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Boron (B) toxicity has been found to be a common nutritional disorder in dry areas, where excessive B tends to accumulate in the subsoil. However, in many of the pot experiments on B toxicity, excess B was mixed uniformly in soil or added from the soil surface, a method that does not detect the subsoil B effect. The objective of this plastic-house experiment was to study the effects of different patterns and timings of B application on the development of B toxicity (BT) symptoms, tissue B concentrations, and growth and yield of barley. A three-factor factorial design with three patterns of B application, three timings of B application, and two barley lines was used. A moderately BT-tolerant barley line from the cross 'Arar/Arabic Aswad' (abbreviated as AA) and the moderately BT-susceptible variety 'Harmal' were compared. Each pot consisted of two sections of 20-cm-tall PVC pipe. The second section was watered to field capacity and joined to the bottom of the top section on 25 January (tillering; roots reached the bottom of top section), 15 February (stem elongation), and 20 March (flag leaf emerged for 'Harmal'; booting stage for AA). The three patterns of B application were (1) no B added (-/-), (2) B added to the bottom section only (-/+), and (3) B added to both sections (+/+). The addition of 50 mg B/kg soil increased the hot-water-soluble B level from 0.7 to 21 mg kg-1. Interaction between pattern of B application and timing of B application had significant effects on BT symptom scores and straw B concentrations but not on grain yield. Grain yield of the +/+ treatment was much less than that of -/+, which in turn was less than -/-, but different timings of joining of the pot sections did not have a significant effect on grain yield. Relative to -/-, the percentage of grain yield reduction (8%) sustained by AA when grown in -/+ was less than half of the reduction (18%) suffered by 'Harmal.' In conclusion, subjecting plants to high-B soils from germination to maturity exaggerates the effects of B on rainfed crops in the field. High subsoil B levels can cause significant yield reduction even when roots reach it as late as the boot stage, suggesting the need to screen/breed and grow BT-tolerant cultivars. The use of subsectioned pots introduced in this study is an effective way to simulate BT in the field. [ABSTRACT FROM AUTHOR]