NITRATE REDUCTASE ACTIVITY AND PROLINE ACCUMULATION IN CASHEW IN RESPONSE TO NaCl SALT SHOCK 1

Abstract

The molecular mechanisms of salt effects on plant growth and plant tolerance to salinity, including uptake and NO - 3 assimilatory reduction, are not well understood. In addition, their effects on the synthesis of both proline and other free nitrogenous compounds, which might be utilized in plant-osmotic adjustment, is still unknown. In this study, young cashew plants (Anacardium occidentale L.), a non- halophyte woody species, were utilized. Twenty-five day old seedlings were grown in a nutrient solution with or without addition of 100 mM NaCl. After 8 days of treatment the plants exhibited a decrease on the shoot mass and increase in the root to shoot ratio. The transpiration rate decreased 50% after the first day of salinity treatment started. Leaf NO 3 - content was only reduced after 8 days of NaCl treatment, whereas the in vivo leaf-nitrate reductase (NR) activity decreased twofold after one day of salt treatment. On the other hand, the in vitro leaf-NR activity was not changed in salt treated plants. Moreover, the direct effects of NaCl and Na 2 SO 4 salts added to the enzyme reaction medium were not significant on the NR activity. The young cashew plants exhibited progressive increase both in proline and leaf free amino acid concentrations during the salt treatment. The leaf-soluble protein was higher in salt treated plants. These results suggest that NaCl effect on in vivo NR activity is to a large extent an osmotic effect, rather than ionic toxicity, and support the hypothesis that decrease in transpiration rate resulting in a reduction in NO - 3 flux from roots to leaves are responsible for leaf NR activity decrease, particularly at beginning of salinity treatment. Cashew seedlings exhibited an efficient protective mechanism of osmotic adjustment through accumulation of free nitrogenous compounds in the leaves. Additional index terms- Anacardium occidentale, nitrate assimilation, nitrate reductase, nitrate uptake, proline, salinity.