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Fact sheet 6: Plant defense response
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Drought has also increased the vulnerability of trees to charcoal disease . The survival and growth of Persian oaks depend on their tolerance to drought stress .
Therefore, conservation activities aimed at preserving Persian oaks in the damaged Zagros forests might greatly benefit from the selection of individuals or provenances more tolerant to drought. Drought is one of the most important limitation factors for growth and plant production.
Under drought stress, some disorder generally occurs in most physiological processes, such as decreases in photosynthesis rate and growth  , stomatal conductance  , as well as cell dehydration  and chlorophyll degradation . Plants respond to drought stress using a combination of biochemical processes . Proline and soluble sugars are overproduced in plants in response to drought stress.
Proline and carbohydrates act as osmolytes to maintain water in the cytoplasm . Furthermore, they prevent protein denaturation and cell membrane damage, and induce stability in the structure of enzymatic proteins, thereby preserving their activity . The production of reactive oxygen species ROS , including superoxide radicals and hydrogen peroxide, is an important biochemical response to oxidative stress . High ROS concentrations can disrupt normal plant metabolism damaging lipids, proteins, chlorophyll, and nucleic acids .
ROS directly damage cell membrane phospholipids and increases lipid peroxidation, which can be determined by the content of malondialdehyde MDA by-products . On the other hand, oxidative damage to cell membranes results in electrolyte leakage EL and, ultimately, in cell death . Plants use different enzymatic and non-enzymatic systems to control ROS production induced by drought stress. The most important antioxidant enzymes include superoxide dismutase SOD , catalase and peroxidases POXs , glutathione reductase, and ascorbate peroxidase APX to prevent cell damage .
In addition, phenolic components - especially flavonoids and phenylalanine ammonia lyase PAL - form another class of plant defense molecules that respond to drought stress . In fact, PAL is the primer of the phenylpropanoid pathway that finally leads to phenolic components biosynthesis.
Although previous studies focused on growth and physiological responses of oak species to drought stress  ,  , there is limited data concerning the effect of drought stress on morphological, physiological, and biochemical responses of the Persian oak. In this study, Persian oak seedlings were selected from two provinces in the Zagros region with different precipitation regimes, and subjected to different levels of drought stress. The growth, physiological, and biochemical responses of the seedlings were measured to compare the level of drought tolerance between and within the individuals of the two populations.
Here, we focus more on biochemical reactions of the species in response to drought stress since there was no comprehensive information about the subject. The aim was to evaluate the diversity of Persian oak populations in the response to water limitation and elucidate whether climatic conditions occurring at Persian oak stands are related with the resistance to drought stress of their seedlings. Different precipitation regimes occur on the Zagros mountains, as the mean annual rainfall declines from north to south.
Southern Zagros region has a longer dry season period and higher mean annual temperature compared to the northern area. We selected Ilam and Lorestan provinces from the southern and central Zagros Mountains, respectively. Ilam province has a lower average annual precipitation, higher mean temperature, and longer dry season, compared with Lorestan province. The general climatic characteristics of both provinces are presented in Tab.
Ten healthy Persian oak individuals were randomly selected from Chegeni Lorestan province and Melasyah Ilam province provenances, and their healthy and mature seeds were collected . The sampled seeds were surface-sterilized with 3. In late May of , uniformly sized seedlings underwent different drought stress treatments.
At the beginning of the experiment, the average of diameter and height of the seedlings were 0. The experiment was carried out in a completely randomized design with two factors two provenances and three watering regimes.
Plant Metabolites and Regulation under Environmental Stress
Each treatment was conducted in four replications and 10 seedlings were used in each replication. The stress treatments continued for three months. At the end of the experiment, the stem diameter and height of each seedling were measured. Then, the seedlings were removed from their pots and the soil around the roots was washed away. Each plantlet was divided into roots, stems, and leaves. The maximum photochemical efficiency of photosystem II, xylem water potential, and gas exchanges were measured three times once every month. Fully-developed leaves from the lower part of the stem the same in all seedlings were selected for gas exchange measurements .
