Effects of exogenous NO and MJ on relative growth of Fraxinus mandshurica under drought stress
By measuring plant height (Fig. 2.1.a) and ground diameter (Fig. 2.1.b) of hybrid combinations after drought stress, it was found that drought stress had a significant inhibitory effect on plant height growth of hybrid combinations. However, there was no obvious effect on the increment of ground diameter. The plant height growth of hybrid D110 was higher than that of non-hybrid D113 and 4-3, which indicated that the growth of hybrid D110 was better than that of parent under drought stress, and the drought resistance of hybrid D110 was stronger than that of non-hybrid D113 and 4-3.The results showed that the plant height growth of D110 hybrid was 1.3 times higher than that of non-hybrid D113 under drought stress or drought stress, while exogenous MJ, was added at the same time as drought stress. The plant height growth of the hybrid D110 was 3.0 times of that of the non-hybrid D113, which indicated that the treatment of MJ could make the plant have stronger drought resistance. After adding exogenous SNP and MJ, the inhibitory effect of drought stress on plant growth was obviously weakened.
Effects of exogenous NO and MJ on photosynthesis of Fraxinus mandshurica under drought stress
Photosynthesis was inhibited and photosynthetic rate decreased under drought condition. It is generally believed that varieties with strong drought resistance can maintain relatively high photosynthetic rate or net photosynthetic rate, so net photosynthetic rate is a reliable indicator for drought resistance identification.
From figure 2.2, the photosynthesis indexes of hybrid D110 and non-hybrid D113 and 4-3 were measured under drought stress and rehydration for every 3 days (12 days) after drought stress and rehydration for 3 days (15 days). It was found that drought stress inhibited the net photosynthetic rate of hybrid D110 ,non-hybrid D113 and 4-3 (Fig. 2.2.a, Fig. 2.2.b).The net photosynthetic rate of the hybrids and their parents decreased gradually with time, and reached the lowest at 12 days, but after rehydration, the inhibition was obviously weakened, the net photosynthetic rate was obviously increased, and the intercellular CO2 concentration was significantly increased. Stomatal conductance and transpiration rate were similar to net photosynthetic rate.
Under drought stress, the net photosynthetic rate of hybrid D110 decreased to some extent when exogenous SNP or MJ was added, but the decrease range of D110 was lower than that of control, and the net photosynthetic rate of hybrid D110 was higher than that of control. It shows the advantage of drought resistance.
Under drought stress, the net photosynthetic rate of progeny D110 added with exogenous SNP or MJ, was 6.4% or 24.1% higher than that of drought stress, indicating that hybrid progenies were more sensitive to exogenous MJ signals. However, when exogenous SNP and MJ were applied to parents, the variation of photosynthesis was not as obvious as that of hybrid progeny, which indicated that hybrid progenies were more sensitive to exogenous signals and had drought resistance advantages than their parents.
Effects of exogenous NO and MJ on the content of malondialdehyde (MDA) in water under drought stress
The damage (or senescence) of plants under stress is closely related to the membrane lipid peroxidation induced by the accumulation of reactive oxygen species. Malondialdehyde (MDA) is one of the most important products of membrane lipid peroxidation. Enzyme binding or crosslinking inactivates the biofilm, thus destroying the structure and function of the biofilm. Therefore, the degree of membrane lipid peroxidation can be understood by measuring the content of MDA, and the damage degree of membrane system and plant resistance to stress can be indirectly measured.
From Fig. 2.3,the content of MDA (Fig. 2.3) of D113 and 4-3 of hybrid progeny D110 and non-hybrid progeny was determined under drought stress and exogenous application of SNP and MJ. The results showed that the MDA content of D110 and D113 and 4-3 increased in different degree under drought stress, but with the rehydration, the MDA content decreased, and the D110 decreased 36.3%. Parent D113 decreased 30.7% and parental 4-3 decreased 27.1%.The results showed that drought stress caused damage to the membrane system of all hybrid combinations to varying degrees, but with the rehydration, the damage degree decreased to a certain extent, and the degree of decline of hybrid progenies was greater than that of parental control. The results showed that the damage degree of membrane system of hybrid progenies was lower and the drought resistance was higher. Under drought stress or exogenous application of SNP and MJ, the MDA content of the hybrid D110 was more stable than that of its parent D113 and 4-3, which indicated that the hybrid progenies had obvious drought-resistance advantage over their parents.
Effects of exogenous NO and MJ on antioxidant enzyme activity of Fraxinus mandshurica under drought stress
When plants are in bad environment or senescence, there will be a large amount of reactive oxygen species accumulation in plants, which will lead to damage to the membrane system, and then affect the normal growth of plants. Superoxide dismutase (SOD) combined with peroxidase (POD) can scavenge oxygen free radicals and reduce oxidative damage of active oxygen groups to plants. The determination of SOD, POD content can indirectly show the degree of oxidative damage in plants. Which shows the drought resistance of the plant.
