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a Univ. of Florida, Range Cattle Res. Educ. Ctr., Ona, FL. 33865-9706 USA
b Dep. of Agronomy, Univ. of Florida, Gainesville, FL 3261-0770 USA
c Statistics Dep., Univ. of Florida, Gainesville, FL 32611-0339 USA
grassdr{at}gnv.ifas.ufl.edu
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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1 mo compared with air drying seed. Chilling air-dried, ambient stored seed at 3°C for 48 h before testing reduced germination at 84 d after harvest (compared with 28 d and 6 mo). Neither chilling oven-dried seed nor heat (40°C for 24 h before testing) treatment of oven- and air-dried seed affected germination. Removal of the lemma and palea at harvest resulted in 96% germination compared with 26% for intact seed. Treatment of freshly harvested seed with H2SO4 for 6 min and accelerated aging (48 h at 41°C, 100% relative humidity) increased germination from 29 to 68% and 30 to 48%, respectively. Dormancy in atra paspalum is short-lived and should be of minor agronomic importance with fall seed harvest and spring sowing. Consideration must be given to seed storage conditions to assure viability beyond 1 yr.
Abbreviations: GLM, general linear model • RH, relative humidity • TZ tetrazolium
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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is a persistent, palatable pasture grass for use in the humid tropics, where it may be valuable for young cattle (Bos spp.) (Kalmbacher et al., 1997b). The first cultivar, Suerte, which was sold under the name `HiGane' in Australia, was released by the University of Florida in 1995. Other cultivars have been released in Thailand (`Ubon') and Argentina (`Camba FCA'). Although pasture establishment and management for atra paspalum have been studied (Kalmbacher et al., 1997a), little is known about seed dormancy and aging except for an observation that its seed underwent afterripening. In preliminary experiments conducted in 1992, 28-d germination at 65 d after harvest was only 7%, but it increased to 59% at 106 d.
Knowledge about dormancy and aging is needed because seedsmen must maintain viability of seed in storage and livestock producers need maximum germination at sowing to enhance pasture establishment. The purpose of this research was to measure the effects that year of harvest, aging, and storage environment have on germination of atra paspalum.
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Materials and methods |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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Storage and Poststorage Temperature Treatments
The 500 g of fresh seed from 1994 to 1996 was divided into two equal portions, one of which was divided and placed into two paper envelopes that were stored under two conditions where it air dried (Fig. 1)
. One envelope was stored in a wood-frame house that was not heated, cooled, or dehumidified (ambient). Temperature ranged from 5°C in winter to 30°C in summer with 40 to 90% relative humidity (RH), a condition similar to warehouses used by Florida seedsmen. The second envelope was stored at 3°C, 25% RH. Relative humidity was measured with a Weathertronic, model 5020 hygrothermograph (Weathertronic, Sacramento, CA).
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Regular and Accelerated Aging
Germination tests were performed on seed lots with and without heating and chilling (Fig. 1, Treatments 1–6) at 28, 84, and 168 d after seed harvest. This will be referred to as 1, 3, and 6 mo after harvest. Oven-dried seed stored at 3°C was also tested at these times, but without heating or chilling (Treatment 7). Seed stored without heating and chilling were tested at 4, 3, and 2 yr after 1994, 1995, and 1996 harvests, respectively.
On 8 May 1997, 185 d after 1996 harvest, air-dried seed from ambient (Treatment 3) and 3°C (Treatment 6) storage were divided. Half was returned to the original storage, and half was placed in a dehumidified room (40% RH) at ambient temperature.
Accelerated aging (41°C, 100% RH for 48 h; West, 1993) was conducted in January and February 1998 (two replicates in each experiment) on three seed lots: 1996 air-dried seed stored at 3°C; 1997 oven- and air-dried seed from ambient storage.
