Edward P. Glenn

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.

Integrating Remote Sensing and Ground Methods to Estimate Evapotranspiration

Evapotranspiraton (ET) is the second largest term in the terrestrial water budget after precipitation, and ET is expected to increase with global warming. ET studies are relevant to the plant sciences because over 80% of terrestrial ET is due to transpiration by plants. Remote sensing is the only feasible means for projecting ET over large landscape units. In the past decade or so, new ground and remote sensing tools have dramatically increased our ability to measure ET at the plot scale and to scale it over larger regions. Moisture flux towers and micrometeorological stations have been deployed in numerous natural and agricultural biomes and provide continuous measurements of actual ET or potential ET with an accuracy or uncertainty of 10–30%. These measurements can be scaled to larger landscape units using remotely-sensed vegetation indices (VIs), Land Surface Temperature (LST), and other satellite data. Two types of methods have been developed. Empirical methods use time-series VIs and micrometeorological data to project ET measured on the ground to larger landscape units. Physically-based methods use remote sensing data to determine the components of the surface energy balance, including latent heat flux, which determines ET. Errors in predicting ET by both types of methods are within the error bounds of the flux towers by which they are calibrated or validated. However, the error bounds need to be reduced to 10% or less for applications that require precise wide-area ET estimates. The high fidelity between ET and VIs over agricultural fields and natural ecosystems where precise ground estimates of ET are available suggests that this might be an achievable goal if ground methods for measuring ET continue to improve.

Salicornia bigelovii Torr.: An Oilseed Halophyte for Seawater Irrigation

The terrestrial halophyte, Salicornia bigelovii Torr., was evaluated as an oilseed crop for direct seawater irrigation during 6 years of field trials in an extreme coastal desert environment. Yields of seed and biomass equated or exceeded freshwater oilseed crops such as soybean and sunflower. The seed contained 26 to 33 percent oil, 31 percent protein, and was low in fiber and ash (5 to 7 percent). The oil and meal were extracted by normal milling equipment, and the oil was high in linoleic acid (73 to 75 percent) and could replace soybean oil in chicken diets. The meal had antigrowth factors, attributed to saponins, but could replace soybean meal in chicken diets amended with the saponin antagonist, cholesterol. Salicornia bigelovii appears to be a potentially valuable new oilseed crop for subtropical coastal deserts.

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.

Assessment of C budget for grasslands and drylands of the world

Intergovernmental Panel on Climate Change (IPCC) estimates indicate that potential changes in seasonal rainfall and temperature patterns in central North America and the African Sahel will have a greater impact on biological response (such as plant production and biogeochemical cycling) and feedback to climate than changes in the overall amount of annual rainfall. Simulation of grassland and dryland ecosystem responses to climate and CO2 changes demonstrates the sensitivity of plant productivity and soil C storage to projected changes in precipitation, temperature and atmospheric CO2. Using three different land cover projections, changes in C levels in the grassland and dryland regions from 1800 to 1990 were estimated to be −13.2, −25.5 and −14.7 Pg, i.e., a net source of C due to land cover removal resulting from cropland conversion. Projections into the future based on a double-CO2 climate including climate-driven shifts in biome areas by the year 2040 resulted in a net sink of +5.6, +27.4 and +26.8 Pg, respectively, based upon sustainable grassland management. The increase in C storage resulted mainly from an increase in area for the warm grassland sub-biome, together with increased soil organic matter. Preliminary modeling estimates of soil C losses due to 50 yr of regressive land management in these grassland and dryland ecoregions result in a 11 Pg loss relative to current conditions, and a potential loss of 37 Pg during a 50 yr period relative to sustainable land-use practices, an average source of 0.7 Pg C yr−1. Estimates of the cost of a 20 yr rehabilitation program are 5 to 8×109 US

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

yr−1, for a C sequestering cost of approximately 10 US

Comparison of transpiration rates among saltcedar, cottonwood and willow trees by sap flow and canopy temperature methods

per tC.

Cultivation of Gracilaria parvispora (Rhodophyta) in shrimp-farm effluent ditches and floating cages in Hawaii : a two-phase polyculture system

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.

Growth of the seaweeds Kappaphycus alvarezii, K. striatum and Eucheuma denticulatum as affected by environment in Hawaii

Transpiration (Et), measured by stem sap flow gauges, and canopy and air temperature differential ( Tc − Ta )o fPopulus fremontii (cottonwood), Salix gooddingii (willow) and Tamarix ramosissima (saltcedar) were compared to determine if remotely sensed canopy temperatures could be used to estimate Et or water stress in these trees in desert riparian zones of the United States and Mexico. Controlled experiments were conducted in which containerized plants were placed closely together and allowed to grow into a single, dense canopy over a summer in a desert climate. At the end of the growth period, two canopies of each species were measured for Et and Tc − Ta over 11 days, first under unstressed conditions then under water or salt stress. Et and Tc − Ta were significantly ( P< 0.05) correlated for all species. Correlation coefficients improved when a radiation term was included in the equation predicting Et from Tc − Ta. During the non-stress part of the experiment, canopies of all three species had similar rates of Et, but saltcedar maintained higher Et rates and lower rsv than the native trees on the stress treatments. For each species, models were developed, using both meteorological data and a canopy, energy-balance equation, to predict daily Et and stomatal resistance (rsv); these models had standard errors of 15–22% when compared with measured Et over the unstressed portion of the experiment.