Responses of the halophyte C. maritima to long term salinity and K or Fe deficiency
Growth of shoots and roots after 15 days of treatments was very much reduced by salinity, followed by Fe deficiency and then by K deficiency (Fig. 1). In the treatments that combined salinity with both deficiencies, the effects were equivalent to those of salinity alone (data not shown).
The examination of root hair proliferation showed that, independently of K and Fe status, the application of 400 mM NaCl inhibited the development of root hairs, with less density and especially lower length (Fig. 2).
Figure 2 Effect of salinity, K and Fe deficiency on root hair proliferation in 37 days-old C. maritima plants. Plants were grown for 15 days in presence or absence of Fe and K, and supplied or not with NaCl 400 mM
In the treatment with Fe deficiency plants exhibited chlorosis symptoms on their young leaves but these ones were not detected when Fe deficiency was combined with salt stress (not shown). Chlorophyll concentration was much more reduced by Fe deficiency than by salinity or K deficiency (Fig. 3). However, in the treatment with Fe deficiency combined with salinity, values were higher than with Fe deficiency alone (Fig. 3).
The concentration of Na in shoots and roots was higher, as expected, in all treatments with salinity, being somewhat higher when combined with Fe deficiency (Fig. 4). Higher values were found in shoots than in roots. In the treatments without salinity, the highest values corresponded to the K deficiency treatment whose nutrient solution contained 2.5 mM NaCl (Fig. 4).
The K concentration in shoots and roots was much lower in the treatments with K deficiency, but also decreased with salinity and with Fe deficiency (Fig. 5A). In the treatment with Fe deficiency combined with salinity, root K concentration was lower than with Fe deficiency alone.
The Fe concentration in shoots and roots was greatly reduced in the treatments with Fe deficiency, while salinity or K deficiency had little effect (Fig. 5B). When Fe deficiency was combined with salinity, shoots concentration was higher than with Fe deficiency alone. In K deficiency, the combination with salinity decreased the Fe concentration in shoots while increased it in roots (Fig. 5B).
Ethylene production by roots was increased by K or Fe deficiency while salinity reduced it when applied alone or in combination with the deficiencies (Fig. 6).
The FCR activity in roots was only stimulated by Fe deficiency, but when Fe deficiency was combined with salinity it was reduced to values similar to the rest of treatments (Fig. 7).
Responses of the halophyte C. maritima and the glycophyte A. thaliana to short term salinity and K or Fe deficiency
In this kind of experiment, treatments were imposed during a shorter period than in experiment I. K deficiency for 7 days and Fe deficiency for 2 days were separately applied or in combination with salinity during the last 2 days.
Growth of shoots and roots of both species was reduced in the treatments containing salinity (Fig. 8). K deficiency reduced root growth in both species while Fe deficiency reduced the root growth only in A. thaliana.
Chlorophyll and carotenoid concentrations showed little variations in C. maritima (Table 2). In A. thaliana, chlorophyll was reduced with salinity and with Fe deficiency, while carotenoids increased with salinity and with Fe deficiency, and decreased with K deficiency (Table 2).
The concentration of Na in shoots and roots was higher, as expected, in all treatments with salinity (Table 3). Shoots accumulated more Na than roots in both species, but values in shoots of C. maritima were lower than in A. thaliana; in roots, the opposite occurred. In the treatments without salinity, the highest values corresponded to the K deficiency treatment whose nutrient solution contained 2.5 mM NaCl. The water content in shoots showed little variations in C. maritima while in A. thaliana it was very much reduced in all treatments containing salinity (Table 3). A slight decrease was also obtained in C. maritima under Fe deficiency combined with salinity, and in A. thaliana under K deficiency (Table 3).
All treatments resulted in significant reduction in K concentration in both species except in A. thaliana plants subjected to Fe deficiency where not effect was detected (Table 4). Interestingly, the effect of high salinity was greatly pronounced regardless K and Fe availability. Indeed, reductions were 41%, 89% and 90% in A. thaliana and 38%, 53% and 79% in C. maritima, respectively under salinity, K deprivation and their interaction. Root Fe content was not affected by Fe deficiency in C. maritima while it was significantly reduced in A. thaliana (Table 4).
Ethylene production by roots in both species was increased by K or Fe deficiency while salinity reduced it when applied alone or in combination with the deficiencies (Fig. 9).
The FCR activity in roots of both species was only stimulated by Fe deficiency, but when Fe deficiency was combined with salinity it was greatly reduced (Fig. 10). In A. thaliana, salinity also inhibited the reductase activity of plants grown with complete nutrient solution.
The effects of high salinity, K or Fe deficiency, and their interactions on the acid phosphatase activity was studied in A. thaliana and C. maritima (Fig. 11). High salinity, either applied under optima K and Fe supply or under K or Fe deficiency, caused induction of the phosphatase activity in A. thaliana while any effect was shown by C. maritima (Fig. 11).
The expression of several genes related to some stress responses was analyzed, but only in A. thaliana because primers were only available for this species.
The expression of the gene AtHAK5, encoding a high affinity K transporter, was induced by K deficiency and by salinity (Fig. 12). But when K deficiency and salinity were combined, the induction was practically annulled.
The expression of key genes for Fe acquisition, AtFRO2, AtIRT1 and AtFIT was induced by Fe deficiency (Fig. 13). Nevertheless, the application of 200 mM NaCl practically abolished their expression.
The expression of ethylene biosynthesis or signaling genes was affected by the treatments but in different way depending on the gen (Fig. 14). An increased expression was only found for AtEIN2 and AtEIN3 under K deficiency. A reduced expression was found for AtCO2, AtEIN2 and AtEIN3 in all treatments containing salinity, and furthermore for AtEIN2 under Fe deficiency. The expression of AtCS6 was not affected by any of the treatments (Fig. 14).