No abstract

Physiological and biochemical responses of Hordeum maritimum and H. vulgare to salt stress were studied over a 60-h period. Growth at increasing salinity levels (0, 100, 200 and 300 mM NaCl) was assessed in hydroponic culture. H. maritimum was shown to be a true halophyte via its typical behaviour at high salinity. Shoot growth of cultivated barley was gradually reduced with increasing salinity, whereas that of wild barley was enhanced at 100 and 200 mm NaCl then slightly reduced at 300 mM NaCl. The higher salt tolerance of H. maritimum as compared to H. vulgare was due to its higher capacity to maintain cell turgor under severe salinity. Furthermore, H. maritimum exhibited fine regulation of Na(+) transport from roots to shoots and, unlike H. vulgare, it accumulated less Na(+) in shoots than in roots. In addition, H. maritimum can accumulate more Na(+) than K(+) in both roots and shoots without the appearance of toxicity symptoms, indicating that Na(+) was well compartmentalized within cells and substituted K(+) in osmotic adjustment. The higher degree of salt tolerance of H. maritimum is further demonstrated by its economic strategy: at moderate salt treatment (100 mm NaCl), it used inorganic solutes (such as Na(+)) for osmotic adjustment and kept organic solutes and a large part of the K(+) for metabolic activities. Indeed, K(+) use efficiency in H. maritimum was about twofold that in H. vulgare; the former started to use organic solutes as osmotica only at high salinity (200 and 300 mm NaCl). These results suggest that the differences in salt tolerance between H. maritimum and H. vulgare are partly due to (i) differences in control of Na(+) transport from roots to shoots, and (ii) H. maritimum uses Na(+) as an osmoticum instead of K(+) and organic solutes. These factors are differently reflected in growth.

Phytosiderophores (PS) are Fe(III)-solubilizing compounds released by Poaceae roots under iron deficiency conditions. Several studies focused on the capacity of these plants to secrete PS as a center of their iron deficiency tolerance, and little information is available on other traits such as root/shoot biomass ratios, iron use efficiency, photosynthetic activity, and iron mobilization capacity that might also contribute to iron deficiency tolerance. In this study, we evaluated some traits other than PS release capacity that could be responsible for differences in iron deficiency tolerance in two barley species, Hordeum maritimum and Hordeum vulgare. Results showed that under iron starvation, biomass production was affected in both species, but H. maritimum kept higher root/shoot ratios due to the distribution efficiency of carbohydrates within the plant and the growth flexibility of its organs. Both species responded to iron starvation by an early release of PS, but they differed in their secretion capacity. In cultivated barley, the PS release rate was 1.5-2-fold higher than that of wild barley. This behavior was also concomitant with no modification in shoot iron concentration of the latter, which may lead to a low stimulation of its PS release as compared to the former. The amount of Fe 3+ mobilized by root exudates was determined at different pH values (between 5.6 and 8.6). Results showed a decrease in the mobilization capacity with the increasing pH, mainly in H. vulgare. At 8.6, it was reduced by 50% in H. vulgare and 30% in H. maritimum. These data suggest that differences in Poaceae tolerance to iron deficiency is attributed not only to PS secretion capacity, but also to carbohydrate distribution within the plant, Fe use efficiency, and root exudates capacity to mobilize Fe(III).

Literature on the separate effects of salinity and inadequate Fe supply on plant growth and nutrient uptake, concentration, and distribution is abundant but little is known about the interactive effects of these two abiotic constraints. Here, we investigated the interactive effect of iron availability and salinity on physiological responses of cultivated and wild barley (Hordeum vulgare and H. maritimum resp.). Seedlings of both species were grown for 9 days, under complete nutrient solution with or without iron supply. Then, NaCl treatment was applied at different concentrations (0, 100, 200, and 300 mM) for 60 hours. After salt exposure, shoot water content of H. vulgare was significantly reduced as compared to H. maritimum. Furthermore, Na + accumulation in shoots increased parallel to increasing NaCl concentration in the medium. However, the increase was significantly higher in H. vulgare than in H. maritimum. These responses were associated with lower Fe absorption efficiency photosynthetic parameters in both species. The reduction was significantly higher in cultivated than in wild barley. Moreover, phytosiderophore exudation was enhanced in both species by direct (iron free medium) or indirect iron limitation (salt-induced iron limitation). Such a stimulation of phytosiderophore release was genotype and salt level dependant.

Physiological responses of the halophyte Suaeda fruticosa cultivated under combined effects of salinity and iron deficiency were addressed. Plants were grown hydroponically for one month with a complete nutrient solution containing iron (Fe) of 0 or 30 μM, without or with salinity (100 and 400 mM NaCl). Growth parameters, chlorophyll concentration, mineral status, Fe(III) chelate reductase (FCR) activity and the acidification capacity were evaluated. Fe deficiency restricted significantly the plant biomass production as well as Fe uptake. Interestingly, the exposure to salinity mitigated the impact of Fe shortage on the plant growth whereas the impacts of salinity and Fe deficiency were not additive with respect to the plant Fe status. Salinity had no effect on the root proton release except a slight reduction at 400 mM and the FCR activity. Interestingly, both Fe absorption and use efficiencies were significantly higher in salt‐treated plants under optimal Fe supply.