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

Abstract

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.

Enhanced Salt Tolerance of Torreya grandis Genders Is Related to Nitric Oxide Level and Antioxidant Capacity

“…Decreases in total chlorophyll content of both female and male T. grandis were observed under salt treatment, suggesting that salt stress caused damage to photosynthetic pigments. Similar results were also reported in Mentha spicata and Oryza sativa ( Amri et al, 2011 ; Ounoki et al, 2021 ). However, compared with salt treatment, NO treatment in both female and male T .…”

Etioplasts are more susceptible to salinity stress than chloroplasts and photosynthetically active etio‐chloroplasts of wheat (Triticum aestivum L.)

“…Giza 168) on 600 mM KCl:NaCl (1:1) solution in the dark for 4,5 h resulted in swelling of the PT lumen and strong inhibition of greening, Chl accumulation and etioplast-to-chloroplast transformation (Abdelkader et al, 2007a(Abdelkader et al, , 2007b(Abdelkader et al, , 2010. Salt stress induced either swelling (e.g., Yamane et al, 2004Yamane et al, , 2008Ounoki et al 2021) or no swelling (e.g., Hasan et al, 2006;Hernández et al, 1995) of the intrathylakoidal space in the chloroplast. NaCl stabilized granum structure (Hasan et al, 2006;Ünnep et al, 2014), while KCl and KNO 3 treatment of isolated thylakoids resulted in shrinkage of the grana as revealed by SANS (Herdean et al, 2016).…”

“…Plant leaf segments were placed in glass tubes and submerged in liquid nitrogen. The 77 K fluorescence spectroscopic measurements and subsequent correction of the spectra were performed as in Ounoki et al (2021) using FluoroMax‐3 spectrofluorometer (Jobin‐Yvon Horiba) and SPSERV V3.14 software (copyright: C. Bagyinka, Biological Research Center, Hungary). At least 3 independent experiments were carried out, and at least 3 different plants were sampled in each experiment (n = 9–29).…”

“…Leaf sections (1 mm  1 mm) were cut from the middle of treated leaf segments. These sections were fixed, dehydrated, embedded, sectioned, stained and analyzed by transmission electron microscopy (TEM) as described in Ounoki et al (2021)…”

“…However, the accumulation of salt within cells may also disrupt several processes of plant metabolism, resulting in negative physiological alterations in plants, such as decrease in photosynthesis (Ounoki et al, 2021), and in ultrastructural changes (Hameed et al, 2021). Ultrastructural data in the literature are somewhat controversial: some authors observed the swelling of the thylakoid lumen in chloroplasts (Hasan et al, 2006; Ounoki et al, 2021; Rahman et al, 2000), while others do not report any alterations in plastid structure under salt stress (Hasan et al, 2006; Ounoki et al, 2021). However, even within the same species, depending on the type or length of treatment (Ounoki et al, 2021) or the studied cell type (Hasan et al, 2006), plastid structure was either affected or not affected by salt stress.…”