Effects of salt stress on ion balance and nitrogen metabolism of old and young leaves in rice (Oryza sativa L.).

BACKGROUND: It is well known that salt stress has different effects on old and young tissues. However, it remains largely unexplored whether old and young tissues have different regulatory mechanism during adaptation of plants to salt stress. The aim of this study was to investigate whether salt stress has different effects on the ion balance and nitrogen metabolism in the old and young leaves of rice, and to compare functions of both organs in rice salt tolerance. RESULTS: Rice protected young leaves from ion harm via the large accumulation of Na+ and Cl- in old leaves. The up-regulation of OsHKT1;1, OsHAK10 and OsHAK16 might contribute to accumulation of Na+ in old leaves under salt stress. In addition, lower expression of OsHKT1;5 and OsSOS1 in old leaves may decrease frequency of retrieving Na+ from old leaf cells. Under salt stress, old leaves showed higher concentration of NO3- content than young leaves. Up-regulation of OsNRT1;2, a gene coding nitrate transporter, might contribute to the accumulation of NO3- in the old leaves of salt stressed-rice. Salt stress clearly up-regulated the expression of OsGDH2 and OsGDH3 in old leaves, while strongly down-regulated expression of OsGS2 and OsFd-GOGAT in old leaves. CONCLUSIONS: The down-regulation of OsGS2 and OsFd-GOGAT in old leaves might be a harmful response to excesses of Na+ and Cl-. Under salt stress, rice might accumulate Na+ and Cl- to toxic levels in old leaves. This might influence photorespiration process, reduce NH4+ production from photorespiration, and immediately down-regulate the expression of OsGS2 and OsFd-GOGAT in old leaves of salt stressed rice. Excesses of Na+ and Cl- also might change the pathway of NH4+ assimilation in old leaves of salt stressed rice plants, weaken GOGAT/GS pathway and elevate GDH pathway.

Tissue-specific and cation/anion-specific DNA methylation variations occurred in C. virgata in response to salinity stress.

Salinity is a widespread environmental problem limiting productivity and growth of plants. Halophytes which can adapt and resist certain salt stress have various mechanisms to defend the higher salinity and alkalinity, and epigenetic mechanisms especially DNA methylation may play important roles in plant adaptability and plasticity. In this study, we aimed to investigate the different influences of various single salts (NaCl, Na2SO4, NaHCO3, Na2CO3) and their mixed salts on halophyte Chloris. virgata from the DNA methylation prospective, and discover the underlying relationships between specific DNA methylation variations and specific cations/anions through the methylation-sensitive amplification polymorphism analysis. The results showed that the effects on DNA methylation variations of single salts were ranked as follows: Na2CO3> NaHCO3> Na2SO4> NaCl, and their mixed salts exerted tissue-specific effects on C. virgata seedlings. Eight types of DNA methylation variations were detected and defined in C. virgata according to the specific cations/anions existed in stressful solutions; in addition, mix-specific and higher pH-specific bands were the main type in leaves and roots independently. These findings suggested that mixed salts were not the simple combination of single salts. Furthermore, not only single salts but also mixed salts showed tissue-specific and cations/anions-specific DNA methylation variations.

[pH and ion balance in wheat-wheatgrass under salt- or alkali stress].

Different intensities of salt- or alkali stress were established by mixing different concentrations of NaCl and Na2SO4 or NaHCO3 and Na2CO3, respectively, and wheat-wheatgrass (Triticum aestivum L. -Agropyron intermedium) seedlings were grown under the stresses for 12 days. The pH value and the Na(+), K(+), free Ca(2+), Cl(-), SO4(2-), NO3(-), H2PO4(-), and organic acid concentrations in the fresh shoots of stressed wheat-wheatgrass seedlings were determined, aimed to approach the characteristics of pH and ion balance in wheat-wheatgrass under salt- or alkali stress. The results showed that intracellular pH was relatively stable under both stresses. Cl(-) concentration increased sharply and organic acid concentration changed less under increasing intensity of salt stress, while it was, in adverse under increasing intensity of alkali stress. Under both stresses, the cations in the fresh shoots were mainly Na(+) and K(+), but the anions were different. Under salt stress, inorganic anions were the dominant components and contributed 61.3 %-66.7% to the total negative charge, while under alkali stress, the contribution of organic acid to total negative charge increased from 38.35% to 61.6% with increasing stress intensity, and became the dominant component. It was concluded that organic acid accumulation might be a key physiological response of wheat-wheatgrass for its keeping pH and ion balance under alkali stress.

[Effects of alkali-stress on Aneurolepidium chinense and Helianthus annuus].

Employing monocotyledon Aneurolepidium chinense and dicotyledon Helianthus annuus, the two species with high alkali-saline resistance as test materials, and stressing them with neutral, alkalic, and mixed salts, this paper studied the characteristics of various stresses and their interrelations, with relative growth rate (RGR) as the main strain index. The results showed that under the same concentration, alkalic salt had a stronger effect than neutral salt, and the RGR of A. chinense and H. annuus was decreased with increasing salt concentration under the same kind of salt stress and pH conditions. When the pH was higher, the RGR decreased more obviously. In addition, the proline and citric acid contents in test plants were increased with increasing stress. The transformation of proline in A. chinense was greater than that in H. annuus, while that of citric acid was in adverse. In a word, alkalic salt stress was different from neutral salt stress in stressing plants and in plant responses. Alkalic salt stress and neutral salt stress were actually two distinct kinds of stresses. The former was better called "alkali-stress", while "salt-stress" only meant neutral salt stress. The key difference between them was their different pH value. It was reasonable to consider the buffer capacity as the strength value of alkali-stress, and the salinity as the strength value of salt-stress. An interactive effect between salt-stress and alkali-stress could be seen under mixed saline and alkali stress.