Data_Sheet_1_Salt Stress Affects Plastid Ultrastructure and Photosynthetic Activity but Not the Essential Oil Composition in Spearmint (Mentha spicata L. var. crispa “Moroccan”).docx

High levels of soil salinity affect plant growth, reproduction, water and ion uptake, and plant metabolism in a complex manner. In this work, the effect of salt stress on vegetative growth, photosynthetic activity, and chloroplast ultrastructure of spearmint (Mentha spicata L. var. crispa “Moroccan”) was investigated. After 2 weeks of low concentration treatments (5, 25, and 50 mM NaCl) of freshly cut shoots, we observed that the stem-derived adventitious root formation, which is a major mean for vegetative reproduction among mints, was completely inhibited at 50 mM NaCl concentration. One-week-long, high concentration (150 mM NaCl) salt stress, and isosmotic polyethylene glycol (PEG) 6000 treatments were compared in intact (rooted) plants and freshly cut, i.e., rootless shoots. Our data showed that roots have an important role in mitigating the deleterious effects of both the osmotic (PEG treatment) and specific ionic components of high salinity stress. At 50 mM NaCl or above, the ionic component of salt stress caused strong and irreversible physiological alterations. The effects include a decrease in relative water content, the maximal and actual quantum efficiency of photosystem II, relative chlorophyll content, as well as disorganization of the native chlorophyll-protein complexes as revealed by 77 K fluorescence spectroscopy. In addition, important ultrastructural damage was observed by transmission electron microscopy such as the swelling of the thylakoid lumen at 50 mM NaCl treatment. Interestingly, in almost fully dry leaf regions and leaves, granum structure was relatively well retained, however, their disorganization occurred in leaf chloroplasts of rooted spearmint treated with 150 mM NaCl. This loss of granum regularity was also confirmed in the leaves of these plants using small-angle neutron scattering measurements of intact leaves of 150 mM NaCl-stressed rooted plants. At the same time, solid-phase microextraction of spearmint leaves followed by gas chromatography and mass spectrometry (GC/MS) analyses revealed that the essential oil composition of spearmint was unaffected by the treatments applied in this work. Taken together, the used spearmint cultivar tolerates low salinity levels. However, at 50 mM NaCl concentration and above, the ionic components of the stress strongly inhibit adventitious root formation and thus their clonal propagation, and severely damage the photosynthetic apparatus.

土壤高盐浓度会以复杂的方式影响植物生长、繁殖、水分与离子吸收以及植物代谢。本研究探究了盐胁迫对摩洛哥卷叶型留兰香薄荷（Mentha spicata L. var. crispa "Moroccan"）营养生长、光合活性以及叶绿体超微结构的影响。对新鲜切段嫩梢开展为期2周的低浓度（5、25、50 mM 氯化钠（NaCl））处理后，我们观察到：作为薄荷类植物营养繁殖的主要途径，茎部不定根的形成在50 mM NaCl处理下完全受到抑制。我们对完整生根植株与新鲜切段（无根）嫩梢分别进行了为期1周的高浓度（150 mM NaCl）盐胁迫及等渗聚乙二醇（PEG）6000处理，并对两组材料开展对比分析。实验数据表明，根系在缓解高盐胁迫带来的渗透胁迫（PEG处理模拟）与特异性离子胁迫两种有害效应中均发挥了关键作用。当NaCl浓度达到50 mM及以上时，盐胁迫中的离子组分将引发强烈且不可逆的生理变化，具体包括相对含水量、光系统II的最大与实际量子效率、相对叶绿素含量的下降，以及通过77 K荧光光谱法检测到的天然叶绿素-蛋白复合物解离。此外，通过透射电子显微镜可观察到显著的超微结构损伤，例如50 mM NaCl处理下类囊体腔发生肿胀。值得注意的是，在近乎完全干燥的叶片区域与叶片中，叶绿体基粒结构仍能较好保留；但在经150 mM NaCl处理的生根留兰香薄荷（spearmint）叶片叶绿体中，基粒结构发生了解离。通过对150 mM NaCl胁迫下生根植株的完整叶片进行小角中子散射检测，我们进一步证实了此类基粒规整性的丧失。与此同时，对留兰香薄荷叶片开展固相微萃取后，再进行气相色谱-质谱联用（GC/MS）分析，结果显示本研究采用的各类处理并未改变留兰香薄荷的精油成分组成。综合来看，本研究所用的留兰香薄荷品种能够耐受低浓度盐胁迫。但当NaCl浓度达到50 mM及以上时，胁迫中的离子组分将强烈抑制不定根形成，进而阻碍其无性繁殖，并对光合机构造成严重损伤。