What Is the Limiting Factor? The Key Question for Grain Yield of Maize as a Renewable Resource Under Salt Stress

During 4 years, maize (Zea mays L.) experiments were conducted to investigate the effects of salt stress on grain yield performance and to identify limiting factors. The plants were grown in large containers, filled with 145 kg soil to a depth of 0.9 m, in a vegetation hall with controlled water supply. Grain yield and its determinants, i.e., kernel number and single kernel weight, were recorded at maturity and related to physiologically relevant parameters 2 days after controlled pollination. The time around pollination is decisive for kernel setting and thus kernel number at maturity, as this parameter was almost exclusively responsible for grain yield reductions under salt stress. Single kernel weight was unaffected by saline conditions. The decreased number of kernels was not caused by source limitation, because the availability of sucrose as main transport metabolite was always higher in developing kernels under salt stress than under control conditions. Although acid invertase activity as a key factor for sink activity was reduced or unchanged under salt stress, the hexose concentrations were always higher in the developing kernels pointing to no limitation. Another key enzyme in the plasma membrane of the sink cells is H-ATPase, which provides the pH gradient necessary for H-cotransport of hexoses from the apoplast into the cytosol. The hexoses are needed for metabolic processes and energy supply in order to enable cell division and extension growth. Plasma membrane H-ATPase activity was significantly reduced in the salt-stressed kernels, resulting in a smaller pH gradient. Thus, kernel development and hence grain yield performance under salt stress seem to be limited by a transport problem, caused by inhibition of plasma membrane H-ATPase.