| Fingered citron (Citrus medica var. sarcodactylis Swingle) is a variation of Citrus medica L. with high ornamental value and medicinal value, which belongs to Citrus of Rutaceae. At present, C. medica is mainly cultivated in Guangdong, Guangxi. Sichuan, Zhejiang, Jiangxi, Fujian, and Yunnan province. In addition, the variety grown in Central-Zhejiang is especially suitable for ornamental usage because of its dwarfed trunk, spectacular fruit shape, and ease to cultivate. In recent years, the cultivation area of Fingered citron in Central-Zhejiang had increased significantly, but plants were faced with numerous unanswered questions about its agronomy practices, as we do not have even the basic biological knowledge about this plant species. The chilling condition falling to less than 5℃or the thermal condition up to more than 40℃, Fingered citron regularly appears chilling-injury with growth blocking up, watering and yellowing leaf, organell death, and burn-injury. We imitated the temperature environment of Central-Zhejiang in order to examine the effects of stress conditions on basic physiology and bio-chemistry of the Fingered Citron. The results and the thermal durability dataobtained in this study will increase our knowledge of the plants'biological properties and can be applied in the field to improve production techniques.We studied the changes of photo synthetic physiology under a simulated chilling condition which happens frequently in mid-Zhejiang area. The results showed temperature below 15℃significantly decreased the net photo synthetic rate, stomatal conductance and increased the intercellular CO2 concentrations. The 10℃was the inflection pointwhere the Fv/Fm began to decline rap idly and Fo began to increase. However, the 15℃decreased the Fv/Fm significantly if the treatment length was postponed to 72 h. Chilling stress also led to declines of carboxylation efficiency, the maximum photosynthetic rate, and the PFD when the photoinhibition was observed. In contrast to the control, the chilling treatment generally led to a rise of relative electric conductivity (REC) in leaves and declines of SOD, POD and CAT activities. In brief, short term chilling stress p rimarily weakened the activity of key enzyme in photosynthetic carbon fixation, such as Rubisco, caused an accumulation of reactive oxygen species, and then led to photoinhibition and a decline in photosynthetic rate.Our study aimed to improve the current summer planting methods of Fingered Citron by studying the effects of short-term high-temperature stress on its photosynthetic capacity, chlorophyll fluorescence, chloroplast ultrastructure, and antioxidant response. The results showed that extreme treatments (those above 40℃, such as 45℃for six hours) decreased the photosynthetic rate (Pn), the carboxylation efficiency (CE), and the saturated photosynthetic rate. These negative effects were primarily achieved through non-stomatal limitation mechanisms. In the chloroplasts, the extreme treatments inactivated the photosynthetic systemⅡ(PSⅡ) and modified the cell ultrastructures. These changes were most likely caused by membrane lipid peroxidation as a result of over-accumulation of reactive oxygen species (ROSs). Although the aforementioned extreme heat treatments,45℃, significantly declined the plant's photosynthetic capacity and severely damaged its photosynthesis structures, the 40℃treatments only caused minor damages to the plant and the 35℃treatments did not harm its photosynthetic capacity at all. These results show that the Fingered Citron is a thermal-durable species. Since the daily high-temperature in the Central-Zhejiang area seldom exceeds 40℃for more than four hours, the Fingered Citron do not need shade protection in the summer season and can be cultivated under full-sun conditions. In addition, since the high temperature in film-covered greenhouses in the summer season often exceeds 45℃, we suggest planters to either use shade protection for greenhouse cultivations or to grow the plants in uncovered settings. In order to study the oxidative stress of ROS that induced by chilling temperature, which happens frequently in Mid-Zhengjiang areas, in Fingered citron, the cell death percentage, lipid membrane peroxidation, distribution, histochemical analysis and content of ROS were investigated. The results showed that temperature below 5℃made the leaves cells died and brought irreversible damage to plants. In contrast to the control, the chilling generally led to a rise of relative electric conductivity (REC) in leaves and the membrane system was oxidatived continuously following with over-accumulation of reactive oxygen species (ROS). Analysized by the technology of frozen section, the H2O2 was apparently fixed in the interior mesophyll cells, while the habitation of O2- was extended to lots of position of leaves, like epidermal cells, palisade tissue and other skeleton structure. The most pronounced increase in H2O2 and O2- production was found in chloroplasts. In addition, the histochemical localization of O2- determined by paraffin section were bounded to four different parts of leaves tissue:(1) plasma membrane of leaves cells, such as epidermal cells, mesophyll cells; (2) internal cell of palisade tissue; (3) cell wall of all leaves cells; (4) vascular bundle and bundle sheath cells. |
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