Irwin A. Ungar

Effects of thermoperiod on recovery of seed germination of halophytes from saline conditions.

Recovery of seed germination from NaCl salinity of desert shrubs (Haloxylon recurvum and Suaeda fruticosa, and the herbs Zygophyllum simplex and Triglochin maritima was studied under various thermoperiods. The percentage of ungerminated seeds that recovered when they were transferred to distilled water varied significantly with variation in species and thermoperiods. Zygophyllum simplex had little recovery from all NaCl concentrations in all thermoperiods. Haloxylon recurvum, S. fruticosa, and T. maritima showed substantial recovery. Percentage recovery was highest in S. fruticosa, followed by T. maritima, and H. recurvum. Thermoperiodic effects varied with the species investigated. There was little thermoperodic effect on the percentage recovery of S. fruticosa, except in the higher salinity treatment at higher thermoperiods. Variation in thermoperiod appears to play an important role in recovery of germination of halophytes from salt stress when seeds are transferred to distilled water.

Are biotic factors significant in influencing the distribution of halophytes in saline habitats

The influence of biotic factors on the distribution and establishment of halophytes is being considered in this review. Physicochemical factors, such as salinity and flooding, often are considered to be the determining factors controlling the establishment and zonational patterns of species in salt marsh and salt desert environments. Sharp boundaries commonly are found between halophyte communities even though there is a gradual change in the physicochemical environment, which indicates that biotic interactions may play a significant role in deterining the distribution pattern of species and the composition of zonal communities. Competition is hypothesized to play a key role in determining both the upper and lower limits of species distribution along a salinity gradient. Field and laboratory experiments indicate that the upper limits of distribution of halophytes into less saline or nonsaline habitats is often determined by competition. There appears to be a reciprocal relationship between the level of salt tolerance of species and their ability to compete with glycophytes in less saline habitats. Halophytes are not competitive in nonsaline habitats, but their competitive ability increases sharply in saline habitats. Allelopathic effects have been reported in salt desert habitats, but have not been reported along salinity gradients in salt marshes. Some species of halophytes that are salt accumulators have the ability to change soil chemistry. Chemical inhibition of intolerant species occurs when high concentrations of sodium are concentrated in the surface soils of salt desert plant communities that are dominated by salt-accumulating species. Establishment of less salt-tolerant species is inhibited in the vicinity of these salt-accumulating species. Herbivory is reported to cause both an increase and a decrease in plant diversity in salt marsh habitats. Heavy grazing is reported to eliminate sensitive species and produce a dense cover of graminoids in high marsh coastal habitats. However, in other marshes, grazing produced bare patches that allowed annuals and other low marsh species to invade upper marsh zonal communities. A retrogression in plant succession may occur in salt marshes and salt deserts because of heavy grazing. Intermediate levels of grazing by sheep, cattle, and horses could produce communities with the highest species richness and heterogeneity. Grazing by geese produced bare areas that had soils with higher salinity and lower soil moisture than vegetated areas, allowing only the more salt-tolerant species to persist. Removal of geese from areas by use of inclosures caused an increase in species richness in subarctic salt marshes. Invertebrate herbivores could also inhibit the survival of seeds and the ability of plants to establish in marshes. Parasites could play a significant role in determining the species composition of zonal communities, because uninfected rarer species are able to establish in the gaps produced by the death of parasitized species.

The effect of extended exposure to hypersaline conditions on the germination of five inland halophyte species

In order to determine how salinity and exposure time affect seed viability and germination, seeds of five halophytes, Atriplex prostrata, Hordeum jubatum, Salicornia europaea, Spergularia marina, and Suaeda calceoliformis were exposed to 3.0, 5.0, and 10.0% NaCl solutions for 30, 60, 90, 365, and 730 d. Recovery experiments in distilled water indicated significantly different species responses to salinity over time. Percentage germination and rate of germination in H. jubatum were dramatically reduced following extended exposure and all seeds exposed to 10% NaCl for > 1 yr failed to germinate. Spergularia marina seeds were stimulated following short-term exposure to 3% NaCl; however, germination was delayed and overall germination was significantly reduced with exposure time in the two higher salinity levels. Percentage germination in A. prostrata decreased over time, but salinity level was not related to this reduction. Germination of S. europaea and S. calceoliformis, the most salt-tolerant species being tested, was stimulated by exposure to high salinity. Both species had a significant increase in percentage germination and in the germination rate when compared to seeds germinated in distilled water. Baseline germination data from seeds placed in 0, 1, 2, and 3% NaCl solutions indicated that S. europaea and S. calceoliformis were the only species to germinate in the 3% NaCl solution. Spergularia marina failed to germinate in the 2% NaCl treatment, and germination of A. prostrata and H. jubatum was significantly reduced at this salinity level. It is concluded that prolonged exposure to saline solutions can inhibit or stimulate germination in certain species, and the resulting germination and recovery responses are related to the duration and intensity of their exposure to salt in their natural habitats.

