Effect of different light conditions on the growth response of sugar beet to NaCl

It is known that sugar beet, derived from a halophytic wild ancestor, is highly salt-tolerant and that moderate concentrations of NaCl enhance its growth. However, the mechanisms of this improved growth performance by NaCl are not fully understood. In this study, we investigated the effects of NaCl on sugar beet growth under different light conditions and examined the relationship between light-induced oxidative stress and the beneficial effects of NaCl. Sugar beet seedlings in pots filled with fertilized soil were grown with different light intensities (low: 150-250 mu mol m-2s-1 or high: 600-1,000 mu mol m-2s-1) and NaCl treatments (irrigation with water with or without 50 mM NaCl) for 10 days. The enhanced sugar beet growth by NaCl application was only observed in plants with high light intensity treatments. Moreover, high light intensity treatment increased the shoot concentration of malondialdehyde, an indicator of oxidative stress, while NaCl application decreased it. While there was a positive correlation between Na concentration and the compatible solute glycine betaine in the shoots and roots under both light intensities, a significant negative correlation between the concentrations of glycine betaine and malondialdehyde was found only in the shoots under high light intensity. Because it is known that glycine betaine activates antioxidant enzyme activities and reduces oxidative stress responses, glycine betaine synthesized in response to NaCl accumulation may have reduced the oxidative stress caused by high light intensity in the shoots, which may have caused the enhanced growth in sugar beet. Furthermore, the growth promotion by NaCl accumulation was accompanied by a significant decrease in NO3-N concentration, which suggested that NaCl application affects nitrogen metabolism in sugar beet.