土壤干旱胁迫对沙棘叶片光合作用和抗氧化酶活性的影响

Effects of soil drought stress on photosynthetic characteristics and antioxidant enzyme activities in Hippophae rhamnoides Linn.seedings

在半干旱黄土丘陵区,采用盆栽控水试验,通过测定3年生沙棘苗木在8个土壤水分梯度下的光合速率、叶绿素荧光、抗氧化酶活性等光合生理生化指标,探讨沙棘叶片光合作用在土壤干旱逐渐加重过程中的变化过程、机制及其与土壤水分的定量关系。结果表明:(1)土壤相对含水量(RWC)在38.9%—70.5%范围内,随干旱胁迫加重,沙棘的净光合速率(Pn)、气孔导度(Gs)和胞间CO2浓度(Ci)明显下降,而气孔限制值(Ls)显著上升,即Pn下降主要是由气孔限制造成的;当RWC<38.9%时,干旱胁迫继续加剧,Pn和Ls都降低,而Ci显著上升,即Pn下降的主要原因已经转变为非气孔因素的限制。(2)土壤适度水分胁迫能够提高沙棘叶片的水分利用效率(WUE),维持沙棘Pn和WUE处于较高水平的RWC范围为58.6%—82.9%和48.3%—70.5%。(3)土壤干旱加重过程中,沙棘的最大荧光(Fm)、PSⅡ最大光化学效率(Fv/Fm)、PSⅡ实际光化学效率ΦPSⅡ,光化学猝灭(qP)均表现出逐渐降低的趋势,而初始荧光(Fo)显著升高,非光化学猝灭(NPQ)则表现出先上升后下降的趋势;RWC在38.9%—70.5%的范围内,热耗散是沙棘重要光保护机制;RWC<38.9%时,PSⅡ受到损伤,电子传递受阻。(4)土壤干旱加重过程中,沙棘叶片的超氧化物歧化酶(SOD)、过氧化物酶(POD)、过氧化氢酶(CAT)活性表现出先升高后降低的趋势,丙二醛(MDA)含量则表现出逐级递增趋势;土壤干旱程度在RWC为48.3%—70.5%时,对沙棘叶片的抗氧化酶系统活性有诱导作用;而土壤干旱到严重胁迫(RWC<38.9%)时,沙棘叶片的抗氧化酶系统损伤,抗氧化酶活性下降,细胞膜遭到破坏。土壤干旱程度在RWC为48.3%—70.5%时,沙棘叶片可以通过热耗散和酶活性调节协同作用,稳定光合机构的正常功能,Pn下降的主要原因是气孔限制;而干旱到严重胁迫(RWC<38.9%)时,PSⅡ系统和抗氧化酶系统损伤,是光合作用发生非气孔限制的主要原因。在半干旱黄土丘陵区,沙棘生长所允许的最大土壤水分亏缺在RWC为38.9%,维持沙棘具有较高WUE和Pn的土壤水分阈值为RWC在58.6%—70.5%之间。

Water deficit is one of the major limiting factors in vegetation recovery and reconstruction in loess hilly region in semi-arid area. Hippophae rhamnoides Linn. is one of the common trees grown in this region. However, the mechanisms for the decline of photosynthesis in leaves of Hippophae rhamnoides Linn. under water stress are not very clear. The objective of this study is to investigate the processes and mechanisms of drought stress on photosynthesis and the relationship between photosynthesis and soil water content, which will help us to further understand the photosynthetic characteristics of Hippophae rhamnoides Linn. under Water stress and to guide cultivation in the loess hilly region. The gas exchange, chlorophyll fluorescence and antioxidant enzyme activities in leaves of three-year-old Linn. grown in pots were tested under 8 different soil water treatments. The results showed that： （1） When relative soil water content （RWC） was 70.5%, the net photosynthetic rate （Pn） was maximal. When RWC was between 38.9% and 70.5%, Pn, stomatal conductance （ Gs ） and intercellular CO/concentration （ Ci） significantly decreased with increasing drought stress, meanwhile, stomatal limitation （Ls） significantly increased, indicating that stomatal limitation was responsible for reduction in Pn. When RWC was lower than 38.9% , P. and Ls decreased and Ci increased with increasing drought stress, showing that the main reason for restricting Po had turn into non-stomatal limitation. Turning point of RWC from stomatal to non- stomatal limitation was 38.9% for leaf photosynthesis. （2） Moderate water stress can improve the water use efficiency （WUE）. When RWC was between 58.6% and 82.9%, Pn was maintained at a high level, and when RWC was between 48.3% and 70.5% , the WUE was maintained high. （ 3 ） The maximal fluorescence （ Fm ） , maximal photochemical efficiency（ Fv/Fm ） of PSII, actual photochemical efficiency （~PsD） and photochemical quenching （qP） all showed declining trends under continuous drought stress, while minimal fluorescence （Fo） increased significantly, non photochemical quenching （NPQ） increased and then decreased. When RWC was between 38.9% and 70.5%, thermal energy dissipation was the significant photoprotective mechanism. And RWC lower than 38.9% caused the inhibition of photosynthetic electron transport and damage of PSII. （ 4 ） The activities of superoxide dismutase （ SOD ）, peroxidase （POD） and catalase （CAT） increased and then decreased with the deepening of drought stress, while malondialdehyde （MDA） contents increased continuously. These showed that the activities of the antioxidant enzyme were activated in the region where SWC were from 48.3% to 70.5%. Under severe water stress was damaged, the activities of the antioxidant enzymes declined and cell （ RWC〈38.9% ）, the antioxidant enzyme system membrane was destroyed. These results implied that： the function of photosynthetic apparatus in Hippophae rhamnoides Linn. seedling leaves was maintained by a synergistic effect of thermal energy dissipation and regulating the antioxidant enzymatic activity jointly in the region where SWC were from 48. 3% to 70. 5%, and the decreased Pn were mainly due to stomatal limitations through stomatal regulation. Major non-stomatal limitation responsible for reduction in P. was associated with the impairment of PS I1 and antioxidant enzyme system under severe water stress （RWC〈38.9%）. RWC 38.9% was the soil water maximum deficit level allowed in the semiarid loess hilly region for Hippophae rhamnoides Linn. , and RWC from 58.6% to 70.5% was economy water threshold value maintaining the largest WUE and the higher Pn