The role of lupin shoot P and Zn status in root allocation and nutrient uptake in soil with heterogeneous P and Zn distribution

Many plants have been found to show preferential root growth allocation to soil patches with locally increased nutrient availability. Little is known about the influence of the plants’ nutrient status on these responses. Addressing this question, we conducted a climate chamber experiment to investigate how foliar phosphorus (P) and zinc (Zn) application (alone or in combination) would affect root allocation and nutrient uptake by white lupin (Lupinus albus L.) seedlings growing on soil with constructed heterogeneity in P and Zn distribution. The same soil packing was used in all pots. One quarter section of each packing was enriched with Zn, one with P, one with Zn and P, while neither Zn nor P was added to the fourth section (opposite to the section with the combined Zn and P amendment). Neither foliar P nor foliar Zn application had a fertilization effect on plant growth. The allocation of root length growth showed a clear preference for P-enriched soil sections, when no foliar fertilizer was applied. With foliar P or Zn application these preferences disappeared. Cluster root allocation and root P concentration showed a similar behavior. In the treatments with foliar Zn application, root length growth was also preferentially allocated in Zn-enriched soil, while without foliar Zn there was only a tendency for such a response. Root Zn concentrations were always much higher in soil sections with than without Zn amendment. The foliar treatments had no effect on root Zn concentration. While the responses to foliar P application can be understood in terms of an optimization strategy in root allocation for P foraging, the effects of foliar Zn application on root growth allocation cannot be interpreted in this way and require further investigation.