| Calcium is not only the essential macro-element in plant growth and development, but also a cellular second messenger, which takes part in signal perception and transduction as well as stress resistance. Many researches have shown that exogenous calcium application can alleviate the damage to the plants caused by adverse conditions. But the underlying mechanism still remains unclear.Two-month-old in vitro seedlings were cultured in MT added with or without 10 mmol/L Ca(NO3)2 for 2 days, which were then subjected to dehydration at ambient environment. Plant morphology, physiological and biochemical indexes and expression of stress-related genes were investigated after dehydration for 0 h, 1 h, 3 h, 6 h. The main results are as follows:1. Measurement of Ca concentration showed that two-day pretreatment with Ca(NO3)2 increased calcium content in the leaves of Poncirus trifoliata L. Under dehydration, the seedlings pretreated with Ca(NO3)2 had higher Ca than the control.2. Under dehydration, the seedlings withered as time extended. Seedlings pretreated with Ca(NO3)2 showed less wilting and lower relative water loss than the control.3. Under dehydration, DAB and NBT indicated that O2- and H2O2 contents in pretreated seedlings were less than in the controls, implying that pretreatment with Ca(NO3)2 decreased dehydration-derived damage.4. Activities of two antioxidant enzymes, SOD and POD, had different trend in Poncirus trifoliata L. under dehydration, but seedlings pretreated with Ca(NO3)2 had higher activities than controls, which indicated that calcium improves dehydration tolerance via enhancing antioxidant enzymes.5. Expressions of stress-responsive genes, CDPK1, CDPK2 and a polyamine biosynthetic gene, ADC, were more profoundly up-regulated, if applicable, in the pretreated seedling leaves, which may be one reason responsible for dehydration tolerance.
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