J. Jed Brown

Salt Tolerance and Crop Potential of Halophytes

Although they represent only 2% of terrestrial plant species, halophytes are present in about half the higher plant families and represent a wide diversity of plant forms. Despite their polyphyletic origins, halophytes appear to have evolved the same basic method of osmotic adjustment: accumulation of inorganic salts, mainly NaCl, in the vacuole and accumulation of organic solutes in the cytoplasm. Differences between halophyte and glycophyte ion transport systems are becoming apparent. The pathways by which Na+ and Cl– enters halophyte cells are not well understood but may involve ion channels and pinocytosis, in addition to Na+ and Cl– transporters. Na+ uptake into vacuoles requires Na+/H+ antiporters in the tonoplast and H+ ATPases and perhaps PPi ases to provide the proton motive force. Tonoplast antiporters are constitutive in halophytes, whereas they must be activated by NaCl in salt-tolerant glycophytes, and they may be absent from salt-sensitive glycophytes. Halophyte vacuoles may have a modified lipid composition to prevent leakage of Na+ back to the cytoplasm. Becuase of their diversity, halophytes have been regarded as a rich source of potential new crops. Halophytes have been tested as vegetable, forage, and oilseed crops in agronomic field trials. The most productive species yield 10 to 20 ton/ha of biomass on seawater irrigation, equivalent to conventional crops. The oilseed halophyte, Salicornia bigelovii, yields 2 t/ha of seed containing 28% oil and 31% protein, similar to soybean yield and seed quality. Halophytes grown on seawater require a leaching fraction to control soil salts, but at lower salinities they outperform conventional crops in yield and water use efficiency. Halophyte forage and seed products can replace conventional ingredients in animal feeding systems, with some restrictions on their use due to high salt content and antinutritional compounds present in some species. Halophytes have applications in recycling saline agricultural wastewater and reclaiming salt-affected soil in arid-zone irrigation districts.

Salt tolerance and osmotic adjustment of Spartina alterniflora (Poaceae) and the invasive M haplotype of Phragmites australis (Poaceae) along a salinity gradient.

An invasive variety of Phragmites australis (Poaceae, common reed), the M haplotype, has been implicated in the spread of this species into North American salt marshes that are normally dominated by the salt marsh grass Spartina alterniflora (Poaceae, smooth cordgrass). In some European marshes, on the other hand, Spartina spp. derived from S. alterniflora have spread into brackish P. australis marshes. In both cases, the non-native grass is thought to degrade the habitat value of the marsh for wildlife, and it is important to understand the physiological processes that lead to these species replacements. We compared the growth, salt tolerance, and osmotic adjustment of M haplotype P. australis and S. alterniflora along a salinity gradient in greenhouse experiments. Spartina alterniflora produced new biomass up to 0.6 M NaCl, whereas P. australis did not grow well above 0.2 M NaCl. The greater salt tolerance of S. alterniflora compared with P. australis was due to its ability to use Na(+) for osmotic adjustment in the shoots. On the other hand, at low salinities P. australis produced more shoots per gram of rhizome tissue than did S. alterniflora. This study illustrates how ecophysiological differences can shift the competitive advantage from one species to another along a stress gradient. Phragmites australis is spreading into North American coastal marshes that are experiencing reduced salinities, while Spartina spp. are spreading into northern European brackish marshes that are experiencing increased salinities as land use patterns change on the two continents.

Halophytes for the treatment of saline aquaculture effluent

We determined the feasibility of using salt-tolerant plants (halophytes) as biofilters to remove nutrients from saline aquaculture wastewater. Suaeda esteroa, Salicornia bigelovii and Atriplex barclayana (Chenopodiaceae), species with potential as forage and oil seed crops, were grown in sand in draining containers (lysimeters) in a greenhouse experiment. They were irrigated to meet evapotranspiration demand and to produce a 0.3 leaching fraction, using aquaculture effluent generated from an intensive tilapia culture system. The effluent salinity was increased with NaCl to make salinity treatments of 0.5, 10 and 35 ppt. The plant–soil system removed 98% and 94% of the applied total and inorganic nitrogen, respectively. It removed 99% and 97% of the applied total and soluble reactive phosphorus, respectively. High removal rates occurred despite the high leaching fraction. Salt inhibited (P<0.05) the growth rate, nutrient removal, and volume of water that all three plant species could process. Suaeda and Salicornia, which are succulent salt marsh species, performed better than the desert saltbush, Atriplex, at the higher salinities.

