Photosynthetic Physiological Response to Drought Stress of Populus euphratica at Different Ages in Minqin

Chunxiu GUO Jianping MA Junmei MA Duoqing MAN Fei WANG Yunian ZHANG Peng ZHAO

Abstract [Objectives] This study was conducted to investigate the photosynthetic physiological characteristics of Populus euphratica trees at different forest ages in the desert area of Minqin in response to drought stress.

[Methods] With P. euphratica trees of different ages in Minqin as the research object， the water characteristics and photosynthetic physiological indexes （chlorophyll， soluble sugar， POD， SOD and MDA） were compared under different conditions.

[Results] On the time gradient， the soil water contents of P. euphratica of different ages decreased continuously with the extension of irrigation stop time， but the decreases were small. Under the same stress treatment， the soil water contents of P. euphratica of different ages increased with the deepening of soil layer， but the differences were not significant （P>0.05）. Under normal condition， the chlorophyll contents of P. euphratica at three ages gradually increased with the increase of age. The chlorophyll contents in leaves of P. euphratica at different ages were all lower under normal condition and higher under water stress. The changes of POD and SOD activity in leaves of P. euphratica at different ages under different conditions were basically the same， showing that the enzyme activity was higher under water stress than under normal under. The MDA contents in leaves of the young and middle P. euphratica forests were higher under water stress at 7 and 21 d than under normal condition， and the differences were not significant （P>0.05）; and the MDA content in leaves of the mature P. euphratica forest was higher under water stress at 21 and 35 d than under normal condition. Drought stress has a certain effect on the photosynthetic physiological characteristics of P. euphratica. In summary， under drought stress， the chlorophyll content， SOD and POD activity， and MDA molar concentration in the leaves of P. euphratica were basically higher than under normal condition， indicating that P. euphratica could resist drought environment through osmotic adjustment and showed strong drought resistance.

[Conclusions] This study provides a theoretical reference for the restoration， protection and reconstruction of natural P. euphratica forests in the desert area of Minqin.

Key words Minqin; Different ages; P. euphratica; Drought stress; Photosynthetic physiological characteristics

In arid desert areas， water is an important factor restricting plant growth and development[1]. Under drought stress， plants will adapt or alleviate drought stress through a series of changes in photosynthetic physiological characteristics such as photosynthetic mechanism， osmotic regulation and antioxidant defense[2-3]. Meanwhile， these changes in plants can reflect the state and degree of environmental stress. Therefore， studying the changes of photosynthetic physiological characteristics of plants under drought stress can help grasp the physiological health status of Populus euphratica and provide a theoretical basis for the identification of drought resistance and conservation measures of plants.

P. euphratica is a tree species with strong stress resistance distributed in arid areas[4]， which has the characteristics of good drought tolerance， strong salt and alkali resistance and the function of wind and sand fixation[3]. Its leaf shape is variable， and it is known as the "living fossil" in the desert. P. euphratica forest is a very important type of vegetation in desert areas， and plays a very important role in maintaining the ecological balance of local desert areas. In recent years， with the drcreasing population of P. euphratica declining and the declining distribution area declining， the potential environmental and ecological role of P. euphratica， the value of scientific research and the value of development and application have received more and more attention[5-7]. Relevant scholars mainly studied the relationship of P. euphratica photosynthesis[8-9]， fluorescence[10]， ecological water demand[11-12]， population structure and dynamics[13-15] with groundwater[16-18]. However， there are few related studies on the photosynthetic physiological characteristics of P. euphratica at different forest ages in the desert area of Minqin under drought stress. In this study， by comparing the photosynthetic physiological indexes of P. euphratica at different ages under different water conditions in Minqin， the response of P. euphratica to water stress （characteristics of adaptation to drought environment） was revealed from the aspects of photosynthetic characteristics and stress-resistance physiological characteristics， aiming to provide a reference for the restoration， protection and reconstruction of natural P. euphratica forests in the desert area of Minqin.

