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Physiology and Biochemistry |
The University of Arizona, Department of Plant Sciences, Tucson, Arizona 85721 USA
Received for publication January 17, 2003. Accepted for publication May 9, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: cation accumulation • chloridoid grasses • halophyte • osmotic adjustment • Na : K ratio • Na+ secretion • Poaceae • salt-stimulated growth • Sporobolus virginicus
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Flowers et al., 1986
). Greenway and Munns (1980)
divide the growth response to salinity of plants into four groups. Group IA (euhalophytes) have optimal growth (i.e., salt-stimulated growth) at moderate salinities (100–300 mmol/L NaCl) and continue to grow and survive at salinities up to 700 mmol/L. Group IB (miohalophytes) have optimal growth at very low salinities and continue to grow at reduced rates even at higher salinities. In group II halophytes and nonhalophytes, growth is greatly reduced even at moderate salinity, and salinities over 300 mmol/L are lethal. Group III are very salt-sensitive nonhalophytes that cannot survive when grown at salinities over 100 mmol/L.
Halophytes occur in three subclasses of angiosperms (Kremer and Van Andel, 1995
). In monocotyledons, halophytes are rare in the Alismatidae and clustered in a few related families in the Commelinidae; in the remaining angiosperms (dicotyledons, sensu lato), halophytes are found in fewer than 50% of families in the Carophyllidae (primarily within the Chenopodiaceae).
Euhalophytic members of the Chenopodiaceae include Suaeda (Yeo and Flowers, 1980
), Atriplex (Greenway, 1968
; Miyamoto et al., 1996
), and Salicornia (Ayala and O'Leary, 1996; Pfister, 1999
). In these genera, it is well established that maximum or optimal growth occurs at salinities ranging from 100 to 300 mmol/L and that growth is less under freshwater conditions (Chapman, 1960
; Yeo and Flowers, 1980
; Flowers et al., 1986
). Optimal growth of dicotyledonous euhalophytes is associated with succulence of leaves and stems (Pfister, 1999
), Na+ accumulation in (and, in some cases, secretion from) leaves (Munns et al., 1983
; Miyamoto et al., 1996
), and a high ratio of Na : K in plant tissues (at least 5.0 to 10.0) (Gorham et al., 1980
; Rozema, 1991
). Reports of salt-stimulated growth (by the definition of Greenway and Munns [1980]
) are rare among grasses, and optimal growth of halophytic grasses is not associated with high Na+ accumulation (Gorham et al., 1980
; Glenn, 1987
).
Optimal growth under saline conditions has been observed to be nitrogen dependent in both halophytic dicotyledons (Rozema et al., 1983
) and monocotyledons (Smart and Barko, 1980
). The nitrogen dependence of halophytes is associated with production of quaternary ammonium compounds and free amino acids that are believed to contribute to osmotic adjustment and act as nitrogen sinks (Flowers et al., 1977
).
Marcum and Murdoch (1992)
evaluated Sporobolus virginicus (L.) Kunth for use as a turf grass or ground cover. At 150 mmol/L NaCl, the growth rate was 13% greater than at 1 mmol/L; however, at 450 mmol/L, the growth rate decreased to 66% of the growth at 1 mmol/L. Concentrations of shoot Na+, root Na+, and root K+ increased with increasing salinity. Shoot K+ declined although shoot K+ selectivity increased. Marcum and Murdoch (1992)
and Naidoo and Naidoo (1998)
have described salt secretion by salt glands on leaves of S. virginicus. Ramadan (2001)
observed similar patterns of salt secretion in S. spicatus.
Sporobolus virginicus (Poaceae) is a perennial, rhizomatous, C4 chloridoid grass with a broad distribution along subtropical shorelines (Hitchcock, 1971
). Growth has been studied in ecotypes of S. virginicus collected from populations in Georgia, Florida, and Hawaii, USA (Gallagher, 1979
; Blits and Gallagher, 1991
; Marcum and Murdoch, 1992
) and Durban, South Africa (Breen et al., 1977
; Naidoo and Mundree, 1993
; Naidoo and Naidoo, 1998
). The source of plant material, experimental design, and growth conditions differed in each study. No salt-stimulated growth of S. virginicus was observed in any study except the small increase found by Marcum and Murdoch (1992)
.
