HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Full Text (PDF) Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via ISI Web of Science (3) Citing Articles via Google Scholar Google Scholar Articles by Horne, F. R. Articles by Kahn, A. Search for Related Content PubMed PubMed Citation Articles by Horne, F. R. Articles by Kahn, A. Agricola Articles by Horne, F. R. Articles by Kahn, A.

Water loss and viability in Zizania (Poaceae) seeds during short-term desiccation¹

⁰ Department of Biology, Southwest Texas State University, San Marcos, Texas 78666 USA

Received for publication December 15, 1998. Accepted for publication February 1, 2000.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
How Texas wild rice, Zizania texana, became isolated in the San Marcos River of Central Texas, hundreds of kilometres from other wild rice populations is not known. Zizania seeds are intolerant of short-term desiccation. Seeds desiccated at 14% relative humidity (RH) and 75% RH do not survive after only 5–6 d and 2–3 wk of drying. Water loss is rapid and reaches a maximum at the time of seed death due to drying. And although all Zizania seeds germinate well following a long, cold dormancy period, Z. texana seeds readily germinate in the isothermic water (22°C) of the San Marcos River and Springs without an obligate, cold dormant period. Within 30–60 d of collection, Z. texana seeds germinate in substantial numbers, unlike seeds of Z. palustris, which require a long, cold dormant period. The Texas population of Z. texana may represent a relict population of a once more widely dispersed wild rice population, since the San Marcos springs probably have never gone dry.

Key Words: desiccation • Poaceae • San Marcos River • seeds • viability • wild rice • Zizania.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
In North America two species of wild rice inhabit north-central United States and Canada, Zizania aquatica L., southern wild rice, and Zizania palustris L., northern wild rice. Southern wild rice also occurs along the Atlantic and Gulf coasts from New Brunswick to Louisiana. A third species (Zizania texana Hitchc., Texas wild rice) consists of a small disjunct population that is endemic to the San Marcos River of Central Texas (Terrell et al., 1997 ). The isolation of one small population of Zizania texana in the upper 4 km of a spring-fed river system in Central Texas, an area that includes numerous spring-fed streams along the Balcones fault zone, has been an enigma. The only other species in the genus, Zizania latifolia (Griseb.) Turcz ex Stapf, is found in southern Asia (Dore, 1969 ; Aiken et al., 1988 ; Chen et al., 1990 ).

That Zizania seeds are extremely sensitive to drying and thus to drought is well known (Brown and Scofield, 1903 ; Duvel, 1906 ; Leggatt, 1923 ; Muenscher, 1936 ; Barton, 1939 ; Simpson, 1966 ; Cardwell, Oelke, and Elliot, 1978 ; Atkins, Thomas, and Stewart, 1987 ; Probert and Brierley, 1989 ; Probert and Longley, 1989 ; Kovach and Bradford, 1992 ; Vertucci et al., 1994 ). Many factors influence seed viability during desiccation (Kermode et al., 1990 ). Relative vapor pressure, temperature, and seed maturity all influence seed survival during drying. To retain viability during drying, seed water content must be high enough to prevent damage from desiccation and low enough to prevent freezing. Because of the increased vapor pressure at higher temperatures, seeds can survive with lower water contents at higher temperatures than at lower temperatures (Vertucci et al., 1994 ).

High susceptibility of seeds to drying may have contributed to the geographic isolation of Z. texana. The San Marcos population of wild rice may have survived because the spring and associated marsh is fed from deep within the Edwards Underground Aquifer and therefore has never gone dry (Horne and Kahn, 1997 ). The lone Central Texas wild rice population may be the last remnant of a much more expansive Zizania population of drought-sensitive wild rice that inhabited the area in the past.