Population ecology of halophyte seeds

Some aspects of the population biology of halophytes are considered in this review. Persistent seed banks have been reported for a number of inland- and coastal-salt marsh plant communities. Seeds of perennial grasses are often under-represented, while annuals and some perennial forbs may be over-represented in the seed bank. The persistent seed bank of annual halophytes appears adaptive, and provides multiple seed germination opportunities which may prevent local extinction when environmental stress increases. Somatic seed polymorphism provides a mechanism by which parent plants can respond to changing environments by partitioning their resources into reproductive units which have distinct germination responses. Parental effects may influence either seed morphology and/or physiological requirements of seeds when they are exposed to environmental stress. A prolonged germination period can provide plant populations with numerous opportunities to establish seedling cohorts. Early cohorts will have a selective advantage under moderate conditions because mortality will be low and plants will survive until maturity. However, fluctuations in salinity levels and tidal activity can cause high mortality in early cohorts in salt marsh habitats, providing later cohorts with an opportunity for establishment. Resource allocation to reproductive structures is related to plant size, which itself can be affected by both abiotic and biotic factors. Larger plants were found to produce more seeds than smaller plants in a population, but the mean seed weight was greater in small plants.RésuméCette revue examine quelques aspects de la biologie des peuplements des halophytes. On a observé qu’il y a des réserves permanentes de graines pour certaines populations de plantes de marais salants, tant sur le littoral qu’ à l’intérieur des terres. Les réserves de graines tendent à présenter insuffisamment de graines de plantes vivaces et trop d’annuelles et de certaines herbacées vivaces. La reserve permanente des halophytes annuelles semble être adaptive, et offre aux graines de multiples occasions de germination, ce qui peut empêcher l’extinction locale quand le stress de l’environnement augmente. Le polymorphisme somatique de la graine offre un mécanisme par lequel les plantes-parents peuvent réagir à des environnements variables en subdivisant leurs ressources en des unités reproductrices qui ont des réactions de germination distinctes. Les plants précoces auront un avantage sélectif dans des conditions modérées parce que la mortalité sera faible et que les plantes survivront jusqu’à la maturité. Une période de germination prolongée peut fournir aux populations de plantes de nombreuses occasions d’établir des ordres de plants. Cependant les fluctuations des niveaux de la salinité et de l’activité des marées peut produire une forte mortalité parmi les plants précoces dans ces marais salant, ce qui donne aux plants suivant l’occasion de bien s’établir. Combien la plante possède de structures reproductives est lié à sa taille, qui, elle-même peut être affectée et par les facteurs abiotiques et par les facteurs biotiques. On a constaté que les plantes plus grosses produisent plus de graines que les plantes plus petites dans une population, mais que le poids moyen des graines est plus élevé dans les plantes plus petites.

Germination ecology of halophytes

Germination is an important stage in the life cycle of species growing in saline environments because it determines the soil conditions later stages in the life cycle will be exposed to. Laboratory investigations of seed germination indicate that seeds of most halophytic species reach their maximum germination in distilled water (Seneca, 1969; Onnis and Bellettato, 1972; Breen et al. 1977; Okusanya, 1977; Ungar, 1977a, 1978a; Zid and Boukhris, 1977; Dietert and Shontz, 1978). Seed germination in saline environments ordinarily occurs during the spring or in a season with high precipitation, when soil salinity levels are usually reduced (Ward, 1967; Chapman, 1974; McMahon and Ungar, 1978). Several studies have shown that seeds of many halophytes remain dormant when exposed to low water potentials (Ungar, 1962, 1974a, b, 1975, 1978a; Williams and Ungar, 1972).

Effects of salinity on growth, ion content, and osmotic relations in Halopyrum mucronatum (L.) Stapf.

Abstract Halopyrum mucronatum (L.) Stapf. is a perennial grass found on the coastal dunes of Karachi, Pakistan. Halopyrum mucronatum plants were grown in 0, 90,180, and 360 mol m‐3 NaCl in a sand culture using a sub‐irrigation method. Fresh and dry weight of roots and shoots peaked at 90 mol m‐3 NaCl. A further increase in salinity inhibited plant growth, ultimately resulting in plant death at 360 mol m‐3 NaCl. The relative growth rate of plants was highest between 60 and 90 days after final salinity concentrations were reached. Maximum succulence was noted in 90 mol m‐3 NaCl. Water potential and osmotic potential of plants became more negative with an increase in salinity, while plants lost turgor with increasing salinity. Time of harvest did not have any significant effect on the water relations of plants. Sodium (Na) and chloride (Cl) content of plants increased with an increase in salinity, while calcium (Ca), magnesium (Mg), and potassium (K) content decreased. Glycinebetaine content of shoots increa...