From figure 2.4, under the conditions of drought stress and exogenous application of SNP and MJ, the activities of antioxidant enzymes SOD (Fig. 2.4.a) and POD (Fig. 2.4.cnd) were significantly increased compared with those of the control group without drought treatment. However, the content of MDA decreased significantly. The increase of the activity of SOD and POD and the decrease of MDA content in D110 were higher than those in D113 and 4-3 of their parents, which indicated that the ability of each hybrid to resist drought environment was stronger than that of its parents. After 3 days of rehydration, D110 could maintain a high level of SOD and POD activity, especially in the activity of POD. After drought, the POD activity of hybrid progenies treated with SNP and MJ increased respectively 41.5%, 120.4%, 173.5%, which may indicate that after rehydration, there are still a large number of active oxygen free radicals need to be removed, maintaining a higher SOD enzyme activity and POD enzyme activity can help plants recover from stress better.
After treated with SNP and MJ for 9 days, the activity of SOD enzyme of hybrid D110 was higher than that of drought stress and control, and increased by 12.2% and 25.1% than that of drought stress, respectively. The non-hybrid progeny D113 was treated with SNP and MJ for 9 days. Compared with drought stress, the activity of SOD enzyme increased by 4% and 8%, respectively. After 9 days of treatment with SNP and MJ, the activity of SOD enzyme in 4-3 hybrid was increased respectively by 7.7% and 10.1%. After treated with SNP and MJ for 12 days, the enzyme activity of POD was increased respectively by 10% and 33. 3% than that of drought stress. The enzyme activity of POD was increased by 15. 7% and 19. 9% than that of drought stress after SNP and MJ treatment for 12 days. After 12 days of treatment with SNP and MJ, the activity of POD enzyme in the 4-3 hybrid was increased respectively by 8.9% and 11.33%, compared with drought stress. The preliminary results showed that the hybrids could produce more antioxidant enzymes and enhance the survival rate of plants under stress, and the plants treated with MJ had more drought-resistant advantages than those treated with SNP.
Effects of exogenous NO and MJ on the expression of Rhythm Gene in Fraxinus mandshurica under drought stress
As a "big switch", rhythm gene can recognize the original regulation of gene upstream of stress signaling pathway and regulate the response of plants to abiotic stress. The determination of gene expression can reflect the drought resistance of plants at molecular level.
After drought stress and exogenous SNP and MJ treatment, the expression of rhythm gene LHY,TOC1,LOX,NIR was determined by drought stress on D110 hybrid and D113 parent.
LOX can synthesize the precursor of MJ and its metabolites are oxygen free radicals and reactive oxygen species which can destroy the membrane structure of cells and participate in the process of plant senescence. The results showed that LOX genes synthesized during germination could increase its expression level through abscisic acid pathway.In order to cope with the environmental pressure caused by global climate change and excessive nitrogen application, it is very important to improve the water and nitrogen use efficiency of crops. It is reported that higher nitrogen uptake can mitigate the destructive effects of drought stress. After drought stress, the nitrogen and proline contents of transgenic plants were higher than that of wild type control, and the activity of nitrite reductase (NIR) was also higher during nitrate assimilation. The results showed that higher nitrate transport and assimilation activity were helpful to improve drought resistance of transgenic plants . Nitrogen (N) is the main plant nutrient and plays an important role in determining plant growth and productivity. Plants need nitrogen to synthesize important molecules, such as protein, nucleic acid and chlorophyll. Most plant species can absorb nitrate (NO3-) and ammonium (NH4). Overexpression of some genes in N pathway can increase plant tolerance to abiotic stress. For example, salinity, drought and extreme temperature all affect the absorption and assimilation of nitrogen (N) in plants. Most of the genes encoding nitrate transporter proteins and enzymes responsible for N assimilation and reactivation (such as nitrite reductase NIR) are down-regulated under abiotic stress. Especially under long-term stress (24 hours), this may be one of the reasons for the decrease of plant growth and development under abiotic stress .
