TURGOR-REGULATION DURING EXTENSION GROWTH AND OSMOTIC-STRESS OF MAIZEROOTS - AN EXAMPLE OF SINGLE-CELL MAPPING

The growing cells of hydroponic maize roots expand at constant turgor pressure (0.48 MPa) both when grown in low- (0.5 mol m(-3) CaCl2) or f ull-nutrient (Hoagland's) solution and also when seedlings are stresse d osmotically (0.96 MPa mannitol). Cell osmotic pressure decreases by 0.1-0.2 MPa during expansion. Despite this, total solute influx largel y matches the continuously-varying volume expansion-rate of each cell. K+ in the non-osmotically stressed roots is a significant exception-i ts concentration dropping by 50% regardless of the presence or absence of K+ in the nutrient medium. This corresponds to the drop in osmotic pressure. Nitrate appears to replace Cl- in the Hoagland-grown cells. Analogous insensitivity of solute gradients to external solutes is ob served in the radial distribution of water and solutes in the cortex 1 2 mm from the tip. Uniform turgor and osmotic pressures are accompanie d by opposite gradients of K+ and Cl-, outwards, and hexoses and amino acids, inwards, for plants grown in either 0.5 mol m(-3) CaCl2 or Hoa gland's solution (with negligible Cl-). K+ and Cl- levels within both gradients were slightly higher when the ions were available in the med ium. The gradients themselves are independent of the direction of solu te supply. In CaCl2 solution all other nutrients must come from the st ele, in Hoagland's solution inorganic solutes are available in the med ium. 24 h after osmotic stress, turgor pressure is recovered at all po ints in each gradient by osmotic adjustment using organic solutes. Rem arkably, K+ and Cl- levels hardly change, despite their ready availabi lity. Hexoses are responsible for some 50% of the adjustment with mann itol for a further 30%. Some 20% of the final osmotic pressure remains to be accounted for. Proline and sucrose are not significantly involv ed. Under all conditions a standing water potential step of 0.2 MPa be tween the rhizodermis and its hydroponic medium was found. We est that this is due to solute leakage.