Winter survival of the unicellular green alga Micrasterias denticulata: insights from field monitoring and simulation experiments

Peat bog pools around Tamsweg (Lungau, Austria) are typical habitats of the unicellular green alga Micrasterias denticulata. By measurement of water temperature and irradiation throughout a 1-year period (2018/2019), it was intended to assess the natural environmental strain in winter. Freezing resistance of Micrasterias cells and their ability to frost harden and become tolerant to ice encasement were determined after natural hardening and exposure to a cold acclimation treatment that simulated the natural temperature decrease in autumn. Transmission electron microscopy (TEM) was performed in laboratory-cultivated cells, after artificial cold acclimation treatment and in cells collected from field. Throughout winter, the peat bog pools inhabited by Micrasterias remained unfrozen. Despite air temperature minima down to −17.3 °C, the water temperature was mostly close to +0.8 °C. The alga was unable to frost harden, and upon ice encasement, the cells showed successive frost damage. Despite an unchanged freezing stress tolerance, significant ultrastructural changes were observed in field-sampled cells and in response to the artificial cold acclimation treatment: organelles such as the endoplasmic reticulum and thylakoids of the chloroplast showed distinct membrane bloating. Still, in the field samples, the Golgi apparatus appeared in an impeccable condition, and multivesicular bodies were less frequently observed suggesting a lower overall stress strain. The observed ultrastructural changes in winter and after cold acclimation are interpreted as cytological adjustments to winter or a resting state but are not related to frost hardening as Micrasterias cells were unable to improve their freezing stress tolerance.

Arbuscular mycorrhizal fungi alleviate low-temperature stress and increase freezing resistance as a substitute for acclimation treatment in barley

Barley (Hordeum vulgare L.) is cultivated globally under a wide range of climatic conditions and is subjected to chilling and freezing stresses under temperate and cold climatic conditions. As a mycorrhizal crop, barley may benefit from this association for increasing cold resistance. In order to investigate the effects of inoculation with arbuscular mycorrhizal fungi (AMF) on cold-stress resistance in barley plants, one winter and one spring cultivar were grown under control (25°C day, 17°C night) and low, non-freezing (LT: 5°C day, 3°C night) temperatures for 3 weeks in the absence (−AMF) or presence (+AMF) of two species of AMF, Glomus versiforme and Rhizophagus irregularis. In addition, the influence of LT (as an acclimation treatment) was studied on plant survival after a 2-day exposure to freezing temperature (FT: −5°C in dark). Biomass production, membrane integrity and survival rate of plants indicated that the winter cultivar was more tolerant than the spring cultivar. Inoculation with AMF resulted in improved growth, photosynthesis, osmotic and water homeostasis, and potassium uptake under both control and LT conditions, whereas the effect on membrane integrity, antioxidative defence and phenolics metabolism was mainly observed in LT plants. AMF inoculation substituted partially or completely for acclimation treatment and increased the survival rate of FT plants, with the highest survival achieved in a combination of AMF and LT. Mycorrhizal responsiveness was higher in LT plants. Despite the lower AMF colonisation, G. versiforme was often more effective than R. irregularis for the alleviation of low temperature stress in both cultivars, whereas R. irregularis was more effective in increasing the survival rate. Our data suggest that the right combination of fungus species and host-plant cultivar is important for successful utilisation of AMF under cold conditions.

The Effects of NPR1 Dependent Salicylic Acid Change in Increasing Salt Tolerance of Soybean Leaves by Acclimation

Non-expressor of Pathogen Related 1 (NPR1) is a regulatory gene of the salicylic acid (SA) signaling pathway, the detailed mechanism of which is still not well understood. This study investigated the effects of NPR1-dependent SA level change on increasing salt tolerance of soybean leaves with acclimation. Salt-sensitive (‘SA88’) and salt-tolerant (‘Erensoy’) soybean (Glycine max L.) plants were treated with increasing NaCl concentrations (25, 50, 75, and 100 mM; acclimation) and with 100 mM NaCl directly (non-acclimation) in two groups. The results showed that acclimation treatment alleviated salt-induced damage in the sensitive cultivar with increasing superoxide anion radical scavenging activity, and decreasing hydrogen peroxide (H2O2) and malondialdehyde (MDA) content. However, the APX, CAT, and GST enzyme activities were increased by acclimation treatment, with the highest increase observed in GST enzymes. Interestingly, the Gmnpr1 gene expression was upregulated in all treatments but was more pronounced in non-acclimation. Furthermore, the highest increase in endogenous SA level was under acclimation treatment in ‘SA88’. In conclusion, the results firstly showed that an acclimation process is useful for increasing salt tolerance in sensitive soybean plants with only ROS-inducted NPR1-independent SA accumulation but not through the NPR1-dependent SA signaling pathway.