Materials and Methods

General situation of research area

The study area is located near the Minqin Desert Control Comprehensive Test Station on the southeastern edge of the Badain Jaran Desert， with a geographic location of 103°05′E， 38°35′N and an altitude of 1 378.5 m. It has a typical arid desert climate， with an average annual temperature of 7.7 ℃， an average annual relative humidity of 50%， and an average annual water vapor pressure of 0.59 kPa. It has strong light intensity and abundant heat resources. The average annual sunshine hours are 2 651.9 h， and the average percentage of sunshine is 60%. The area is dry and has little rain. The average annual precipitation is 115.56 mm， the evaporation is 2 643 mm， which is more than 23 times of the precipitation， and the dryness is 5.85. This area is windy， with an average annual wind speed of 2.4 m/s; and the vegetation is sparse and of a single type， and includes both natural vegetation and artificial plants. The zonal soil is gray-brown desert soil and aeolian sand soil， with a soil organic matter content of 0.15%-0.8% and a pH value of 8.5.

Research methods

Plot setting and sample collection

In this study， P. euphratica of different forest ages near Minqin Desert Control Comprehensive Test Station was selected as a research object. The age class of P. euphratica was divided based on the study of Wang et al.[19]： young forest 5 cm≤D<12 cm， middle-aged forest 12 cm≤D<25 cm and mature forest 25 cm≤D<50 cm. In June 2018， three P. euphratica trees with normal growth， good growth vigor， straight trunk and moderate crown， free of pests and diseases， were selected， and 6-9 normal growing leaves were selected as analysis samples on the sunny side of each tree. Three replicates were taken from each part. The samples were then wrapped with aluminum foil， frozen with liquid nitrogen， and brought back to the laboratory for analysis of chlorophyll content and POD activity.

Physiological index determination methods

For the content of chlorophyll （Chl）， the sample was extracted with a mixed solution of 95% acetone∶anhydrous ethanol=2∶1， obtaining an extract， the absorbance of which was determined by a spectrophotometer at 645 nm （A645） and 663 nm （A663）， and the chlorophyll content was calculated by the modified Arnon formula： Chla=12.63A663-2.59A645， Chlb=22.88A645-4.67A663. The soluble sugar content was determined by anthrone colorimetry. The POD activity was determined by guaiacol method. The SOD activity was determined by nitroblue tetrazolium （NBT） photoreduction method. The malondialdehyde （MDA） content was determined by TBA colorimetric method[1].

Data processing and statistical methods

Simple data reduction and analysis and chart plotting were performed with Microsoft Excel 2010. One-way （ANOVA） analysis was performed using SPASS21.0 software on various indexes of P. euphratica forests at different ages， with a significant level of P<0.05.

Results and Analysis

Changes of soil water content and degree of soil drought stress

It could be seen from Table 1 that on the time gradient， with the increase of the irrigation stop time， the soil moisture contents of P. euphratica trees of different forest ages continued to decrease， but the decreases were small. After stopping irrigation for 35 d， the soil water contents of different soil layers were significantly lower than those at 7 d before stopping irrigation. Through analysis of variance， under the same stress treatment， the soil moisture contents of P. euphratica trees of different forest ages tended to increase with the deepening of the soil layer， but the differences were not significant （P>0.05）. The soil moisture content of the 0-40 cm soil layer was significantly lower （P<0.05） than that of the 40-180 cm soil layer after 35 d of cessation of irrigation in the mature forest.

Study on the photosynthetic physiological indexes of P. euphratica

Change characteristics of chlorophyll in P. euphratica trees of different forest ages

It could be seen from Fig. 1 that the chlorophyll contents in leaves of P. euphratica at different ages were in the range of 0.129-0.171 mg/g. Under normal condition， the P. euphratica trees of different ages showed a gradual increase in chlorophyll content with the increase of age level， and the chlorophyll contents ranked as mature forest>middle-aged forest>young forest. The chlorophyll contents in the leaves of P. euphratica trees of different ages exhibited basically the same trend in different conditions， showing that the chlorophyll content in the leaves of P. euphratica was lower under normal condition， and higher under water stress. According to the analysis of variance， the young and middle-aged forests showed significant differences in chlorophyll content between normal condition and stress condition （P<0.05）. The chlorophyll content of the young forest at 35 d of stress was significantly different from that at 7 and 21 d of stress （P<0.05）， and the chlorophyll content at 7 d of stress was not significantly different from that at 21 d of stress （P>0.05）. The chlorophyll content of the middle-aged forest at 7 and 21 d of stress was significantly different from that at 35 d of stress （P<0.05）. The chlorophyll content of the mature forest under normal condition was significantly different from that under stress at 21 d （P<0.05）， while there were no significant differences between different stress time （P>0.05）.

