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Role of the pigmented seed coat of proso millet ( Panicum miliaceum L.)in imbibition, germination and seed persistence

Mumtaz Khan, Paul B. Cavers, Marguerite Kane and Ken Thompson

Seed Science Research / Volume 7 / Issue 01 / March 1997, pp 21 - 26DOI: 10.1017/S0960258500003329, Published online: 19 September 2008

Link to this article: http://journals.cambridge.org/abstract_S0960258500003329

How to cite this article:Mumtaz Khan, Paul B. Cavers, Marguerite Kane and Ken Thompson (1997). Role of the pigmented seed coat of proso millet( Panicum miliaceum L.) in imbibition, germination and seed persistence. Seed Science Research, 7, pp 21-26 doi:10.1017/S0960258500003329

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Seed Science Research (1996) 7, 21-25 21

Role of the pigmented seed coat of proso millet (Panicummiliaceum L.) in imbibition, germination and seed persistence

Mumtaz Khan1, Paul B. Cavers2, Marguerite Kane2 and Ken Thompson1*1NERC Unit of Comparative Plant Ecology, Department of Animal and Plant Sciences, The University, SheffieldS10 2TN.UK

department of Plant Sciences, University of Western Ontario, 1151 Richmond St N, London, Ontario, N6A 5B7,Canada

Abstract

Proso millet (Panicum miliaceum L) is both a crop anda weed in many parts of the world. The weedy biotypesexhibit a wide range of seed colours, but the strainswith the most persistent seeds are distinguished bydarker seeds. This paper compares the seed biology ofa range of biotypes from Canada and demonstrates thatdarker seeds have heavier seed coats, imbibe andgerminate more slowly, and suffer less imbibitiondamage (measured as electrolyte leakage). It isconcluded that all these attributes contribute to theincreased persistence in the soil of the dark-seededweedy biotypes. Imbibition damage is widely implicatedin poor emergence and low vigour of crop seeds, buthas not previously been considered in the context ofweed seed persistence.

Keywords: seed bank, weed, seed coat, proso millet (Panicummiliaceum L), imbibition, germination, seed persistence.

Introduction

Proso millet (Panicum miliaceuniL.), an old world cereal,forms a crop-weed complex in many areas of the world(Cavers and Bough, 1985). While grown in NorthAmerica mainly for livestock and bird feed, somepopulations of proso millet, including crop-likevarieties, form serious weed infestations.

Weed populations of proso millet have beenrecognized in North America for over 100 years.Serious weed infestations were first observed in theMidwestern United States in the early 1970s. Thesewere attributable to the black-seeded biotype, whichhad not hitherto been identified in North America(Wilkins and Robertson, 1981). This biotype wasintroduced to Canada (Southern Ontario) within the

"Correspondence

following ten years, almost certainly on used farmequipment, and has since spread throughoutSouthern Ontario and into Quebec (Dekker et al.,1981; Cavers, 1985; Cavers and Bough, 1985). Thelarge infestations found over an extensive area ofSouthern Manitoba in the late 1970s and early 1980swere all caused by the 'crown' biotype (see Table 1).Crown was the only licensed crop variety in Canadaat that time.

Several biotypes of the weed exist and one of themore obvious features by which they can be recognizedis by colour of the palea and lemma, hereafter calledthe 'seed coat'. Between each biotype there can beconsiderable variation in seed size, weight, colour andpersistence in the soil. The black-seeded biotype isconsidered to be the most troublesome and persistentstrain of proso millet (Bough et al., 1986).

Despite the economic cost to agriculture,horticulture and forestry of seed persistence in weedspecies, the mechanisms that determine long-termseed persistence in the soil are largely unknown. Somephysical attributes, such as seed size, shape andchemical composition have already been discussed inrelation to persistence (Thompson, 1987; Thompsonet al, 1993; Hendry et al, 1994).

Differences in seed coat colour have beenassociated with the incidence of imbibition damageand seed vigour in grain legumes. In dwarf Frenchbeans (Phaseolus vulgaris) differences in vigour andimbibition damage have been associated with thecolour of the testa. Cultivars with white testas tend toimbibe rapidly and show extensive imbibition damageand high solute leakage (Powell et al, 1986; Legesseand Powell, 1992). These responses suggest that seedcoat colour in proso millet may be correlated with thecapacity to maintain a persistent seed bank in the soil.The objective of this study was therefore, to explorethe possible role of seed coat colour in water uptake,imbibition damage, seedling growth and seedpersistence in the soil.