Germination Testing
Because nothing was known about the germination requirements of P. atratum, procedures for germination of bahiagrass (P. notatum Flugge) were used (Association of Official Seed Analysts, 1994). Germination tests were conducted using a Stults model 12 seed germinator (Stults Scientific Engineering, Springfield, IL) with 9 h at 35°C (light) and 15 h at 20°C (dark). Each of four petri dishes (replicates) contained a single sheet of blotter paper on which 100 seed were placed. At the start of each trial, blotter paper was moistened with distilled water and rewetted as needed. Germinating seed (coleorhiza protruding) were counted and removed Monday, Wednesday, and Friday (beginning on Day 5) for a 28-d period. To determine viability, tetrazolium tests (TZ) (Grabe, 1970) were conducted in March 1997 on 1994 air-dried seed that was stored at 3°C and 1995 and 1996 air-dried seed that was in ambient storage. Before bisecting laterally above the embryo, seeds were moistened for 16 h. After bisecting, seeds were placed embryo-down on a 30 mmol L-1 TZ solution. After 4 h, the TZ solution was removed and each seed was evaluated under 10 x magnification using Group C (small seeded grasses) guidelines (Grabe, 1970). To determine if atra paspalum had a light requirement for germination, 1994 oven-dried seed that was in ambient storage for 1 mo was germinated in petri dishes covered with black plastic film.
Removal of Integuments and H2SO4 Treatment
Using a dissecting microscope, lemmas and paleas were removed by hand from 200, 1997 oven-dried seed (four replicates with 50 seed each). For acid treatment, four lots of 10 g each were treated in concentrated H2SO4 for 0, 2, 4, and 6 min. Acid-treated seed were washed in distilled water, dried overnight at 40°C, and 400 seed (four replicates with 100 seed each) from each of the four durations of acid treatment were subjected to 28-d germination tests along with previously fresh-frozen seed and seed with their lemmas and paleas removed.
Seed Imbibition
To determine if seed dormancy was associated with water impermeability of the integuments, imbibition was measured in freshly harvested, oven-dried 1998 seed (high dormancy), and 1997 oven-dried seed (low dormancy). High- and low-dormancy seed were tested for germination. Duplicate lots each containing 1000 fresh seed and duplicate lots containing 1000 1997 seed were dried for 24 h at 100°C, cooled in a desiccator, and weighed to determine moisture concentration. The four 1000-seed lots and 50 g each of high- and low-dormancy seed were allowed to reach moisture equilibrium with ambient air for 8 d (seed water content = 76 g kg-1); then they were reweighed. Sixteen dishes were filled with seed from the two 50-g lots based on the 1000-seed weight (eight with 2.460 g dish-1 of 1998 seed and eight with 2.356 g dish-1 of 1997 seed). Seed were covered top and bottom with blotter paper. Four of the dishes for both low and high-dormancy seed received 10 mL of distilled water, and four dishes of each received no water (referred to as wet and dry seed, respectively). After 48 h in the germinator, seed was removed, dried at 100°C for 1 min to remove surface water, weighed, and the amount of water imbibed was calculated on a dry matter basis.
Statistical Analyses
Percentage of germination without arcsin transformation, which was found to be not necessary, was used in all analyses because our main interest was in treatment comparisons after a specified time (Scott et al., 1984). Analyses started with a general linear model (GLM) (SAS Institute, 1985) that included effects due to year of harvest, aging, and storage (Fig. 1, Treatments 3, 4, 6, and 7). Days within the 28-d test period were examined with the repeated-measures option (SAS Institute, 1985). We found that there were significant interactions involving all four of these factors and that the majority of the seed that germinated in the 28-d test period did so in the first 14 d. We reanalyzed the data by year for the 0- to 7- and 8- to 14-d periods. Differences were determined by applying the p-diff option (SAS Institute, 1985) to the age x storage treatment x period interaction.
For total germination across the 28-d test period, GLM analyses began with a model that included year of harvest, aging, and storage (Fig. 1, Treatments 3, 4, 6, and 7). There was a significant year of harvest x aging x storage treatment interaction which led to GLM analyses for each year of harvest. For significant aging x treatment interactions, storage treatment differences for 1, 3, and 6 mo after harvest were examined with orthogonal polynomial contrasts to determine whether effects were linear or quadratic. Differences among treatments were determined by GLM analyses within year of harvest and age interval, and Duncan's multiple range test was used to separate treatments.