The present study was designed to verify and to amplify the results of Marcum and Murdoch (1992)
and to answer the question, Is S. virginicus a euhalophyte? Four aspects of the response to salinity were investigated: (1) growth under a range of concentrations of NaCl, (2) the interaction of nitrate levels with levels of NaCl, (3) the relative accumulation of cations in plant tissues grown at different concentrations of NaCl, and (4) the water content and osmotic adjustment of S. virginicus grown at different concentrations of NaCl.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Growth conditions and protocols
Rhizomes were planted in shallow trays filled with half sand/half Sunshine commercial planting mix (Sun Gro Horticulture, Bellevue, WA, USA) and watered weekly with half strength Hoagland's solution #2 (Hoagland and Arnon, 1950
) supplemented with 5 mmol/L NaCl (Brownell and Crossland, 1972
). Plant material was periodically divided and planted in fresh sand/Sunshine commercial planting mix to increase stock.
Plants were grown in a greenhouse at the University of Arizona Campus Agricultural Center, Tucson, Arizona, USA. Temperatures were maintained between 20° and 31°C in the winter and 24° and 35°C in the summer. Relative humidity ranged from 25 to 50% (days) and 70 to 85% (nights). Mean photosynthetically active radiation (at noon, within the greenhouse) was 450 µmol · m–2 · s–1 in the winter and 1350 µmol · m–2 · s–1 in the summer. The roof and west side of the greenhouse were covered with a shade cloth that excluded 45% of natural light from June through September. The electrical conductivities (given in decisiemens per meter) of the nutrient solutions plus the following concentrations (in millimoles per liter) of NaCl were 1.9 (5), 6.8 (50), 11.7 (100), 16.1 (150), 30.3 (300), and 44.8 (450). Electrical conductivity was monitored daily until it was determined that changes in electrical conductivity from immediately after the change of solutions until the next weekly change were under 5%.
The hydroponic system of Marcum and Murdoch (1992)
was used. Plastic tubs (10 L) were covered with duct tape to minimize algal growth in the nutrient solution. Tubs were topped with wooden boards each containing eight holes to support plastic cups filled with coarse sand. Bases of the cups were removed and replaced with pliable screening that was permeable to roots and the nutrient solution. Individual ramets were transferred from soil culture to the cups and allowed to acclimate to hydroponics. During acclimation and before the start of treatments, plants were maintained with half strength Hoagland's solution #2 (Hoagland and Arnon, 1950
) supplemented with 5 mmol/L NaCl (Brownell and Crossland, 1972
). Nutrient solutions were prepared with tap water and were aerated constantly. As necessary, volumes were brought back to 10 L with the addition of tap water to replace losses to evaporation and transpiration.
At the beginning of an experiment, NaCl levels were increased by 50 mmol/L increments daily until the final level was reached. Solutions were changed every 2 wk during acclimation and once per week during experiments. Tubs were arranged in a randomized complete block design. Blocks were assigned to fixed positions within the greenhouse and treatments were randomly assigned to tubs within each block. At harvest, plant material was divided into above- and belowground portions and was dried to constant mass at 70°C. Plant material from within a tub was pooled as a replicate. In experiments 2 and 3, blocks were photographed at the beginning of treatments and again just before harvest.
Two methods were used to equalize starting amounts of plant material in each tub. In experiment 1 during acclimation to hydroponics, shoots were cut back to the sand surface and allowed to regrow for 2 wk. Regrowth was cut back to the sand surface and dried to constant mass at 70°C. Based on the dry biomass harvested from each cup within tubs, cups were redistributed between tubs to equalize the total plant material in each tub and block. In experiments 2 and 3, the same procedure of shoot trimming and regrowth was followed but there was no redistribution of cups between tubs (S. Smith, University of Arizona, personal communication). The total dry biomass was pooled for each tub; analysis of variance was used to compare the values for tubs and blocks. There was no statistically significant difference in the starting total dry aboveground biomass between tubs or blocks in either experiment 2 or 3. It was assumed that these procedures, along with the use of a randomized complete block design, minimized the influence of random differences in potential for growth between individual ramets.
Experimental design
In experiment 1, the nutrient solution was changed to the formula used by Rozema et al. (1983)
(supplemented with 5 mmol/L NaCl) 3 mo before the start of treatments. During acclimation, 3.5 mmol/L NaNO3, equivalent to quarter strength Hoagland's #2 (Hoagland and Arnon, 1950
), was used for maintenance of plants. All other nutrients were supplied in amounts similar to half-strength Hoagland's solution #2 except that N was only supplied as NO32–. For experiments 2 and 3, the nutrient solution was half-strength Hoagland's solution #2 supplemented with the designated concentration of NaCl.
