Browsing by Subject "Salzstress"

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Aspects of stomatal physiology during salt-stress-related disturbances of ion homeostasis
(2020) Franzisky, Bastian Leander; Zörb, Christian
Soil salinity is a major challenge for agriculture, because most crop plants are sensitive to high salt concentrations in soil, an environment that results in reduced growth and yield. One major constraint imposed by salinity is the disruption of ion homeostasis attributable to the uptake competition of salts and nutrients and the accumulation of deleterious ions, which are toxic to plants at high concentrations. For a better understanding of ion-homeostasis-associated traits contributing to salt tolerance in salt-sensitive crops, such as Vicia faba and Zea mays, the capabilities of ion exclusion and tissue tolerance were assessed in diverse genotype selections under saline conditions. In addition, the impact of increased salt ion concentrations in leaves and in the apoplast on stomatal physiology and guard cell integrity was characterized in V. faba exposed to long term salinity in order to improve our knowledge of stomatal physiology and functioning under conditions of NaCl stress. The treatment of diverse V. faba varieties with 100 mM NaCl demonstrated that ion homeostasis-associated tolerance mechanisms are differentially managed for Na+ and Cl-. The longer-withstanding varieties were tolerant to the accumulation of Na+ suggesting that tolerance to Na+ predominantly occurred at the level of tissue tolerance after Na+ had entered the leaves. Conversely, tissue tolerance for Cl- was weak throughout all varieties suggesting that the tolerance to Cl- was facilitated instead by the restriction of the intrusion of Cl- into the plant’s shoots; this process might be crucial for the ability of V. faba to withstand NaCl salinity. The treatment of diverse Z. mays hybrids with mild and high doses of Cl- added to the soil revealed that most genotypes restricted Cl- root to shoot translocation. This suggests that Z. mays effectively prevents Cl- from entering the xylem and, thus, the acropetal transport of Cl-, thereby hindering harmful Cl- accumulations building up in the photosynthetically active leaf blades. A detailed analysis of guard cell physiology under long-term NaCl demonstrated that guard cell primary metabolism differentially responds to altered ion composition resulting from salt stress in comparison with whole leaf tissue in V. faba; such a differential response might be a prerequisite for the maintenance of guard cell functionality under conditions of stress, i.e. the adjustment of guard cell turgor that affects stomatal aperture and water loss. Moreover, the shift from a photoperiod dependent accumulation of sucrose in guard cells and the apoplast to a photoperiod independent under salinity suggests that a metabolic sucrose-mediated feedforward mechanism is involved in coordinating stomatal closure under conditions of long term NaCl and might be beneficial for reducing water loss under conditions of stress related carbon partitioning. In summary, this work shows that ion-homeostasis associated tolerance traits vary between crop species and that the differential metabolic acclimatisation of guard cells to disturbed ion homeostasis might represent an important aspect of tissue tolerance enabling the maintenance of stomatal regulation during long term salinity.
Physiological and metabolic adaptation of Beta vulgaris and Suaeda maritima to salinity and hypoxia
(2022) Behr, Jan Helge; Zörb, Christian
Soils with high salinity are often also affected by waterlogging with hypoxic conditions in the root zone, which severely reduces plant growth and crop yield. The combination of salinity and hypoxia generates an intense stress for the plant: On the one hand, hypoxic conditions at the root level cause a severe energy deficit due to the inhibition of oxidative phosphorylation, on the other hand, energy-consuming tolerance mechanisms have to be maintained to cope with salt stress. To better understand the tolerance mechanisms to combined saline and hypoxic conditions, the metabolic and physiological adaptation capacity of the model halophyte Suaeda maritima, typically found in flooded saline soils, and the closely related sugar beet (Beta vulgaris L.) were analysed. Salt tolerant plants are characterised by their ability to tolerate high Na+ and Cl- concentrations without being damaged by ion toxicity. The basis of this tolerance is primarily osmotic adaptation, the compartmentalisation of ions in cell organelles and the ability to replace K+ with Na+ in important cellular processes. Li+ has similar physico-chemical properties to Na+ and K+, but forms complexes with organic and inorganic anions more readily than other alkali metals. Therefore, Li+ can displace metals during the uptake and translocation by the plant and at enzymatic binding sites, which impairs enzyme activity and can lead to toxic effects. The effects of different cations with similar physicochemical properties on their accumulation pattern at high and low osmolarity were investigated to determine whether Li+ toxicity could be mitigated by competitive uptake of K+ and Na+. Hydroponic culture experiments with increasing salt concentration demonstrated the ability of S. maritima and B. vulgaris to tolerate high salt concentrations by maintaining ion homeostasis and high tissue tolerance to Na+ accumulation. An increased Na+/K+ ratio under hypoxic conditions indicates that an energy shortage caused by oxygen depletion in the root impairs Na+ exclusion and K+ uptake, thereby increasing the ionic imbalance under hypoxic conditions. The metabolic profile showed a tissue-specific response to salinity and hypoxia: The root metabolism is mainly influenced by hypoxia, inhibiting oxidative phosphorylation, while at the same time glycolysis is enhanced to maintain ATP production. The enhanced accumulation of amino acids and TCA cycle intermediates suggests that a partial flow of the TCA cycle fuelled by the GABA shunt may play a crucial role in the recovery of reduction equivalents for ATP production by glycolysis, thereby sustaining energy-intensive