Glycinebetaine Improved Photosynthesis in Canola under Salt Stress: Evaluation of Chlorophyll Fluorescence Parameters as Potential Indicators

Salinity is a widespread soil problem limiting productivity of oilseed crops worldwide. Reduction in growth generally associated with decline in photosynthesis. Accumulation of osmo-protectants such as glycinebetaine (GB) permits the plant to cope with salt stress. This study was aimed to understand the mechanism by which exogenous GB application improves photosynthetic capacity and salt tolerance in two diverse lines of canola differing in salt tolerance using chlorophyll fluorescence technique. Glycinebetaine was applied as foliarly or through rooting medium to two canola lines Dunkeld and Cyclone that were grown under non-saline or saline conditions. Root-applied GB caused adverse effects whereas foliar application of GB was effective in improving growth of canola cultivars. However, the response of salt-tolerant cultivar Dunkeld was stronger than that of salt sensitive Cyclone. Foliar application of GB improved accumulation of proline and plant water status. Application of GB enhanced the photosynthetic CO2 fixation, stomatal conductance (g(s)) and water-use efficiency. The tolerant lines Dunkeld had more responsive to GB application. Chlorophyll fluorescence measurements (F-o, F-m, F-v, F-o/F-m, F-v/F-o and F-v/F-m ratios) revealed that salt stress reduced energy trapping efficiency by damaging oxygen evolving complex, over reduction of Q(A) resulting in occurrence of chronic photoinhibition. However, exogenous GB protected the oxygen evolving centre of PSII and maintains activity of PSII. Although root-applied GB adversely affected the growth of canola plants, it did not have any adverse effect on PSII photochemistry. Moreover, fluorescence parameters could provide a rapid means for determining salt tolerance in canola and could be a rapid and sensitive test to identify genotypes highly tolerant to salt stress.