In experiment 1, three levels of salinity were tested (5, 100, and 300 mmol/L NaCl) as well as three levels of nitrate (0.5, 3.5, and 14.0 mmol/L NaNO3) (Rozema et al., 1983
), giving nine treatment combinations. (Note: Since nitrate was supplied as the sodium salt, the treatment combination with the highest level of nitrate and lowest level of NaCL had 14.0 mmol/L Na+.) There were two tubs for each treatment combination and two control tubs harvested at the beginning of treatments. Treatments were started on 25 May 1999, and plants were harvested after 4 wk.
For experiment 2, there were six blocks and six levels of treatments: 5, 50, 100, 150, 300, and 450 mmol/L NaCl. At each of three harvests (the beginning of treatments = control, 4 wk, and 8 wk) two cups were selected at random from each tub. A replicate consisted of the pooled dry biomass of the two cups from each tub for each harvest. At the 8-wk harvest, the aboveground plant material was rinsed with tap water and allowed to dry before harvesting. The roots and rhizomes were rinsed with an isotonic solution of LiCl (to remove surface cations) with 1.0 mmol/L Ca(NO3)2 added to maintain cell membrane integrity. The number of tillers per cup and the width of the two widest fully expanded leaves per cup were recorded at each harvest. Treatments were begun on 31 August 1999.
Sporobolus virginicus bloomed in the fall; the first panicle appeared on 27 September 1999 during experiment 2. As panicles were exerted from the sheath, they were cut off just below the second subtending leaf because when the panicles mature the tiller bearing the panicle senesces (K. Marcum, University of Arizona, personal communication). Panicle dry mass was added to that of the tub from which they were removed.
In experiment 3, there were 12 blocks and three levels of treatment: 5, 125, and 450 mmol/L NaCl. Plants were scored for tiller number and leaf width (as in experiment 2) at the beginning and end of treatments. Treatments were started on 29 March 2000 and plants were harvested after 4 wk. The same procedure of rinsing plants prior to harvest (from experiment 2) was used in experiment 3.
Cation content and secretion
In experiments 2 and 3, concentrations of Na+, K+, Ca2+, and Mg2+ in leaves, rhizomes, and roots were determined by atomic absorption spectrophotometry (Perkin-Elmer 560, Perkin Elmer, Norwalk, Connecticut, USA). In experiment 2, dried plant samples were finely ground; in experiment 3, whole plant tissues were used. Tissue was digested in nitric acid and diluted to 25 mL with 0.25% La in 2.5% nitric acid prepared with deionized water. This solution was designated as full strength. If required to read within the range of the standards, samples were diluted 10x or 100x. The nitric acid concentration was brought to about 10% to match that of the blank and standards. La was added to the samples to improve the sensitivity of detection for Ca and Mg (Perkin-Elmer, 1976
). The readings in parts per million (ppm) were converted to micromoles per gram dry biomass using the following formula: parts per million · dilution volume · sample mass–1 · atomic mass–1.
In experiment 3, secretion of Na+, K+, Ca2+, and Mg2+ was determined by the difference in concentration of those cations in rinsed leaves and leaves that had been rinsed and allowed to secrete for 3 d. Plants were rinsed thoroughly with tap water and allowed to dry for 6 h. (It would have been preferable to rinse with distilled water but that was not available at the Campus Agricultural Center.) Four leaf blades per tub were removed, placed in tared Erlenmeyer flasks, sealed with Parafilm (American National Can, Greenwich, CT, USA), and transported to the laboratory in an ice chest. Fresh and dry biomass of leaves were obtained. The tissue was digested in the Erlenmeyer flasks and processed as in experiment 2. The procedure was repeated 72 h later. Salt crystals were visible on the leaves of plants grown at 125 and 450 mmol/L NaCl. Care was taken not to disturb the leaf surface during harvesting of leaves. The difference in cation concentration between the two samples of leaves was attributed to secretion. The cation concentrations of leaves were expressed both as micromoles per gram dry biomass and as micromoles per gram H2O. Water content was determined as [(fresh biomass – dry biomass)/dry biomass] and expressed as gram H2O per gram dry biomass.
