A comprehensive investigation was carried out to determine the changes that occurred in water-stressed cucumber (Cucumis sativus L.) in response to melatonin treatment. We examined the potential roles of melatonin during seed germination and root generation and measured its effect on reactive oxygen species (ROS) levels, antioxidant enzyme activities, and photosynthesis. Melatonin alleviated polyethylene glycol induced inhibition of seed germination, with 100 μm melatonin-treated seeds showing the greatest germination rate. Melatonin stimulated root generation and vitality and increased the root:shoot ratio; therefore, melatonin may have an effect on strengthening cucumber roots. Melatonin treatment significantly reduced chlorophyll degradation. Seedlings treated with 100 μm melatonin clearly showed a higher photosynthetic rate, thus reversing the effect of water stress. Furthermore, the ultrastructure of chloroplasts in water-stressed cucumber leaves was maintained after melatonin treatment. The antioxidant levels and activities of the ROS scavenging enzymes, i.e., superoxide dismutase, peroxidase, and catalase, were also increased by melatonin. These results suggest that the adverse effects of water stress can be minimized by the application of melatonin.

Kochia sieversiana (Pall.) C. A. M., a naturally alkali-resistant halophyte, was chosen as the test organism for our research. The seedlings of K. sieversiana were treated with varying (0-400 mM) salt stress (1:1 molar ratio of NaCl to Na 2 SO 4 ) and alkali stress (1:1 molar ratio of NaHCO 3 to Na 2 CO 3 ). The concentrations of various solutes in fresh shoots, including Na + , K + , Ca 2+ , Mg 2+ , Cl À , SO 4 2À , NO 3 À , H 2 PO 3 À , betaine, proline, soluble sugar (SS), and organic acid (OA), were determined. The water content (WC) of the shoots was calculated and the OA components were analyzed. Finally, the osmotic adjustment and ion balance traits in the shoots of K. sieversiana were explored. The results showed that the WC of K. sieversiana remained higher than 6 [g g À1 Dry weight (DW)] even under the highest salt or alkali stress. At salinity levels >240 mM, proline concentrations increased dramatically, with rising salinity. We proposed that this was not a simple response to osmotic stress. The concentrations of Na + and K + all increased with increasing salinity, which implies that there was no competitive inhibition for absorption of either in K. sieversiana. Based on our results, the osmotic adjustment feature of salt stress was similar to that of alkali stress in the shoots of K. sieversiana. The shared essential features were that the shoots maintained a state of high WC, OA, Na + , K + and other inorganic ions, accumulated largely in the vacuoles, and betaine, accumulated in cytoplasm.On the other hand, the ionic balance mechanisms under both stresses were different. Under salt stress, K. sieversiana accumulated OA and inorganic ions to maintain the intracellular ionic equilibrium, with close to equal contributions of OA and inorganic ions to anion. However, under alkali stress, OA was the dominant factor in maintaining ionic equilibrium. The contribution of OA to anion was as high as 84.2%, and the contribution of inorganic anions to anion was only 15.8%. We found that the physiological responses of K. sieversiana to salt and alkali stresses were unique, and that mechanisms existed in it that were different from other naturally alkaliresistant gramineous plants, such as Aneurolepidium chinense, Puccinellia tenuiflora.

Effects of salt and alkali stresses on growth, osmotic adjustment and ionic balance of Suaeda glauca (Bge.), an alkali-resistant succulent halophyte, were compared. The results showed that alkali stress clearly inhibited the growth of S. glauca. Moreover, the concentrations of Na + and K + both increased with increasing salinity under both stresses, suggesting no competitive inhibition between absorptions of Na + and K + . The mechanism underlying osmotic adjustment during salt stress was similar to alkali stress in shoots. The shared essential features were that organic acids, betaine and inorganic ions (dominated by Na + ) mostly accumulated. On the other hand, the mechanisms governing ionic balance under both stresses were different. Under salt stress, S. glauca accumulated organic acids and inorganic anions to maintain the intracellular ionic equilibrium, but the anion contribution of inorganic ions was greater than that of organic acids. However, the concentrations of inorganic anions under alkali stress were significantly lower than those under salt stress of the same intensity, suggesting that alkali stress might inhibit uptake of anions, such as NO 3 -and H 2 PO 4 -. Under alkali stress, organic acids were the dominant factor in maintaining ionic equilibrium. The contribution of organic acids to anions was 74.1%, while that of inorganic anions was only 25.9%. S. glauca enhanced the synthesis of organic acids, dominated by oxalic acid, to compensate for the shortage of inorganic anions.

