The effect of drought stress and elicitor of chitosan on photosynthetic pigments, proline, soluble sugars and lipid peroxidation in Thymus deanensis Celak. in Shahrekord climate

 
Introduction
Thymus daenensis as a medicinal plant dispersed in high altitudes in Zagros Mountains range, western and south western Iran and has an extensive diversity of chemical composition of essential oil (Ghasemi Pirbalouti et al., 2014). Water stress has been considered as one of the limit factors for plant production (Passioura, 2007). Drought condition frequently leads to oxidative stress primarily due to chloroplast damaging and injury, leading to over-reduction of the chlorophyll content and enzyme activity in calvin cycle and finally it can reduce grow and yield of plant (Monakhova and Chernyadev, 2002).
 
Chitosan categorized as a biotic elicitor in plants which activates gene expression for secondary metabolites, increasing of plant production, conservation of plant against microorganisms, seed germination and for growing in plants (Yin et al., 2011).
The objective of this experiment was to investigate the effect of drought stress and chitosan application on physiological traits including photosynthetic pigments, proline, soluble sugars, and lipid peroxidation in T.deanensis in Shahrekord climate condition.
Materials and methods
In order to determine, the effect of drought stress and foliar application of chitosan on photosynthetic pigments, proline, soluble sugar, lipid peroxidation and cell membrane permeability of T. daenensis an experiment was conducted in Shahrekord climate in 2014. The experiment was laid out as factorial arrangement in complete randomized with three replications. Drought stress levels included well-watered (field capacity), irrigated daily to 50% field capacity (mild stress), and irrigated daily to 25% field capacity (severe stress), combined with three chitosan levels 0, 0.2, 0.4 g L^-1 and acetic acid. At first seeds were planted in spring of 2014 in greenhouse conditions and then transferred in climate conditions. Chitosan was sprayed three times, just prior to flowering stage, at 50% flowering and at full flowering.
Photosynthetic Pigments. Chlorophyll and carotenoid content was determined by Arnon method (1967).
Proline. Proline concentration was measured by Bates et al., (1973). Leaf soluble sugars content. Soluble sugars content was determined by Fsles, (1951). Lipid peroxide (MDA). The lipid peroxidation level in plant tissues was determined in terms of MDA concentration (Robbert et al., 1980). Cell membrane permeability. Membrane permeability was determined by measuring (Lutts et al.,1998).
Statistical analysis. SAS software and MSTATC were used for data analysis and means were compared using least significant difference.
Results and discussion
Photosynthetic pigments: The effect of drought stress had significantly (p≤0.01) on chlorophyll a. It seems that, when water stress intensified, activity of reactive oxidative species and chlorophylls’ enzyme increased. In general, foliar application of chitosan did not change on photosynthetic pigments.
Proline: The effect of drought stress, chitosan and interaction between drought stress and chitosan had significantly (p≤0.01) on proline. When water stress intensified, proline increased. The highest proline content was obtained (3.85) µmol g^-1 from severe stress and 0.4 g L^-1 chitosan. By decreasing of water potential, the synthesis of proline from glutamic acid increased (Liang et al., 2013).
 Soluble Sugars: The interaction between drought stress and chitosan did not significant on soluble sugars but when water stress intensified, soluble sugars increased.
Lipid peroxide (MDA content): The effect of drought stress, chitosan and interaction between drought stress and chitosan had significantly (p≤0.01) on lipid peroxide. In the study the effect of water stress on apple seedling, MDA content increased (Yang et al., 2009). In this study, spraying with 4 g L^-1 chitosan could compensated damage caused by drought stress.
Membrane permeability: The effect of drought stress and chitosan had significantly (p≤0.01) on membrane permeability.
Conclusion
Drought stress provokes ROS production in thyme plant, the compensatory effect of chitosan on reducing the negative impact of stressful conditions on plant was mainly due to stimulation of osmotic adjustment through proline accumulation, and reduction of lipid peroxidase level and therefore improvement of the integrity of cell membranes.