Agricultural Biotechnology2020

Changes of total soluble sugar contents in P. euphratica at different sample plots

It could be seen from Fig. 2 that the soluble sugar contents in leaves of P. euphratica at different ages were in the range of 0.669-5.063 μg/g. The analysis of variance showed that the soluble sugar content in the leaves of the young forest was significantly higher under stress at 7 and 35 d （P<0.05） than under normal condition; the soluble sugar content in the middle-aged forest was significantly higher under stress at 7 d than under normal condition （P<0.05）; the soluble sugar content in the mature forest was significantly higher under stress at 21 d than under normal condition （P<0.05）; and the young and middle-aged P. euphratica forests had a soluble sugar content significantly lower under 21 d of stress than under normal condition （P<0.05）. The changes in soluble sugar content in leaves varied during different stress periods for P. euphratica forests at different ages.

Study on drought-resistance physiological indexes of P. euphratica

Changes in the activity of peroxidase （POD）

It could be seen from Fig. 3 that the POD activity in leaves of P. euphratica at different ages was in the range of 9.352-180.448 △A470 nm/min·g. According to the analysis of variance， the POD activity in leaves of P. euphratica at different ages under different conditions all showed basically the same trend， i.e.， significantly higher under water stress than under normal condition （P<0.05）. Specifically， the POD activity in leaves of the middle-aged forest showed a significant increasing trend with the extension of stress time （P<0.05）， and the changes in POD activity in leaves of the young and mature forests were different with the extension of stress time.

Changes of superoxide dismutase （SOD） activity

It could be seen from Fig. 4 that the SOD activity in leaves of P. euphratica at different ages was in the range of 0.178-0.466 U/ml·FW， and the change trends of POD activity in leaves of P. euphratica at different ages under different conditions were basically the same， all of which were higher under water stress than under normal condition. Through analysis of variance， the SOD activity in leaves of the young P. euphratica forest was significantly different between the water stress and normal conditions （P<0.05）， and the SOD activity in leaves of the young and mature P. euphratica forests was on the decrease with the extension of stress time.

Changes in malondialdehyde （MDA） activity

It could be seen from Fig. 5 that the MDA contents in leaves of P. euphratica at different ages were in the range of 0.001 0-0.003 0 mmol/g·FW under different conditions. Through analysis of variance， the MDA contents in leaves of P. euphratica in the young and middle-aged forests under water stress at 7 and 21 d were all higher than those under normal condition， and the differences were not significant （P>0.05）. At 35 d of stress， the MDA contents of P. euphratica leaves were all lower than those in normal condition. Specifically， the young forest showed significant differences （P<0.05）; the middle-age forest had no significant differences （P>0.05）; and the MDA content of the mature forest was lower under stress at 7 d than under normal condition， but higher under water stress at 7 and 21 d than under normal condition， while the differences were not significant （P>0.05）.

Discussion and Conclusions

Water is a necessary condition for plant survival， and soil moisture is the main ecological factor restricting plant growth in arid desert areas. The growth of P. euphratica is closely related to soil moisture. The water resources in P. euphratica growing area mainly come from underwater diving systems and natural precipitation. Due to the arid climate in Minqin desert area and the sparse rainfall， when the groundwater drops to the condition under which plants cannot absorb available water from the soil to maintain their normal plant growth， soil drought will form. In this study， three stress gradients were set in the growing area of P. euphratica. On the time gradient， with the increase of irrigation stop time， the soil water contents of P. euphratica forests of different ages continued to decrease， but the decrease was small. After 35 d of irrigation stop， the soil water contents of different soil layers were significantly lower than those at 7 d of irrigation stop. With the deepening of the soil layer， the soil water contents of P. euphratica of different ages under the same stress treatment tended to increase. It was mainly because the soil surface had low soil water content due to strong evaporation， while in the deep layer， because P. euphratica is a perennial tall tree species with deep root system and has a very developed lateral root system， a thick skin layer on the outer surface of the roots prevented the tree roots from losing water under drought conditions. The main root of P. euphratica usually contains more than 50% of water， and a lot of water is also accumulated in the trunk tissue. When the soil is dry， P. euphratica can still maintain normal physiological activities[20].

