Title of Thesis

Influence of exogenous application of salicylic acid on physiological and biochemical attributes of sunflower (Helianthus annuus L.) under salinity stress

Author(s)

Sibgha Noreen

Abstract
To examine whether salicylic acid (SA) can mitigate the deleterious effects of salt stress on vegetative and reproductive development in sunflower, a greenhouse experiment was conducted. The treatments consisted of two sunflower lines (Hysun-33 and SF-187), two salt levels (0 mM and 120 mM NaCl), three doses of salicylic acid (0, 100, 200, and 300 mgL-1) and four replicates arranged in a completely randomized design. Varying NaCl levels were raised at day 19 and SA was applied foliarly at day 24 after sowing.The growth and development of the sunflower lines was significantly affected under salinity stress. However, foliar spray of 200 mg L-1 of SA resulted in improved growth and achene yield. In addition, salt-induced reduction in achene yield and improvement in achene yield with SA application in both sunflower lines was found to be attributable to alteration in the size of both achene and capitulum. Photosynthetic rate of both lines increased due to foliar applied SA both under normal and salt regimes, specifically in line SF-187. Moreover, net CO2 assimilation rate was enhanced by 200 mg L-1 SA applied exogenously under salt treatment, which was positively associated with SA-induced increase in growth and yield. However, SA-induced changes in photosynthesis were not associated with stomatal conductance, but partially associated with photosynthetic pigments. Salt stress up-regulated the activities of leaf superoxide dismutase (SOD),catalase (CAT) and peroxidase (POD) in both sunflower lines. The SOD and POD activities were further improved both in stressed and non-stressed plants due to applied SA. However, the activity of leaf CAT was found to be unaffected due to SA applied exogenously. specifically being prominent in SF-187. In addition, SA-induced improvement in growth and photosynthetic performance in both lines was probably due to up-regulation in peroxidase activity caused by SA. Exogenously applied SA restricted the sunflower lines to absorb higher quantity of Na+ in their leaf tissues under saline conditions.The foliar applied SA induced both sunflower lines to absorb higher quantity of Ca2+ in their leaves and roots, notwithstanding, K+ accumulation remained almost unaffected under salt stress. In addition, SA application also caused accumulation of high amount of proline by the plants of both sunflower lines grown under saline substrate.Salt stress considerably reduced both achene oil quantity and quality of both sunflower lines.Contrarily, foliar applied SA caused improvement in achene oil percentage as well as quality of achene oil in terms of linolenic acid in both sunflower lines.Exogenous application of SA did not alter palmitic acid, stearic acid or oleic acid. Overall, salt tolerance in sunflower could be improved by foliar application of SA.

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