Harvest Index of Maize ( Zea mays L.): Are There Possibilities for Improvement?

Abstract

Abstract During the last decades, the harvest index (HI) of many agricultural crops (e.g., wheat) was genetically or agronomically increased, whereas the HI of hybrid maize (Zea mays L.) remained almost unchanged in the major growing areas. In maize, opposite to wheat, a close positive correlation exists between dry matter accumulation and grain yield. However, the production of vegetative biomass which is usually not utilized requires high amounts of water and nutrients. In this review two main ways for improvement of maize HI are suggested: Increase in kernel setting and decrease of vegetative shoot biomass. In salt-stressed maize plants which are particularly prone to kernel abortion, a decrease in plasma membrane H+-ATPase activity in kernels was found around pollination. It is hypothesized that with application of phytohormones, specifically auxins, H+-ATPase activity can be enhanced, leading to an improved transport of hexoses into the developing kernels and thus to a better kernel set. For the reduction of vegetative biomass, the focus lies on the the phytohormones gibberellins (GAs). We demonstrated that after shoot growth reduction, achieved either by application of a GA biosynthesis inhibitor or under drought stress, the HI was significantly increased with a concomitant improvement of resource use efficiencies. After this proof of concept (increase of maize HI is possible without grain yield losses), the identification of the responsible physiological mechanisms is necessary in order to apply the knowledge to other maize genotypes and to a broader range of growth conditions, eventually contributing to a more sustainable crop production.