driving force for cations by 1974; Reed and Tukey, 1982). These a factor of 2. rates suggest a different mechanism may be operating for these cations. It is pos- This work showed that cuticular permea- sible in this case that prerinsing the leaves tion and ion exchange between precipita- may have removed most of the ex- tion and the leaf apoplast do not occur at changeable K+, Ca and Mg from the + 2 + 2 biologically significant rates. Instead, ion surface, so that cuticular permeation was exchange processes with the leaf cuticle
surface dominate. Electrophysiology stu- Reed D.W. & Tu4;ey H.B. (1982) Permeability of Brussels sprouts and carnation cuticles from dies showed that significant diffusion leaves developed in different temperatures and potentials can arise across cuticles and light intensities. In: The Plant Cuticle. (Cutler these can have a significant effect on ionic D.F., Alvin K.L. & Price C.E., eds.), Linnean flux. Furthermore, ionic permeability in Society of London International Symposium, London, 8-11 Sept. 1980. Academic Press, cuticles varied with both ionic strength and London, pp. 267-278 cuticle structure. Sch6nherr J. (1979) Transcuticular movement of xenobiotics. In: Advances in Pesticide Science (Geissbuhler H., Brooks G.T & Kearne P.C., References eds.), Papers from the Fourth International Congress of Pesticide Chemistry, Zurich, 24-28 July 1978. Pergamon, Oxford, pp. 392-400 McFarlane J.C. & Berry W.L. (1974) Cation pene- tration through isolated leaf cuticles. Plant Phy Seymour V.A. (1980) Leaf cuticle: an investiga- siol. 53, 723-727 tion of some physical and chemical properties Orgell W.H. (1955) The isolation of plant cuticle derived from a study of Berberis. Ph.D. Thesis, with pectic enzymes. Plant PhysioL 30, 78-80 University of Washington, Seattle