INFLUENCE OF ROOT TEMPERATURE ON THE POTASSIUM REQUIREMENTS OF YOUNG TOMATO PLANTS

The effect of low root temperature on the growth and K requirements of young tomato (Lycopersicon esculentum Mill. cv Sonatine) plants was investigated. When K was supplied in solution at high concentration (5 mM), lowering the temperature of the root system from 25-degrees to 15-degrees-C reduced the relative growth rate so that after then days plant dry weight was 60% and leaf area 44% of that of controls maintained at 25-degrees-C. Shoot:root dry weight ratio was initially increased by cooling, but declined to 84% of controls after ten days. In spite of these changes in shoot:root ratio the concentration of K in whole plants, expressed on the basis of tissue water, was stable throughout the experiment and was significantly higher than that of controls. Further, the critical concentration of K for shoots (the concentration in the shoot associated with 90% maximum growth) was also increased at root temperatures of 15-degrees and 30-degrees-C compared with 24-degrees-C. It is suggested that the higher concentration of K at low root temperature may reflect a real increase in requirement for the element at the physiological level. Preliminary measurements of the solute potential demonstrated a less osmotically active sap in leaves of root-cooled plants, thus there may be a greater reliance on K as an osmoticum in these individuals. When supplies of K limited growth, root-cooling had no effect on any of the parameters determining the efficiency of its use; the minimum concentration to which roots could deplete the solution of K was identical for cooled and control plants and at the same stage of visible deficiency there was no significant difference in the efficiency ratio (mg DW, mg-1 K) or utilization efficiency (mg DW mM-1 K), in spite of large differences in the partitioning of dry matter.