Osmotic adjustment
About eight rinsed, dry leaves from plants in each tub were packed into 5-mL syringes and quickly frozen at –80°C. After thawing, the cell sap was expressed by the plunger and collected for measurement. The osmolality of the cell sap was determined by vapor pressure osmometry (Wescor Model 5500, Vapor Pressure Osmometer, Wescor, Logan, Utah, USA). Osmolality was multiplied by 2.48 to give osmotic pressure (Taiz and Zeiger, 1991
, pp. 68–69). For monovalent cations, a balancing anion and dissociation factor of 92% were assumed; molality was multiplied by 1.84 to give osmolality (Kramer and Boyer, 1995
, p. 34).
Data and statistical analysis
Relative growth rate (RGR) was calculated using the classical method following Chiariello et al. (1989)
and expressed as grams per gram per day; (ln[final dry biomass] – ln[initial dry biomass])/(final time – initial time).
The root : shoot ratio was calculated as the ratio of the belowground to aboveground dry biomass. One- and two-way analyses of variance (ANOVA) were performed on the results using SigmaStat (Jandel Scientific, San Rafael, California, USA); in those cases in which data did not meet the requirements for normality and/or equal variances, a Kruskal-Wallis analysis of variance on ranks was performed (Zar, 1984
). Differences between means were determined using a Student-Newman-Keuls (S-N-K) multiple comparison test.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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In experiment 2, two-way ANOVA for above- and belowground dry masses showed no significant differences in RGR due to blocks or salinity treatments at 0–4 wk (data not shown). However, at 4–8 wk (data not shown) and 0–8 wk, there were highly significant differences in RGR due to levels of NaCl. Aboveground dry biomass of plants grown at 100 and 150 mmol/L NaCl was significantly greater than that of plants grown at 5, 50, and 450 mmol/L NaCl (Table 2). Belowground biomass of plants grown at 50 and 300 mmol/L was significantly greater than that of plants grown at 5 and 450 mmol/L, but could not be distinguished statistically from one another because of the high variance of the data. Growth was highly variable between individuals, possibly due to a short period of acclimation in hydroponics. At 4 wk, 8% of individuals were no larger than at the average at the start of treatments, and at 8 wk, 5% were no larger. No effects of blocking or interaction between blocks and and levels of NaCl were observed on growth patterns in experiment 2.
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There was a fourfold increase in above- and belowground biomass between plants grown at 50 mmol/L and those grown at 100 and 150 mmol/L. Although not statistically significant, the difference in dry biomass between plants grown at 300 mmol/L and those grown at 50 mmol/L is striking (Table 2).
The increase in biomass at 100 and 150 mmol/L was associated with increases in leaf width, numbers of tillers, and panicles produced (Table 3). The increase in tiller number was particularly dramatic. At 8 wk, on average, tubs grown at 5 mmol/L NaCl had produced only two more tillers than they had at the start of treatments, while tubs grown at 100 and 150 mmol/L produced 46.7 and 48.7 more tillers than they had at the start of treatments. The widest leaves were significantly wider in plants grown at 100, 150, and 300 mmol/L NaCl than they were in plants grown at 5, 50, or 450 mmol/L. A total of 10 panicles were produced in tubs grown at 5 mmol/L; a total of 43 panicles was produced in tubs grown at 100 mmol/L. Only two panicles were produced in tubs grown at 450 mmol/L.
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A similar pattern (as in the previous two experiments) was observed in the ratio of belowground to aboveground dry biomass. The highest ratio was at 5 mmol/L, followed by 450 mmol/L and 125 mmol/L. No effect of blocking or interaction between blocks and levels of NaCl was observed in growth parameters in experiment 3.
The mean of the widest leaves was about 1.5 times greater in plants grown at 125 mmol/l that at 5 mmol/L (Table 3). For plants grown at 450 mmol/L, the mean of the widest leaves was 1.2 times greater than the mean of plants grown at 5 mmol/L. Tiller number increased more than fourfold in plants grown at 125 mmol/L vs. those grown at 5 mmol/L (Table 3).
Cation content
The results of cation analysis in experiment 2 were highly similar to the results from experiment 3 except that overall slightly higher concentrations of K+ were observed in experiment 2. For space and simplicity, we will present only the results from experiment 3. The results from experiment 2 are available in Bell (2000)
.
