Salt tolerance in Aster tripolium L. II. Ionic regulation

Abstract Measurements of tissue ion contents (Na, K and Cl) were carried out at frequent intervals on plants of Aster tripolium L. grown at a range of salinities for 36 d. Aster tripolium behaved as a typical halophyte showing high levels of inorganic ion accumulation even at low salinities. As salinity increased Na replaced K to a large extent in the shoot but root K was unaffected up to 500 mol m^?3 external NaCl. Shoot (Na + K) concentration on a tissue water basis was maintained constant in all treatments throughout the experiment, whereas shoot (Na + K) on a dry weight basis showed marked fluctuations in some treatments. An increase in (Na + K) per gram dry weight was, however, accompanied by a parallel increase in fresh weight: dry weight (FW : DW) ratio. Transport of (Na + K) to the shoot per unit root weight changed during the experiment in the manner expected, given the observed changes in shoot relative growth rate and FW : DW to result in a constant shoot (Na + K) concentration on a water basis. Chloride was the major balancing anion in the shoot at high salinity, but never accounted for more than 38% of the (Na + K) found in the root tissue. At all salinities (Na + K) salts accounted for the majority of the measured shoot sap osmotic potential. The interactions between salinity, growth, ion transport and osmotic adjustment are discussed.     

Salt tolerance in Aster tripolium L. I. The effect of salinity on growth

Abstract A study of the growth of the maritime halophyte Aster tripolium L. has been carried out over a range of salinity treatments. The regression approach to growth analysis using frequent small harvests has been used to allow ‘continuous’ measurement of growth over a period of 36 d. Salinity was applied with the major ions present in ratios typical of those found in seawater. Growth was inhibited in terms of both dry weight production and leaf expansion at salinity levels equivalent to 0.625 strength sea water (full culture solution 300) and above, with the greatest effect being seen in terms of leaf area. Aster tripolium did not show increased succulence at high salinity, leaf fresh weight to dry weight ratio in fact declined, whilst leaf fresh weight per unit area remained constant. It should be noted that the plants exhibit low growth rates due to the low light intensity used.     

Genetic diversity in natural and anthropogenic inland populations of salt-tolerant plants: random amplified polymorphic DNA analyses of Aster tripolium L. (Compositae) and Salicornia ramosissima Woods (Chenopodiaceae)

Eight populations of Aster tripolium (Compositae) and six of Salicornia ramosissima (Chenopodiaceae) from inland, naturally salt-contaminated habitats and anthropogenic salt-polluted sites in central Germany (Thuringia, Anhalt-Saxony) were analysed using random amplified polymorphic DNA (RAPD) markers to investigate the patterns of genetic variation. In both species, the genetic diversity observed in the younger, anthropogenic sites caused by potash mines during the last century was found to be not significantly lower than in the older, naturally salt-contaminated habitats. Therefore, it is speculated that the loss of genetic diversity caused by founder effects on the anthropogenic habitats was balanced by successive colonization events, actual gene flow between populations, or the rapid growth of populations on the secondary habitats after colonization. Analyses of molecular variance (amova) of the RAPD markers, neighbour-joining clustering of populations based on Reynolds' co-ancestry distances, and Mantel tests indicate that: (i) anthropogenic habitats were colonized independently; (ii) genetic differentiation among populations of S. ramosissima is more pronounced than in A. tripolium, which is considered to be mainly due to biological differences between the two species; and (iii) the geographical pattern of genetic diversity was considerably modulated by historical events and/or population genetic effects.     

Growth and protein profile responses in the halophyte sea aster (Aster tripolium L.) suspension-cultured cells to salinity

We established salt tolerant cell lines, which survived and grew under high salinity conditions with 150 mM (S-150) and 300 mM (S-300) NaCl, to study the effects of salt stress on the proliferation and protein profile of these cells in the halophyte sea aster,Aster tripolium L. These salt-adapted cell lines were produced from leaves and selected by repeated suspension subculture in media containing NaCl every 25 days for five cycles. S-150 cells displayed no inhibition in their growth compared to control cells maintained under non-stressed conditions. S-150 cells exhibited approximately a 15-fold increase in both fresh and dry weight during the 25 days under saline conditions. S-300 cells showed positive growth under severe salt stress, but their dry matter gain was significantly less than that of the S-150 cells, with only a 2.5-fold increase in dry weight. We also detected changes in the protein profile of salt-adapted cells with two specifically induced polypeptides (basic 58.4 and acidic 24.8 kDa) and one enhanced polypeptide (basic 15.1 kDa) in the soluble fraction, and one specifically induced polypeptide (42.0 kDa) in the insoluble fraction.     

