Physiological responses in Neolitsea sericea seedlings to drought stress

During development process of the Zhoushan archipelago, the original old-growth broadleaved forests on the islands have been severely destroyed due to intensive human activity. Neolitsea sericea (Lauraceae), distributed on a few islands of the Zhoushan archipelago, is facing the danger of extinction due to rapid degradation and destruction of original habitats. So far, plenty of studies on N. sericea have been focused on its population genetic diversity and genetic structure. Neolitsea sericea is well adaptable to the environment, thus can survive in the ravine on the islands. The strong tolerance of N. sericea to drought, wind, salt and barren soil also makes it optimal species for afforestation. However, few studies on drought tolerance of N. sericea have been reported yet.Plant response to drought stress at a molecular level is a complex biological process, involving consideration of the stress effects and regulation events. Thus, it is of great importance to analyze the underlying mechanism systematically at the physiological level.In this study, one-year-old seedlings of N. sericea were selected as test materials. To obtain a more complete physiological mechanism in responses to drought stress, N. sericea was exposed under the conditions of four different relative water contents (normal water supply, light drought, moderate drought and severe drought) in soil for 45 days. The relative water content in soil of normal water supply, light drought, moderate drought, and severe drought was controlled in 75%-80%, 55%-60%, 40%-45% and 30%-35% of field capacity, respectively. All plants were rehydrated after drought treatments by normal water supply for 10 d. The relative water content in soil was controlled by pot- weighing method through the experiment. The dynamic changes of the physiological and photosynthetic traits in leaves were measured every 15 days of stress treatments and at the end of rehydration. The traits measured in this study include net photosynthetic rate (Pn), stomatal conductance (Gs), water use efficiency (WUE), intercellular CO2 concentration (Ci), transpiration rate (Tr), total content of chlorophyll (Chl), superoxide dismutase (SOD) and peroxidase (POD) activities, proline content, malondialdehyde (MDA) content, soluble sugar content and relative conductivity (Rc).The results showed that the contents of Chl, Pn, Tr and Gs decreased constantly and significantly during severe drought, and the content of Ci decreased in 0-30 d but enhanced in 30-45 d during the severe drought, which indicated that the decrease of Pn was caused by stomatal limitation during the 0-30 d of severe drought, and by non-stomatal limitation during 30-45 d of severe drought. The continuous inhibition of Tr after rehydration from severe drought suggested the afunction of stomatal regulation. The drought stress increased the overall WUE of the leaves, although the WUE dropped significantly in 30-45 d of severe drought. The trend of MDA content, SOD and POD activities in N. sericea first increased and then decreased during the drought treatments, indicating the production and clearance of MDA had reached a dynamic balance. The significant increase of Rc indicated the elevation of cell membrane permeability. The overexpression of soluble sugar and proline in leaves during 30-45 d of drought treatment resulted in the reduction of Rc, which relieved the osmotic pressure in N. sericea. However, Rc staying at a high level after rehydration from severe drought indicated the damage of cell membrane.In conclusion, N. sericea can conduct the instant stomatal regulation, eliminate active oxygen and regulate the osmotic pressure effectively, decrease the Tr and MDA content, increase the cell membrane permeability and enhance the WUE of leaves, to reduce the damage under the drought stress. However, severe and persistent droughts lead to irreversible damage in leaves, such as the afunction of stomatal regulation and damage in cell membrane.