空气湿度与土壤水分胁迫对紫花苜蓿叶表皮蜡质特性的影响

选用2个抗旱性不同的紫花苜蓿品种,敖汉(强抗旱)和三得利(弱抗旱),设置空气湿度(45%-55%和75%-85%)和土壤水分胁迫(75%和35%田间持水量)处理,分析紫花苜蓿叶表皮蜡质含量、组分及晶体结构、气体交换参数、水势及脯氨酸含量的变化规律。结果表明,单独土壤水分胁迫时,紫花苜蓿叶表皮蜡质晶体结构及蜡质总量无显著变化;敖汉蜡质组分中烷类、酯类含量增加,醇类含量下降;三得利醇类含量下降,烷类、酯类含量变化不显著。低空气湿度胁迫时,两品种蜡质总量无显著变化,烷类和酯类含量显著增加,醇类含量显著下降,叶表皮片状蜡质晶体结构熔融呈弥漫性,扩大了对叶表面积的覆盖,其蒸腾速率显著低于正常湿度。复合胁迫处理时,叶表皮片状蜡质晶体结构继续呈弥漫性,烷类、酯类、未知蜡质组分含量均高于单独胁迫处理,醇类含量最低,而蜡质总量除三得利显著高于对照外,其余均无显著差异。紫花苜蓿叶表皮蜡质各组分含量(除醇类)及蜡质总量与光合速率呈显著负相关,与蒸腾速率无显著相关关系。蜡质总量与叶水势呈显著正相关。总体上,敖汉蜡质总量显著高于三得利,蜡质组分中烷类物质的增加有助于提高植株的抗旱性。在复合胁迫下,强抗旱品种主要通过气孔因素控制水分散失,而弱抗旱品种通过气孔和非气孔因素共同控制植物水分散失。

Effects of air humidity and soil water deficit on characteristics of leaf cuticular waxes in alfalfa (Medicago staiva)

Leaf cuticular waxes are the outmost of plant surface, protecting plants from disease, ultraviolet radiation, physical damage, and water loss. The current study aims to explore the mechanism of cuticular waxes in controlling water loss in response to environmental stresses under different air humidity (RH 45%-55% and 75%-85%) and soil water deficit (75% and 35% of soil water capacity). Two alfalfa (Medicago sativa) cultivars with different drought resistance, Aohan (high resistance) and Sanditi (low resistance), were selected as testing materials. A range of parameters, contents, constituents and crystalloid structure of leaf cuticular waxes, gas exchange indices, leaf water potential and proline content, were measured to assess the responses to different stresses. Results showed that alfalfa leaf surface was covered with a thick layer of vertically distributing wax crystalloid platelets with 14%-25% alkanes, 30%-60% alcohols, 8%-13% terpenes and 16%-29% unknown components. The wax crystalloid structure on leaf surface did not changed under soil water deficit, but melted and dispersed over leaf surface under low air humidity conditions, resulting in a significant reduction of water loss due to decreased transpiration rate. Soil water deficit had no effect on the content of total leaf cuticular waxes, but significantly increased the contents of alkanes and terpenes, and decreased the contents of alcohols in Aohan. In contrast, soil water deficit decreased the contents of alcohols, but no effect on the contents of alkanes and terpenes in Sanditi. Low air humidity increased the contents of alkanes and terpenes and decreased the contents of alcohols in both cultivars. When two stresses applied together (low air humidity and soil water deficit), both cultivars had the highest contents of alkanes, terpenes and unknown wax components, and the lowest content of alcohols. The contents of total leaf cuticular waxes remained unchanged except for Sanditi where content of total waxes was higher. Correlation analysis showed that the leaf photosynthesis rate was negatively correlated with the contents of total leaf cuticular waxes, alkanes, terpenes, and unknown components. The contents of total cuticular waxes were positively correlated with leaf water potential, indicating that waxes coverage on leaf surface played important role in keeping relatively higher water potential to adapt to water loss under drought conditions. The total wax content in leaves of Aohan was significantly higher than that of Sanditi as expected. It appeared that the increase of alkanes content in stressed plants was related to plant drought resistance. Under the conditions of low air humidity and soil water deficit, Aohan mainly was relying on stomatal adjustment (with wax) to control water loss, while Sanditi was depending upon both stomatal and non-stomatal (with osmotic adjustment materials) factors to control water loss. It is suggested that leaf cuticular waxes could be used as an index in selecting and breeding drought tolerant alfalfa cultivars.