We compared the effects of salt-stresses (SS, 1 : 1 molar ratio of NaCl to Na 2 SO 4 ) and alkali-stresses (AS, 1 : 1 molar ratio of NaHCO 3 to Na 2 CO 3 ) on the growth, photosynthesis, solute accumulation, and ion balance of barley seedlings, to elucidate the mechanism of AS (high-pH) damage to plants and the physiological adaptive mechanism of plants to AS. The effects of SS on the water content, root system activity, membrane permeability, and the content of photosynthetic pigments were much less than those of AS. However, AS damaged root function, photosynthetic pigments, and the membrane system, led to the severe reductions in water content, root system activity, content of photosynthetic pigments, and net photosynthetic rate, and a sharp increase in electrolyte leakage rate. Moreover, with salinity higher than 60 mM, Na + content increased slowly under SS and sharply under AS. This indicates that high-pH caused by AS might interfere with control of Na + uptake in roots and increase intracellular Na + to a toxic level, which may be the main cause of some damage emerging under higher AS. Under SS, barley accumulated organic acids, Cl − , SO 4 2− , and NO 3 − to balance the massive influx of cations, the contribution of inorganic ions to ion balance was greater than that of organic acids. However, AS might inhibit absorptions of NO 3 -and Cl -, enhance organic acid synthesis, and SO 4 2− absorption to maintain intracellular ion balance and stable pH.

Hexaploid bread wheat (Triticum aestivum L., genome BBAADD) is generally more salt tolerant than its tetraploid wheat progenitor (Triticum turgidum L.). However, little is known about the physiological basis of this trait or about the relative contributions of allohexaploidization and subsequent evolutionary genetic changes on the trait development. Here, we compared the salt tolerance of a synthetic allohexaploid wheat (neo-6x) with its tetraploid (T. turgidum; BBAA) and diploid (Aegilops tauschii; DD) parents, as well as a natural hexaploid bread wheat (nat-6x). We studied 92 morphophysiological traits and analyzed homeologous gene expression of a major salt-tolerance gene High-Affinity K + Transporter 1;5 (HKT1;5). We observed that under salt stress, neo-6x exhibited higher fitness than both of its parental genotypes due to inheritance of favorable traits like higher germination rate from the 4x parent and the stronger root Na + retention capacity from the 2x parent. Moreover, expression of the D-subgenome HKT1;5 homeolog, which is responsible for Na + removal from the xylem vessels, showed an immediate transcriptional reprogramming following allohexaploidization, i.e., from constitutive high basal expression in Ae. tauschii (2x) to salt-induced expression in neo-6x. This phenomenon was also witnessed in the nat-6x. An integrated analysis of 92 traits showed that, under salt-stress conditions, neo-6x resembled more closely the 2x than the 4x parent, suggesting that the salt stress induces enhanced expressivity of the D-subgenome homeologs in the synthetic hexaploid wheat. Collectively, the results suggest that condition-dependent functionalization of the subgenomes might have contributed to the wide-ranging adaptability of natural hexaploid wheat.transcriptional rewiring | Na + homeostasis | salinity tolerance P olyploidy or whole genome duplication (WGD) is a pervasive, driving force in plant and vertebrate evolution, which has fascinated biologists for more than a century (1, 2). The common occurrence of WGD suggests an evolutionary advantage of having multiple genomes at least in certain circumstances, which might have enabled the polyploid organisms to be better adapted to some adverse environmental conditions than their diploid progenitors (3, 4). Polyploidy can instantaneously develop novel features that allow them to invade new territories or expand their parental niche (3). Polyploids may also exhibit higher evolvability than their diploid progenitors, which allows them to adapt to capricious environmental conditions (4). Thus, polyploidy has been demonstrated as a process that may lead to saltational speciation especially when novel ecological niches are available for colonization.Although abrupt genome duplication often produces adverse effects on physiology at both cellular and organismal levels (4, 5), it has been shown that polyploidy in plants may result in favorable physiological consequences such as increased photosynthetic capacity and enhanced tolerance to biotic and abiotic stresses, which ...