Cold Acclimation Treatment–induced Changes in Abscisic Acid, Cytokinin, and Antioxidant Metabolism in Zoysiagrass (Zoysia japonica)

Zoysiagrass (Zoysia spp.), a warm-season turfgrass species, experiences freezing damage in many regions. The mechanisms of its cold acclimation and freezing tolerance have not been well understood. This study was designed to investigate effects of cold acclimation treatment on leaf abscisic acid (ABA), cytokinin (transzeatin riboside (t-ZR), and antioxidant metabolism associated with freezing tolerance in zoysiagrass (Zoysia japonica). ‘Chinese Common’ zoysiagrass was subjected to either cold acclimation treatment with temperature at 8/2 °C (day/night) and a photosynthetically active radiation (PAR) of 250 µmol·m−2·s−1 over a 10-hour photoperiod or normal environments (temperature at 28/24 °C (day/night), PAR at 400 µmol·m−2·s−1 and 14-hour photoperiod) for 21 days in growth chambers. Cold treatment caused cell membrane injury as indicated by increased leaf cell membrane electrolyte leakage (EL) and malondialdehyde (MDA) content after 7 days of cold treatment. Cold treatment increased leaf ABA and hydrogen peroxide content and reduced t-ZR content. Leaf superoxide dismutase (SOD), ascorbate peroxidase (APX) activity, and proline content increased, whereas catalase (CAT) and peroxidase (POD) activity declined in response to cold treatment. Cold treatment increased freezing tolerance as LT50 declined from −4.8 to −12.5 °C. The results of this study indicated that cold acclimation treatment might result in increases in ABA and H2O2, which induce antioxidant metabolism responses and improved freezing tolerance in zoysiagrass.

Generalist–specialist trade-off during thermal acclimation

The shape of performance curves and their plasticity define how individuals and populations respond to environmental variability. In theory, maximum performance decreases with an increase in performance breadth. However, reversible acclimation may counteract this generalist–specialist trade-off, because performance optima track environmental conditions so that there is no benefit of generalist phenotypes. We tested this hypothesis by acclimating individual mosquitofish ( Gambusia holbrooki ) to cool and warm temperatures consecutively and measuring performance curves of swimming performance after each acclimation treatment. Individuals from the same population differed significantly in performance maxima, performance breadth and the capacity for acclimation. As predicted, acclimation resulted in a shift of the temperature at which maximal performance occurred. Within acclimation treatments, there was a significant generalist–specialist trade-off in responses to acute temperature change. Surprisingly, however, there was also a trade-off across acclimation treatments, and animals with greater capacity for cold acclimation had lower performance maxima under warm conditions. Hence, cold acclimation may be viewed as a generalist strategy that extends performance breadth at the colder seasons, but comes at the cost of reduced performance at the warmer time of year. Acclimation therefore does not counteract a generalist–specialist trade-off and, at least in mosquitofish, the trade-off seems to be a system property that persists despite phenotypic plasticity.

Ultrasonic irradiation as pretreatment for the reduction of excess sludge by Fenton-acclimation treatment

Ultrasonic irradiation as a pretreatment of Fenton-acclimation treatment was investigated to enhance the efficiency of sludge reduction and effectiveness of operating cost. A series of batch experiments were conducted to optimize the reaction conditions for ultrasonic–Fenton treatment. Separate ultrasonic treatment suggested that input power of 0.4 W/mL and ultrasonic time of 10 min were the optimal conditions for sludge disintegration, and the efficiency was reduced with the increase of sludge mixed liquor suspended solids. Separate Fenton treatment revealed that 9 g/L and 40 mg/L were the optimal dosages of H2O2 and Fe2+ respectively for sludge lysis under pH of 3. Particle distribution (75.49% of the particles distributed between 7.18 and 31.11 μm after Fenton treatment while 93.35% of the particles distributed between 4.62 and 18.50 μm after ultrasonic–Fenton treatment) and chemical oxygen demand (51.89% higher in ultrasonic–Fenton treatment than that in Fenton treatment) demonstrated that combined ultrasonic–Fenton treatment was effective in sludge disintegration compared to separate Fenton treatment. With ultrasound as pretreatment, sludge reduction rate increased from 26.53 to 63.59% with operating cost reduction of 51.46%, indicating ultrasonic irradiation was effective in improving both sludge reduction efficiency and operating cost-effectiveness.

Photoperiod, Temperature, and Plant Age Interact to Affect Short-day Onion Cold Hardiness

Proper acclimation of onion (Allium cepa L.) seedlings can enhance winter freeze survival; therefore, the effects of photoperiod-temperature combinations, photoperiod, and plant age on the cold hardiness of short-day onions were investigated. Following acclimation at various photoperiod-temperature regimes, different-aged plants were frozen to various subzero temperatures in an ethylene glycol bath and evaluated for cold hardiness. Older plants were more cold hardy than younger plants. An 11-hour photoperiod-decreasing temperature (20/15 to 10/5C day/night) treatment improved plant cold hardiness over other photoperiod-temperature regimes. Various photoperiods (8-, 11-, 14-, and 24-hour) applied during a 14-day, 3C acclimation treatment before freezing had little effect on plant cold hardiness. However, day 7 foliar and day 14 root evaluations indicated that 81-day-old plants given an 8- or 11-hour photoperiod during the 3C acclimation treatment were less cold hardy than older plants (91 or 112 days) given the same acclimation photoperiod.