Response of Bur and Red oak seedlings to NaCl-induced salinity

Growth performance of Bur oak (Q. macrocarpa Michx.) and Red oak (Q. rubra L.) under salinity conditions was assessed by growing seedlings in the presence of increasing levels of NaCl. Salinity reduced root growth in both species, although its repressive effect was more pronounced in Red oak. Exposure to 75 mM NaCl for 3 weeks almost arrested root growth in Red oak, while it reduced it only by 40 % in Bur oak. Red oak roots showed extensive necrosis and limited branching. Salinity also induced leaf injury, which at a NaCl level of 25 mM was less severe in Bur oak possibly due the higher activity of catalase, superoxide dismutase, and dehydroascorbate reductase enzymes participating in the detoxification of reactive oxygen species. Relative to Red oak, a higher retention of photosynthetic rate was observed in Bur oak grown under saline conditions. Salinity also altered nutrient uptake and accumulation in root and leaf tissue. Compared to Red oak, the content of magnesium and calcium in Bur oak leaves exposed to NaCl remained elevated, while an opposite trend was observed in root tissue. This was in contrast to nitrogen which was not altered by salinity in Bur oak, while it increased in roots of Red oak. The better performance of Bur oak root tissue under salinity conditions was ascribed to structural modifications of the root system with maturation of casparian bands and suberinization occurring closer to the root tip. These structures are known to act as barriers enhancing ion selectivity. Collectively this study demonstrates that relative to Red oak, Bur oak is more tolerant to NaCl-induced salinity conditions.