Using six bread wheat genotypes (Triticum aestivum L. cvs. Dagdas-94, Gerek-79, BDME-10, SBVD 1-21, SBVD 2-22 and Partizanka Niska) and one durum wheat genotype (Triticum durum L. cv. Kunduru-1149) experiments were carried out to study the relationship between the rate of phytosiderophore release and susceptibility of genotypes to zinc (Zn) deficiency during 15 days of growth in nutrient solution with (1 mu M Zn) and without Zn supply. Among the genotypes, Dagdas-94 and Gerek-79 are Zn efficient, while the others are highly susceptible to Zn deficiency, when grown on severely Zn deficient calcareous soils in Turkey. Similar to the field observations, visual Zn deficiency symptoms, such as whitish-brown lesions on leaf blades occurred first and severely in durum wheat Kunduru-1149 and bread wheats Partizanka Niska, BDME-10, SBVD 1-21 and SBVD 2-22. Visual Zn deficiency symptoms were less severe in the bread wheats Gerek-79 and particularly Dagdas-94. These genotypic differences in susceptibility to Zn deficiency were not related to the concentrations of Zn in shoots or roots. All bread wheat genotypes contained similar Zn concentration in the dry matter. In all genotypes supplied adequately with Zn, the rate of phytosiderophore release was very low and did not exceed 0.5 mu mol/48 plants/3 h. However, under Zn deficiency the release of phytosiderophores increased in all bread wheat genotypes, but not in the durum wheat genotype. The corresponding rates of phytosiderophore release in Zn deficient durum wheat genotype were 1.2 mu mol and in Zn deficient bread wheat genotypes ranged between 8.6 mu mol for Partizanka Niska to 17.4 mu mol for SBVD 2-22. In Dagdas-94, the most Zn efficient genotype, the highest rate of phytosiderophore release was 14.8 mu mol. The results indicate that the release rate of phytosiderophores does not relate well with the susceptibility of bread wheat genotypes to Zn deficiency. Root uptake and root-to-shoot transport of Zn and particularly internal utilization of Zn may be more important mechanisms involved in expression of Zn efficiency in bread wheat genotypes than release of phytosiderophores.