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Crop Science 42:185-189 (2002)
© 2002 Crop Science Society of America

SEED PHYSIOLOGY, PRODUCTION & TECHNOLOGY

Cupule Removal and Caryopsis Scarification Improves Germination of Eastern Gamagrass Seed

X. Tiana, A. D. Knapp*,a, K. J. Moorea, E. C. Brummera and T. B. Baileyb

a Dep. of Agronomy, Iowa State Univ., Ames, IA 50011
b Dep. of Statistics, Iowa State Univ., Ames, IA 50011

* Corresponding author (adknapp{at}iastate.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Eastern gamagrass (Tripsacum dactyloides L.) is a warm-season, perennial grass with high palatability and productivity. However, poor stand establishment, often due to seed dormancy, limits its widespread use. Seed dormancy is often caused by structures surrounding the embryo, the physiological state of the embryo itself, or a combination of these factors. The eastern gamagrass dispersal unit is a floret within a thick, hard cupule. The objective of this study was to evaluate effects of cupule (including lemma and palea) removal and caryopsis scarification on germination of eastern gamagrass by means of different commercial seed lots produced in different locations and years. Germination tests were conducted at 20/30°C alternating temperature with light during 30°C for 8 h daily. Germination counts were made every 7 d. After 28 d, the germination of decupulated caryopses from different seed lots germinated from 16 to 49% across seed lots, compared with 5 to 18% germination for caryopses with cupule intact. Scarifying the pericarp over the embryo, however, resulted in germination of all dormant seeds. We conclude that while the cupule (including the lemma and palea) contributes to the dormancy of eastern gamagrass, the pericarp and/or testa are the main factors restricting germination of this species. In addition, caryopsis scarification increased the germination rate and the germination test could be shortened to 21 or even 14 d depending on the seed lot.

Abbreviations: 2, 3, 5-triphenyl tetrazolium chloride, TZ


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
EASTERN GAMAGRASS is a warm-season, perennial grass with high palatability and productivity (Polk and Adcock, 1964). Recently, interest in eastern gamagrass has increased because of its potential use for high quality forage, soil conservation, and wildlife habitat (Burns et al., 1992; Hardin, 1994). Poor stand establishment, often due to seed dormancy, restricts its widespread use, affects planting practices, and complicates seed inventory management.

Structures encompassing the embryo, the physiological state of the embryo, itself or a combination of these factors often cause seed dormancy. Toole et al. (1956) stated that seed dormancy in many species is caused by the inhibitory influence of structures covering the embryo rather than by factors within the embryo itself. In wild oat (Avena fatua L.), the lemma and palea contributed to reduced germination (Hay, 1960; Hsiao and Quick, 1985) and pricking the outer layers of the caryopses promoted germination of dormant seeds (Crocker, 1906). Woods and Gutek (1974) reported that freshly harvested wild rice (Zizania palustris L.) germinated following removal of the lemma and palea and scraping the pericarp covering the embryo. Simpson (1990) summarized the effect of removing the lemma and palea on germination in 25 species that normally exhibit a high degree of dormancy at the time of seed maturation. He observed that in one of the 54 comparisons, removing the lemma and palea gave 100% germination and in the others removal of hulls improved germination from 25 to 54%, on average. Simpson (1990) also summarized the influence of puncturing the pericarp and testa on overcoming dormancy in 31 grass species. He reported that scarifying the coat produced a significant increase in germination and 30% of the puncturing treatments achieved germination greater than 90%.

The influence of encompassing structures on germination and dormancy in several warm-season grasses has also been studied. Coukos (1944) indicated that hammer mill processing tended to induce germination of dormant caryopses of little bluestem (Andropogon scoparius Michx.), big bluestem (Andropogon gerardii Vitman.), side-oats grama (Bouteloua curtipendula Michx.), indiangrass (Sorghastrum nutans L.), and smooth bromegrass (Bromus inermis Leyss.). Sautter (1962) scarified switchgrass (Panicum virgatum L.) with emery cloth to remove the lemma and palea and total germination was 84% with 74% of the seed germinating in 3 d. Ahring and Todd (1977) demonstrated that buffalograss [Buchle dactyloides (Nutt.) Engelm.] caryopses extracted from the seed-burs germinated readily.