The purpose of this paper is to measure the short-term drought sensitivity of the seeds of closely related Z. texana and Z. palustris. To accomplish this, two desiccation experiments in humidity-controlled environments were used to test seed viability and germination.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Wild rice seeds of Z. palustris were obtained from a commercial supplier in Northern Wisconsin (Wildlife Nurseries Inc., Oshkosh, Wisconsin, USA) and Dennis Brovold, Clearwater County, Bagley, Minnesota, in mid-September (47°32'N, 95°23'W; vouchers at SWT Herbarium). Seeds were shipped moist and had been chilled a few days before shipping. Upon arrival or collection all Zizania seeds were wetted and then after-ripened either at 22°C or at 4°C for at least 90 d prior to use.

Some freshly collected Z. palustris seeds also from Clearwater County, Minnesota (47°32'N, 95°23'W) and a few Z. texana seeds collected from locally cultivated plants (29°54'N, 97°57'W) during the period from May through July were kept wet at room temperature (22°C). These seeds were used within 7 d after harvest with no after-ripening cold period (4°C). Unlike the commercially purchased seeds, the Minnesota seeds had never been chilled or stored wet. Seeds from both species were used to test the comparative germination differences of the effects of no cold after-ripening period on seed germination. All collected Z. palustris seeds were in the hard, brown stage of maturity, while those of Z. texana were hard dark green to tan. Mature seeds of Z. texana are green rather than brown (Vertucci et al., 1994 ). With storage at either room temperature or 4°C, Z. texana turn brown.

Seeds collected from greenhouse-reared plants from a Louisiana population of Z. aquatica (West Pearl River, southeast of Slidell, Louisiana: 30°15'N, 89°41'W) were used to show that these seeds do not readily germinate at room temperature, but do after cold storage.

For each of two desiccation experiments, (1) germination and (2) water loss, intact, nonhulled seeds with paleas and lemmas were blotted to remove adhering water, checked for firmness, and divided into two groups. In an attempt to simulate natural conditions intact seeds with paleas and lemmas were used in germination experiments rather than those with hulled or surgically slit pericarps. To test the effects of desiccation, seeds then were placed in (1) a 14% Relative Humidity (RH) atmosphere in a desiccator (maintained with a saturated solution of LiCl) or (2) a 75% RH atmosphere in a desiccator (maintained with a saturated solution of NaCl; Winston and Bates, 1960 ). The rate and percentage of seed germination were determined by removal of seeds from the desiccators at daily intervals and then placing them in de-ionized water. Although germination is usually assumed when the lemma splits and the epiblast and cotyledon completely emerge (Aiken, 1986 ), many such seeds with an emerged epiblast and cotyledon died and were quickly covered with fungi. Until further seedling growth was apparent, seeds were not scored as having successfully germinated. The tetrazolium test for seed viability, which measures tissue metabolic activity, was used to estimate seed viability and death (Ellis, Hong, and Roberts, 1985 ).

To establish the influence of short-term desiccation on seed water loss, the rate of water loss of two groups (14% RH = 0.32 kPa and 75% RH = 1.75 kPa) of replicates of 100 intact seeds each was determined gravimetrically by weighing seeds to the nearest 0.01 mg every 6–12 h. Once the intact seed masses stabilized, seeds were placed in a 130°C oven for 1 h for final drying to calculate total water content (ISTA, 1985 ). The quantity of water loss at 130°C was referred to as unbound water. The concept of unbound water is confusing and consists of three broad categories of water—strongly, weakly, and loosely bound water (Vertucci and Leopold, 1987 ).

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Intact seeds of Z. palustris, stored previously at 4°C for at least 90 d, which were subjected to a relative humidity of 14% (22°C), germinated with the protrusion of the epiblast and cotyledon within 48 h after wetting (Fig. 1). The percentage of seeds germinating and developing into viable seedlings was >90% for drying times of up to 48 h (undesiccated controls = 92%), and then declined to 0% by 144 h or 5–6 d of desiccation. Zizania texana seeds responded in a similar manner to those of Z. palustris, and also ceased to germinate after 144 h of desiccation (Fig. 1). At the lower RH of 14%, seeds of both Z. palustris and Z. texana died by the 5th–6th d of desiccation.