From Fig. 2.5, it can be seen that the gene expression of the hybrid D110 with the exception of LOX has a certain change with the passage of time, reaching the peak value at 12 days. The expression of LOX gene was very low under drought stress and increased rapidly after rehydration. Under drought stress, the expression of LHY in hybrid D110 was 400 times higher than that of parent D113 when exogenous SNP and MJ were applied to the hybrid. The NIR was 8.5 times higher, and the TOC1 was 2.3 times higher, which indicated that the drought resistance advantage of the hybrid was much greater. The LHY gene of the hybrid D110 continued to increase under SNP treatment and reached its peak at 12 days. The MJ treatment decreased first and then increased, reached a low point at 6 days and a peak at 12 days. When exogenous SNP was applied, the expression of LHY was 17 times higher than that of MJ. The expression of TOC1 gene decreased at first and increased at the beginning of SNP treatment, and had a low point at 9 days and reached its peak at 12 days after SNP treatment. The expression of TOC1 gene decreased at first, reached a low point at 6 days and reached a peak at 12 days after treatment with exogenous MJ. The difference between the expression of LHY and TOC1 was that the expression of TOC1 decreased to a low point when the expression of LHY gene reached its peak from 3 to 9 days, and they formed a form of mutual inhibition. The expression of LHY and TOC1 genes in parent D113 increased at first and reached its peak at 12 days. When SNP and MJ were applied , the expression of LHY and TOC1 genes was higher than that of drought, which indicated that exogenous hormones had a certain effect on the improvement of drought resistance of plants.
The gene expression of the hybrid D113 is higher than that of the parent, and the mechanism of the drought-resistant advantage of the hybrid is preliminarily explained, and the mechanism of the different physiological conditions of the hybrid and the parent gene is preliminarily explained, and the reason for the drought-resistant advantage of the hybrid is also disclosed.
Effects of exogenous NO and MJ on hormone content in Fraxinus mandshurica under drought stress
Plant hormones are organic compounds that naturally exist in plants. Even at low concentrations, plant hormones can coordinate a wide range of physiological processes, including growth and development, as well as responses to abiotic and biological stresses.
Abscisic acid (ABA) plays a key role in the response of plants to drought stress. ABA is known to mediate drought response by regulating the expression of drought response genes and stomatal closure. These hormones regulate signal transduction pathways and gene expression through rapid induction or inhibition of transcription . Exogenous ABA, which increases the content of free polyunsaturated fatty acid (PUFA) under stress, can stimulate the activity of lipoxygenase and promote the production of JA. However, JA binds or methylates with amino acid isoleucine to produce methyl jasmonate MJ. Peroxidation of membrane lipids was activated to promote the formation of tolerance when rice leaves were damaged . Therefore, the increase of ABA concentration under drought stress is beneficial to improve the survival rate of plants.
Auxin IAA is a common chemical signal in all vascular plants. IAA can regulate many aspects of plant growth and development, including cell division and elongation, and organ development at cell and whole plant level . This physiological regulation is achieved by signal transduction that results in changes in the expression of many genes. Gibberellin GA can inhibit cell proliferation by increasing the action intensity of cell cycle inhibitor. GA can not only inhibit the growth, but also promote the survival of plants under pressure by limiting the accumulation of reactive oxygen (ROS). Thus delaying cell death .
It can be seen from chart. a, b and c that the variation trend of IAA, ABA and GA is similar, with the increase of the number of days of drought stress, and the concentration is obviously higher than that of the control group, and the content of the three hormones is almost the highest at 12d. And remained at a higher concentration at the time of the hydration 3d (15d). Under drought stress, after 12 days of treatment with exogenous SNP and MJ, the ABA, IAA, GA in the hybrid D110 was significantly higher than that in the control group, and the increase of ABA content was the most obvious, the average increase was 37.9%, 33.9%, 51.2% in comparison with the control group. The content of ABA in parent D113 was increased by 3.3%, 13.1%, 0.9% in comparison with the control group. The content of ABA in the hybrid D110 was much higher than that of the parent D113, which indicated that the hybrid progeny could respond to drought stress more quickly at the hormone level, and had the advantage of drought resistance, and maintained a high concentration after rehydration. It shows that the plant needs a certain time to recover after rehydration.
Cytokinin is known as a plant hormone that modifies root morphology and increases root biomass to enhance drought resistance by reducing root-crown-axis ratio (CTK level) . From figure d, it was found that the concentration of CTK decreased under drought stress and was significantly lower than that of control group, and reached the lowest value after 12 days of drought treatment, and the content of CTK increased obviously after rehydration. And after 12 days of drought treatment, compared with the control group, the CTK content of the hybrid D110 decreased by 10.4%, and that of the parent D113 decreased by 8%. The content of CTK in the hybrid progenies decreased significantly at 9 days, while that of the parent D113 decreased slightly at 12 days, which proved that the hybrids could respond more quickly and strongly to drought stress. Increase plant survival in arid environments by adjusting hormone content. The difference of hormone between hybrids and their parents is one of the reasons why hybrids have the advantage of drought resistance.