Physiological Ecological Studies On Adaptation To Water Stress In Leersia Hexandra As A Drought-flood Co-tolerant Plant

Overabundant moisture or insufficient moisture are deleterious to the terrestrial plants. Recently a series of waterlogged injury and drought injury resulted from the acute variation of the moisture environment. At present, more and more studies are focused on the plant adaptability to water stress and the adaptation mechanism of stress. Leersia hexandra could adapt to the drought-flood conditions,and L. hexandra was an amphibious plants,living in the water-fluctuation belt such as rivers or lakes. However, the little information was available about its adaptation mechanism.Under the conditions of potted plants and water stress,including drought, drought-water, submerged and waterlogged, the study dealt with L.hexandra morphological, anatomical structural and physiological strategies to fit drought-flood environment. Through field investigation and control experiment, we studied the rudimental biological characteristics of L. hexandra which grow in normal and water stress environment. The results showed that L. hexandra held greater ability in the tolerance to drought-flood conditions. The dynamic response change to water stress were studied, which included the anatomical structure of nourish organ, the roots parameter, the peroxidase activity and the endogenous hormones concentration. Moreover, the mechanism of physiological ecological adaptation to adversity of L. hexandra was founded. Furthermore we researched a higher reproductive efficiency technique of L. hexandra and it was proved to have many advantages in the natural vegetation restoration schemes of disturbed habitats such as the water-fluctuation belt. The results are as follows:(1) According to the dormancy rate on ground, the plant adaptability to water stress was estimated. The dormancy rate was 10.0% after being durative waterlogged (120d). Throughout a 45_day persistent drought or drought-water period (lowest content of soil moisture was 2.74%), the survival rates remained 30.0% and 50.0% respectively. The results indicated that the ability of L. hexandra in the tolerance to drought-flood conditions was greater.(2)The anatomy structure responses extent of roots,stems and leaves under waterlogged condition was bigger than that under submerged condition. The variation extent of stems and leaves were less when L. hexandra in submerged stress. Only in the final stage of submerged stress, a vacuum formed in the root with some organ disassembling. Under waterlogged condition the vascular bundle amount reduced, but was green. With the time of waterlogged stress prolonging, the amount of folium cell in stem declined and a vacuum formed in the root. Under drought condition, the dense stomas decreased and the upper epidermis keratinization, while the cortex thickness of root increased. The duct and vascular cylinder of L. hexandra was more sturdier than that of hydrophilic plant. All of the root length, root diameter, root project area and root volume increased under submerged condition, while only the root diameter significantly increased under waterlogged condition. In contrast, although the roots parameter was initially affected, it could recover the level of control with water stress being released. All in all, it was to useful understand the relation between L .hexandra and environment, especially how L. hexandra adapted to water stress.(3)In natural environment, the diurnal courses of air relative humidity (RH) surrounding leaves was higher both in June and in October, while the transpiration rate(Tr) was highest in April. The order of light use efficiency(LUE) was in October >in April >in June. The chlorophyll a (Chla) content in flooding condition was higher compared to the chlorophyll b (Chlb) content. The total chlorophyll content was higher compared to the control (0d) and ascended wavelike from the 0d in the period of submerged treatments. The Chla/Chlb ratio was 3:1 in natural environment, so it was under submerged condition. Not only the total chlorophyll content reduced under waterlogged condition, but also Chla/Chlb ratio steadily declined to lower value.(4) Both drought and flooding could result in the variation of photosynthetic character. Under submerged condition, the chlorophyll fluoresce parameters varied little, such as the coefficient of photochemical quantum(qP) and the effective photochemical quantum yield of PSⅡ(Yield).The photosynthesis of L. hexandra was not affected almost because its photosynthetic apparatus remained integrality by non-photochemical quantum (qN). But under waterlogged condition, the Yield values and qP values decreased obviously and the qP values was small. At the same time, the Yield values and qP values could recover the level of control with water stress being released, indicated that L. hexandra remained photosynthetic ability. Under drought condition, the Yield values and qP values first increased and then decreased, and the range of change was little. The qP values had a increasing trend. All results show that the physiological functions were not damaged because of qN avoiding the injury from superfluous light energy.(5) With the time of drought and waterlogged being prolonged, the activity of super-oxide dismutase (SOD) increased, higher 20% than control, but it was lower than control in the submerged treatments. Under water stress, the activity of peroxidase (POD) increased, and the order was the activity of peroxidase (POD) in leaf>that in stem>that in root. Especially under drought condition, the activity of peroxides (POD) in root was same as control. Furthermore the activity of cataloes (CAT) increased response to water stress, being 5th than control. As such the activity of dissocialitive praline (Pro) raised with drought stress. Comparation of the activity of dissociative proline(Pro) in roots,stems and leaves indicated it was different in the different parts. Pro value descended from on 40th days on the 30th days (highest), but higher than control. There was not significant difference between drought-dehydration and control. Under drought condition, all of the stem, root and leaf were affected by drought stress. Compared with stem and leaf, root was less injured from waterlogged condition. There was a system depressing the extent of peroxide and avoiding the damage resulting from overabundant moisture. (6)The endogenous hormones played an important role, which receipted the variation of internal or external environment, adjusted growth and remained normal state to resist water stress. The content of Eth first increased and then decreased under flooding condition, and the value under submerged condition was higher than that under waterlogged condition. In the early period of water stress, both the content of IAA and GA3 increased obviously. By contrast, the content of ABA first increased and then decreased, higher than control. With flooding stress being released, the content of IAA and ABA decreased, yet higher than control. The concentration of endogenous hormones was an important factor, whether promoting or restraining plant growth.(7)Usually plant owned the specific reproduce strategy to adopt all kinds of different environment in its life history. Without any pretreatments, the potentional livingness of L. hexandra seeds was near 69.5%, but germination rate was lower, only 3.8%. The average ratio of rhizomes with new bud appearance was 82.5% under the normal condition. Annual increases of tiller number and accumulated height of L. hexandra populations established in different years was significantly different. We supported that there was a physiologic dormancy and it was a perfect ways to reproduce L. hexandra by rhizomes germination.In conclusion, the plant adaptability to stress was determined by its traits and its surrounding environment. The systems of L. hexandra about photosynthesis, enzyme and physiological growth adjustment could vary along with water stress. After stress being released, the metabolize intensity could recovery the level of control. Furthermore, L. hexandra exhibited a higher reproductive effort that might be good at surviving and multiplying. In a word, L. hexandra was an ideal plant to conserve soil and water in the water-fluctuation belt such as rivers or lakes. For the development and application of L. hexandra, further studies on the mechanism of stress resistance, especially the transportation, anabolism and reciprocity of endogenous hormones by way of external hormones are needed. And study about the molecular mechanism and genetic basis is also needed.