Critical deficiency concentration of zinc in barley genotypes differing in zinc efficiency and its relation to growth responses

A growth room study was conducted to compare responses to zinc of two barley (Hordeum vulgare L. cw. Tarm and Hamidiye) genotypes differing in zinc (Zn) efficiency and to determine a critical deficiency concentration of Zn in tissue. Two genotypes of barley, Tarm (Zn efficient) and Hamidiye (Zn inefficient), were grown in a Zn deficient siliceous sand with Zn added at 10 Zn rates (0, 0.04, 0.08, 0.1, 0.2, 0.4, 0.8, 1.6, 3.2, and 6.4 mg Zn/kg dry soil). Visual Zn deficiency symptoms, such as inhibition of shoot elongation and development of chlorotic areas on leaves, appeared more rapidly and severely in Hamidiye when Zn supply was deficient (<0.08 mg/kg), while those symptoms in Tarm were slightly visible only when no Zn was applied. Tarm had greater shoot and root dry matter than Hamidiye until Zn fertilization reached 0.8 mg/kg fertilization rate at which both genotypes achieved their maximal growth. Beyond this rate, both genotypes had similar yields. Zn concentration and content were increased progressively by Zn fertilization, but significant genetic differences occurred only in Zn content under severe Zn deficiency (0.04 mg/kg). Similarly, Zn concentration in youngest emerged leaf blades (YEBs) increased as a result of Zn fertilization; however, there was no significant genotypic difference. The critical deficiency concentration of Zn estimated by Mitscherlich plant growth model was similar for both genotypes (20 mg Zn/kg DW). The results demonstrated that under Zn deficiency, Tarm and Hamidiye exhibited a differential response to Zn fertilization. The greater efficiency of Tarm over Hamidiye may be attributed to its higher uptake of Zn from the soil and more efficient utilization at a cellular level. The differences found in the field between the two genotypes were expressed in visual symptoms, dry matter production, Zn efficiency, and Zn content in shoots under controlled conditions, suggesting that these parameters can be used as criteria when assessing for tolerance to Zn deficiency. The results also established a good correlation between Zn concentration in the YEBs and plant growth, indicating that critical deficiency concentration can be used as a reference point in the assessment of Zn status in barley.