Poststorage temperature treatments were analyzed in a GLM model that included year of harvest, aging, and chilling vs. no chilling (Treatments 1, 2, 3, and 4) or heat vs. no heat (Treatments 5 and 6). Significant interactions were examined with orthogonal polynomial contrasts for each treatment across age intervals and with contrasts for comparison of chilling vs. no chilling or heat vs. no heat within the same age interval.
Integument removal, H2SO4 treatment, and imbibition data were analyzed with GLM, and orthogonal contrasts were used to determine differences due to integument removal and imbibition treatments. Linear and quadratic contrasts were used to determine effects due to duration of acid treatment. Effects due to regular vs. accelerated aging and storage treatment were determined with a GLM model that included experiment, method of aging, and storage treatment. Differences in storage treatment were determined with Duncan's multiple range test.
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Results |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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In each of 5 yr, germination of fresh seed before and after oven drying was zero during the 0- to 7-d germination period (data not shown). Germination in the 8- to 14-d period and 28-d germination depended on year and drying treatments, which interacted
. For the 8- to 14-d period, seed harvested in 1998 had 14% germination for oven-dried seed vs. 5% for fresh seed
. There were no differences between fresh and dried seed in the other 4 yr for 8- to 14-d germination, which averaged 6% for fresh and 5% for oven-dried seed. The 28-d germination of fresh seed was 8% in 1994 and 36% in 1998, compared with 42 and 45% for oven-dried seed, respectively
. In 1995 to 1997, germination of fresh (29%) and dried seed (27%) was not significantly different. Fresh-frozen 1997 seed had zero germination.
Seed from the 1994 harvest dried at 40°C and stored for 1 mo at ambient conditions did not have a light requirement as 28-d germination of seed incubated in the dark was 76%, compared with 74% for similar seed germinated under alternating light and dark conditions. However, alternating light and dark during germination was used in subsequent experiments because initial work begun in 1992 was under these conditions.
Storage and Aging Treatment Interactions
Germination in 0- to 7- and 8- to 14-Day Periods
Seed stored in ambient conditions (Fig. 1, Treatments 3 and 4), whether oven (Fig. 2)
or air dried (Fig. 3)
, had greater percentages of germination in the 0- to 7-d period compared with the 8- to 14-d period at 1, 3, and 6 mo after 1994 harvest. In 1995 and 1996, differences in germination between periods depended on age. At 1 mo after harvest, there was a lower percentage germination in oven- and air-dried seed in the 0- to 7- compared with 8- to 14-d periods in both years. At 6 mo, germination was usually greater in the 0- to 7-d period than in the 8- to 14-d periods for oven- and air-dried seed.
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28-Day Germination
When comparing air- and oven-dried seed within the same storage treatment, there was generally no difference (P > 0.05) in 28-d germination at any age (1994 example, 74 vs. 75%, etc.) (Table 1) . There were exceptions, as 1995 oven-dried seed stored in ambient conditions had greater germination than air-dried seed stored in ambient conditions at 3- and 6-mo of age (66 vs. 41%, etc.). Comparisons across storage treatments within oven- or air-dried seed indicated that seed that was in ambient storage generally had greater germination than seed stored at 3°C at 1- to 6-mo of age. Seed harvested in 1995 provided a few exceptions.
In 1994, 28-d germination of oven-dried seed that was in ambient storage did not change from 1 to 6 mo after harvest, but germination increased linearly from 1 to 6 mo after harvest in 1995 and 1996 (Table 1). Air-dried seed from ambient storage did not change for 1 to 6 mo after harvest in 1994, decreased linearly in 1995, and changed quadratically in 1996. Germination of oven- and air-dried seed stored at 3°C did not change from 1 to 6 mo after harvest in 1994 and 1995, but changed quadratically in 1996.