The Na+ content of roots grown at 5 mmol/L was very low and increased almost 10-fold in roots grown at 125 and 450 mmol/L (Table 5). The K+ content of roots was not significantly different between roots grown at 5 and 450 mmol/L but was significantly higher in roots grown at 125 mmol/L. The Mg2+ content was lowest in roots grown at 450 mmol/L and was significantly higher in roots grown at 5 and 125 mmol/L. The Ca2+ content was significantly higher in roots grown at 5 mmol/L than in roots grown at 125 or 450 mmol/L. Total cation content was lowest in roots grown at 5 mmol/L, significantly higher in roots grown at 450 mmol/L, and highest in roots grown at 125 mmol/L. The Na : K ratio of roots was highest in roots grown at 450 mmol/L and significantly lower in roots grown at 5 and 125 mmol/L.
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Cation secretion
Salt crystals were visible on leaves of plants grown at 125 and 450 mmol/L NaCl. The most noticeable difference between the cation contents of freshly rinsed leaves and leaves that have secreted salts for 72 h is the large increase in Na+ content (Table 5). Leaves grown at 450 mmol/L secreted Na+ at the rate of 336.7 µmol · g dry biomass–1 · d–1; leaves grown at 125 mmol/L secreted Na+ at the rate of 166.5 µmol · g dry biomass–1 · d–1. Negligible amounts of Na+ were secreted from leaves grown at 5 mmol/L NaCl. Much smaller and highly variable amounts of K+, Mg2+, and Ca2+ were detected. The Na : K ratio of leaves harvested immediately after rinsing was not significantly different from 1 in plants grown at 5, 125, or 450 mmol/L NaCl. However, in leaves that had secreted for 72 h, the Na : K ratio increased significantly as the salinity of the nutrient solution increased.
Two-way ANOVA was performed on the cation contents of leaves harvested soon after rinsing and 72 h later with NaCl concentration and time as the fixed variables (Table 6). NaCl concentration in the nutrient medium and time and the interaction of NaCl and time were highly significant factors in the Na+ content of leaves. Both NaCl concentration and time were significant factors affecting K+ content, but there was no interaction between them. NaCl concentration had a highly significant effect on Ca2+ content and the interaction between NaCl and time was barely significant. There was no effect of NaCl, time, or the interaction of factors on Mg2+ content.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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. Growth stimulation was similar to that observed with euhalophytic dicotyledons (Flowers et al., 1986
); plants of S. virginicus grown at 100–150 mmol/L NaCl were visibly larger than those grown at 5 mmol/L. In experiment 3, RGR was 100% greater in plants grown at 125 mmol/L NaCl than in those grown at 5 mmol/L, accumulation of aboveground dry biomass was four times greater, and tiller production was about 23 times greater. These results support the assertion that 100–150 mmol/L NaCl is optimal for growth of S. virginicus. Not only can S. virginicus survive at NaCl levels of 450 mmol/L, biomass accumulation and tiller production were greater at 450 mmol/L than in plants grown at 5 mmol/L.
Other researchers have not observed growth stimulation by NaCl in S. virginicus (Gallagher, 1979
; Blits and Gallagher, 1991
; Naidoo and Mundree, 1993
; Naidoo and Naidoo, 1998
), except for the small increase found by Marcum and Murdoch (1992)
. Marcum and Murdoch (1992)
did not measure the entire aboveground increase in biomass from tiller production, but only the growth above 10 cm. Blits and Gallagher (1991)
used dune and marsh ecotypes of S. virginicus in their study and found significant differences in the response to salinity between the ecotypes. Rhizome biomass increased strikingly in the dune ecotype grown with full seawater.
