Four foliar applications of glycinebetaine did not alleviate adverse effects of salt stress on growth of sunflower

In response to salt stress, plants have evolved some adaptations such as osmotic adjustment to acclimatize salt stress. Glycinebetaine (GB) is known to have a role in osmotic adjustment. The present investigation was focused to understand the role of fortnightly exogenous foliar application of GB in inducing salt tolerance in sunflower through osmotic adjustment or by modulating plant water relations. Three levels of GB solutions (0, 0.1% Tween 20 solution; 50 and 100 mM GB in 0.1% Tween 20 solution) were foliarly applied to three-week old plants of sunflower grown at 0, 60, and 120 mM NaCl. Salt stress reduced the growth of sunflower plants. However, four foliar applications of 50 mM GB on weekly basis improved the growth of sunflower plants at intermediate level of salt stress, whereas higher level of GB did not improve the growth or even reduced the growth. Although exogenously applied GB reduced the leaf water potential and osmotic potential that resulted in enhanced leaf turgor potential, it did not improve the growth. Salt induced reduction in photosynthetic rate was partially improved by four applications of GB at intermediate level of salt stress. Furthermore, changes in photosynthetic capacity mainly occurred due to stomatal limitations. Finally, it was concluded that four applications of GB partially alleviated adverse effects of salt stress, which was associated with stomatal factors.