Changes in variable charge and acidity of rye (<Emphasis Type="Italic">Secale cereale</Emphasis> L<Emphasis Type="Italic">.</Emphasis>) roots surface under Zn-stress

The plants of winter rye variety Rostockie grown in nutrient solution prepared according to Marschner and Romheld (1983) were stressed at shooting stage with different zinc (ZnCl₂) concentrations of 0, 20, 200 and 400 mgXdm⁻³ during 10 days at pH = 4.5. The control plants were grown continuously at pH = 7, without Zn²⁺. The roots of all plants were titrated with 0.1 molXdm⁻³ NaOH in 1 molXdm⁻³ NaCl solution with a rate 0.01 ml/min using auto-titrator Titrino 702 (Metrohm). The amount of the base consumed between pH 3 and 10 was recorded with a step of 0.1 pH unit. From the titration data root surface charge was characterized. Roots grown at pH = 4.5 without Zn²⁺ addition, had apparently the same charge properties as the control roots. Under 200 and 400 mgXdm⁻³ of zinc addition, variable charge, Q, and the cation exchange capacity, CEC, of the roots significantly decreased as did the fractions of surface acidic functional groups of strong and medium acidity i.e. the groups having apparent surface dissociation constants, Kapp, between 10^−3.5 and 10^−5.5. For 20 mgXdm⁻³ zinc stress the fraction of weaker surface groups (surface dissociation constants around 10^−6.5) decreased also. The surface charge density, SCD, of roots decreased under the Zn stress.