Department of Plant Sciences

To improve the salt resistance of maize, a breeding program was initiated that combines physiological knowledge with classical breeding methods. According to the two-phase model by Munns, growth is decreased due to osmotic stress in a first phase whereas Na+ toxicity develops in a second phase. Therefore, an efficiently Na+-excluding inbred line (NaExIl) was established that combines low Na+ uptake at the root surface with low Na+ root-to-shoot translocation. Screening for osmotic resistance yielded 20 inbred lines, which were crossed with NaExIl. Based on grain yield, the resulting F1 hybrids (SR hybrids) revealed improved salt resistance compared to the parental hybrid. Enhanced salt resistance of the newly established SR hybrids was primarily due to efficient Na+ exclusion, whereas osmotic resistance was improved to a small extent. Osmotic stress was not due to a lack of plant water uptake as concluded from unchanged transpiration rates but apparently limited cell-wall extensibility and not turgor maintenance. Abscisic acid was accumulated under salt stress to a higher degree in the resistant SR hybrids and thus may play a role in osmotic stress resistance. An important resistance trait was the ability of cell-wall acidification by plasma membrane H+ ATPase. Whereas in the sensitive hybrid Pioneer 3906 H+-pumping was decreased probably due to the expression of inefficient H+ ATPase isoforms, in the resistant hybrid SR03 H+-pumping and cell-wall acidification were maintained despite growth reduction, indicating that additional limitations of cell extension must be overcome to further improve the salt resistance of maize.