From each seedling, five fully-developed leaves were collected. The maximum PSII photochemical efficiency was calculated as eqn. Free proline content in leaves was quantified in accordance with Bates et al. The soluble obtained was placed in a boiling water bath for 10 minutes and then cooled in an ice bath. Lipid peroxidation was measured by the concentration of MDA byproducts.
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First, 0. The absorbance of the supernatant was read by spectrophotometer at nm .
To determine the electrolyte leakage EL , mg fresh leaf samples were cut into 5 mm length and placed in test tubes containing 10 mL distilled deionized water. After two hours, the initial electrical conductivity of the medium EC 1 was measured using an electrical conductivity meter. The EL was calculated by the following equation  - eqn. Hydrogen peroxide H 2 O 2 content was measured as described by Velikova et al.
Leaf samples 0. The absorbance of the supernatant was read using a spectrophotometer at nm. Briefly, 1 g of fresh leaf was homogenized in 4 mL of 65 mM phosphate buffer pH 7. Then, 1 mL of the obtained supernatant was mixed with 0. The absorbance of supernatant was read at nm. Nitrogen dioxide radical was used as a standard. Leaf tissue 1 g was homogenized with 3 mL 0. The supernatant was used to determine enzyme activities.
CAT activity was determined as reported by Bergmeyer . PAL activity was determined by trans-cinnamic acid production rate in accordance with Wang et al. Total leaf phenol content was determined by the Folin-Ciocalteu method  and leaf flavonoid content was determined in accordance with the method suggested by Zhishen et al. Typically, Chegeni seedlings had larger diameter compared with Melasyah seedlings. No significant difference were found between stem diameter of seedlings under control W and moderate M drought conditions Fig.
However, severe S drought significantly reduced the diameter of Chegeni seedlings, but not that of Melasyah seedlings. Drought stress significantly reduced the height of seedlings in both populations Fig. Severe stress S significantly reduced the height of seedlings when compared to control W and moderate M stress. The total biomass of Chegeni seedlings was significantly influenced by drought stress, but Melasyah population was not Fig. A significant interaction effect between population and drought stress was also detected for total biomass.click here
Fact sheet 6: Plant defense response | Epidiverse
As a result, the total seedling biomass for the Chegeni provenance was higher than that for Melasyah under control condition. In addition, severe stress S decreased the total biomass of Chegeni seedlings compared to moderate stress M. In June, one month after the drought stress was applied, the net photosynthesis rate, stomatal conductance, and V cmax in moderate and severe drought-stressed seedlings from both provenances declined, in comparison with unstressed seedlings, and there was no significant difference between the two provenances Fig. After two months of drought stress, in July, the gas exchange parameters were similar to those observed in June.
In August after three month of drought stress , the net photosynthesis rate declined considerably in seedlings subjected to severe drought stress SM and SCh ; however, we did not find any significant difference between moderate stress MM and MCh and control WM and WCh seedlings. Furthermore, we did not observe a significant difference of net photosynthesis after three months of drought among the two provenances. In August, stomatal conductance extensively decreased in the seedlings from Melasyah only under severe drought stress SM.
On the other hand, moderate and severe drought stress negatively affected stomatal conductance of Chegeni seedlings MCh and SCh, respectivey. The results of V cmax were the same for all three months Fig. In June and July, the xylem water potential of seedlings from both provenances declined in response to moderate and severe drought stress. In July, Chegeni seedlings under severe drought stress showed the lowest mean value of xylem water potential and the decreasing rate was greater in this provenance in comparison with the Melasyah provenance.
On the other hand, moderate drought stress only affected the xylem water potential of Chegeni provenance seedlings Fig. The maximum PSII photochemical efficiency in both provenances significantly decreased in response to moderate and severe drought stress when they were measured in June and July. In August, PSII in Melasyah seedlings was affected only by severe drought stress, while both moderate and severe drought stress led to a decrease in the parameter values of Chegeni seedlings at the same time Fig.
Chlorophyll and carotenoid content of the seedlings decreased in response to drought stress Fig.