Seed dormancy in eastern gamagrass has also been studied. Ahring and Frank (1968) found that prechill stimulated the germination of dormant cupules. Kindiger (1994) improved germination of caryopses by applying a 300 g kg-1 hydrogen peroxide solution. Anderson (1985) evaluated several aspects of seed dormancy in this species. He found that cupule removal improved germination of eastern gamagrass from 5 (cupule intact) to 40%. The residual dormancy, after cupule removal, was not investigated. The objective of this study was to evaluate effects of cupule (including lemma and palea) removal and caryopsis scarification on the germination of eastern gamagrass seed.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
The study consisted of (i) a cupule removal experiment with four seed lots combined with two cupule conditions (cupule intact, cupule removed) and two fungicide treatments (untreated and treated) using approximately 8-mo-old seed produced in 1995, (ii) a scarification experiment with three seed lots combined with three seed conditions (cupule intact, cupule removed and caryopsis scarified) using approximately 7-mo-old seed produced in 1996, and (iii) a scarification experiment of four seed lots combined with three seed conditions (cupule intact, cupule removed and caryopsis scarified) using approximately 18-mo-old seed produced in 1995.

Seed Lots
Seed lots used in these experiments were produced in 1995 and 1996 and purchased in November 1995 and 1996, respectively. The seeds were stored in a cold room at 4.4°C and 46% relative humidity until use. Detailed information on the seed lots is given in Table 1.


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  		Table 1. Origin and mean viability (±SE) of the eastern gamagrass seed lots used in the study.

 
Viability Test
A tetrazolium test was used to determine seed viability of the 1995 and 1996 seed lots in June 1996 and May 1997, respectively. Two replications of 50 seeds each were removed randomly from each sample and imbibed between folded germination paper towels moistened with distilled water. The towels with seeds were kept moist in a plastic bag for 24 h at room temperature. Subsequently, the cupule, lemma, and palea were removed. The caryopsis was bisected longitudinally with a razor blade to expose the main structures of the embryo. Half of each caryopsis was placed in a 20-mL petri dish and stained in 1 g kg-1 TZ solution for 4 h at room temperature. Following staining, the embryo was examined under magnification and germinability was determined according to standard tetrazolium testing procedures for grasses (AOSA, 1970).

Seed Treatments
The cupule (including the lemma and palea) was removed by cutting through it at the juncture of glume and rachis with a razor blade. Care was taken to ensure that scarification of the pericarp did not occur. Caryopsis scarification was accomplished by scraping the pericarp covering the scutellum area of the embryo to minimize damage to the embryonic axis.

In experiment one, fungicide treatments consisted of dusting dry seeds with Thiram (tetramethylthiuram disulfide) 50 WP (Gustafson Inc., Plano, TX) consistent with the recommended dosage rate of 5 g kg-1 of seeds.

Germination Test
Plastic boxes, 13 by 13 by 3.5 cm, with tight fitting lids were used as germinating containers. The substrate for each box was two layers of Anchor Steel blue seed germination blotter paper (Anchor Paper Co., St. Paul, MN), moistened with distilled water. One box, containing 50 randomly sampled seeds, was considered as an experimental unit. Four replications were used in all tests. All germination tests were carried out at 20/30°C alternating temperature (Ahring and Frank, 1968) with light (two 40 W, cool-white fluorescent lights placed vertically on left and right sides of the germinator) during 30°C for 8 h daily. Germination counts were made every 7 d for 28 d. Seeds were considered germinated if both the radicle and coleoptile exceeded the seed in length and the seedling was normal according to the seedling evaluation criteria of the AOSA for comparable grasses (AOSA, 1992). Normal seedlings were removed as they were counted. After 28 d of incubation, ungerminated seeds were examined by TZ tests and classified as dormant or dead.

Experimental Design and Statistical Analysis
All experiments were designed as a randomized complete block with four planting dates serving as blocks. The treatments were factorial combinations of 4 (seed lot) x 2 (cupule condition) x 2 (fungicide treatment) in the cupule removal experiment. In the caryopsis scarification experiments, the combinations were 3 (seed lot) x 3 (seed treatment) and 4 (seed lot) x 3 (seed treatment). Germination percentages were collected from each experimental unit and adjusted based on the viability of each seed lot. The germination data from each week then were subjected to individual analysis of variance according to the general linear model (GLM) procedure of the Statistical Analysis System (SAS Inst., Cary, NC). Duncan's Multiple Range Test at the 5% level of probability was used for comparisons between means. Analysis of germination over time, from 7 to 28 d, was accomplished using Proc Mixed of SAS with the auto regressive (ar(1)) covariance structure and the Satterthwaite approximation of degrees of freedom.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Seed lot viability, as determined by tetrazolium analysis, varied from 79 to 90% (Table 1). In May 1997, viability tests were repeated for the seed lots harvested in 1995 prior to the caryopsis scarification experiment. No significant differences were found between the 1997 test results and those of 1996; therefore, only the 1996 test results are reported for 1995 seed lots.