View larger version (21K): [in this window] [in a new window]   Fig. 1. Effects of desiccation time (days) at two relative humidities (RH, 14 and 75%) on Zizania palustris and Z. texana seed germination and growth to viable seedlings after desiccation (grown for 21 d)

At higher RH (75%), Zizania seed tolerance time to desiccation was more than doubled. Although only a small percentage of seeds could survive for more than 20 d, a few Z. palustris seeds did endure drying for 20–30 d vs. 5–6 d for relative humidities of 14%. Germination and seedling survival remained >50% for the first 7 d of drying and decreased to 0% survival by the 30th d. Longer periods of desiccation, and water loss associated with it, tended to interfere with seedling development as did exposure to 14% RH (Fig. 1). Protrusion of the epiblast and cotyledon was as far as germination progressed before fungal growth developed on the dead seeds.

Control seeds of Z. palustris kept in de-ionized water at 22°C had not started germinating by the 110th d, whereas those kept at 4° or 7°C for at least 110 d began germinating upon warming to room temperature (22°C). Similar results were observed for Z. aquatica seeds. After 12 mo at room temperature, <1% of the room temperature (22°C) seeds had germinated, yet 80% of the Z. palustris seeds still tested viable with tetrazolium. After 110 d, >80% of the cold-treated controls germinated and developed normally into seedlings. In contrast, a few freshly collected seeds of Z. texana began germinating after 17 d at room temperature and continued to readily germinate for 60 d. At 22°C maximum germination occurred between 30 and 60 d after wetting. Although some seeds still germinated, the germination rate declined after 60 d with a total germination for the 120-d period of 67% (Fig. 2). Less than 1% of the Z. palustris or Z. aquatica seeds (22°C) germinated during this same 120-d period (Fig. 2). After 200 d at room temperature (data not shown) when the last recording was made, germination rates of 67% and <1% were noted for Z. texana and Z. palustris, respectively (data not shown).

View larger version (15K): [in this window] [in a new window]   Fig. 2. Comparison of germination at 22°C of Zizania aquatica, Z. palustris, and Z. texana seeds following collection without refrigeration. For Z. aquatica N = 105, for Z. palustris N = 400, and for Z. texana N=133.

View larger version (16K): [in this window] [in a new window]   Fig. 3. Effects of exposure to 14% RH on Z. palustris and Z. texana seed water loss

View larger version (16K): [in this window] [in a new window]   Fig. 4. Effects of exposure to 75% RH on Z. palustris and Z. texana seed water loss

View larger version (16K): [in this window] [in a new window]   Fig. 5. Effects of desiccation at 14% RH on Zizania palustris seed water loss and germination. N = 200

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

Once detached from the plant Zizania seeds must be stored moist to retain long-term seed viability. Not only are Zizania seeds extremely sensitive to drying and thus drought, the seeds also enter a dormant period between seed maturation and germination (Brown and Scofield, 1903 ; Duvel, 1906 ; Leggatt, 1923; Muenscher, 1936 ; Barton, 1939 ; Simpson, 1966 ; Cardwell, Oelke, and Elliot, 1978 ; Atkins, Thomas, and Stewart, 1987 ; Probert and Brierley, 1989 ; Probert and Longley, 1989 ). For instance, after 4 mo of cold storage (2°C), Z. palustris seeds begin germinating in the cold and readily germinate (>90%) upon warming at room temperatures. But if stored at 16°C for up to 80 d, only a few seeds will germinate even though the pericarp had been surgically scraped to enhance germination (Probert and Longley, 1989 ). Similar results are reported here for intact Z. palustris seeds from both Minnisota and Wisconsin stored wet at 22°C. At room temperature <1% of these seeds germinated after 200 d and 12 mo, although 80% of the embryos tested viable by staining a bright red with tetrazolium. Seeds collected from locally grown Louisiana Z. aquatica plants also require a long dormant period of 4 mo (personal observations) and do not germinate at 22°C. Why dormancy is terminated by cold temperatures, but not by warm temperatures, is unknown. Cold storage seems essential for significant Z. palustris seed germination, but is not necessary for retention of seed viability (Kovach and Bradford, 1992 ).

