The growth is a comprehensive response of plants to drought stress and a reliable criterion for evaluating drought stress degree and drought resistance of plants. The decrease of plant height and stem diameter is a common phenomenon in drought stress (Wang et al.). The result of this study showed that under drought stress, the shoot growth of Phedimus aizoon L. was inhibited, and the stronger the degree of stress, the longer the time, the more significantly inhibited (Table 1), the main reason was that the lack of water led to clogging of vascular tissue and reduced cell elongation (Abdalla and El-Khoshiban). Roots are the main organs of plants that absorb water and closely related to the drought resistance, the root system can adapt to drought stress by regulating its own growth and development, water absorption and transportation. The root length proliferation of Phedimus aizoon L. was promoted under mild and moderate drought stress (Table 1), which was beneficial for the root system to absorb the deep water of the soil and improve the utilization rate, thus improving the drought tolerance of the plant (Hufstetler et al.)..
The leaves are sensitive organs to drought during the evolution process. Because of their plasticity, the changes of leaf morphological structure will inevitably lead to the change of physiological and biochemical characteristics of plants. Therefore, the changes of leaf morphological traits can reflect the adaptability of plants to drought stress. (Mahajan and Tuteja ). The thicker the plant leaves, the greater the tightness of the tissue, the stronger the water storage capacity and the drought tolerance of the leaves (Pu). Phedimus is a typical phedimus plant with strong drought tolerance, and the leaves belong to special fleshy, which is closely related to its drought tolerance (Gravatt and Martin). Through the observation of leaf anatomical structure, it was found that in the absence of drought stress, the leaves of Phedimus aizoon L. were thicker, the mesophyll cells were large and irregular, arranged loosely, with large intercellular spaces and strong water storage capacity. Under moderate and severe stress, the mesophyll cells gradually aggregated and smaller, the chloroplast gradually aggregated into the middle of the cell, and the upper epidermis thickness became thinner (Fig. 1, Table 2). The purpose of this change is to reduce the direct contact of plants with radiation and transpiration of water, to preserve the limited water and to make full use of it (Xue et al.). Especially under severe stress, the leaf thickness and upper epidermis thickness were significantly lower than that of control, and the tissue arranged from loose to tightly. As the drought deepened, water between the mesophyll cells was consumed, thus the intercellular space was shrinked, the cells were tightly arranged after being squeezed, and the cuticle was thicken gradually (Fig. 1, Table 2), which indicated that the leaf tissue of Phedimus aizoon L. showed the strongest resistance to drought stress under such soil moisture.
The plasma membrane is the original site where plants are firstly damaged in stress. Under drought stress, the plasma membrane is damaged, which is characterized by increased plasma membrane permeability and partial electrolyte leakage (Tan and Blake). At the same time, a large amount of reactive oxide species are produced in plants, which induces membrane lipid peroxidation and produces malondialdehyde, thus causes damage to plant cell membrane systems (Yao et al.). The level of MDA content and changes of plasma membrane permeability are important indicators reflecting the degree of plasma membrane damage. This study showed that although the MDA content and electrolyte leakage increased with the increase of the time and degree of drought stress, throughout the growth phase, only the electrolyte leakage fluctuated significantly at 30d, and there were no significantly difference between mild drought stress and control from beginning to end (Fig. 2), indicating that Phedimus aizoon L. could endure a certain degree and time of water deficit.
The accumulation of osmotic regulatory substances (proline, soluble sugar and soluble protein) is one of the basic characteristics for plants to adapt to drought stress. Under drought stress, plants actively accumulate osmotic regulatory substances to increase the concentration of cell fluid, the main function of which is to maintain cell turgor, balance the infiltration of protoplasm and the external environment, and enable various physiological processes of cells to proceed normally. Moreover, it is generally believed that the drought resistance of plants is positively correlated with its content (Zhang et al.). The content of soluble sugar, proline and trehalose in leaves of Phedimus aizoon L. was significantly increased under drought stress, especially at 30 days of stress (Fig. 3A, B, C).The result of this study was similar to the study of Guo, which also showed that under drought conditions, as the stress time increased, the content of osmotic regulatory substances such as proline and soluble sugar increased sharply. The increase of these osmotic regulatory substances reduced the cell osmotic potential and ensured that Phedimus aizoon L. could continue to absorb water from soil. However, the change of soluble protein content was slightly different from that of other osmotic regulatory substances. The possible reason is that as the drought time prolongs, the degree of stress is aggravated, and the anabolism in plants is inhibited, resulting in the inability of soluble protein to continue to rise, and even decreased.
Oxidative stress is usually accompanied by drought stress, and antioxidant defense system is one of the mechanisms of drought response, which provides aerobic metabolism of energy for plant growth and development. The generation and clearance of intracellular reactive oxygen species (ROS) are in a state of dynamic equilibrium in normal state, but when plants are subjected to drought stress, the dynamic balance is destroyed (Cruz de Carvalho). Antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) are effective in scavenging reactive oxygen species and preventing excessive ROS accumulation, and protect plants from hurt (Harb et al.). This study showed that in order to remove excess ROS and reduce oxidative damage, Phedimus aizoon L. could maintain high antioxidant enzyme activity under different degrees of drought stress. Especially under severe drought stress and in later period of stress, the activities of SOD, POD and CAT increased sharply (Fig. 4), the main reason is that the production of ROS in the plant increased sharply under this condition, although which caused serious damage to normal metabolism Phedimus aizoon L., the plant could still remove these ROS by increasing antioxidant enzymes accordingly, showing its strong resistance to drought stress.
The life activities of plants depend on stomata for gas exchange, and stomata is also a channel of transpiration. The size, opening degree and density of stomata directly affect the transpiration rate of plants(Dong J and Bergmann D C.). In this study, stomata closed during the day and slightly opened at night under severe stress, which reduced the evaporation of water and hindered the absorption of CO2, which may be one of the factors causing the reduction of intercellular CO2 concentration (Ci) (Fig. 5). At this time, stomatal conductance (Gs) and transpiration rate (Tr) of leaves significantly decreased, and stomatal limit value Ls significantly increased, leading to a decline in photosynthetic capacity, indicating that stomatal status directly affected the photosynthetic capacity and dark reaction metabolism of plants. For every gram of carbohydrate produced by the CAM pathway, crassulaceae plant consume one-tenth as much water as the C3 cycle.CAM pathway not only reduces water loss through stomata, but also promotes water absorption by plants(Winter K et al.). Under drought stress, Phedimus aizoon L. can reduce water loss by regulating stomatal state and dark reaction metabolism, which is another important reason for the improvement of drought resistance.
Chloroplasts and mitochondria are the two most important sites for photosynthesis and respiration. Drought stress can change the microstructure of plants. In this study, chloroplasts gradually converge to the middle of cells under moderate and severe stress, and the chloroplasts shrink under severe stress, resulting in fragmentation of the granule lamella, internal dissolution, and destruction of membrane structure, which are consistent with the rise of the MDA content and electrolyte leakage of leaves.The increase of osmiophilic granule, the fragmentation and dissolution of starch granules increased the osmotic potential of cells and contributed to water absorption.The inner lamella of the mitochondria dissolves and become swollen and faded.In this case, the life activities of plants are seriously affected.