Interaction effects of phosphorus and zinc on photosynthesis, growth and yield of dwarf bean grown in two environments

Two experiments were conducted in a factorial combination of three Zn levels (0, 10 and 40 mg Zn kg⁻¹ soil) and two P levels (0 and 200 mg P kg⁻¹ soil). Experiment 1 was carried out during winter in a heated glasshouse, and experiment 2 during summer under a rain shelter. Plants of dwarf bean (Phaseolus vulgaris L., cv. Borlotto nano) were grown in pots filled with sandy soil. In both experiments, leaf Zn concentration was reduced by the addition of P to plants grown at low Zn supply. However, leaf Zn concentration lower than the critical level was observed only during experiment 2, and the main effects of low Zn were reductions of internode length, light use efficiency and maximum photosynthetic rate. In plants with leaf Zn concentration lower than the critical level, saturating irradiance levels fell from ~1000 µmol m⁻² s⁻¹ PPFD to ~300-400 µmol m⁻² s⁻¹ PPFD. Reduction of net photosynthesis was observed from the beginning of flowering and led to decreased seed production.