Diferulic acids in the cell wall may contribute to the suppression of shoot growth in the first phase of salt stress in maize

In the first phase of salt stress the elongation growth of maize shoots is severely affected. The fixation of shape at the end of the elongation phase in Poaceae leaves has frequently been attributed to the formation of phenolic cross-links in the cell wall. In the present work it was investigated whether this process is accelerated under salt stress in different maize hybrids. Plants were grown in nutrient solution in a growth chamber. Reduction of shoot fresh mass was 50% for two hybrids which have recently been developed for improved salt resistance (SR 03, SR 12) and 60% for their parental genotype (Pioneer 3906). For SR 12 and Pioneer 3906, the upper three leaves were divided into elongated and elongating tissue and cell walls were isolated from which phenolic substances and neutral sugars were determined. Furthermore, for the newly developed hybrids the activity of phenolic peroxidase in the cell wall was analysed in apoplastic washing fluids and after sequential extraction of cell-wall material with CaCl2 and LiCl. The concentration of ferulic acid, the predominant phenolic cross-linker in the grass cell wall, was about 5mgg(-1) dry cell wall in elongating and in elongated tissue. The concentration of diferulic acids (DFA) was 2-3mgg(-1) dry cell wall in both tissues. Salt stress increased the concentration of ferulic acid (FA) and DFA in the parental genotype Pioneer 3906, but not in SR 12. Both genotypes showed an increase in arabinose, which is the molecule at which FA and DFA are coupled to interlocking arabinoxylan polymers. In SR 12, the activity of phenolic peroxidase was not influenced by salt stress. However, in SR 03 salt stress clearly increased the phenolic peroxidase activity. Results are consistent with the hypothesis that accelerated oxidative fixation of shape contributes to growth suppression in the first phase of salt stress in a genotype-specific manner.