Bioavailability of zinc to aerobic rice

Keywords:</span><span lang=EN-US>Arbuscular mycorrhiza, Exudation, Oryza sativa , Rhizosphere, Rice, Soil, Zinc</span>Zinc deficiency is a wide-spread constraint for crop production and human health. This thesis should contribute to alleviation of Zn deficiency problems and aimed at identifying soil and plant factors affecting Zn bioavailability in rice ( Oryza sativa L.). Two main research questions were: 1) what is the consequence of a cultivation shift from flooded to aerobic rice on Zn bioavailability and 2) which mechanisms control Zn mobilization by aerobic rice? Field experiments demonstrated that the cultivation shift from flooded to aerobic may increase Zn deficiency problems. Lower Zn uptake and mass fraction in shoot and brown rice were observed in aerobic fields. Results of a soil incubation experiment and modeling showed that the difference found in plant Zn uptake between the two cultivation systems was orders of magnitude lower compared to what can be expected based on chemical equilibria in the bulk soil. This discrepancy suggests that soil chemical properties such as pH, DOC level and redox conditions in rhizosphere need consideration. We observed considerable variation among aerobic rice genotypes in tolerance to Zn deficiency under both field and pot conditions. In a pot experiment we found that variation in tolerance was mainly associated with Zn uptake. Plant Zn uptake from low Zn soils can be increased by both Zn-mobilizing rhizosphere processes and inoculation with arbuscular mycorrhizal fungi. We demonstrated that mycorrhizal inoculation significantly increased Zn uptake, but only in genotypes with a low inherent Zn uptake. High Zn mobilization by a combination of chemical rhizosphere and mycorrhizal effects seems impossible and may not be feasible as a target for breeders. In a rhizotron experiment and a nutrient solution experiment, aerobic rice genotypes responded to Zn deficiency with increased root exudation of malate.</span><span lang=EN-US style='font-size:13.0pt; line-height:120%;font-family:Garamond;mso-bidi-font-family:Arial'>Genotypes with a higher Zn uptake showed a stronger increase in malate exudation in response to Zn deficiency.</span><span lang=EN-US style='font-size:13.0pt; line-height:120%;font-family:Garamond'>These results confirm our hypothesis that genotypic variation in Zn uptake of aerobic rice can partly be explained by root exudation of malate