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Vitex trifolia which grows in the sandy coast of Yantai,China is a plant with high resistance to sand burial and a good ground cover.It was chosen as the plant material to study how a plant adapts physiologically to sand burial.Four levels of sand burial treatment were set up: no sand burial,light(1/3 plant height),moderate(2/3 plant height) and severe sand burial(3/3 plant height).Changes in plasma membrane permeability,MDA content,osmotic regulation(proline and soluble sugar contents),and activities of antioxidant enzymes(SOD,POD,CAT) in the leaves from different levels of sand burials were monitored. On the 5th day under light and moderate levels of sand burial,leaves of both young and mature plants of V.trifolia showed increased membrane permeability,MDA content,activities of POD and SOD,proline content,and RWC while soluble sugar content as compared to controls.We also compared leaves of the same plant above and below sand burial: leaves above sand had higher membrane permeability,MDA content,activities of SOD and POD,and soluble sugar than those below sand burial.In contrast,proline content of leaves below sand was higher than the content of those above sand. With prolonged time of sand burial(10 days),membrane permeability,MDA,soluble sugar content and activity of POD decreased and SOD activity and proline content increased in leaves above sand.After 10 days of light and modest sand burial,leaves above sand also grew well and were stronger than those of control plants.These results indicate that antioxidant enzymes and osmotic matter are involved in adaptation of V.trifolia to sand burial through physiological regulation.The sand burial not only intensified drought and ground thermal radiation stress to leaves above sand,but also made the leaves below sand encounter stress due to deficiency of light,O2 and CO2,all of which interrupted normal metabolism and induced the production of active oxygen free radicals resulting in membrane lipid peroxidation.At the same time,sand burial stress activated antioxidant enzymes and promoted the accumulation of proline,eliminating the accumulation of free oxygen radicals and lipid peroxidation,thus maintaining the integrity of the membranes.Therefore,under sand burial,rapidly activated antioxidant enzymes(SOD and POD) hindered membrane lipid peroxidation and kept a dynamic balance between oxygen free radical production and antioxidant enzyme scavenging.This played a very important role in the rapid response of V.trifolia to sand burial stress,keeping plants growing well under light and modest sand burials.This may be the main mechanism of protection during sand burial of the whole plant,allowing the shoot apex to grow fast and emerge from the sand.

Perennial plants that normally experience dormancy during winter may be subjected to more cycles of freeze-thaw caused by warming winter.We explored the physiological mechanisms of adaptation of white clover,Trifolium repens Linn,to freeze-thaw stress.In late fall of 2009,uniform plots of white clover grown in a natural environment(NE) were covered with a plastic house(pH) to simulate the effect of a warmer winter or left uncovered(control).During the winter and following spring,growth capacity and physiological indices related to resistance to adverse environment were measured in leaves of white clover grown in both conditions.In the winter of 2009,the average temperature ranged from below zero(-10℃) to 7℃ in the NE during which leaves experienced thawing-freezing-thawing-freezing,and above zero(from 1 ℃ to 7 ℃) in the pH where leaves were never frozen.In late fall of 2009,all white clover was of uniform height.But in spring 2010,plants grown in the PH(26 cm) were 3 times taller than those in the NE(8 cm).This difference in height disappeared by June 2010.In winter,the relative membrane permeability,MDA(malondialdehyde),proline,and soluble sugar contents,and the activities of catalase(CAT) and superoxide dismutase(SOD) and peroxidase(POD) increased with decreasing temperature in leaves of white clover grown in both NE and PH.The physiological index values in the white clover grown in the NE experiencing freezing-thaw stress was greater than those in leaves grown in PH(no freezing).In spring,two days after removing the plastic housing,the relative membrane permeability and content of MDA in the leaves of white clover rose quickly to 127% of values before removing the PH,and even higher than that in the leaves of white clover grown in the NE.Activities of SOD and CAT in white clover after removing the PH also increased quickly to 33% and 84.6% higher than in white clover grown in NE.In addition,during late spring coldness,plants in both treatments showed the same trend of increasing proline and soluble sugar content and activities of POD and SOD in response to lower temperature.Our results suggest that Trifolium repens has a high capacity to activate antioxidant enzyme activity to control lipid peroxidation and increase osmolytes in response to freezing temperature and that these mechanisms play a key role in surviving warm winter.While a warming winter delays the greening period,it does not retard white clover growth in spring,making white clover an excellent plant to withstand climate changes that include warmer winters.

Messerschmidia sibirica Linn,Siberian sea rosemary,may be useful in the restoration and protection of sand dunes.We explored its ability to withstand varying levels of sand burial in the Yan Tai coastal dune in the spring and summer of 2009 and 2010.Four levels of sand burial treatments were set up: no-sand burial(control),light sand burial(1/3 of plant height),moderate sand burial(2/3 of plant height) and severe sand burial(plant fully covered by sand).Changes in soil temperature,soil moisture,fresh weight(FW),dry weight(DW),relative plant height,plasma membrane permeability,MDA content,osmotic regulation,and activity of antioxidant enzymes in the leaves of plants in different levels of sand burial were investigated to elucidate the strategies used to maintain physiological and biochemical processes during growth in the different environments.Soil temperatures below sand decreased while soil water content increased with increasing levels of sand burial.Plants in light and moderate sand burials were taller than controls.In individual plants,the FW and DW of leaves above sand increased while FW and DW of leaves below sand decreased.On the 5th day after sand burial,the leaves of whole plants in each of the three levels of sand burial had lower membrane permeability,MDA and proline contents and higher levels of activities of SOD and CAT than controls.Also,in individual plants,membrane permeability,MDA and proline contents,and activities of SOD and CAT in the leaves below sand were lower than in leaves above sand.The decrease in these physiological indicators was therefore correlated with increases in soil moisture and lower soil temperatures.Leaves above sand appeared to suffer from stronger ground thermal radiation,drought stress,and high temperature. These findings suggest that under light and moderate sand burial,antioxidant enzymes and proline play an important physiological regulating role in retarding lipid peroxidation and protect the leaves above sand,allowing them to grow rapidly and helping the plant avoid further sand burial.Moreover,under severe sand burial,although darkness and low oxygen limit photosynthesis and respiration of the buried leaves,those leaves can keep more nutrients to transfer to the apex of the shoot to make it grow and emerge above sand.Our results confirm that Messerschmidia sibirica Linn has a high resistance to even severe sand burial and uncovers some of the physiological strategies employed in this resistance.Thus it may be a very suitable plant for vegetation restoration,sand fixation,and protecting beaches in coastal dunes.