Reports of growth stimulation by NaCl in halophytic monocotyledons are rare and have usually involved small changes relative to the increases observed for halophytic dicotyledons (Rozema, 1991
). For Distichlis palmerii, Glenn (1987)
observed a 15% increase in RGR in plants grown at 180 mmol/L NaCl vs. those grown with no NaCl. In that study, 15 species of grasses survived at 540 mmol/L NaCl although in all cases RGR was reduced to 50% or less of RGR with no NaCl. Distichlis spicata (Kemp and Cunningham, 1981
), Cynodon dactylon (Ramakrishnan and Nagpal, 1973
; Dudeck et al., 1983
), Paspalum vaginatum (Dudeck and Peacock, 1993
; Marcum and Murdoch, 1994
), Spartina alterniflora (Bradley and Morris, 1991
), and Stenotaphrum secundatum (Marcum and Murdoch, 1994
) are grass species that have been reported to survive at high salinities. Greipsson and Davy (1996)
observed an increase in tiller number as NaCl concentrations increased but a decrease in aboveground biomass in a study of Leymus arenarius. Stelzer and Läuchli (1977)
report a spike of growth stimulation of Puccinellia peisonis; optimal growth occurs at 100 mmol/L NaCl but plants cannot survive at salinities greater than 300 mmol/L NaCl. Puccinellia peisonis is a C3 grass lacking salt glands. Similar results have been found for P. maritima (Munns et al., 1983
). Interestingly, in these experiments, as in Kemp and Cunningham (1981)
where no growth stimulation of Distichlis spicata by NaCl was observed, the optimal growth of these grasses occurred in nonaerated hydroponic culture. Hester et al. (1996)
observed wide intraspecific variation in salt tolerance among clones of Spartina patens. Macke and Ungar (1971)
observed optimal growth of Puccinellia nuttallinana seedlings in 100 mmol/L NaSO4; growth was greatly reduced by 350 mmol/L and no plants survived at 550 mmol/L. Halopyrum mucronatum showed a similar pattern of growth (Khan et al., 1999
); after 90 d, seedlings showed a maximum RGR at 90 mmol/L NaCl. However, no plants survived at 360 mmol/L. There were genotypic, ecotypic, and developmental stage differences in the actual plant material used in these studies as well as differences in experimental protocols that may, in part, account for the differences in growth responses observed.
There are reports of growth stimulation by NaCl in two other monocotyledons. Bourn (1935)
reported that Ruppia maritima (Potamogetonaceae), an aquatic species found in salt marshes of temperate zones, required about 10% seawater for optimal growth and "appeared quite healthy" at 150% seawater. Growth in seawater positively correlated with increased production and retention of leaves of Triglochin maritima (Juncaginaceae) (Rudmik, 1983
). The cation content and Na : K ratio of fully expanded young leaves of T. maritima is similar to that of the halophytic grasses; older leaves with high Na+ contents are shed.
Allocation of biomass to plant organs forms characteristic patterns in different species. These patterns are subject to perturbations by environmental conditions (Brouwer, 1983
). Higher allocations to roots and belowground structures are symptomatic of nonoptimal nutritional conditions in the root environment and of preparation for dormancy (i.e., storage of carbohydrates). The pattern of allocation observed in these three experiments showed that the lowest ratio of belowground : aboveground biomass was found in plants grown at 100–150 mmol/L NaCl.
In contrast to the results of Rozema et al. (1983)
, no significant effect of nitrate levels or interaction between nitrate and NaCl was observed in this study. Sporobolus virginicus may have a lower requirement for nitrate than the three dicotyledons studied by Rozema et al. (1983)
. Smart and Barko (1980)
found that growth of Distichlis spicata and Spartina alterniflora were nitrogen limited in freshwater, brackish, and marine sediment culture. Limitations in nitrogen stimulated root growth in S. alterniflora but not in D. spicata. Both species showed the greatest biomass accumulation on the freshwater sediments. Field studies by Gallagher (1979)
did not find that pulses of NH4NO3 influenced the growth of S. virginicus in saline soil. However, both Marcum and Murdoch (1992)
and Naidoo and Naidoo (1998)
observed a high production of compatible solutes (which would require N) in S. virginicus. It is possible that the large volume of nutrient solution in the present study (10 L) and weekly changing of solutions was sufficient to meet the nitrogen needs of S. virginicus.
When expressed in terms of dry biomass, the cation contents of halophytic dicotyledons were higher than those observed for S. virginicus. In Salicornia bigelovii grown at optimal salinity of 200 mmol/L NaCl, Pfister (1999)
found over 3000 µmol Na+/g dry biomass. These results are consistent with other studies on halophytic dicotyledons (Gorham et al., 1980
; Glenn and O'Leary, 1984
). However, when expressed in terms of water content, the differences between halophytic dicotyledons and monocotyledons are not as great. The more significant difference appears to be in the relative proportions of Na+ and K+. Among dicotyledons, the cation contribution to osmolality comes primarily from Na+ (Glenn and O'Leary, 1984
), while in the present study, Na+ and K+ made roughly equivalent contributions to osmolality.
