干旱-复水对桂西北喀斯特地区青冈栎叶片光合能力、叶绿素荧光和显微结构的影响

为了探讨喀斯特地区适生种青冈栎幼苗对“干旱－复水”环境的适应机制，以当年生青冈栎实生苗为材料，通过盆栽控水试验，研究了4 种土壤干旱胁迫强度[对照（–0.1 MPa）、轻度干旱（–0.5 MPa））、中度干旱（–0.9 MPa）和重度干旱（–1.5 MPa）]及复水处理对叶片的水分状况、光合、叶绿素荧光和解剖结构参数的影响。结果表明：（1）随干旱胁迫加剧，叶片相对含水率、水势、净光合速率（Pn）、蒸腾速率（Tr）、气孔导度（Gs）和胞间CO2浓度（Ci）均显著降低，而气孔限制值（Ls）显著增加；轻度胁迫下各光合参数以及轻中度胁迫下瞬时水分利用效率（WUE）均不受显著影响。复水后，各干旱处理叶片水分参数、Pn、Tr、Gs、Ci、WUE均比复水前提高，Ls比复水前降低；轻度胁迫复水后叶片水分和光合参数均优于对照，中度胁迫仅Ls未恢复到对照，重度胁迫复水后叶片水分和光合参数均未恢复。（2）随干旱胁迫加剧，叶片初始荧光（Fo）显著增加，而最大荧光（Fm）、最大光化学量子产量（Fv/Fm）和潜在光化学效率（Fv/Fo）均显著下降，且在轻度胁迫下均与对照显著差异。复水后，各干旱胁迫Fm、Fv/Fm和Fv/Fo比复水前提高，而Fo均略低于复水前，轻度胁迫复水后各叶绿素荧光参数均恢复到或优于对照，中度和重度胁迫复水后Fo未恢复到对照，且重度胁迫复水后Fv /Fm仅为0.75。（3）随干旱胁迫加剧，叶片厚度、上下表皮厚度、气孔密度、主脉导管直径均显著增加，叶片气孔器长度、宽度、开口面积、海绵组织厚度均显著降低，而栅栏组织厚度、栅海比和主脉厚度均表现为中度＞轻度＞对照＞重度。复水后，仅各干旱胁迫处理的气孔开口面积和主脉厚度比复水前显著提高；轻度胁迫复水后叶片结构参数也均恢复到或优于对照，中度胁迫复水后气孔开口面积仍显著低于对照，重度胁迫复水后气孔开口未能恢复打开，主脉厚度也低于对照。因此，青冈栎幼苗有耐旱性和旱后恢复能力，适合作为喀斯特地区的生态恢复树种，但在青冈栎幼苗抚育阶段应免受中度以上干旱胁迫（–0.9 MPa），以利于其旱后恢复生长。

Effects of drought-rehydration on photosynthetic capacity, chlorophyll fluorescence and microstructure of Cyclobanopsis glauca leaves in karst area of northwest Guangxi

In order to explore the adaptation mechanism of Cyclobanopsis glauca seedlings to "drought-rehydration" environment in karst area, using the current growth seedlings of C. glauca as materials, the pot water control experiment was carried out. The effects of four soil drought stress treatments (control (–0.1 MPa), mild drought (－0.5 MPa)), moderate drought (–0.9 MPa) and severe drought (–1.5 MPa)) and rehydration treatments on water status, photosynthesis, chlorophyll fluorescence and anatomical structure parameters of leaves were studied. Results show that: (1) With the increase of drought stress intensity, leaf relative water content, water potential, net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs) and intercellular CO2 concentration (Ci) were significantly decreased, and stomatal limitation (Ls) increased significantly. Photosynthetic parameters under mild stress and instantaneous water use efficiency (WUE) under mild and moderate drought stress were not significantly affected. After rehydration, leaf water parameters, Pn, Tr, Gs, Ci and WUE under each drought stress treatment were were higher than those before rehydration, while Ls was lower than those before rehydration.. The leaf water conditions and photosynthetic parameters under mild drought stress were significantly better than those of the control after mild stress rehydration, only Ls under moderate drought stress did not recover to the level of the control after rehydration, and neither leaf water content nor photosynthetic parameters under severe drought stress recovered to the level of the control after rehydration. (2) With the increase of drought stress intensity, the initial fluorescence (Fo) of leaves increased significantly, while the maximum fluorescence (Fm), maximum photochemical quantum yield (Fv/Fm) and potential photochemical efficiency (Fv/Fo) of leaves decreased significantly. There were significant differences in these parameters between mild drought stress and control. After rehydration, Fm, Fv/Fm and Fv/Fo under each drought stress were higher than those before rehydration, whereas Fo was slightly lower than that before rehydration. All the chlorophyll fluorescence parameters after rehydration under mild stress recovered to or better than those of the control. Fo under moderate and severe drought stress did not return to the control level after rehydration, and Fv/Fm under severe stress was only 0.75 after rehydration. (3) With the increase of drought stress intensity, leaf thickness, upper and lower epidermis thickness, stomatal density, and main vein conduit diameter increased significantly, length and width of stomatal apparatus, stomatal opening area and sponge tissue thickness decreased significantly, while palisade tissue thickness, ratio of palisade tissue thickness to spongy tissue thickness and main vein thickness were moderate > mild > control > severe drought stress. After rehydration, only the stomatal opening area and the thickness of main leaf veins of each drought stress increased significantly compared with that before rehydration. After rehydration, the leaf structural parameters under mild drought stress were restored to or better than those in the control. The stomatal opening area after rehydration in moderate drought stress was still significantly lower than that in the control group. After rehydration, the stomatal opening area in severe drought stress failed to recover, and the thickness of main veins in severe drought stress was lower than that in the control. Therefore, the seedlings of C. glauca have drought tolerance and post-drought recovery ability, which is suitable for ecological restoration tree in karst area, but the seedlings should be protected from moderate drought stress (-0.9mpa) in the nurturing stage, so as to facilitate their post-drought recovery.