No escape: The influence of substrate sodium on plant growth and tissue sodium responses

1. As an essential micronutrient for many organisms, sodium plays an important role in ecological and evolutionary dynamics. Although plants mediate trophic fluxes of sodium, from substrates to higher trophic levels, relatively little comparative research has been published about plant growth and sodium accumulation in response to variation in substrate sodium. Accordingly, we carried out a systematic review of plants'' responses to variation in substrate sodium concentrations.
2. We compared biomass and tissue‐sodium accumulation among 107 cultivars or populations (67 species in 20 plant families), broadly expanding beyond the agricultural and model taxa for which several generalizations previously had been made. We hypothesized a priori response models for each population''s growth and sodium accumulation as a function of increasing substrate NaCl and used Bayesian Information Criterion to choose the best model. Additionally, using a phylogenetic signal analysis, we tested for phylogenetic patterning of responses across taxa.
3. The influence of substrate sodium on growth differed across taxa, with most populations experiencing detrimental effects at high concentrations. Irrespective of growth responses, tissue sodium concentrations for most taxa increased as sodium concentration in the substrate increased. We found no strong associations between the type of growth response and the type of sodium accumulation response across taxa. Although experiments often fail to test plants across a sufficiently broad range of substrate salinities, non‐crop species tended toward higher sodium tolerance than domesticated species. Moreover, some phylogenetic conservatism was apparent, in that evolutionary history helped predict the distribution of total‐plant growth responses across the phylogeny, but not sodium accumulation responses.
4. Our study reveals that saltier plants in saltier soils proves to be a broadly general pattern for sodium across plant taxa. Regardless of growth responses, sodium accumulation mostly followed an increasing trend as substrate sodium levels increased.

     

Accumulation of free proline and glycine betaine in Aster tripolium subjected to a saline shock: A kinetic study related to light period

On transferring three-week-old plants of Aster tripolium L. growing in a half strength Hoagland's medium to the same medium containing 333 m M NaCl a very quick uptake of salt and, after a lag phase of 3 to 5 h, an increase in free proline level was observed. During the time course of imino acid storage, the accumulation rates were higher in the light than in the dark, thereby suggesting some kind of photocontrol on solute metabolism. At zero time, high levels of glycine betaine were present in young plants grown without salt. However, after the application of saline shock, the betaine level also increased significantly. The highest rate of betaine accumulation was detected during the third day of treatment when the rate of proline storage decreased. Glycine betaine storage could also be linked to light dependent processes; whatever its importance in response to salt shock was, the levels observed were lower than those of plants directly grown on 333 m M NaCl for three weeks. When saline stressed plants were transferred to a medium without NaCl, the proline level quickly decreaed while that of glycine betaine remained stable.     

Comparison of Distichlis spicata and Suaeda aegyptiaca in response to water salinity: Candidate halophytic species for saline soils remediation

Distichlis spicata and Suaeda aegyptiaca are two potential halophytic plant species for bioremediation of salt degraded soils, and development of saline agriculture. The physiological responses of the species to different levels of salinity (EC 0, 12, 24, 36, and 48 dS/m) in a controlled environment experiment were studied. Both species showed a high level of tolerance to elevated concentrations of salt in the irrigation water. The shoot fresh and dry weights in S. aegyptiaca increased till 36 dS/m and were sustained under 48 dS/m while in D. spicata, both parameters decreased as salinity increased. Glycine betaine accumulation did not change in D. spicata with increasing salinity, whereas proline content revealed a marked increase of 7.13 fold in 48 dS/m salinity compared to the control, which showed its critical osmoprotection role in the plant. In S. aegyptiaca, both osmolytes content significantly increased at high salinity levels (36 and 48 dS/m) up to 3.22 and 2.0 folds, respectively. Overall, S. aegyptiaca had a better potential of Na⁺ phytoremediation, and tolerated higher salinity compared to D. spicata. In contrast, the vigorous root and rhizome growth in D. spicata made it a proper solution for protecting the soils against further erosion under saline conditions.     