In the study by Kovach and Bradford (1992) , low temperatures and high humidities enhanced stored seed viability (tetrozolium test). No Z. palustris seeds were viable (tetrozolium test) after 4 wk in <79% RH (5°C) and whole-seed moisture contents of 6–8% (79% water loss). Loss of viability (50%) occurred at moisture contents of 20% (64% initial water content). Vertucci et al. (1995) reported that desiccation rather than rate of water imbibition was responsible for damage in dried Zizania seeds and that dormancy breaking and desiccation tolerance were independent. As seeds matured, resistance to drying was enhanced. During cold dormancy seed water content must be high enough to prevent desiccation damage, but low enough to prevent freeze damage.

Freshly collected seeds of Z. texana differ from those of Z. palustris by beginning to germinate as early as 17 d after collection and continue to germinate for 120 d with ultimately 67% of the seeds germinating when daily tracking of germination ceased (Fig. 2). After cold storage for 4 mo, seed germination percentages are high for all three species, Z. aquatica, Z. palustris, and Z. texana. The hulled seeds of Z. texana had germination percentages ranging from 50 to 90% (Power and Fonteyn, 1995 ). Seeds of Z. palustris have an obligate cold dormancy period, whereas seeds of Z. texana do not. Zizania texana seeds require a short period of dormancy since only 25% germinate within 45 d at 22°C. Both species have seeds that readily survive 4 mo of cold storage and then germinate in high percentages. Seeds collected from locally grown Louisiana Z. aquatica plants are similar to those of Z. palustris in requiring a long dormant period for germination (personal observations). In the West Pearl River near Slidell, Louisiana, Z. aquatica seeds begin germinating in mid-January. Specific differences in seed temperature and dormancy requirements are apparent.

Zizania seeds tolerate drying only for a few days. After they have been desiccated for only 5–6 d (14% RH) or 12–30 d (75% RH), few Zizania seeds survive to germinate (Fig. 1). Earlier studies demonstrated that air-dried seeds survive for only 2–3 wk (Simpson, 1966 ; Probert and Longley, 1989 ). Following short periods of desiccation, seed damage frequently can be so great that the germinating seed dies after protrusion of the epiblast and cotyledon. Probert and Brierley (1989) noted that dried Zizania seeds often did not have uniform metabolic activity as shown by tetrazolium staining, suggesting that a portion of the embryo was already dead. The shoot end of the embryo axis stained pink and might still have been viable as was the epiblast mentioned in the present study. Wild rice is unusual among grasses in that the epiblast emerges before the root and apparently functions to absorb water (Aiken, 1986 ).

Water evaporates faster at both greater vapor pressure deficits where the difference in saturation vapor pressure and actual vapor pressure is large (air's dry) and at higher temperatures. Vertucci et al. (1994, 1995) found this to be true for Zizania embryos. When stored below a water potential () of -2 MPa or at 98% RH, Z. palustris seeds lose water and eventually their viability (Probert and Longley, 1989 ). Therefore, to estimate and compare how rapidly Zizania seeds dehydrate and fail to germinate, wild rice seeds were exposed to two different relative humidities. Zizania seeds lost at least 95% of their unbound water (retain 5%) within 6 d at 14% RH or at least 80% of their unbound water (retain 20%) within 12–15 d at 75% RH. At both humidities seeds lose substantial quantities of water quickly (Figs. 3 and 4). Once such water losses are attained, seed survival was either lost or significantly reduced (Fig. 5). Below an embryonic critical water content (45%), Zizania seeds showed reduced viability, while an embryonic water content of 10% (equivalent to about a 80% embryo total water loss) was considered lethal (Probert and Brierley, 1989 ).

