Effects of chloride on growth, photosynthetic traits of canola seedlings

Soil salinization not only is a serious environmental problem but also poses a severe threat to plant growth and agricultural production. The ions of Na+ and Cl- are mainly responsible for salinization. People generally pay close attention to study the Na+ effects on grow, productivity and quality of crop plant, however, relatively much less studies are on Cl- effects on crop plant. The effects of different concentrations of Cl- (0, 25, 50, 75, 100 and 200 mmol/L) on plant growth, photosynthetic pigment, photosynthesis traits and water use efficiency in canola (Brassica napus L.) seedlings for 9 and 18 days were investigated. The results show that the dry matter accumulation rates are increased under the 25-100 mmol/L Cl- treatments for 9 and 18 d, and the highest values are under the treatment of 50 mmol/L Cl-, while the plant dry matter accumulation rate is decreased significantly under the 200 mmol/L Cl-. The chlorophyll (Chl) contents are ascended in first and descended under different concentrations of Cl- treatment for 9 d, and the highest values are under the treatments of 100 mmol/L Cl-. Nevertheless, for 18 d, the Chl contents are decreased under the Cl- treatments, and the more the applied Cl- concentration is, the more the decrease is. The chloride concentrations of canola leaves are markedly increased under the Cl- treatments. Compared with the control, the Cl- contents in canola leaves are increased by 814%, 805%, 1043%, 1083% and 2157% for 9 d and by 745%, 1270%, 1340%, 1900 and 3386% for 18 d, under 25, 50, 75, 100 and 200 mmol/L Cl- treatments, respectively. Apart from Ci under the Cl- treatments for 18 d, canola leaves are able to maintain high values of Pn, Gs, Ci, and Tr under 25-100 mmol/L Cl- treatments. For 9 and 18 d, Pn both achieve the highest values under the 25 mmol/L Cl- treatment, and are increased by 12% and 13% compared with each control, respectively. Pn, Gs, Ci and Tr of canola leaves are significantly decreased under the 200 mmol/L Cl- treatment for 9 and 18 d. As Cl- concentration increased, WUE is descended in first and ascended followed. However, Ls is descended in first and ascended followed under the Cl- treatments for 9 d, and increased gradually for 18 d. Under 200 mmol/L Cl-, Ls of canola leaves are increased by 48 % for 9 d, and by 76 % for 18 d. Correlation analysis indicates that for the 9 d treatments, the DMAR of canola is insignificantly related to root/shoot, Chl content, Cl- content and Ls, very negatively related to WUE, positively related to Gs, and very positively related to Pn, Ci and Tr, respectively. For 18 d treatment, the DMAR of canola is insignificantly related to root/shoot, Chl content, Cl- content, Ls and WUE, positively related to Gs, and very positively related to Pn, and Tr, respectively. Our results implicate that photosynthesis and transpiration of canola seedlings are significantly increased under the low Cl- treatments, and thus make a contribution to plant growth, while the high Cl- treatments show opposite situations, and the photosynthetic limitation is mostly due to stomatal limitation. Pn, Gs and Tr can be regarded as value indexes of chloride adaptation of canola growth.