Pfister (1999)
observed a Na : K ratio of 10 in shoots of Salicornia bigelovii grown at optimal salinity; in contrast the present study found a Na : K ratio of about 1 in leaves of S. virginicus grown at optimal salinity. The Na : K ratio in roots grown at optimal salinity was 1.6 to 2.5. Sodium increased dramatically in leaves that secreted for 3 d. These observations taken together suggest that Na+ is actively secreted from leaves and retained in roots. Further, it appears that there is discrimination in the transport of K+ from roots to leaves and rhizomes.
Results of the present study regarding cation content in tissues of S. virginicus are in general agreement with those made by Marcum and Murdoch (1992)
and Naidoo and Naidoo (1998)
and with studies of other halophytic grasses (Stelzer and Läuchli, 1977; Gorham et al., 1980
; Miyamoto et al., 1996
; Ramadan, 2001
). Secretion of Na+ from leaves of S. virginicus increased from 166.5 to 336.7 µmol · g dry biomass–1 · d–1 as the NaCl concentration in the nutrient solution increased from 125 to 450 mmol/L. In contrast, secretion of K+ was about 10 times lower than Na+ and decreased in a nonsignificant manner. The patterns for secretion observed by Naidoo and Naidoo (1998)
were similar to the present study; however, the results of Marcum and Murdoch (1992)
differ from both. Marcum and Murdoch (1992)
observed slightly higher levels of secretion for both Na+ and K+.
As a percentage of fresh biomass, water content of tillers of S. virginicus ranged from 61.7% at 5 mmol/L to 69.8% at 125 mmol/L and 63.3% at 450 mmol/L. These figures are at the low end of the normal range for grasses (65–85%) (Tiku and Snaydon, 1971
; Howard and Mendelssohn, 1999
) and considerably lower than those of succulent halophytic dicotyledons (Glenn and O'Leary, 1984
). Pfister (1999)
observed water contents of 84% for S. bigelovii grown at 5 mmol/L NaCl and 90% when grown at 200 mmol/L. The water content of whole tillers of S. virginicus was slightly higher than that of leaves alone.
When expressed in terms of µmol/g H2O, there was no significant difference between the Na+ or K+ concentrations in leaves of S. virginicus grown at 5 or 125 mmol/L NaCl. It is possible that the increase in water content serves as a mechanism to dilute Na+. The Na+ concentration in leaves grown at 450 mmol/L was 1.7 times higher and the K+ concentration was 1.5 times higher. The absolute amounts of Na+ and K+ were higher in plants grown at 450 mmol/L. When expressed as the Na+ and K+ contributions to osmolality, in plants grown at 5 mmol/L, the contribution was 62.7%; at 125 mmol/L, 67.5%; and at 450 mmol/L, 54.2%. Marcum and Murdoch (1992)
estimated the contributions of Na+ and K+ to molality to be between 40 and 45%.
The structure of the C4 leaves and the relatively rigid cell wall (Esau, 1977
) of grasses limits the ability of halophytic grasses to change cell volumes in response to salinity. When grown at optimal salinity, S. virginicus increases its RGR and tillering. Within the time frame of these experiments, the water content of leaves increases by 18% and that of shoots by 46% when grown at optimal salinity. The increase in water content can be attributed to an increase in actively growing young tissues. Sporobolus virginicus appears to regulate cation uptake and retention; K+ is selectively taken up and retained in leaves, while Na+ is retained in roots and secreted from leaves. Both Na+ and K+ contribute to osmotic adjustment. Secretion of Na+ allows S. virginicus to maintain transpiration while avoiding toxic buildup of Na+. Sporobolus virginicus is a euhalophytic grass that does not accumulate Na+ in response to salinity.
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FOOTNOTES |
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2 Current address: Rancho Santa Ana Botanic Garden, 1500 N. College Avenue, Claremont, California 91711, USA (hester.bell{at}cgu.edu
)
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Bell H. L. 2000 Response of Sporobolus virginicus (L.) Kunth to salinity. M.S. thesis, University of Arizona, Tucson, Arizona, USA
Blits K. C. J. L. Gallagher 1991 Morphological and physiological responses to increased salinity in marsh and dune ecotypes of Sporobolus virginicus (L.) Kunth. Oecologia 87: 330-335[CrossRef][ISI]
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0.05 as separated by the S-N-K test (NS, not significant). N = 6