22 M. Khan el al.

Table 1. Description, origin and seed persistence of 11 proso millet populations

Biotype description

White (DF)Golden (GD)Golden (GL)Crown (weed) (CWC)Crown (weed) (CWP)Crown (crop) (CCP)Elgin (EC)Orange-red (OR)Dark red (DR)Black 1 (BG1)Black 2 (BG2)

Seed colour

whitegolden yellowgolden yellowgreen/greyyellow/greyyellow/greydark yelloworange-reddark redblackblack

Location

Durant farm near Ottawa, OntarioDurham County, OntarioLaval, QuebecCarman, ManitobaSte. Polycarpe, QuebecPlum Coulee, ManitobaElgin Co., OntarioCarleton Co., OntarioSt. Jacques, comte de QuebecGlengarry Co., OntarioGlengarry Co., Ontario

Seed persistence

transienttransient to persistenttransienttransienttransienttransienttransienttransientpersistentpersistentpersistent

Table 2. Seed coat colour, absorbance and percentage of total seed mass of 11 proso millet populations.Results are expressed as mean values (± SE)

Biotype description

White (DF)Golden (GD)Golden (GL)Crown (weed) (CWC)Crown (weed) (CWP)Crown (crop) (CCP)Elgin (EC)Orange-red (OR)Dark red (DR)Black 1 (BG1)Black 2 (BG2)

Seed coat colour

whitegolden yellowgolden yellowgreen/greyyellow/greyyellow/greydark yelloworange-reddark redblackblack

A m a X4,»,«K,

0.124 ± 0.0070.151 ± 0.0050.166 ± 0.0160.167 ± 0.0040.169 ± 0.0010.182 ± 0.0030.197 ± 0.0210.213 ± 0.0100.285 ± 0.0210.307 ± 0.0070.339 ± 0.004

Seed coat %*

12.6 ±0.418.2 ±0.318.1 ±0.319.0 ± 0.517.5 ± 0.416.3 ± 0.417.2 ± 0.716.7 ±0.519.3 ± 0.624.5 ± 0.921.9 ±1.1

*10 replicates each of 1 seed

Materials and methods

Plant material

Seeds of weedy biotypes of proso millet (Panicum miliaceumL.) were obtained from throughout Canada. The biotypesand collection locations are shown in Table 1. Seedpersistence information is from Cavers el al. (1992). Allbiotypes had a similar initial moisture content of 9-11%.

Seed colour determination

Random samples, each of three seeds, were ground andhomogenized in minimum volumes of dimethyl-formamide. The homogenate was centrifuged at 12 000g for 3 minutes and maximum absorption wasmeasured between 400 and 500 nm.

Proportion of seed coat

The percentage of seed mass accounted for by seed coatwas determined using 10 replicates each of one seed.

Germination test

Three replicates, each of 20 seeds, were germinated inPetri dishes on Whatman No. 1 filter paper. Just enoughdeionized water (1.5 ml) to moisten the filter paper wasprovided initially. Moisture level was checked daily andtopped up as necessary. Radicle emergence wasrecorded daily for 21 days at 22°C.

Rate of water uptake and electrolyte leakage

The rate of imbibition at 20°C was measured for twentyindividually weighed seeds placed separately in 2 mlof deionized water. At predetermined intervals, seedswere removed from the water, blotted dry, weighed andreturned to the water. Changes due to imbibition wereexpressed in terms of percentage weight increase.Leakage of electrolytes from individually weighedseeds in 2 ml of deionized water was determined after2,6,18 and 24 hby measuring the conductivity (fjS/cm)of the seed soak water, using a Horiba compactconductivity meter.

Seed coat colour and seed persistence in proso millet 23

DF —•—GD- - i r - G L G CWC

CWP — • — C C PEC * — O R

— -A- - DR — -O — BG1- -O- •BG2

36 48 60 72 84

Incubation time (hours)96 108

Figure 1. Relationship between proso millet seed colour and rate of seed germination incubatedfor 108 h.

Seedling relative growth rate

Seedlings of each biotype were transplanted into pots4 days after radicle emergence. The seedlings weregrown under the standard ISP environment (Hendryand Grime, 1993). Total plant dry weight was measuredat 7 and 21 days as described by Hunt et al. (1993), withseven replicate seedlings at each harvest.

Results

Maximum absorbance of the seed coat colour between400 and 500 nm showed that the white seeds had thelowest absorbance (0.124) and the black seeds thehighest (0.339) (Table 2). Absorbance data closelyparalleled the subjective ranking of seed coat pigments.Dark pigmented seeds showed a higher percentage ofseed coat, e.g. 24.5% for black- and 12.6% for white-pigmented seeds (Table 2).

Rate of seed germination was recorded for the 11proso millet collections. The results showed that dark-pigmented seeds had a slower rate of seed germinationthan light-coloured seeds (Fig. 1). White-pigmentedseeds took the shortest time (36 h) to achieve maximumgermination, while one of the black-pigmentedcollections took the longest (108 h).