Cupule Removal Experiment
The three main effects, seed lot, cupule condition, and fungicide treatment produced significant differences (P < 0.01) in germination after 28 d. The seed lot x cupule condition interaction was not significant (P = 0.1); however, the seed lot x fungicide treatment (P = 0.04) and cupule condition x fungicide treatment interactions were significant (P < 0.01).

Cupule removal improved seed germination of eastern gamagrass over the cupule intact treatment for all seed lots without fungicide treatment (Table 2). Cupule removal, however, did not induce germination of all viable seed after 28 d of the germination test. Increases in germination due to cupule removal ranged from two-fold for Iuka-ISE to four-fold for PMK-24. Cupule removal, on the other hand, did not improve germination over the cupule intact in any of the seed lots that received fungicide treatment.


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  		Table 2. Germination percentage of eastern gamagrass (1995 seed lots) as influenced by cupule removal and fungicide treatment after 28 d of germination test.

 
The beneficial effects of cupule removal are in agreement with the findings of Anderson (1985), who investigated the effect of cupule removal on germination of eastern gamagrass using one seed lot collected near Carbondale, IL. Anderson (1985) reported that cupule removal improved germination approximately eight-fold, from 5 (cupule intact) to 40% after 30 d incubation at the average daily minimum and maximum temperatures of 19.8 ± 1.5°C and 22.6 ± 1.5°C, respectively. Since seed dormancy can be influenced by genetic background, year of harvest, location of seed production, and germination temperature, it is not surprising that the relative improvement in germination due to cupule removal reported by Anderson (1985) is not the same as reported here.

The negative effects of fungicide treatment, when applied to caryopses (after cupule removal) on seed germination were pronounced. The mean germination of fungicide treated caryopses, across seed lots, was 12%, while the mean germination of untreated caryopses was 42%. Reductions in caryopsis germination due to fungicide treatment varied from two-fold for PMK-24, four-fold for Iuka-ISE, six-fold for Pete-PS, to seven-fold for Iuka-IS.

Cupules of different seed lots responded differently to fungicide treatment. While there was a tendency for the germination of fungicide treated cupules to be lower than that of untreated cupules, these differences were not significant except for the Iuka-ISE seed lot.

The lower germination of fungicide treated caryopses compared with the germination of untreated caryopses could be due to phytotoxicity or some interaction of fungicide treatment with seed dormancy causing mechanisms. Since abnormal seedlings were not observed from the seeds treated with fungicide and, moreover, viability, as determined by tetrazolium test at the end of germination test, was not affected (data not presented), it appears that this inhibition may have been due to alterations in water uptake and/or gas exchange as a result of the application of the dry wettable powder to eastern gamagrass caryopses.

Scarification Experiments
For seed lots harvested in 1995, the influences of seed condition (P < 0.01) and seed lot x seed condition interaction (P = 0.03) on germination were significant at the end of the germination test. The effect of seed lot, however, was not significant (P = 0.12).

After 28 d, cupule removal again improved seed germination, with the exception of Iuka-ISE. Caryopsis scarification essentially overcame dormancy in all seed lots (Table 3).


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  		Table 3. Germination percentage of eastern gamagrass as influenced by cupule removal and caryopsis scarification after 28 d of germination test.

 
Cupule removal had no significant effect on 7- or 14-d germination (Fig. 1) . Cupule removal doubled the 21-d germination percentages compared with the cupule intact treatment in all seed lots with the exception of Iuka-ISE. Caryopsis scarification, however, induced a rapid increase in germination by 7 d in all seed lots, especially in the Pete-PS where the germination reached 46%. Germination at 14 d was more than twice as high as germination at 7 d in all seed lots. The 21-d germination was significantly higher than that at 14 d. The 28-d germination was not significantly different from the 21-d germination in any seed lots. No viable seeds, as determined by TZ, were found after 28 d of germination in the caryopsis scarification treatments.



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  		Fig. 1. Influence of cupule removal and caryopsis scarification on seed germination of different eastern gamagrass seed lots harvested in 1995. The standard error of the means are shown as vertical bars.

