To avoid mortality in the greenhouse, plants received water every three to four days, which differs greatly from natural rainfall patterns, even in the wet season. During the dry season, soil moisture is often between 30% – 80% of sample weight for marsh and low ecotone soil cores in the high ecotone and upland locations, soil moisture accounts for 0% – 30% of sample weight. We have no data showing whether we achieved similar conditions with potting soil in the greenhouse, and regardless, we would expect more rapid drying in pots than for in situ field soil. Thus, as for all greenhouse studies, results presented here should be used with caution when predicting performance in the field. To expand on these results, the greenhouse experiment should be repeated using native marsh soil as the substrate and including higher salinity treatments . Response to treatment in marsh soil should provide a more accurate prediction of response to field conditions. Surprisingly, measured differences in water potential did not translate to plant performance. Neither growth nor survival were visibly affected by watering treatment, even in potentially stressful low volume / high salinity treatments. Existing literature suggests that halophytes concentrate solutes to generate low tissue water potential, allowing continued passive uptake of water. In this case, low tissue water potential is not detrimental, since it prevents or reduces water deficits that can impair growth. Another possible reason for the lack of effect on growth was timing of the experiment. We began the experiment in June, when most individuals were beginning to reproduce. Beyond this point, energy is less likely to be allocated to vegetative growth and more likely to be allocated towards reproduction or survival strategies,aeroponic tower garden system like salt management . In contrast, younger plants allocate the majority of their energy to vegetative growth .
Adaptations, such as salt glands or specialized vacuoles, are energy expensive and require energy normally allocated to growth . Additionally, decreasing water potential has been shown to inhibit cell expansion , which would disproportionately affect young plants, since the rate of cell expansion in mature plants is reduced. Therefore, by better aligning the experimental period with the natural growth period, and focusing on young plants, treatment effects on growth might become more apparent. D. spicata displayed the greatest variability in tissue water potential, and this variability may have been influenced by factors other than watering treatment. D. spicata was grown in shallower, wider pots in a sandier potting medium. In both volume treatments, water would drain quickly through the pots, leading to uneven soil saturation that likely affected treatment efficacy and making it difficult to draw definitive conclusions regarding the large range in water potential. However, low water potential values are not uncommon for D. spicata. Other authors have observed sustained, highly negative water potential used to compensate for soil salinity . The highest D. spicata mortality in our experiment occurred in the drought treatments, with three out of four deaths in the 60% seawater drought treatment. Nonetheless, increased drainage and evaporation rates likely contributed to mortality for this species. E. californica was affected by both the drought and salinity treatments, causing lower water potential and a slight negative effect on growth. Interestingly, our results contrast with those from another study. Jong measured E. californica net dry weight when irrigated with a saline Hoagland solution in sandy soil, using artificial sea salt instead of seawater. The water potential of their maximum salinity treatment was similar to our 60% seawater treatment, but the authors found that dry weight of E. californica decreased significantly as salinity increased.
This experiment used young E. californica seedlings – the first tissue harvest occurred when seedlings were one month old and continued every 8 days until all plants were harvested, with the authors noting a difference in dry weight between treatments . Since we did not observe a difference in above ground biomass, the contrasting results may be due to the misalignment of experiment start time with the natural growth period. F. salina did not show an effect of salinity and drought stress on total plant growth, since biomass was maintained across treatments. In contrast, Barbour’s and Davis’s results showed a decrease in F. salina’s growth as salinity increased, with total mortality at approximately 89% seawater Hoagland solution . Plants in their non-saline control showed the most growth, measured by the length of the main and lateral shoots . The majority of our plants remained constant in size. The high mortality rate across treatments was driven by aphid infestation, despite attempts to control aphids with Botanigard . The highest mortality occurred in the drought, 60% seawater treatment, suggesting that stringent growing conditions may have made plants more susceptible to aphid-induced mortality. J. carnosa was the only species that added biomass between the first and final surveys. However, growth did not differ across treatments . Other studies have found mixed effects of salinity and drought treatments on growth. One study found that J. carnosa grew best in non-saline or minimal saline environments , using recently germinated individuals with stalks that extended 1-10cm above the growing substrate . In contrast, two other studies found that J. carnosa can tolerate salinities twice as concentrated as seawater, but moderate salinity conditions were ideal . St. Omer and Schlesinger used Hoagland solution in a greenhouse experiment to determine that maximum J. carnosa growth, measured by total dry weight, occurred at about 30% – 60% NaCl, with growth decreasing above 60% salinity.
