Kinetics of active uptake of boron, zinc, copper and manganese in barley and sugarcane

Active uptake of boron (B), zinc (Zn2+), copper (Cu2+) and manganese (Mn2+) in excised barley roots and sugarcane leaf tissue is characterized by multiphasic kinetics in every case. Two concentration‐dependent phases were observed in the Cu2+ ‐Zn2+ system and also in the Mn2+ system of each tissue. The transitions for each cation occurred at 9.6 × 10‐5M in barley roots and 7.5 × 10‐5M in sugarcane leaf tissue. The boron uptake isotherm was multiphasic and quite complex in both barley roots and sugarcane leaves but it was strictly monophasic in sugarcane parenchyma cells. The fit of these seemingly multiphasic data was compared statistically to three other models; i.e., the dual model, the model incorporating a single Michaelis‐Menten term and a diffusion term, and the negative cooperativity model. The data were best represented by the multiphasic model in every case except that of B uptake in the sugarcane cells. Thus, although multiphasic kinetics are common in plant tissues and cells, such is not universal. No conclusions can be drawn about the functional role of multiphasic behavior, if any, from these data.