EFFECTS OF SOIL SALT LEVELS ON THE GROWTH AND WATER USE EFFICIENCY OF ATRIPLEX CANESCENS (CHENOPODIACEAE) VARIETIES IN DRYING SOIL

The effect of salt stress on the growth and water use efficiency of the xerohalophyte Atriplex canescens (Pursh.) Nutt. in drying soil was determined by growing plants to the wilting point in soils receiving a one-time irrigation of nutrient solution containing low, medium, and high levels of NaCl. The experiment compared three varieties of A. canescens that differed in salt tolerance and capacity for Na and K uptake in previous research. Contrary to expectations, we did not find that water and salt stress were strictly additive in reducing plant performance. Soil salts enhanced the growth performance of the plants in drying soil by increasing their days to wilting, ability to extract water from the soil, organic matter production, and water use efficiency. The variety with the highest salt tolerance also had the highest growth rates and water use efficiency on drying soils. We conclude that tolerances to water and salt stress are linked through a common mechanism of Na uptake for osmotic adjustment in this species.

Reuse of highly saline aquaculture effluent to irrigate a potential forage halophyte, Suaeda esteroa

We tested the feasibility of reusing saline aquaculture effluent to produce a salt-tolerant shrub (Suaeda esteroa) with potential as a forage crop. Plants were grown in sandy loam soil, in drainage lysimeters to determine forage yield, water use and capacity for nitrogen and phosphorus uptake when irrigated with highly saline (31 ppt NaCl) effluent from a tilapia culture system. Water was applied to soil three times per week at five rates, ranging in volume from 50 to 250% of the pan evaporation rate. Plant biomass increased significantly with increasing irrigation volume (P<0.05). Due to higher plant growth, water consumption also increased with increasing irrigation volume (P<0.05). Nitrate concentrations in water draining from the lysimeters decreased during the experiment and decreased with increasing irrigation volume (P<0.05). Toward the end of the experiment, concentrations of nitrate in the leachate in the high volume treatments were below the mean limits set by the US Environmental Protection Agency for effluent discharge. Phosphorus concentrations in the leachate water increased during the experiment and increased with increasing irrigation volume (P<0.05). We conclude that using high salinity aquaculture wastewater to irrigate halophyte crops can be a viable strategy for disposal of effluent, especially where phosphorus is not a limiting nutrient.

A sustainable culture system for Gracilaria parvispora (Rhodophyta) using sporelings, reef growout and floating cages in Hawaii

Abstract A culture system for the edible, red seaweed, Gracilaria parvispora Abbott (long ogo), was developed in Hawaii that utilized a hatchery to produce tetrasporophyte and gametophyte life stages of the seaweed, reef growout of sporelings to harvest size adults, and multiplication of the harvested thalli in floating cages prior to sale. A central cooperative operated the hatchery and floating cages, and marketed the product. Sporelings from the hatchery were distributed to coastal residents who established patches of seaweed on the reef and sold their harvest to the cooperative. Mean relative growth rate of seaweed in the cages over 52 weeks was 2.64% d −1 and productivity was 14.8 g m −2 d −1 (dry weight), within the range of intensive culture systems. Cage cultures were not sensitive to water motion over the range of 4–14 cm s −1 but growth and productivity tended to be higher in summer and spring than in winter. The culture system potentially overcomes problems that have hindered development of a sustainable supply of this species: low availability of wild stocks due to overharvesting; low productivity of spore cultures; and deterioration of vegetative cultures over time. Some of the elements may be applicable to other areas where wild stocks of Gracilaria have been overharvested.

Halophytes for the Production of Liquid Biofuels

We discuss the potential of using halophytes as a source for producing liquid biofuels. We review the potential pathways for converting oilseeds into biodiesel and bio-derived synthetic paraffinic kerosene and presents some preliminary data on biomass composition and pretreatment of the halophyte Salicornia bigelovii. Six samples of S. bigelovii cultivated at three fertilizer levels (F1: 1 gN/m2, F2: 1.5 gN/m2 and F3: 2 gN/m2) and two salinity levels (S1: 10 ppt and S5: 50 ppt salt) were analyzed with regard to chemical composition and bioethanol potential. Chemical characterization showed that S. bigelovii contained, 16.31–55.67 g/100gTS (total solids) of carbohydrates, 5.42–16.60 g/100gTS of lignin, 27.85–66.37 g/100gTS of total extractives (including extractable ash), and 2.18–9.68 g/100gTS of structural ash, depending on the plant fraction and cultivation conditions. Enzymatic hydrolysis of the pretreated samples revealed high glucose recoveries of up to 90 % (of glucose in raw S. bigelovii) corresponding to ethanol yield of 111 kg ethanol/dry ton S. bigelovii.

Feasibility of Halophyte Domestication for High-Salinity Agriculture

We discuss the process of domesticating wild halophytes to serve as crop plants using seawater irrigation. First steps in this domestication involve determining whether halophyte species exist that may produce significant amounts of a usable product under seawater irrigation and that this is a sustainable agronomic practice. This is followed by development of strategies to improve crop productivity via selecting appropriate species for domestication and then affecting agronomic traits through plant breeding. We demonstrate that halophytes may be productive under seawater irrigation, that this management system may be sustainable, and there are demonstrated pathways toward domestication.