These differences in rate of germination were relatedto the rate of water uptake and electrolyte leakage ofthe different biotypes. White-pigmented seeds absorbedmore water during the first 4 h of imbibition than dark-coloured seeds. These differences continued to besignificant after all measurement intervals. One group

of biotypes imbibed rapidly and were all completelywhite/light-pigmented, while a second group whichimbibed slowly had a range of colour from dark golden,orange-red to black (Fig. 2). The seeds of rapid imbibers(white) took up water quickly, increasing in weight byover 23% after 6 h (Fig. 2). In contrast, the slowlyimbibing coloured seeds showed an average increaseof only 12% after 6 h imbibition. The white/light-coloured seeds also exhibited high levels of electrolyteleakage, implying that rapid imbibition was responsiblefor more damage to the tissues of these biotypes(Fig. 3).

Relative growth rate (RGR) of each of the 11populations was determined, but one of the populations(CCP) established and grew very poorly so we haveignored the data in this case. RGR for the remainingten populations showed no consistent relationship withseed coat colour (data not shown). The correlationbetween RGR and germination rate was positive butnon-significant.

Discussion

Differences in water uptake and incidence of imbibitiondamage in proso millet biotypes were associated withseed coat colour. Biotypes that had entirely or partlylight-coloured seed coats imbibed rapidly and sufferedmajor imbibition damage as revealed by high soluteleakage. Dark-coloured seeds imbibed slowly andsustained less imbibition damage. Previous work hasalso suggested that in snap beans (Wyatt, 1977), limabeans (Kannenberg and Allard, 1964), dwarf French

24 M. Khan et al

ase

(

CDOc*->. cD)CD

45 r

40 h|

35 L

30 |-

25 1-

2 0 ^

1 5 -

10 []

5

oJDF GL CWP EC DR

Proso millet populations

BG2

Figure 2. Relationship between proso millet seed colour and water uptake (expressed aspercentage weight increase).

u

•d

§U

DF GL

Figure

CWP EC DR

Proso millet populations

3. Relationship between proso millet seed colour and electrolyte leakage.

BG2

beans (Powell et al, 1986) and cowpeas (Legesse andPowell, 1992) pigmentation is associated with a slowrate of water uptake.

The major factor influencing water uptake waspossibly the permeability of the seed coat. Colouredseed coats are heavier and therefore presumably thickerthan those of light-coloured seeds. This is in agreementwith earlier studies (Morrisetal., 1968) on permeabilityof bean seed coats. Permeability of dark-coloured seedcoats may be reduced by a physical barrier of greater

cell numbers, by differences in cell density or by somechemical reaction (phenolic oxidation) unique tocoloured seeds. Lignin content of lima bean seed coats(Kannenberg and Allard, 1964) has been shown toinfluence both testa thickness and rate of waterabsorption. In Sorghum halepense (another crop weed),the normally dormant, pigmented seeds could bemodified by drought during development, to producenon-pigmented, thinner-coated, more-germinableseeds (Benech Arnold et al., 1992).

Seed coat colour and seed persistence in proso millet 25

The pigmented seeds of proso millet biotypes, allweeds, show a wide range of seed bank behaviour(Cavers et ah, 1992). At one extreme, weed biotypesresembling crop varieties form transient seed banks. Incontrast, black and dark red seeded biotypes formpersistent seed banks that survive for several years inmoist soil. The present study suggests that the thick anddark-coloured seed coats of persistent proso milletbiotypes slow the absorption of water and may reduceimbibition damage and solute leakage. Thick, strongly-pigmented seed coats may also form a chemical barrierto pathogens. Hendry et al. (1994) found a significantcorrelation between the concentration of ortho-dihydroxyphenols in seeds and persistence in the soil,and concluded that orf/jo-dihydroxyphenols act as adefence against herbivory and decomposition in cooltemperate, relatively moist soils. We provisionallyconclude that the imbibitional damage and soluteleakage experienced by light-coloured seeds contributedirectly and indirectly (via pathogen infection) to theinability of these biotypes to maintain persistent seedbanks in the soil.

Acknowledgements

This study was supported by the Government ofPakistan and the Natural Environment ResearchCouncil. We are very grateful for the assistance of HansCornelissen in determining relative growth rates. Adraft of this paper was much improved by theconstructive comments of two referees.

References

Benech Arnold, R.L., Fenner, M. and Edwards, P.J. (1992)Changes in dormancy level in Sorghum halepense seedsinduced by water stress during seed development.Functional Ecology 6, 596-605.

Bough, M., Colosi, J.C. and Cavers, P.B. (1986) The majorweedy biotypes of proso millet (Panicum miliaceum) inCanada. Canadian Journal of Botany 64, 1188-1198.

Cavers, P.B. (1985) Intractable weeds — intraspecific variationmust be considered in formulating control measures, pp367-376 in 1985 British Crop Protection Conference — Weeds.