Without an obligate seed cold dormancy requirement and with acute seed sensitivity to desiccation, it seems plausible that Z. texana thrives as an endemic species in the San Marcos River because of the river's constant temperature (22°C year-round) and spring flow. At these temperatures many Z. texana seeds readily germinate without a long dormancy. After 4 mo a few seeds (<1%) of Z. palustris or Z. aquatica will germinate while maintained at 22°C once dormancy has been broken. Many young Texas wild rice plants are seen growing on the aquatic moss, Amblystegium riparium (Hedw.) B.S.P., which grows on the walls of the Southwest Texas State University (SWTSU) campus raceways where Z. texana has been grown since the late 1960s (Emery, 1966 ). Here in the constant temperature water (20°–22°C) from a SWTSU artesian water well, Z. texana seeds germinate and grow into young plants (personal observations). Threats of seed desiccation and low temperatures do not exist in the San Marcos River today. In more northern latitudes, where aquatic habitats are frozen for a few months each year, a long, cold, dormant period would be highly adaptive for Z. palustris seeds.

In the constant temperature of the San Marcos River, which has probably never gone dry since the Late Pleistocene (Horne and Kahn, 1997 ), Z. texana is a perennial. Texas wild rice may represent a relict population of a much larger population that inhabited Central Texas during Wisconsin glaciation. As the glaciers retreated northward, the expanding warmer and more arid climates of lower western latitudes may have eliminated populations of Zizania that were isolated in the many spring systems of Central Texas that eventually failed because of drought. Only the San Marcos population survived because the spring and associated marsh was fed from deep within the Edwards Underground Aquifer and therefore presumably never went dry (Horne and Kahn, 1997 ). The absence of wild rice in other similar, local, spring-fed systems along the Balcones Fault Zone of Central Texas may indicate that they probably have gone dry in the last 15 000 yr (Horne and Kahn, 1997 ). Recolonization of spring systems via seed dispersal from neighboring populations would have been difficult because of seed sensitivity to drying.

In conclusion, we believe that the intolerance of Zizania seeds to short-term desiccation as shown by the decline of seed viability and germination with drying is closely related to seed water loss. It also seems feasible that the occurrence of the disjunct and endemic population of Texas wild rice (Z. texana) in the San Marcos River may be related to the sensitivity of seeds to desiccation.

	FOOTNOTES

 
¹ The authors thank the Office of Sponsored Projects, Southwest Texas State University, for financially supporting this research, and O. W. Van Auken, Carl Haux, and unknown reviewers for comments on the manuscript. The many discussions on Texas wild rice with W. P. Emery deceased are acknowledged.

² Author for reprint requests (FH01{at}swt.edu ).

	LITERATURE CITED

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Aiken, S. G. 1986 The distinct morphology and germination of grains of two species of wild rice (Zizania, Poaceae). Canadian Field Naturalist 100(2): 237–240.

———, P. F. Lee, D. Punter, and J. M. Stewart. 1988 Wild rice in Canada. New Canada Press Ltd, Toronto, Ontario, Canada.

Atkins, T. A., A. G. Thomas, and J. M. Stewart. 1987 The germination of wild rice seed in response to diurnally fluctuating temperatures and after-ripening period. Aquatic Botany 29: 245–259.[CrossRef][ISI]

Barton, L. V. 1939 Experiments at Boyce Thompson Institute on germination and dormancy in seeds. Science Horticulture 7: 186–193.

Brown, E., and C. S. Scofield. 1903 Wild rice: its use and properties. United States Department Agriculture Bureau of Plant Industry Bulletin 50: 1–24.

Cardwell, V. B., E. A. Oelke, and E. A. Elliot. 1978 Seed dormancy mechanisms in wild rice (Zizania aquatica). Agronomy Journal 70: 481–488.

