Physiological, biochemical and molecular responses of different barley varieties to drought and salinity

Abstract

Barley (Hordeum vulgare) is an important member of grass family Poaceae. Among various agricultural crops, barley is considered a model plant due to its important features like short season, tolerance to abiotic stress, large number of varieties and availability of sequenced genome. Ten old barley varieties from different parts of the world; Reisgerste II, Candice, Scarlett, Heilis Frankin, Himalaya USA, Himalaya Nepal, Himalaya Winter, Himalaya Freak, Himalaya Nakt and Himalaya India were selected for studying the drought and salt tolerance mechanisms. All these varieties were grown in hydroponic cultures. Relative water contents (RWC) and water loss rate (WLR) of the plants were measured to have a rough estimation of stress tolerance and water retention rates in plants. The seedlings were subjected to drought stress after two weeks after germination by stopping irrigation. Salt stress was imposed by treating the plants with 200 mM NaCl and 400 mM NaCl solutions for seven days. Although the RWC and WLR in varieties showed that all the barley varieties had a range of oxidative stress tolerance, Himalaya Nakt, Himalaya India and Scarlett were with better water retention capability than others while Himalaya Freak had the least one.<br /> Although the plants were affected by 200 mM salt treatment as well, yet the effect of 400 mM NaCl treatment and drought stress were much more than that of 200 mM NaCl treatment. The amount of total chlorophyll contents estimated from the leaves showed a greater decrease at 400 mM NaCl and drought treatments moreover the decrease was much more in the plants with less water holding capacity, showed the degradation of chlorophyll in the plants. <br /> Proline is an amino acid that contributes in scavenging ROS, hence enhances oxidative stress tolerance in living organisms. Proline content increased in all the varieties on stress treatments. Increase in proline contents in tolerant varieties was more than five times during stress conditions, in less tolerant varieties the observed increase was two times at drought and 400 mM NaCl. <br /> The amounts of MDA and H2O2 in the plants show the susceptibility of the plants towards oxidative stress. MDA level in Himalaya Freak was double than that were found in Himalaya Nakt, Himalaya India and Scarlett in drought stress while even more than double 400 mM NaCl. Similarly, amount of H2O2 in Himalaya Freak was almost 1.6 times higher than in tolerant varieties in drought stress while 2.5 times in case of 400 mM NaCl treatment. <br /> Antioxidants are known to inhibit the oxidation of biological molecules thereby protecting the cells against oxidative damage. The activities of different anti-oxidative enzymes like SOD, catalase, peroxidase and glutathione reductase were measured. The activities of SOD and catalase increased in 200 mM NaCl while in 400 mM NaCl and drought stress their activities were at par with control plants in the majority of the varieties. However, in Himalaya Freak it decreased on all the three treatments. The activities of glutathione reductase and peroxidase increased in all varieties on all treatments except for Himalaya Freak, where no significant difference was found at 200 mM NaCl treatment. The activities of anti-oxidative enzymes in the varieties like Scarlett, Himalaya Nakt and Himalaya India were much more than in Himalaya Freak, Reisgerste II and Candice. <br /> In order to correlate the physiological and biochemical changes with molecular changes, the differences in gene expression levels of different dehydrins were analyzed. The reverse-transcriptase polymerase chain reaction (RT-PCR) was performed to analyze the relative expression levels of different dehydrins as indicator of stress tolerance. Among the 13 dehydrins found in barley, Dhn8 and Dhn13 were constitutively expressed in all varieties. Dhn10 and Dhn11 did not express in any variety. The expressions of Dhn1, Dhn6 and Dhn7 correlate with physiological and biochemical data.<br /> To summarize, the physiological, biochemical and molecular analysis of different varieties of Barley at different stress conditions suggests that out of selected 10 varieties, Scarlett, Himalaya Nakt and Himalaya India were found to be most tolerant varieties and Himalaya freak was found to be the most susceptible variety.