Salt Tolerance of a Coastal Salt Marsh Grass

Growth, water and osmotic relations were examined in the halophytic perennial grass Urochondra setulosa (Trin.) C.E. Hubbard from a population located on the coast of Arabian Sea near Karachi, Pakistan. Urochondra setulosa plants were grown in 0, 200, 400, 600, 800, and 1000 mM NaCl in a sand culture using sub‐irrigation. Number of leaves and tillers, root length and shoot length, fresh weight and dry weight of roots and shoots were highest in the non‐saline control and decreased with an increase in salinity. Some plants survived in salinities above 600 mM NaCl, but their growth was stunted and mortality was high. Maximum succulence as estimated from plant water content was found in 0 mM NaCl. Water potential, osmotic potential and stomatal conductance became more negative with an increase in salinity, while pressure potential decreased with increasing salinity. Percentage ash content of shoots did not vary with increases in salinity. Na+ and Cl− levels increased in shoots while K+ concentration decreased with increases in salinity.

Effects of sodium chloride treatments on growth and ion accumulation of the halophyte Haloxylon recurvum

Abstract Effects of increasing salt concentrations 0, 180, 360 mol im3 sodium chloride (NaCl), on growth, succulence, mineral composition, and glycinebetaine content in Haloxylon recurvum was investigated. Fresh and dry weight of plants increased with an increase in salinity. Succulence of shoots increased at low salinity and decreased at high salinity. Root and shoot Ca+, Mg+, and K+content decreased with increasing salinity while both Na+ and Cl‐ content increased, reaching 4,900 and 5,300 mmol kg‐1 dry weight, respectively. Glycinebetaine (mol m‐3 tissue water) significantly increased in shoots at 360 mol m‐3 NaCl, but did not differ significantly in roots treated with from 0 to 360 mol m‐3 NaCl. Haloxylon recurvum is a highly salt tolerant stem succulent plant which accumulate a high quantity of salt, which makes it a good candidate to use for phytoremediation in highly saline areas of the sub‐tropics.

Effect of salinity on growth, ion content, and cell wall chemistry in Atriplex prostrata (Chenopodiaceae).

Atriplex prostrata Boucher, a facultative halophyte, exhibits significant reduction in height and biomass and in the width of the cortex and vascular tissue under saline conditions. Therefore, the goal of this investigation was to determine the effect of salinity on plant growth as well as on the patterns of lignification, peroxidase activity, and extensin deposition. Biomass, leaf area, internode length, water potential, photosynthesis, transpiration, and ion content were measured. In addition, lignin, peroxidase, and extensin were, respectively, examined via phloroglucinol staining, peroxidase staining, and immunostaining with extensin antibody on tissue prints of free-hand stem sections. Length of internodes and leaf area significantly decreased with increased salinity, and net photosynthesis declined dramatically as well. There was a significant accumulation of Na+ in organs when plants were grown in saline solutions, while the concentration of K+, Ca2+, and Mg2+ decreased. The signals in tissue prints showed that soluble peroxidase and extensin accumulated in the first three internodes of A. prostrata grown under saline conditions. In contrast, lignification was reduced under saline growth conditions in the third and fourth internodes. These results indicate that extensin may replace lignin in providing mechanical support for cells, while stems remain in a juvenile stage because of growth retardation caused by salinity.

Seed Polymorphism and Germination Responses to Salinity Stress in Atriplex triangularis Willd.

Seeds of Atriplex triangularis exhibited a very pronounced morphological and physiological seed polymorphism. Seed size varied from 1.0 to 2.8 mm and predicted the likelihood of successful establishment through its effect on germination and seedling vigor. Large seeds had a mean dry weight of 2.44 ± 0.16 mg and a mean length of 2.45 ± 0.24 mm; medium seeds, mean dry weight of 1.21 ± 0.10 mg and mean length of 1.78 ± 0.19 mm; small seeds, mean dry weight of 0.64 ± 0.04 mg and mean length of 1.27 ± 0.10 mm. The degree of salt tolerance increased progressively with increasing seed size. Seeds from all size classes that were initially treated with 2%-5% NaCl had from 85% to 100% germination after being immersed in distilled water for 6 days, indicating a transitory adverse effect of salt stress on germination. The amount of water absorbed by all seeds is influenced by change in media salinity but not by hormonal treatments. Small seeds contain more Na+ and Cl- than medium and large seeds. Seedling dry weight was related to initial seed size. Salt stress inhibited seedling growth. Gibberellic acid alleviated some of the dormancy in seeds induced by high salt concentrations.