Comparison of tissue culture and whole plant responses to salinity in potato

Salt sensitivities of six potato cultivars using six levels of sodium chloride (0.0 to 0.25M) were studied in a greenhouse. Responses of these cultivars were also determined in tissue culture by studying rooting of stem segments, increase in length of cultured roots and inhibition of growth of cell suspension cultures using similar salt concentrations. Responses of cultured stem segments and cell suspensions differed from those expressed by whole plants. A close similarity was observed between the salt stress response of whole plants and of cultured roots. The latter technique may provide a preliminary screening method for assessing salt tolerance in potato genotypes.     

Inorganic ions modulate the path of phloem loading of sucrose in Ricinus communis L. seedlings

The impact of inorganic ions on sucrose fluxes in the cotyledons and on the pathway of phloem loading was studied in Ricinus communis L. seedlings. The cotyledons were incubated in defined solutions which contained either potassium, sodium, magnesium or calcium as chloride salts, or the sodium salts of sulphate or phosphate. Sucrose uptake from the medium into the cotyledons was only slightly affected by the salts. Sucrose efflux to the medium was increased by phosphate and sulphate and to a lesser extent by sodium and potassium. Phloem loading from the apoplasm and the symplasm was analysed by addition of labelled sucrose to the medium, determination of the specific radioactivity of sucrose in sieve-tube exudate and quantification of export into the seedling axis. Potassium and sodium stimulated the apoplasmic route of phloem loading of sucrose, mostly at the expense of loading from cotyledon sucrose pools. In contrast, sulphate and phosphate strongly inhibited the apoplasmic route whereas the (small) symplasmic flux from the cotyledon sucrose pools was less affected. Magnesium ions inhibited phloem loading by both pathways. The potential of ions in modulating the pathways of sucrose export in day to day operation of plants is discussed.     

Allocation pattern,ion partitioning,and chlorophyll a fluorescence in Arundo donax L. in responses to salinity stress

Biometric and physiological analyses of salt stress responses were performed in two time-course experiments on giant reed (Arundo donax L). Experiment I evaluated biomass production in plants exposed to 128, 256, 512 mM NaCl for 84 days. For Experiment II, plants grown under 256 mM NaCl were further assessed for chlorophyll a fluorescence, ionic partitioning, and proline content at 14 and 49 days after treatment (DAT). Biomass allocation was affected with all the concentrations of NaCl used from 28 DAT onward. Proline biosynthesis in leaves was more stimulated than that in roots after salt stress. Photosynthetic efficiency of photosystem II (PSII) was not affected by salt stress up to 42 DAT, while 49 DAT plants exhibited a significant reduction of both potential (Φ_PSII) and maximal (F_v/F_m) PSII quantum yield. A. donax resulted a moderately sensitive species in response to 256 and 512 mM NaCl, concentrations that are however higher than that commonly found in most marginal lands (such as 128 mM or lower), where the biomass yield is appreciable, especially in short-term cultivation (56 DAT here). Altogether, this study indicates that A. donax can be considered as a promising and valuable energy crop for exploiting the Mediterranean marginal land.     

Salt sensitivity in chickpea (Cicer arietinum L.): ions in reproductive tissues and yield components in contrasting genotypes

The reproductive phase in chickpea (Cicer arietinum L.) is affected by salinity, but little is known about the underlying cause. We investigated whether high concentrations of Na⁺ and Cl^– in the reproductive structures influence reproductive processes. Chickpea genotypes contrasting in tolerance were subjected to 0, 35 or 50 mm NaCl applied to soil in pots. Flower production and abortion, pod number, percentage of empty pods, seed number and size were evaluated. The concentrations of Na⁺, K⁺ and Cl^– were measured in various plant tissues and, using X‐ray microanalysis, in specific cells of developing reproductive structures. Genotypic variation in reproductive success measured as seed yield in saline conditions was associated with better maintenance of flower production and higher numbers of filled pods (and thus seed number), whereas seed size decreased in all genotypes. Despite the variation in reproductive success, the accumulation of Na⁺ and Cl^– in the early reproductive tissues of developing pods did not differ between a tolerant (Genesis836) and a sensitive (Rupali) genotype. Similarly, salinity tolerance was not associated with the accumulation of salt ions in leaves at the time of reproduction or in seeds at maturity.     