Germination of seed from ambient storage declined, often to zero, after 1 yr (Table 1). Tetrazolium tests indicated this seed was not viable. An exception was 1996 air-dried seed from ambient storage, with 65% germination after 1 yr, but 0% after 2 yr. Seed stored at 3°C for 1 yr or longer maintained germination better than seed from ambient storage.
Seed that was removed from 3°C storage at 6 mo after 1996 harvest and was stored from 6 mo to 1 yr of age at ambient temperature and low RH had 94% germination for oven-dried and 59% for air-dried seed (P < 0.05). These respective percentages were greater than oven- and air-dried seed that remained in storage at 3°C for 1 yr (Table 1).
Accelerated Aging
Accelerated aging increased
28-d germination from 32% with no accelerated aging to 51% with accelerated aging. Air-dried 1997 seed had greater germination following accelerated aging (64%) than 1996 seed stored at 3°C (34%), and 1997 oven-dried seed (26%) was lowest.
Poststorage Treatments
Chilling air-dried seed resulted in an age interval x treatment interaction
(Fig. 1, Treatments 1 vs. 3) for 28-d germination. Mean (across year of harvest) germination of unchilled seed was not different among 1 (61%), 3 (64%), and 6 (57%) mo after harvest (means can be calculated from Table 1). Means for chilled seed were 65, 51, and 57%, respectively (data not shown). Germination was significantly reduced by chilling at 3 mo after harvest, thus germination changed quadratically over age. Chilling oven-dried seed for 48 h before germination (Fig. 1, Treatments 2 vs. 4) resulted in no significant effects due to treatment and no significant interactions.
Heating air-dried seed (Fig. 1, Treatment 5 vs. 6) resulted in a year of harvest x aging x treatment interaction
(data not shown). Heating had no effect on germination percentage at any time after harvest in 1994 and 1996. In 1995, heating reduced germination at 1 and 3 mo after harvest (29 and 30%, respectively) and increased germination at 6 mo (83%) compared with unheated seed (Fig. 3, Treatment 6).
Removal of Integuments and H2SO4 Treatment
Total 28-d germination was 96% for seed from which the lemmas and paleas had been removed on the day of the 1997 harvest compared with 29% for fresh, intact seed
. Most of the germination (94 percentage points) occurred within the first 4 d of the test.
Treatment of seed with H2SO4 for 0 to 6 min linearly
increased 28-d germination
, where x = minutes in H2SO4). Much of the germination occurred late in the 28-d period. For seed treated for 6 min, 8 and 32 percentage points (of the total 68%) occurred in the 0- to 7- and 8- to 14-d periods, respectively, with 28 percentage points in 15 to 28 d.
Seed Imbibition
Germination in the 0- to 7- and 8- to14-d periods for low-dormancy seed was 54 and 3%, respectively, compared with 0 and 14% for high-dormancy seed. After 48 h in the germinator, low-dormancy, wet seed imbibed 0.84 mg water seed-1, compared with 0.88 mg for high-dormancy, wet seed
. Water imbibition for low-dormancy, dry seed was 0.36 mg water seed-1 compared with 0.34 mg for high-dormancy, dry seed
. Wet seed imbibed more water than dry seed
, and there was no interaction
with dormancy. Water concentration in low-dormancy, wet seed after 48 h in the germinator was 403 g water kg-1 of seed compared with 369 g kg-1 for high-dormancy, wet seed
. Water concentration in dry seed was 154 and 159 g water kg-1 of seed
for low- and high-dormancy seed, respectively.
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Discussion |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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Dormancy in Indian ricegrass (Oryzopsis hymenoides Roem. and Schult.) is affected by lemma thickness, a genetically controlled trait (Zemetra and Cuany, 1984). In kleingrass (Panicum coloratum L.), genotypes differed in the degree, and possibly the mechanism, of dormancy (Tischler and Young, 1983). Germination of fresh kleingrass seed from 107 plants selected for reduced postharvest dormancy ranged from 20 to 90% (Tischler and Young, 1987).
