Introduction of the Most Resistant Juniperus Genotypes to Drought Stress Based on Physiological and Antioxidant Traits

Introduction  Drought is one of the most important environmental stresses that affects various plants such as ornamental plants. The identification and selection of ornamental tolerant genotypes is essential for landscape projects. Understanding the mechanisms that enable plants to adapt to drought stress can help to select the most tolerant genotypes for cultivation in arid and semi-arid regions. Materials and Methods  For this purpose, a research was conducted as a factorial experiment based on completely randomized design with eleven genotypes and two levels of irrigation (irrigation as Control and severe drought) at Ramsar Citrus and Tropical Fruits Research Institute. Results and Discussion  The first symptoms of drought stress were observed after 10 days in the Juniperus horizontalis (G1) genotype (the most sensitive genotype) and were not recovered and dried after 15 days. G3 and G5 genotypes (Ravande-mamouli and Ravande-setarei, respectively) showed drought stress after 28 days (most tolerant genotypes) and recovered at the end of the stress period after irrigation. Drought stress decreased photosynthetic pigments in studied genotypes. The content of soluble sugars, proline, and total soluble protein increased under drought stress conditions and the highest amount was observed in G3 genotype 30.8 mg g-1 DW, 30.5 μg g-1 DW, and 965.2 μg g-1FW, respectively. Under drought stress condition, the highest concentration of hydrogen peroxide, malondialdehyde and electrical conductivity were observed in G11, G4, and G10 genotypes respectively compared to control plants. In addition, the most enzyme activity of superoxide dismutase (85.57%), total phenol (181.09%) and total flavonoid (98.46%) was evaluated in G3, G5 and G8 respectively. Also, chlorophyll changes indicate the response of plants to environmental stresses such as drought during drought stress, the concentration of abscisic acid and ethylene increases, which stimulates the activity of the enzyme chlorophilase and causes chlorophyll degradation. The reduction of photosynthetic pigments under drought stress also seems to be related to changes in nitrogen metabolism to proline production and reduced chlorophyll synthesis because the precursor of chlorophyll and proline is glutamate. Furthermore, one of the biochemical changes that occur in plants under drought stress is the accumulation of ROS. Numerous reports have stated that drought stress increases ROS production. Drought-induced oxidative stress causes lipid peroxidation and membrane damage. It seems that in some genotypes with low levels of malondialdehyde, the membrane damage is severe and leads to more electrical conductivity. Genotypes with more electrical conductivity are more damaged by drought stress. In some genotypes, such as G2 and G11, there was a positive correlation between malondialdehyde content and electrical conductivity, but in others, such as G1, there was a negative correlation. Although the amount of malondialdehyde in this genotype is low, electrical conductivity is very high. In other words, this genotype should be a genotype sensitive to drought stress. The tolerance of the plant to various environmental stresses may be related to the level of activity of the enzymes responsible for scavenging ROS. The antioxidant response to water scarcity depends on the severity of stress and type of plant species. Therefore, different genotypes increased their antioxidant activity to reduce the effects of oxidative stress, and the high antioxidant activity was observed for G5 compared with other genotypes which can be contemplated as drought-tolerant genotype. The accumulation of compatible metabolites such as soluble sugars and proline in plants under drought conditions can help to protect them against stress. The proline and soluble sugars accumulation under stress conditions reduce lipid peroxidation and acts as a free radical scavenger. According to the results, drought stress induced accumulation of proline and soluble sugars in the genotypes of Juniperus and the highest accumulation of proline was related to G3. Therefore, this genotype can be introduced as drought-resistant genotype. Conclusion  The results of the current study showed that drought stress significantly affected some biochemical parameters in all eleven genotypes. However, a variation in drought susceptibility was observed among genotypes. The studied genotypes in this experiment had different responses to drought stress and it seems that they utilized different mechanisms for stress tolerance. Genotype of G3 (Ravande -mamouli) was the most tolerant genotype to drought stress based on the highest levels of superoxide dismutase, soluble sugars, proline, and soluble protein. Genotype of G5 was also tolerant to drought stress with high superoxide dismutase activity and the largest amount of total flavonoid production. Therefore, increasing of compatible metabolites and antioxidant system are effective protective mechanisms against oxidative damage under drought stress.