Ionic partitioning and stomatal regulation: Dissecting functional elements of the genotypic basis of salt stress adaptation in grafted melon

Vegetable grafting is commonly claimed to improve crop’s tolerance to biotic and abiotic stresses, including salinity. Although the use of inter-specific graftings is relatively common, whether the improved salt tolerance should be attributed to the genotypic background rather than the grafting per se is a matter of discussion among scientists. It is clear that most of published research has to date overlooked the issue, with the mutual presence of self-grafted and non-grafted controls resulting to be quite rare within experimental evidences. It was recently demonstrated that the genotype of the rootstock and grafting per se are responsible respectively for the differential ion accumulation and partitioning as well as to the stomatal adaptation to the stress. The present paper contributes to the ongoing discussion with further data on the differences associated to salinity response in a range of grafted melon combinations.     

Action potentials as multifunctional signals in plants: a unifying hypothesis to explain apparently disparate wound responses

Abstract I suggest that action potentials, which occur in most, if not all, plants, play a major role in intercellular and intracellular communication. Long-distance communication is achieved through the transmitted changes in membrane potential, whereas local signalling is achieved through changes in the subcellular localization of ions (K⁺, Cl^?, and Ca²⁺) and of water, and perhaps by membrane depolarization and current flow. These local changes in ion concentration can lead to modified activities of enzymes in the cell wall (e.g. pectinase), the plasma membrane (e.g. cellulose synthetase, callose synthetase), and the cytoplasm (e.g. protein kinase). In particular, the elevated concentration of cytoplasmic Ca²⁺ is shown to have a major, yet complex, role, especially in the modulation of translation. I suggest further that, in addition to their role in wounding, action potentials may also be important signalling mechanisms for chilling injury, mechanical perturbation and invasion by pathogens, as well as for light and gravity.     

Stomatal response to humidity in a sugarcane field: simultaneous porometric and micrometeorological measurements*

Abstract. Gas exchange data obtained with wellventilated leaf cuvettes provide clear evidence of a stomatal response to leaf-air vapour pressure difference (V). In contrast, remotely sensed leaf temperatures with specific assumptions regarding canopy boundary layer characteristics, have been interpreted to mean that stomata do not respond to V. We address this apparent discrepancy in a sugarcane field by simultaneous application of a single-leaf, porometric technique and a whole-canopy, Bowen ratioenergy balance technique. These methods indicated significant stomatal response to V in well-irrigated sugarcane. Stomatal responses to V in the field were obscured by strong covariance of major environmental parameters so that opening responses to light and closing responses to V tended to offset each other. Low boundary layer conductance significantly uncoupled V at the leaf surface (V_s) from V determined in the bulk atmosphere (V_a). This reduced the range of the stimulus, V_s, thereby reducing the range of the stomatal response, without indicating low stomatal sensitivity to V. Stomatal responses to V_a may be smaller than expected from V response curves in cuvettes, since V_s rather than the conventionally measured V_a is analogous to V in a well-stirred cuvette. Recently published conclusions that remotely sensed canopy temperatures are inconsistent with stomatal response to V may be based on erroneous estimates of canopy boundary layer conductance and thus of V_s, use of air saturation deficit rather than V to express evaporative demand, and investigation at higher levels of evaporative demand than those eliciting maximal stomatal response.     

Sodium and potassium transport to the xylem are inherited independently in rice, and the mechanism of sodium: potassium selectivity differs between rice and wheat

The heritability of sodium and potassium transport to the xylem was measured by the regression of F_n+1, on F_n means in two segregating breeding populations of rice (Oryza sativa L.). The narrow-sense heritabilities of shoot sodium concentration were 0.42 and 0.43 in the two populations, respectively, and the corresponding values for the heritability of shoot potassium concentration were 0.46 and 0.52. The sodium: potassium ratio was apparently heritable (0.36 and 0.40) because it was regressed positively on sodium concentration and negatively on potassium concentration. There was no significant relationship between the shoot sodium and potassium concentrations themselves. It is concluded that sodium and potassium uptake in rice are controlled by different genes which segregate independently. The magnitude of the transpirational bypass flow was estimated to be some 10 times greater in rice than in wheat (Triticum aestivum L.) and was found to be highly correlated with sodium uptake in rice but not in wheat. It is concluded that the bypass flow provides an additional pathway for sodium uptake in rice and that this accounts for the functional and genetic independence of sodium and potassium uptake in rice and consequently for the lesser prominence of potassium:sodium discrimination in rice than in wheat.     