Chlorophyll plays an important physiological function in photosynthesis， and its content can reflect plants light use efficiency， photosynthetic potential and ability to produce organic matter and adapt to the environment to a certain extent[18]. Drought stress can hinder the synthesis and decomposition of chlorophyll in plant leaves[3]. Gao et al.[21] reported that under different water conditions， the chlorophyll content of P. euphratica showed a fluctuation trend of first rising and then falling with the decrease of water. Zhao and Huang et al.[22-23] found through the effect of water stress on the physiological characteristics of maize and Brazil banana seedling leaves that dry stress led to a decrease in chlorophyll content and water content in plant leaves. In this study， the trend of photosynthetic capacity of P. euphratica at different ages with different water conditions was basically the same as that of the chlorophyll content. The chlorophyll content was lower in leaves of P. euphratica under normal conditions and higher in leaves of P. euphratica under water stress. It was mainly because excessive water deficit caused the plant leaves to wither and fall， and the amount of leaves was reduced， so that the nitrogen absorbed by P. euphratica was concentrated in a small number of leaves， which made the chlorophyll content in the leaves increase to some extent， indicating that as the drought stress was strengthened， P. euphraticas ability to endure stress was enhanced.

Under drought stress， some plants often actively accumulate soluble sugars and other substances in the cells through osmotic adjustment to reduce the osmotic potential of the plants and increase the water retention capacity， which helps the plants maintain normal metabolism and development in adversity to improve the drought resistance of plants[24]. This study showed that the soluble sugar contents of P. euphratica of different ages varied under different conditions， and the soluble sugar contents in leaves of the young and middle-aged forests were higher under water stress at 7 and 35 d than those under normal condition. The content of soluble sugar was higher in the mature forest under water stress at 21 and 35 d than under normal condition. It showed that under water stress， P. euphratica could reduce the osmotic pressure in cells by accumulating soluble sugar in the body to resist the arid environment.

Peroxidase （POD） and superoxide enzyme （SOD） are important protective enzymes for cells in plants to remove active oxygen. They can eliminate oxidative damage to cell membranes caused by free radicals produced by plants during metabolic processes[25]. Under drought， low temperature and other stress conditions， the level of protective enzyme activity can reflect plants ability to resist drought[26]. The results of this study showed that the activity of SOD and POD in leaves of P. euphratica at different ages was higher under drought stress than under normal condition， indicating that drought stress produced excessive superoxide free radicals in P. euphratica， enhanced the radical scavenging ability of P. euphratica， reduced membrane lipid peroxidation level and improved drought resistance. With the increase of stress time， the SOD activity in the leaves of P. euphratica of different ages showed a gradual decreasing trend， indicating that moderate drought stress could increase the level of membrane lipid peroxidation in plants， but when the drought stress exceeded the tolerance range of P. euphratica， the cytoplasmic membrane of the plant was damaged by reactive oxygen radicals， which is consistent with the research results of Luo et al.[27] on Camellia oleifera under drought conditions.

Malondialdehyde （MDA） is the final product of membrane lipid peroxidation of plant organs under stress[1]， and its mass concentration can reflect the degree of damage of plant cell plasma membrane[24]. This study showed that the MDA content was higher in leaves of the young and middle-aged P. euphratica forests at 7 and 21 d of drought stress than in leaves of normal condition， but the differences were not significant （P>0.05）. The MDA content in leaves of the mature P. euphratica forest at 21 and 35 d of water stress was higher than that in leaves of normal condition， indicating that drought stress promoted membrane lipid peroxidation， and the cell membrane in the plant was severely damaged. The MDA contents in leaves of the young and middle-aged P. euphratica forests were lower under water stress at 35 d than under normal condition， and the MDA content in leaves of the mature forest was also lower under water stress at 7 d than under normal condition， indicating that the leaf membrane lipid peroxidase activity of P. euphratica leaves effectively removed reactive oxygen species in the cells and reduced the MDA content， which meant that the cell membrane system was damaged lighter， and had better membrane permeability.

In summary， drought stress had a certain effect on the photosynthetic physiological characteristics of P. euphratica. The results of this study showed that on the time gradient， with the increase of the irrigation stop time， the soil water contents of P. euphratica forests at different ages continued to decrease， but the decreases were small， and the water content of the same stress treatment tended to increase with the deepening of soil layer. Under drought stress， the chlorophyll content， SOD and POD activity， and MDA molar concentration in the leaves of P. euphratica were basically higher than those under normal condition. It indicated that P. euphratica could resist drought environment through osmotic adjustment and showed strong drought resistance. P. euphratica is an excellent windproof and sand-fixing tree species in the arid desert area of northwest China， and has an irreplaceable role in ecological environment protection.

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