Chen, S. L., H. Z. Qin, Y. Z. Jin, and S. Pu. 1990 Preliminary studies on systematic position and evolution of Zizania L. (Gramineae). Plant Biosystematics 41: 593–605.

Dore, W. G. 1969 Wild rice. Canada Department Agriculture Research Publication 1393, Ottawa, Ontario, Canada.

Duvel, J. W. T. 1906 The germination and storage of wild rice seed. USDA Bureau of Plant Industry Bulletin 90: 1–13.

Ellis, R. H., T. D. Hong, and E. H. Roberts. 1985 Handbook for seed technology for genebanks, vol. 1, Principles and methodology. International Board for Plant Genetic Resources, Rome, Italy.

Emery, W. H. P. 1966 The decline and threatened extinction of Texas wild rice (Zizania texana). Southwestern Naturalist 12: 203–204.

Horne, F. R., and A. B. Kahn. 1997 Phylogeny of North American wild rice, a theory. Southwestern Naturalist 42: 423–434.[ISI]

ISTA (International Rules of Seed Testing). 1985 Determination of moisture content. Seed Science and Technology 13: 338–341.

Kermode, A. R., J. D. Bewley, J. Dasgupta, and S. Misra. 1990 The transition from seed development to germination: a key role for desiccation? Horticultural Science 21: 1113–1118.

Kovach, D. A., and K. J. Bradford. 1992 Imbibitional damage and desiccation tolerance of wild rice (Zizania palustris) seeds. Journal of Experimental Botany 43:747–757.

Leggatt, C. W. 1923 Investigations with wild rice. Seed World. Sept. 7, 1923: 20–21.

Muenscher, W. C. 1936 Storage and germination of seeds of aquatic plants. Cornell Agricultural Experiment Station Bulletion 652: 1–17.

Power, J. P., and P. J. Fonteyn. 1995 Effects of oxygen concentration and substrate on seed germination and seedling growth of Zizania texana (Texas wildrice). Southwestern Naturalist 40: 1–4.[ISI]

Probert, R. J., and E. R. Brierley. 1989 Desiccation intolerance in seeds of Zizania palustris is not related to developmental age or the duration of post-harvest storage. Annals of Botany 64: 669–674.[Abstract/Free Full Text]

———, and P. L. Longley. 1989 Recalcitrant seed storage physiology in three aquatic grasses (Zizania palustris, Spartina anglica and Porteresia coarctata). Annals of Botany 63: 53–63.[Abstract/Free Full Text]

Simpson, G. M. 1966 A study of germination in seed of wild rice (Zizania aquatica). Canadian Journal of Botany 44: 1–9.

Terrell, E. E., P. M. Peterson, J. L. Reveal, and M. R. Duvall. 1997 Taxonomy of North American species of Zizania (Poaceae). Sida 17: 533–549.

Vertucci, C. W., J. Crane, R. A. Porter, and E. A. Oelke. 1994 Physical properties of water in Zizania embryos in relation to maturity status, water content and temperature. Seed Science Research 4: 211–224.

———, ———, ———, and ———. 1995 Survival of Zizania embryos in relation to water content, temperature and maturity status. Seed Science Research 5: 31–40.

———, and A. C. Leopold. 1987 Water binding in legume seeds. Plant Physiology 85: 224–231.[Abstract/Free Full Text]

Winston, P. W., and D. H. Bates. 1960 Saturated solutions for the control of humidity in biological research. Ecology 41: 232–237.[CrossRef][ISI]

This Article Abstract Full Text (PDF) Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via ISI Web of Science (3) Citing Articles via Google Scholar Google Scholar Articles by Horne, F. R. Articles by Kahn, A. Search for Related Content PubMed PubMed Citation Articles by Horne, F. R. Articles by Kahn, A. Agricola Articles by Horne, F. R. Articles by Kahn, A.