Reductions in Maize Root-tip Elongation by Salt and Osmotic Stress do not Correlate with Apoplastic$O_2^{ \bullet - }$Levels

• Background and Aims Experimental evidence in the literature suggests that $O_2^{ \bullet - }$ produced in the elongation zone of roots and leaves by plasma membrane NADPH oxidase activity is required for growth. This study explores whether growth changes along the root tip induced by hyperosmotic treatments in Txa mays are associated with the distribution of apoplastic $O_2^{ \bullet - }$. • Methods Stress treatments were imposed using 150 mM NaCl or 300 mM sorbitol. Root elongation rates and the spatial distribution of growth rates in the root tip were measured. Apoplastic $O_2^{ \bullet - }$ was determined using nitro blue tetrazolium, and H₂O₂ was determined using 2’, 7’-dichlorofluorescin. • Key Results In non-stressed plants, the distribution of accelerating growth and highest $O_2^{ \bullet - }$ levels coincided along the root tip. Salt and osmotic stress of the same intensity had similar inhibitory effects on root elongation, but $O_2^{ \bullet - }$ levels increased in sorbitol-treated roots and decreased in NaCl-treated roots. • Conclusions The lack of association between apoplastic $O_2^{ \bullet - }$ levels and root growth inhibition under hyperosmotic stress leads us to hypothesize that under those conditions the role of apoplastic $O_2^{ \bullet - }$ may be to participate in signalling processes, that convey information on the nature of the substrate that the growing root is exploring.