They did not record plant age . The age of the plants likely impacted the differences in growth among studies due to the difference of energy allocation between mature and immature plants, which would have been exacerbated with higher salinity. Barbour and Davis used younger plants, which may have been more sensitive to treatment effects compared to the St. Omer and Schlesinger experiment , and the results reported here. Our experimental results align more closely with those of St. Omer and Schlesinger , even though our experimental design was more similar to Barbour and Davis . The experiment should also be repeated with younger plants to determine if age has any effect on salinity and drought tolerance. Other experiments that used younger plants observed a decrease in growth or total biomass as salinity levels increased, contrasting with our finding that plants are largely unaffected by salinity. Seedlings are more desirable to use in revegetation operations due to the reduced propagation cost and transplant effort, so it is important to determine the range of conditions young plants can tolerate. Our experiment addressed a knowledge gap regarding halophyte salinity and drought tolerance that could inform the design of future restoration projects and experiments in Pacific coast salt marshes. Revegetation efforts often have low success rates due to the stringent abiotic conditions within the ecotone, which disproportionally affect seedlings . Furthermore, the different natural distributions of halophytes within the ecotone suggest that salinity and drought tolerance could vary among species. In our experiment,dutch buckets for sale treatments had negligible effects on growth or survival – only water potential was affected. These results imply that these five species could survive anywhere within the ecotone, by employing different physiological adaptations – such as succulence, salt glands – to withstand stressful conditions. However, our results are likely not representative of plant performance in the field due to a variety of factors. The timing of our experiment did not align with the natural growth period of the plants, causing us to use mature plants rather than young seedlings. Additionally, our use of 60% seawater is not representative of the tidal inundation that some of the species may experience in the field. Therefore, future experiments will examine how these factors influence outcomes, using lessons learned during this effort. Taken together, findings from this set of experiments will allow us to 1) identify zones within the ecotone maximizing survival and establishment on a by-species basis, or 2) demonstrate that species are flexible enough to compensate for conditions across the ecotone,making careful placement of species unnecessary. In either case, these experiments will provide valuable insight to restoration practitioners. Ultimately, we hope that this work will support rapid and robust strategies to recreate thriving salt marsh systems.The microbial community associated with roots was proposed to be assembled in two steps: first, the rhizosphere is colonized by a subset of the bulk soil community and, second, the rhizoplane and the endosphere are colonized by a subset of the rhizosphere community. Intriguingly, a set of recurring plant-associated microbes has emerged. This review focuses on how plants shape their rhizobiome. On the one hand, common factors among plants likely lead to the assembly of the core microbiome. On the other hand, factors specific to certain plants result in an association with microbes that are not members of the core microbiome. Here, we discuss evidence that plant genetic factors, specifically root morphology and root exudation, shape rhizobiomes.
Initial evidence for an influence of plant genotype on rhizobiome composition was that similar rhizobiomes assembled in association with arabidopsis and barley grown in the same experimental conditions, although they displayed different relative abundances and some specific taxonomic groups. A correlation between phylogenetic host distance and rhizobiome clustering was described for Poaceae species, distant relatives of arabidopsis, rice varieties, and maize lines, but not for closely related arabidopsis species and ecotypes. Distinct rhizobiomes were also described for domesticated plants, such as barley, maize, agave , beet , and lettuce , compared with their respective wild relatives. Interestingly, not all plants have a rhizobiome distinct from bulk soil: some species, such as maize and lotus, have assembled a distinct rhizobiome, whereas other species, such as arabidopsis and rice, assembled a rhizobiome similar to bulk soil. The former species display a strong, and the latter a weak rhizosphere effect . The cause ofthis phenomenon is currently unknown. The strength of the rhizosphere effect varies with the developmental stage of the plant. Similarly, root exudation and microbial communities were found to change with the age of the plant. Furthermore, distinct rhizobiomes were associated with different developmental stages of arabidopsis, rice, and Avena fatua grown during two consecutive seasons. Pioneering studies demonstrated the ability of microbes to alter plant development. Overall, it appears evident that host genotype, domestication, and plant development influence the composition of rhizobiomes. As an alternative to plant developmental stage, residence time of plants in soil was discussed as a hypothesis for successive microbiomes. These contrasting results might be partially explained by differing environmental influences, host plants, or soils, and additional work is needed to resolve these questions. In this review, we discuss root morphology and root exudates as two genetic factors shaping plant–microbiome interactions, and we examine the following aspects: how root morphology and border cells affect rhizobiomes; how plant exudates shape the rhizobiome; and possible plant transport proteins involved in exudation. Figure 1 provides a general overview of exometabolite networks in the rhizosphere, and Box 1 illustrates the interplay between root exudates, border cells, and rhizobiomes in phytoremediation. We conclude by integrating these ideas into a possible scenario of rhizobiome assembly.Rhizobiomes are influenced by their spatial orientation towards roots in two ways. First, the radial proximity of microbial communities to roots defines community complexity and composition, as described in recent publications, and as outlined by the two-step model of microbial root colonization mentioned above. Second, the lateral position of microbes along a root shapes the community, as exemplified by early studies . Importantly, recent microbiome studies take into consideration the former, but not the latter aspect. In this section, we discuss specific microbial associations with various root regions, and the role of spatially distinct root exudation. Root tips are the first tissues that make contact with bulk soil: root tips are associated with the highest numbers of active bacteria compared with other root tissues, and likely select microbes in an active manner. The root elongation zone is specifically colonized by Bacillus subtilis, which suggests a particular role of this zone in plant–microbe interactions. Mature root zones feature a microbial community distinct from root tips. Their community includes decomposers, which could be involved in the degradation of dead cells shedding from old root parts.