cellular processes under hypoxic conditions. As a consequence to the high Na+ accumulation in the shoots, the metabolic profile of young and mature leaves is mainly influenced by salt stress, which triggers the accumulation of compatible solutes for osmotic adjustment and ROS scavenging mechanisms. To achieve tolerance to high salinity, energy consumption rises. Hence, the biomass increase of B. vulgaris stagnates at 200 mM NaCl. In contrast, S. maritima shows its optimal growth at the same salinity range, which reflects the higher adaptability of the halophyte to saline conditions. Different mechanisms in the shoot and root lead to an accumulation of proline, which contributes to the increased tolerance to combined salinity and hypoxia, as proline stabilises membranes and proteins under salt stress and scavenges increased ROS formation induced by hypoxia. High ion accumulation in combination with hypoxic conditions enhances ROS formation in the shoots, leading to light-induced pigment degradation in S. maritima, which is mitigated by enhanced proline biosynthesis in the chloroplasts. In contrast, proline accumulation in the root is not exclusively the result of enhanced proline biosynthesis, but of inhibited proline degradation due to the low availability of reduction equivalents when salinity and hypoxia are combined. The accumulation of Li+ is relatively low in comparison to Na+ and K+, as B. vulgaris strongly limits the Li+ uptake via the transpiration stream to avoid toxic Li+ concentrations in the leaves. High concentrations of Li+ combined with Na+/K+, increase Li+ accumulation in leaves and cause growth inhibition as well as the formation of necrotic tissue, indicating low tissue tolerance to Li+ and severe stress. The application of equimolar concentrations of Na+ and K+ has no effect on Li+ accumulation and ion toxicity, suggesting that Li+ uptake is independent of Na+ and K+ cation channels and that Li+ toxicity is not mainly caused by the displacement of K+ at enzymatic binding sites.
Physiological mechanisms and growth responses of sweet potato subjected to salinity
(2023) Mondal, Shimul; Asch, Folkard
For the development of salt-tolerant sweet potato varieties, either through breeding or biotechnology, an appropriate salinity screening tool is necessary for the identification of tolerant or sensitive genotype. Our overall objectives for this study were to develop a suitable, reliable and rapid salinity screening tool in view of salt tolerance mechanism in sweet potato under salinity. To better understand the tolerance mechanisms; leaf level ion uptake and distribution patterns by transpirational water loss and leaf level ROS scavenging antioxidant enzyme activities were evaluated under salinity. Additionally, different ion extraction methods were tested which will contribute to the development of reliable salinity screening tool in sweet potato genotypes. All the experiments were conducted in the greenhouse and VPD (vapor pressure deficit) chambers of the Hans-Rutenberg Institute of Tropical Agricultural Sciences, University of Hohenheim, Germany, in a hydroponic system. Twelve genotypes of sweet potato were collected from Bangladesh Agricultural Research Institute (BARI) and used to evaluate salt thresholds with salt tolerance mechanisms for a wide range of salinity levels (0, 50, 100, and 150 mM NaCl). First, genotypic thresholds were determined for 12 sweet potato genotypes exposed to salinity, whereupon it was found that 75 mM root zone salinity (NaCl) was the threshold for sweet potato. The genotypic threshold was estimated from the dry matter accumulation that began to decrease under the influence of salinity. It was found that genotypic thresholds were negatively linearly correlated with the difference between tissue K content at 75 mM NaCl and tissue K content at controlled salinity in the root zone. This information is very important for identifying the salt tolerant and sensitive genotype of sweet potato. Second, the uptake and distribution of Na, K, and Cl ions by transpiration, across different-aged leaves, were studied to better understand the mechanisms of salt tolerance in sweet potato. Two different sweet potato genotypes were subjected to salt stress of 0 and 50 mM NaCl in artificially dry (VPD 2.27 kPa) and humid (VPD 0.76 kPa) chambers. We found that cumulative water loss per unit leaf area was twice as high at a VPD of 2.27 kPa, but Na uptake remained the same. No relationship was observed between water loss from individual leaves and Na or Cl uptake. About 30% more Na was distributed in the petioles of salt tolerant genotype compared to leaf blades, while the opposite was observed in salt sensitive sweet potato genotype and VPD had no effect on Na distribution. Third, the activities of ROS scavenging antioxidant enzymes were evaluated with respect to different leaf age, in two different genotypes of sweet potato under 100 mM salinity. In general, antioxidant enzymes in sweet potato do not respond to salt stress but are altered by the effects of leaf position, leaf age, duration of stress, and genotype. No effect of Na on antioxidant enzyme activities was found under salt stress in sweet potato leaves. However, the significant positive correlation between K concentration and the level of SOD (super oxide dismutase) in older leaves suggests that SOD contributes to the maintenance of a high K concentration to protect photosynthetic activity. In summary, this study shows that sweet potato responds differently to salinity depending on the genotype, and that the threshold beyond which yield decreases is 75 mM NaCl. Genotypic threshold strongly linked to high tissue K content under increasing salinity that suggests a salt tolerance mechanisms in sweet potato. Salt-tolerant sweet potatoes distribute significant amounts of Na and K in their petioles. Young leaves of the tolerant genotype contain more K under salt stress. GR and positive relationship between K concentration and SOD in salt tolerant genotypes indicate some tolerance mechanisms. So, a screening tool is proposed for sweet potato based on the genotypic ability to maintain high tissue K levels under increasing salinity level.