The response of potatoes (Solunum tuberosum L.) to salinity: plant growth and tuber yields in the arid desert of Israel

The response of potato cultivars to water salinity was studied under field conditions in sandy loessial soil in the arid desert of Southern Israel. The potatoes were drip-irrigated with water of three different salinities: water commonly used for irrigation (1.0–1.4 dS m^-1); saline water from a local well (6.1–6.9 dS m^-1); and a mixture of the two (3.84.3 dS m^-1). Salinity retarded plant emergence, enhanced haulm senescence and reduced growth of both haulms and tubers. Increasing the salinity progressively reduced tuber yields. Application of the saline water well after plant establishment (Expt A) decreased tuber yields by 615% and 22–31% in the intermediate and the high salinities, respectively. When irrigation with saline water was started soon after planting (Expt B), tuber yields were decreased by 0–17% and 21–79% in the intermediate and the high salinities, respectively. When the tubers emerged in salinised soil, tuber yields were decreased by 21–54% and 42–59% in the intermediate and the high salinities, respectively. A differential response of various cultivars to salinity was observed. None of the potato cultivars or clones exhibited exceptional tolerance to severe salinity. The earlier maturing cvs Atica and Désirée were the least susceptible to the moderate salinity imposed throughout the entire growing season; however, no association was noted between maturation time and the response to salinity.     

The cytosolic Na+ : K+ ratio does not explain salinity-induced growth impairment in barley: a dual-tracer study using 42K+ and 24Na+

It has long been believed that maintenance of low Na+ : K+ ratios in the cytosol of plant cells is critical to the plant's ability to tolerate salinity stress. Direct measurements of such ratios, however, have been few. Here we apply the non-invasive technique of compartmental analysis, using the short-lived radiotracers 42K+ and 22Na+, in intact seedlings of barley (Hordeum vulgare L.), to evaluate unidirectional plasma membrane fluxes and cytosolic concentrations of K+ and Na+ in root tissues, under eight nutritional conditions varying in levels of salinity and K+ supply. We show that Na+ : K+ ratios in the cytosol of root cells adjust significantly across the conditions tested, and that these ratios are poor predictors of the plant's growth response to salinity. Our study further demonstrates that Na+ is subject to rapid and futile cycling at the plasma membrane at all levels of Na+ supply, independently of external K+, while K+ influx is reduced by Na+, from a similar baseline, and to a similar extent, at both low and high K+ supply. We compare our results to those of other groups, and conclude that the maintenance of the cytosolic Na+ : K+ ratio is not central to plant survival under NaCl stress. We offer alternative explanations for sodium sensitivity in relation to the primary acquisition mechanisms of Na+ and K+.     

Changes in ion and water content of individual shoot organs in a salt-tolerant and a salt-sensitive clone of Agrostis stolonifera L. during and subsequent to treatment with sodium chloride

Abstract Individual leaves and stems were analysed for Na⁺, Cl^?, K⁺ and water content in two clones of Agrostis stolonifera differing in salt resistance, during 14 d of treatment with NaCl, 100 and 200 mol m^?3, and a further 7 d in a salt-free medium. Great differences in ion and water content were revealed between individual organs, and organ-by-organ analysis also emphasized the differences between the clones better than whole shoot analysis. In both clones, Na⁺ and Cl^? accumulated to the greatest degree in the older leaves, but for corresponding organs, the concentrations were lower in the more tolerant clone. In the sensitive clone, the lowest leaves dehydrated in 200 mol m^?3 NaCl and failed to recover, while the plants of the more resistant clone maintained viable water content in all organs. In the resistant clone, K⁺ concentration decreased less in response to salt treatment than in the more sensitive clone. For a full appreciation of the plants' reactions, it was found necessary to express the analytical data on several bases, namely, per unit dry-weight, unit water, and total ion-content.