| Orchardgrass (Dactylis glomerata L.) is indigenous to northern Africa, Europe, and temperate Asia, which has been naturalized on nearly every continent. Orchardgrass has higher application value and development prospects and is widely applied to mountain slope improvement and rocky desertification of karst area owing to its good nutrition, high yield, biomass production and good shade tolerance.However, Drought stress, as a worldwide problem, is one of the major abiotic factors limiting plant growth and productivity, and induces various changes at the morphologic, physiological, and genomic levels during adaptation to drought stress in plants.Therefore, studying the mechanisms for drought resistance and the adaptation and regulatory mechanisms to drought in plants is crucial for selecting and breeding drought-tolerant cultivars, which could be an important solution to the water shortage.Drought tolerance of nine Orchardgrass cultivates were evaluated in this study, and then two types of Orchardgrass, which genotypes differing in drought tolerance (drought-tolerant ’Baoxing’ and drought-sensitive ’01998’), were used to investigate different physiological and molecular responses to drought stress and rewatering. The main results were as follows:1. Seed Germination Response to Osmotic Stress and Drought Tolerance Evaluation of OrchardgrassThe results of germination stage showed that the germinating vigor and germinating rate,the growth rate of embryo bud and root were all decreased by PEG water stress. The drought resistance of all germplasm was evaluated comprehensively by membership function method. Based on the Drought tolerant index, plant vigor tolerant index, relative seed germination rate, relative seed germination vigor, relative embryo bud length (dry weight), and relative radical length (dry weight) of nine Orchardgrass germplasm were measured to evaluate their drought tolerant ability.9Dactylis glomerata materials were ordered below based on their drought tolerance:Baoxing>02-116>Sparta> Aldebaran> Gulin> Chuandong> Donata> Proto>01998.2. Drought Resistance Evaluation of Orchardgrass Germplasms at Seedling Stage The results of seedling stage showed that all of the materials decrease to various extent in plant height, leaf relative water content (RWC) under drought stress. Whereas, they showed an increase in free praline accumulation (Pro) and soluble sugar content (TSS). Superoxide dismutase (SOD), Catalase (CAT), Guaiacol peroxidase (POD) as the main antioxidant protection enzymes, the activity increased firstly and then decreased. Analysis of variance, correlation analysis and cluster analysis were used to explore their drought resistance. The materials were divided into three drought resistant types based on Euclidian Distance method. The most drought resistant germplasm were’Baoxing’,’02-116’,’Sparta’and the moderate drought resistant ones were’Aldebaran’,’Gulin’,’Chuandong’,’Donata’ while the drought sensitive ones were ’Proto’ and ’01998’.3. Effects of drought stress on root growth and photosynthetic characters of Orchard grass seedlingsUsing drought-sensitive Orchardgrass genotapy ’01998’ and drought-tolerant genotapy ’Baoxing’ as test materials to study the effects of drought stress on root system morphology, physiological characteristics and leaf photosynthesis of Orchardgrass seedlings, aiming to investigate the differences of drought tolerance mechanism between different cultivars of Orchardgrass seedlings.The test results showed that drought stress decreased the relative water content, leaf chlorophyll content, Pn, gs, and Tr of the two materials, among them, the large r variations in "01998", while at the same time, electrolyte leakage and Intercellular CO2concentration increased, thereinto’01998’changed most obviously while ’Baoxing’ changed only slightly under drought stress. Studies also showed that,stomatal restriction was the main factor for the net photosynthetic rate’s decrease in the leaves of two orchardgrass genotypes under the light soil stress. While non-stomatal restriction was the main factor under serious soil stress. And meanwhile, drought stress decreased the individual leaf area of the two materials and underground and aboveground plant biomass of ’01998’, but had no significant effects on ’Baoxing’. Two materials of the root activity and root number increased significantly when SWC decreased to30%and reached the maximum.therefore, under drought stress, drought-tolerant ’Baoxing’ were able to maintain higher photosynthetic area and photosynthetic rate by improving the root activity, maintaining higher root biomass, enhancing the ability of root water adsorption.lt was also optimal to keep soil relative moisture content in40%-50%in sandy loam for maximum growth of Orchardgrass.4. Effects of Drought Stress on Lipid Peroxidation, Osmotic Accumulation and Activity of Protective Enzymes in Root and Leaf of OrchardgrassIn order to investigate the different drought tolerance mechanism in Orchardgrass, the objective of this research was to study the different responses associating with lipid peroxidation, osmotic adjustment and activity of protective enzymes in root and leaf to drought stress of two different genotypes of Orchardgrass (drought-sensitive ’01998’and drought-tolerance ’Baoxing’).The results showed that with the extension of drought stress, MDA content and electrolyte leakage in root and leaf of two Orchardgrass increased while total soluble sugar content and free proline content were accumulated. At the same time, soluble protein content, SOD, CAT, POD, APX activities increased initially and then decreased. As the results showed, significantly higher cell membrane stability, content of osmotic adjustment substances and activities of SOD, CAT, POD, APX as well as lower lipid peroxidation level in root and leaf of ’Baoxing’ were observed as compared with the ’01998’ under drought stress. In addition, APX and POD activity in root and leaf of "Baoxing" was higher than that of the ’01998’ under24-days drought stress, which indicated that its ability to scavenge for singlet oxygen and H2O2was not weakened by drought stress. Drought tolerant ’Baoxing’ showed more active and stable peroxidation ability under drought stress.5. Antioxidant Enzyme Activities and Gene Expression Patterns in Leaves of Orchardgrass (Dactylis glomerata L.) during drought stress and recoveryThe study was designed to investigate different physiological and molecular responses of two genotypes of Orchardgrass (drought-tolerant ’Baoxing’ and drought-sensitive ’01998’) to drought stress and rewatering. The ’01998’ and ’BaoXing’ were exposed to drought stress for21days and24days in a growth chamber, respectively allowing the leaf relative water content (RWC) of both genotypes to drop to the same level (20%). Significantly higher activities of superoxide dismutase (SOD), catalase (CAT), and guaiacol peroxidase (POD) were observed in the ’Baoxing’ compared to the ’01998’ under drought stress and rewatering. CAT and POD for H2O2scavenging was associated with post-drought recovery on rewatering in Orchardgrass.The maintenance of higher transcripts levels of SOD in ’Baoxing’ versus ’01998’ when both cultivars were exposed to the same level of water deficit suggests that the activation of SOD expression could be involved in the antioxidant protection of Orchardgrass during drought. At both enzymatic activity and gene transcript levels, POD was significantly higher in drought-tolerant ’Baoxing’ than in drought-sensitive ’01998’ under post-drought recovery, suggesting that POD could play critical roles protecting plants from drought-induced oxidative stress and the recovery from drought damages. |
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