以海岛树种舟山新木姜子(Neolitsea sericea)的一年生幼苗为材料,采用盆栽控水模拟干旱胁迫处理,研究苗木叶片在轻度、中度和重度干旱胁迫以及复水(土壤含水量分别为田间持水量的55%~60%、40%~45%、30%~35%和75%~80%)后的光合特性、保护酶活性、渗透调节物质含量等生理指标随时间的动态变化.结果表明:在中度及重度干旱胁迫下叶片的总叶绿素含量、净光合速率、蒸腾速率及气孔导度明显下降,而在重度干旱胁迫下叶片的胞间CO2浓度呈现先降低后升高的趋势,说明在重度干旱胁迫中期以及中度干旱胁迫期间,叶片净光合速率的下降主要受到气孔限制因素的影响,而在重度干旱胁迫后期则是受到非气孔限制因素的影响.复水后蒸腾速率仍处于较低水平,叶片的气孔调节功能有所丧失.干旱胁迫提高了叶片的水分利用效率,但在重度干旱胁迫下叶片的水分利用效率从胁迫中期开始呈明显的下降趋势.在中度及重度干旱胁迫下舟山新木姜子叶片的丙二醛含量、超氧化物歧化酶及过氧化物酶活性均呈先升高后降低的趋势,说明丙二醛的产生与清除达到了一个动态平衡.干旱胁迫使叶片细胞膜透性显著增加,叶片通过提高合成可溶性糖和脯氨酸缓解渗透压,使叶片细胞膜透性在干旱胁迫中后期逐渐降低;但在重度干旱胁迫下叶片的细胞膜透性在复水后仍保持在较高水平,表明细胞膜遭受到了一定的损伤.综上所述,在干旱胁迫下舟山新木姜子能快速调节叶片气孔,清除活性氧,进行渗透调节,降低蒸腾速率、丙二醛含量,增加细胞膜透性,提高水分利用效率,从而减少干旱带来的损伤,但是持久、严重的干旱使叶片的气孔调节功能丧失,细胞膜受损.%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.
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