Zinc uptake kinetics in the low and high-affinity systems of two contrasting rice genotypes

Rice (Oryza sativa L.) cultivars differ widely in their susceptibility to zinc (Zn) deficiency. The physiological basis of Zn efficiency (ZE) is not clearly understood. In this study, the effects of Zn-sufficient and Zn-deficient pretreatments on the time and concentration-dependent uptake kinetics of Zn were examined at low (0-160 nM) and high Zn supply levels (0-80 mu M) in two contrasting rice genotypes (Zn-efficient IR36 and Zn-inefficient IR26). The results show that Zn-65(2+) influx rate was over 10 times greater for the Zn-deficient pretreatment plants than for the Zn-sufficient pretreatment plants. At low Zn supply, significant higher Zn-65(2+) influx rates were found for the Zn-efficient genotype than for the inefficient genotype, with a greater difference (over three-fold) at Zn supply > 80 nM in the Zn-deficient pretreatments. At high Zn supply levels, however, a difference (2.5-fold) in Zn-65(2+) influx rate between the two genotypes was only noted in the Zn-deficient pretreatments. Similarly, the Zn-65(2+) accumulation in the roots and shoots of Zn-efficient IR36 pretreated with Zn-deficiency were sharply increased with time and higher than that in the Zn-inefficient IR26 with an over four-fold difference at 2 h absorption time. However, with Zn-deficient pretreatments, the Zn-efficient genotype showed a higher shoot : root Zn-65 ratio at higher Zn supply. Remarkable differences in root and shoot Zn-65(2+) accumulation were noted between the two genotypes in the Zn-deficiency pretreatment, especially at low Zn level (0.05 mu M), with 2-3 times higher values for IR36 than for IR26 at an uptake time of 120 min. There appear to be two separate Zn transport systems mediating the low and high-affinity Zn influx in the efficient genotype. The low-affinity system showed apparent Michaelis-Menten rate constant (K-m) values ranging from 10 to 20 nM, while the high-affinity uptake system showed apparent K-m values ranging from 6 to 20 mu M. The V-max value was significantly elevated in IR36 and was 3-4-fold greater for IR36 than for IR26 at low Zn levels, indicating that the number of root plasma membrane transporters in low-affinity uptake systems play an important role for the Zn efficiency of rice.