Increase of maize harvest index to improve water und nutrient use efficiency for a more sustainable crop production

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The harvest index (HI = grain yield/total aboveground biomass) of many agricultural crops has been strongly increased during the last decades. However, the HI of maize has only marginally increased since the introduction of hybrids. In maize, opposite to other crops, a close positive correlation exists between biomass production and grain yield. However, the production of vegetative biomass in grain maize requires high amounts of water and nutrients. The increase of the HI may thus improve water and nutrient use efficiency, allowing a more sustainable production. In future, this will be particularly important as water resources become scarce, and as applications of fertilizers have to be reduced to meet environmental limits.We suggest two ways for improvement of maize HI: increase in kernel setting and decrease of vegetative shoot biomass. For better kernel setting, the proposed research focuses on the effect of salt stress and the role of auxins in grain development. Our research group was the first to measure plasma membrane H+-ATPase activity in kernels shortly after pollination. This enzyme is the key driver for hexose uptake from the apoplast into the developing kernels via H+ cotransport. Thus, in the applied project maize plants will be grown under field-like conditions in large containers and treated with elicitors of plasma membrane H+-ATPase activity (auxin, fusicoccin) to improve kernel setting. In the first year, the optimal dosage has to be determined, whereas in the second year additional application dates will be tested. In both years, two days after controlled pollination an intermediate harvest will be conducted and sugar concentrations and H+-ATPase activity parameters in apical and basal parts of the cob will be measured. At final harvest (maturity) yield parameters will be analyzed and water and nutrient use efficiencies determined. In the third year the apoplastic pH in the developing kernels will be measured in vivo, as it is directly affected by plasma membrane H+-ATPase activity.In a second approach the effect of reduced vegetative shoot growth on maize HI and resource use efficiencies will be tested. By application of a gibberellin biosynthesis inhibitor it will be tested whether it is possible to affect vegetative shoot and cob growth differently and thus break the strong correlation between both parameters. To test cultivar-inhibitor interactions, the optimal dosage of paclobutrazol, which has already shown positive effects, will be determined with six maize cultivars. Application time will be varied as well in order to find the optimum growth stage for the treatment. Eventually three cultivars, which performed best in HI and grain yield, will be chosen and the effect of reduced fertilizer amendment on nutrient use efficiencies will be tested.