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 ofOsHKT1;1,OsHAK10andOsHAK16might contribute to accumulation of Na⁺in old leaves under salt stress. In addition, lower expression ofOsHKT1;5andOsSOS1in 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 ofOsNRT1;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 ofOsGDH2andOsGDH3in old leaves, while strongly down-regulated expression ofOsGS2andOsFd-GOGATin old leaves.Conclusions: The down-regulation ofOsGS2andOsFd-GOGATin 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 ofOsGS2andOsFd-GOGATin 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.

Alkali stress is an important agricultural contaminant and has complex effects on plant metabolism. The aim of this study was to investigate whether the alkali stress has different effects on the growth, ion balance, and nitrogen metabolism in old and young leaves of rice plants, and to compare functions of both organs in alkali tolerance. The results showed that alkali stress only produced a small effect on the growth of young leaves, whereas strongly damaged old leaves. Rice protected young leaves from ion harm via the large accumulation of Na⁺ and Cl⁻ in old leaves. The up-regulation of OsHKT1;1, OsAKT1, OsHAK1, OsHAK7, OsHAK10 and OsHAK16 may contribute to the larger accumulation of Na⁺ in old leaves under alkali stress. Alkali stress mightily reduced the NO₃⁻ contents in both organs. As old leaf cells have larger vacuole, under alkali stress these scarce NO₃⁻ was principally stored in old leaves. Accordingly, the expression of OsNRT1;1 and OsNRT1;2 in old leaves was up-regulated by alkali stress, revealing that the two genes might contribute to the accumulation of NO₃⁻ in old leaves. NO₃⁻ deficiency in young leaves under alkali stress might induce the reduction in OsNR1 expression and the subsequent lacking of NH₄⁺, which might be main reason for the larger down-regulation of OsFd-GOGAT and OsGS2 in young leaves. Our results strongly indicated that, during adaptation of rice to alkali stress, young and old leaves have distinct mechanisms of ion balance and nitrogen metabolism regulation. We propose that the comparative studies of young and old tissues may be important for abiotic stress tolerance research.

Background: Introgression as a means of generating phenotypic novelty, including altered stress tolerance, is increasingly being recognized as common. The underlying basis for de novo genesis of phenotypic variation in the introgression lines remains largely unexplored. In this investigation, we used a rice line (RZ35) derived from introgressive hybridization between rice (Oryza sativa L.) and wild rice (Zizania latifolia Griseb.), along with its rice parental line (cv. Matsumae) as the experimental materials. We compared effects of salt stress on growth, ion homeostasis, and relevant gene expression between RZ35 and Matsumae, to explore possible mechanisms of heritable alteration in stress tolerance induced by the introgression. Results: Contrary to our expectation, the results showed that the inhibitory effect of salt stress on growth of RZ35 was significantly greater than that of Matsumae. We further found that a major underlying cause for this outcome is that the introgression process weakened the capacity in Na⁺ exclusion under the salt stress condition, and hence, escalated the injuries of Na⁺ and Cl^- in shoots of RZ35. Accordingly, based on q-RT-PCR analysis, four genes known to be involved in the Na⁺ exclusion, i.e., OsHKT1;5, OsSOS1, OsCIPK24 and OsCBL4, were found to be significantly down-regulated in roots of RZ35 relative to its rice parental line under the salt stress condition, thus implicating a gene expression regulation-based molecular mechanism underlying the difference in salt stress-tolerance between the introgression line and its rice parental line. Conclusions: We show that introgression represents a potent means for rapidly generating de novo heritable variations in physiological traits like stress tolerance in plants, although the direction of the alteration appears unpredictable.

Sorghum (Sorghum bicolor L.) was grown on an alkalinized plot for 90 days. Inorganic ions and compatible solutes were measured to investigate the physiological adaptations by which sorghum tolerates natural alkaline stress. Plant growth was clearly inhibited. The stress led to increased Na⁺ content in roots but had only a small effect on Na⁺ accumulation in leaves; K⁺ content in roots was lower than that in leaves and stems; and Na⁺ content and Na⁺/K⁺ ratio were much higher in roots than in leaves and stems. These suggest that maintaining a substantially higher osmotic potential in roots for greater uptake of water and restricting the spread of Na⁺ to shoots are physiological adaptations to alkalinity. Soluble sugar was the main osmolyte in roots, stems, and leaves; proline, amino acids, and oxalates contributed to osmotic regulation slightly, and it was noteworthy that K⁺ played a major role in osmotic regulation in leaves.