In an apomictic grass, where embryos are genetically similar, seed dormancy can vary from year to year. This has been observed in dallisgrass (Paspalum dilatatum Poir.) (C.R. Tischler, 1998, personal communication). Differences in dormancy among years in P. atratum, also apomictic (Quarin et al., 1997), emphasizes that the expression of dormancy is controlled by environment. Low temperature (5°C) during the postzygotic phases of seed formation in buckhorn plaintain (Plantago lanceolata L.) reduced germination because low temperature increased seed coat mass (Lacey et al., 1997).
It appears that the dormancy mechanism in atra paspalum is not due to a water-impermeable lemma and palea as it is in Pensacola bahiagrass (West and Marousky, 1989). Breaking of fibers in the lemma was the mechanism that allowed water uptake and expansion of the bahiagrass embryo. There are other reasons for seed dormancy in grasses besides water-impermeable integuments. Dormant knotgrass (Paspalum distichum L.) seed imbibed water, but the hull and seed coat membranes did not permit germination (Huang and Hsiao, 1987). In signalgrass (Brachiaria decumbens Stapf), dormancy was due to mechanical restriction and inhibited oxygen diffusion by the seed coats (Whiteman and Mendra, 1982).
Atra paspalum lost dormancy slower when seed was stored at 3°C, compared with ambient storage (Table 1, Fig. 2 and 3). The increase in germination that resulted from removing seed from 3°C storage after 6 mo and storing it at ambient temperature for 6 mo may have been due to a reduction in dormancy as a result of increased metabolism in the warmer storage. Loss of dormancy in guineagrass (Panicum maximum Jacq.) was also slower at 10°C than at 22°C (Smith, 1979).
Three months after harvest seems to be an important point in the aging process. There was an increase in dormancy (reduced germination) due to chilling of air-dried seed stored under ambient conditions for 3 mo. There was also a trend toward increased dormancy in seed stored at 3°C from 1 to 3 mo, followed by a decrease in dormancy at 6 mo (Table 1).
A hallmark of the break in dormancy was a faster rate of germination. Germination of fresh seed was zero in the 0- to 7- and 5% in the 0- to 14-d periods. Germination of seed that was in ambient storage shifted from late in the 28-d germination period at harvest to the 8- to 14-d period at 1 mo of age and then to the 0- to 7-d period at 3- and 6-mo of age. Oven-drying seed at harvest delayed faster germination in 1995 and 1996 for both ambient and 3°C storage. Seedsmen must dry seed on the day of harvest, otherwise it will spoil in storage. This may delay the break in dormancy regardless of storage, but drying should not affect total germination.
Once dormancy was broken, viability depended on storage conditions. Seed handlers should anticipate that atra paspalum seed stored under ambient conditions will have poor viability at 1 yr after harvest. For long-term storage, atra paspalum seed should be refrigerated or stored at low humidity. Cold (5°C), dry (40% RH) storage was superior to other conditions tested for seven grasses (Canode, 1972). Humphreys and Riveros (1986) suggested the primary investment for seedsmen in the tropics should be for seed drying and sealed storage (i.e., to control RH), with refrigeration a secondary investment. Observations not related to this research indicated that atra paspalum seed maintained germination for up to 2 yr when stored at ambient temperature in dehumidified conditions.
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Conclusions |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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1 mo. For seed that had been oven dried at harvest, chilling (3°C for 48 h) at 1, 3, or 6 mo after harvest had no effect on germination. Chilling seed that had been air dried at harvest reduced germination at 3 mo compared with 1 and 6 mo after harvest. Once dormancy was broken, seed remained viable for 1 yr when stored in ambient temperatures and relative humidities. Seedsmen should not be concerned with dormancy because the interval between seed harvest and sowing is at least 4 mo. Seed stored for several years must be stored at low temperature and relative humidity if it is to maintain quality. |
NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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Received for publication July 2, 1998.
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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