Role of K+ in leaf growth:: K+ uptake is required for light-stimulated H+ efflux but not solute accumulation

The stimulation of dicotyledonous leaf growth by light depends on increased H+ efflux, to acidify and loosen the cell walls, and is enhanced by K+ uptake. The role of K+ is generally considered to be osmotic for turgor maintenance. In coleoptiles, auxin-induced cell elongation and wall acidification depend on K+ uptake through tetraethylammonium (TEA)-sensitive channels (Claussen et al., Planta 201, 227-234, 1997), and auxin stimulates the expression of inward-rectifying K+ channels (Philippar et al. 1999). The role of K+ in growing, leaf mesophyll cells has been investigated in the present study by measuring the consequences of blocking K+ uptake on several growth-related processes, including solute accumulation, apoplast acidification, and membrane polarization. The results show that light-stimulated growth and wall acidification of young tobacco leaves is dependent on K+ uptake. Light-stimulated growth is enhanced three-fold over dark levels with increasing external K+, and this effect is blocked by the K+ channel blockers, TEA, Ba++ and Cs+. Incubation in 10 mM TEA reduced light-stimulated growth and K+ uptake by 85%, and completely inhibited light-stimulated wall acidification and membrane polarization. Although K+ uptake is significantly reduced in the presence of TEA, solute accumulation is increased. We suggest that the primary role of K+ in light-stimulated leaf growth is to provide electrical counterbalance to H+ efflux, rather than to contribute to solute accumulation and turgor maintenance.