Physiological response of Vitex trifolia to sand burial in the sand coast

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.