 
For seed lots harvested in 1996, the main effects, seed lot and seed condition, produced highly significant (P < 0.01) differences in seed germination at 28 d. The seed lot x seed condition interaction was also highly significant (P < 0.01).

Similar to 1995 seed lots, cupule removal improved seed germination over the cupule intact treatment, and caryopsis scarification further enhanced germination over cupule removal in all 1996 seed lots studied (Table 3).

Cupule removal did not affect 7-d germination (Fig. 2) . The 14-d germination of Pete-PSZ and Iuka-IBZ were significantly increased by cupule removal, while PMK-24 was not. The 21-d germination of all seed lots also were improved by cupule removal and the final germination (28 d) were three times higher than the cupule intact in all seed lots. Caryopsis scarification again accelerated the germination rate. The germination increased linearly for the first 2 wk. Following the 14-d initial peaks, no further enhancement in germination was apparent in the remaining testing period. After the germination test, ungerminated seeds were examined by TZ to determine their viability. The results showed no dormant seeds in PMK-24; however, 0.5 and 1.5% dormant seeds, because of insufficient scarification, remained in Pete-PSZ and Iuka-IBZ, respectively.



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  		Fig. 2. Influence of cupule removal and caryopsis scarification on seed germination of different eastern gamagrass seed lots harvested in 1996. The standard error of the means are shown as vertical bars.

 
The lower germination of 1996 seed lots compared with 1995 seed lots after caryopsis scarification (Table 3) may be due to heavy fungal infection and subsequent death. It is commonly believed that seed coat damage allows fungal establishment, which contributes to seed deterioration. Interestingly for the 1995 seed lots, which were only slightly infected by fungi, the percentage of dead seeds was 19% for scarified caryopses, 17% for seed with cupule intact, and 16.1% for seed with cupule removed. In 1996 seed lots, heavily infected by fungi, the percentage of dead seeds was 37% for seed with cupule removed, 30% for seed scarified, and 25% for seed with cupule intact. We observed that the fungi developed slowly in the first 2 wk of the germination test but spread more rapidly during the last 2 wk. Caryopsis scarification, however, accelerated the germination process allowing most seeds to germinate within the first 2 wk. This probably reduced the opportunity for fungi to influence seed viability and germination. Therefore, caryopsis scarification reduced the percentage of dead seed.

Germination levels for Iuka-IBZ (47%) and Pete-PSZ (46%) were significantly higher than that of PMK-24 (33%). However, these data do not necessarily reflect genotype differences in dormancy levels among seed lots. For valid comparisons of seed dormancy levels among genotypes, seeds must be produced in uniform environments and experience common conditioning and handling procedures, since the environment during seed formation, harvest protocols, and storage conditions can alter the level of seed dormancy. The seed lots used in all the experiments were commercial seeds and produced at different locations.

It was evident from the experiments that, regardless of year and location of seed production, all seed lots showed similar responses to cupule removal and caryopsis scarification; cupule removal partially relieved dormancy and further treatment by caryopsis scarification overcame all the residual dormancy. This indicates that the cupule (including the lemma and palea), and pericarp, and/or testa are the important factors in restricting germination of eastern gamagrass. These encompassing structures can regulate germination by controlling water uptake, selective permeability to gases, presence of germination inhibitors, or mechanical restriction of embryo growth.

Anderson (1985) attempted to describe the mechanisms by which the cupule inhibited germination and reported that germination was not affected by soaking seeds (cupule intact or cupule removed) in bleach (sodium hypochlorite) for 15 min. In addition, seeds in contact with their removed cupules had higher germination (63%) than seeds with the cupule removed (40%). Anderson (1985) suggested that germination inhibitors were not present. He also found that germinating seed in air with elevated CO2 levels stimulated germination for some populations. Anderson (1985) concluded that it was unlikely that the inhibitory effect of the cupule on germination was related to restricted CO2 diffusion. The mechanism(s) of the inhibitory effect of the encompassing structures could be related to mechanical restraint on germination, oxygen availability, and/or prevention of water uptake. Further research will be necessary to determine the exact mechanism(s).


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Journal Paper No. J-18415 of the Iowa Agric. and Home Economics Exp. Stn., Ames, IA 50011. Project No. 3244, supported by Hatch Act and State of Iowa, and a grant from the Leopold Center for Sustainable Agriculture.

Received for publication June 3, 1999.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 


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