JXB Advance Access originally published online on April 4, 2005
Journal of Experimental Botany 2005 56(415):1369-1378; doi:10.1093/jxb/eri138
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RESEARCH PAPER |
Effect of divalent cations on ion fluxes and leaf photochemistry in salinized barley leaves
1School of Agricultural Science, University of Tasmania, Hobart, Australia
2School of Mathematics and Physics, University of Tasmania, Hobart, Australia
3Department of Botany, University of Washington, Seattle, USA
* To whom correspondence should be addressed. Fax: +613 6226 2642. E-mail: Sergey.Shabala{at}utas.edu.au
Photosynthetic characteristics, leaf ionic content, and net fluxes of Na+, K+, and Cl– were studied in barley (Hordeum vulgare L) plants grown hydroponically at various Na/Ca ratios. Five weeks of moderate (50 mM) or high (100 mM) NaCl stress caused a significant decline in chlorophyll content, chlorophyll fluorescence characteristics, and stomatal conductance (gs) in plant leaves grown at low calcium level. Supplemental Ca2+ enabled normal photochemical efficiency of PSII (Fv/Fm around 0.83), restored chlorophyll content to 80–90% of control, but had a much smaller (50% of control) effect on gs. In experiments on excised leaves, not only Ca2+, but also other divalent cations (in particular, Ba2+ and Mg2+), significantly ameliorated the otherwise toxic effect of NaCl on leaf photochemistry, thus attributing potential targets for such amelioration to leaf tissues. To study the underlying ionic mechanisms of this process, the MIFE technique was used to measure the kinetics of net Na+, K+, and Cl– fluxes from salinized barley leaf mesophyll in response to physiological concentrations of Ca2+, Ba2+, Mg2+, and Zn2+. Addition of 20 mM Na+ as NaCl or Na2SO4 to the bath caused significant uptake of Na+ and efflux of K+. These effects were reversed by adding 1 mM divalent cations to the bath solution, with the relative efficiency Ba2+>Zn2+=Ca2+>Mg2+. Effect of divalent cations on Na+ efflux was transient, while their application caused a prolonged shift towards K+ uptake. This suggests that, in addition to their known ability to block non-selective cation channels (NSCC) responsible for Na+ entry, divalent cations also control the activity or gating properties of K+ transporters at the mesophyll cell plasma membrane, thereby assisting in maintaining the high K/Na ratio required for optimal leaf photosynthesis.
Key words: Barley, chlorophyll fluorescence, divalent cations, ionic fluxes, leaf photochemistry, photosynthesis, stomatal conductance
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