Stevia rebaudiana Bertoni is a sweet medicinal herb that is cultivated worldwide. This study aimed to identify the genotypic responses and function of nine cultivars of S. rebaudiana (accession numbers 1–9 from the EUSTAS Stevia Gene Bank) to low temperature. Plants were grown in vitro and incubated under controlled conditions at 5° or 25°C for 1 month. Cold stress significantly decreased the maximum quantum yield of photosystem II (Fv/Fm) in all cultivars, which was more pronounced in cultivars 5, 6, 8, and 9. The efficiency of photosystems I and II (PIABS) also declined in cold-stressed plants and was accompanied by reductions in net photosynthesis (PN), intercellular CO2 (Ci), water use efficiency (WUE), and chlorophyll a, chlorophyll b and carotenoid contents, more so in cultivars 5, 6, 8, and 9. Regardless of the downregulation of photosynthetic capacity, the cold stress increased water-soluble carbohydrates in all cultivars, which was accompanied by an increase in fresh leaf mass and area, more so in cultivars 5, 6, 8, and 9. Furthermore, cold stress increased the stomatal index and density, epidermal cell density, stem diameter, xylem vessel width, phloem tissue width, and number of sclerenchyma in all cultivars. Even though the nine cultivars of S. rebaudiana had lower PSII efficiencies at low temperatures, the increase in carbohydrates and leaf mass suggests that damage to PSII is not responsible for the reduction in its efficiency.

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Plant survival in response to freezing stress depends on the efficient activation of tolerance mechanisms. Fritillaria imperialis exposure to freezing stress enhanced signalling molecules ca 2+ and H 2 o 2 along with overexpression of ca 2+ signalling proteins (ca 2+ dependent protein kinases, cpK), followed by upregulation of NHX1 (na + /H + antiporter), LEA (late embryogenesis abundant proteins) and P5CS (1-pyrroline-5-carboxylate synthetase). Overexpression of OsCNGC6 was responsible for high accumulation ca 2+ , na + and K +. The NHX1 gene product transported Na + to vacuoles and increased cytosolic K + content to re-establish ionic homeostasis under stress conditions. The reduced water potential of leaves was due to high accumulation of osmolytes and ions. no changes were observed in relative water content of leaves, which might be correlated with overexpression of the LEA gene, which protects against dehydration. High accumulation of H 2 o 2 under freezing stress was responsible for activation of antioxidant systems involving SoD, phenols, anthocyanins, catalase and ascorbate peroxidase. Photosynthesis, suppressed in freezing-stressed plants, returned to normal levels after termination of freezing stress. Taken together, our findings suggest that Fritillaria efficiently tolerated freezing stress through induction of signalling mechanisms and overexpression of cold stressresponsive genes, and prevention of cold-induced water stress, oxidative stress and photosynthetic damage. Fritillaria imperialis is a perennial plant in the Liliaceae family native to Iran with beneficial medicinal qualities and ornamental importance. The orange-red downward facing flowers in combination with a crown of glossy green leaves at the top of stem make Fritillaria a very attractive horticultural species. One of the main habitats of wild Fritillaria in Iran is the protected area of Golestan-Kuh with an area of about 950 hectares and height of 3631 m. It is located 15 kilometres northeast of Khansar city, Isfahan Province which is subject to cold and snowy winters. Fritillaria require a period of cool temperatures to encourage flowering and its blooming happens in April at optimal temperatures of 18 to 20 °C. Fritillaria with other species of rangeland plants, on the southern slopes of Golestan-Kuh, are valuable natural cover vegetation, which attracts thousands of tourists every spring to visit this area 1-3. F. imperialis, despite its tropical origin, is tolerant of cold stress, especially at the bulb stage 4,5. Cold stress is an abiotic stress that plants experience during extended exposure to freezing temperatures. A frost is especially harmful in late spring or early fall when plants are in an active growth stage 6. Each plant has an optimum temperature range for its growth and development and conditions that are optimal for one plant, may be stressful for another 4,6. Exposure to cold stress induced signalling mechanisms through an increase in cytosolic Ca 2+ 7. The increase in cytosolic Ca 2+ can be triggered through activat...

Effect of paclobutrazol (PBZ) treatment on salinity tolerance of wheat (Triticum aestivum) was investigated on a salt-tolerant (Karchia-65) and saltsensitive (Ghods) cultivars. Salinity significantly reduced the investigated growth parameters such as plant height, length and area of sixth leaf, root length, fresh and dry weight of shoot, roots and sixth leaf, water content (WC) of plant and seeds weight in the both cultivars. The negative effect of salinity in Ghods cultivar was more than Karchia cultivar. However, PBZ treatment reduced the growth in both cultivars, the differences in plant growth among various levels of NaCl decreased in PBZ-treated plants. Salt stress resulted in high accumulation of Na + in the sixth leaf and roots in both cultivars, particularly in Ghods cultivar. Against Karchia cultivar, salt stress decreased the storage of K + , P and N in sixth leaf and roots in Ghods cultivar. In the both cultivars, PBZ treatment enhanced the K + , P and N contents in sixth leaf and roots by increasing salinity. Although PBZ treatment decreased the growth of plants, it improved the weight of seeds against stress damage. PBZ treatment reduced the accumulation of harmful Na + ion in plant tissues while increased the K + , P and N contents. These observations suggest that PBZ treatment may increase tolerance by diminishing ionic imbalance caused by salt stress.