模拟喀斯特生境条件下干旱胁迫对青冈栎苗木的影响

为了研究喀斯特"双层"地质结构对植物的影响,以及在干旱环境下,喀斯特地区植物的生理变化与适应策略,建立"土壤层-岩石(石灰岩)层-岩溶水层"水分供应分层模拟柱,对‘土壤层’设置不同水分梯度,种植青冈栎(Cyclobalanopsis glauca)苗木进行干旱胁迫试验,测定青冈栎的叶面积、比叶面积、生物量、渗透调节物质含量、相对叶绿素含量。结果表明:在模拟柱‘岩溶水层’加水条件下,植物根系能下扎至‘岩溶水层’,‘土壤层’干旱胁迫对青冈栎的生理变化没有产生影响;在‘岩溶水层’无水条件下,青冈栎生理变化受‘土壤层’干旱胁迫影响显著,‘土壤层’水分含量越低,青冈栎的叶面积、生物量、叶绿素含量、叶片相对含水量越小,青冈栎干鲜比、根冠比、可溶性糖、丙二醛和脯氨酸含量越高。青冈栎幼苗利用岩溶水层水分是适应喀斯特干旱环境的重要策略。

Effects of drought stress on Cyclobalanopsis glauca seedlings under simulating karst environment condition

The bare karst terrain in South China is one of the largest and most densely-populated tropical karsts in the world. It is distinguished for its harsh natural conditions. Most karst areas have a characteristic "double layer" geological structure; the soil layers and fissures formed after prolonged intensive karstification cause a loss of surface water and subsequent desiccation. Seasonal drought is the main feature of karst ecosystems and is a major constraint upon agricultural production and vegetation restoration. In the South China karst region, particularly the bare-rock karst areas, plants have evolved adaptations to the rocky and xeric conditions of karst soils; the dominant plants commonly have robust root systems. One such plant, Cyclobalanopsis glauca, is an evergreen broadleaf species and a dominant species in the Guilin karst forest. These plants survive well in higher mountain elevations, even in very thin and rocky soils, and karst forests can grow under seasonal drought conditions. We set up a water supply device in combination with a simulated karst structure comprising soil, limestone, and epikarst layers. The setup had a diameter of 100 cm. Soil, limestone, and epikarst layers had heights of 30, 50, and 20 cm, respectively. Epikarst layer had two treatments-watered and unwatered-and the water content of the soil layer was controlled within 30%-40%, 50%-60%, 70%-80%, or 90%-100%. Each treatment was replicated three times. We investigated the effects of this karst structure on the growth, leaf area (LA), specific leaf area (SLA), biomass, osmotic solutes, SPAD value, root-shoot ratio, leaf relative water content (RWC), and ratio of dry weight to fresh weight (RDF) of 2-year-old seedlings of C. glauca that were transplanted from a nursery into the simulated karst substrate. The controlled soil water treatments lasted for 4 months, after which the plant physiological parameters were tested. Results showed that plant roots could extend into the epikarst layer, and seedling growth was not affected by topsoil drought stress when water was added to the epikarst layer. When the epikarst layer had no water, the physiology of the seedlings was greatly influenced by the soil water content (SWC). When the topsoil underwent drought, LA, RWC, and SPAD decreased, while RDF and root-shoot ratio increased. Malondialdehyde (MDA), soluble sugars, and proline are important osmotic solutes; they are products of lipid peroxidation and the most prominent indicators of oxidative stress in plants. Compared with well-watered treatments and treatments watered in the epikarst layer, MDA, soluble sugars, and proline content increased significantly (analysis of variance, Duncan's test; P<0.01) in the leaves of unwatered C. glauca seedlings, suggesting that they suffered drought stress. The results also showed that using water in the epikarst layer was an important adaptive strategy for C. glauca seedlings in drought-stricken karst soils. In this study, we also found that the seedling roots could penetrate through the rock layer to reach the epikarst layer when the latter was watered. However, no roots penetrated this layer when it was not watered. The results suggested that ground water in the epikarst layer plays an important role in the growth and survival of C. glauca seedlings and in their water use.