Cavers, P.B. and Bough, M.A. (1985) Proso millet (Panicummiliaceum L.) — a crop and a weed, pp 143-155 in White,

J. (Ed.) Studies on plant demography: a Festschrift for John L.Harper London, Academic Press.

Cavers, P.B., Kane, M. and O'Toole, J.J. (1992) Importance ofseedbanks for establishment of newly introduced weeds— a case study of proso millet (Panicum miliaceum L.).Weed Science 40, 630-635.

Dekker, J.H., McLaren, R.D., O'Toole, J.J. and Colosi, J.C.(1981) Proso millet. Ontario Ministry of Agriculture FoodFactsheet No. 81-067.

Doersch, R.E. and Harvey, R.G. (1979) Wild proso millet incorn. Proceedings North Central Weed Control Conference 34,58-59.

Hendry, G.A.F. and Grime, J.P. (1993) Methods in comparativeplant ecology. London, Chapman & Hall.

Hendry, G.A.F., Thompson, K., Moss, C.J., Edwards, E. andThorpe, P.C. (1994) Seed persistence: a correlation betweenseed longevity in the soil and orf/io-dihydroxyphenolconcentration. Functional Ecology 8, 658-664.

Hunt, R., Neal, A.M., Laffarga, J., Montserrat-Marti, G.,Stockey, A. and Whitehouse, J. (1993) Mean relativegrowth rate, pp 98-101 in Hendry, G. A. R, Grime, J. P.(Eds) Methods in Comparative Plant Ecology. London,Chapman & Hall.

Kannenberg, L.W. and Allard, R.W. (1964) An associationbetween pigment and lignin formation in the seed coat ofthe lima bean. Crop Science 4, 621-622.

Legesse, N. and Powell, A.A. (1992) Comparisons of wateruptake and imbibition damage in eleven cowpea cultivars.Seed Science and Technology 20, 173-180.

Morris, J.L., Campbell, W.F. and Wiebe, H.H. (1968) Arefractometric method for testing seed coat permeability.Agronomy Journal 60, 79-80.

Powell, A.A., Oliveirra, M. de A. and Matthews, S. (1986)Seed vigour in cultivars of dwarf French bean (Phaseolusvulgaris, L.) in relation to the colour of the testa. Journal ofAgricultural Science 106, 419-425.

Thompson, K. (1987) Seeds and seed banks. New Phytologist106 (suppl.), 23-34.

Thompson, K., Band, S.R. and Hodgson, J.G. (1993) Seedsize and shape predict persistence in soil. FunctionalEcology 7, 236-241.

Wilkins, D. and Robertson, L. (1981) Proso millet: threat tocontinuous corn. Country Guide, 100 (10), S-12, S-14, S-15.

Wyatt, J.E. (1977) Seed coat and water absorption propertiesof seeds of near-isogenic snap bean lines differing in seedcoat color. Journal of the American Society for HorticulturalScience 102, 478^80.

Received 15 July 1996, accepted after revision25 September 1996

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Plant Dormancy: Physiology,Biochemistry and Molecular Biology

Edited by G A Lang, Irrigated Agriculture Research and Extension Center, Washington State University, USA

Understanding plant dormancy is of great consequence to the sustainable and efficient procurement of food and fibre for nutritionaland economic benefits. The term plant dormancy embraces the processes that bring about a programmed inability within a plantto grow and develop in spite of suitable environmental conditions. Dormancy plays a crucial part at a variety of significant stagesin the life cycle of many plants. Seeds, buds and tubers are all included in this book and the relationships between dormancy,development and the environment are explored.Papers have been contributed from leading workers in the field and have been developed from the First International Symposiumon Plant Dormancy held in Oregon. The book brings together work that considers the genetic regulation of plant dormancy and itsphysiological and biochemical manifestations. Molecular techniques to help in the elucidation of processes and paradigms arediscussed and there are also examples of the use of modelling in the study of dormancy. Overall, the book presents a valuablereview of plant dormancy for crop scientists, plant physiologists, plant molecular biologists, horticulturists and all those involvedin investigating and managing plant dormancy.Contents:

. Seed Dormancy Systems and ConceptsNatural history of seed dormancy, A C LeopoldIs failure of seeds to germinate during development adormancy-related phenomenon? J D Bewley andB DownieControl and manipulation of seed dormancy, A A KhanA physiological comparison of seed and bud dormancy,F G Dennis, Jr

. Bud Dormancy Systems and ConceptsDormancy and symplasmic networking at the shootapex, C van der SchoolA new conceptual approach to bud dormancy in woodyplants, J Crabbe and P BarnolaDevelopment of dormancy in tissue-cultured lily bulbletsand apple shoots, G M de Klerk and M M GerritsDormancy in tuberous organs' problems andperspectives, J C Suttle

. Physiology/Temperature, Light, StressA physiological comparison of vernalization anddormancy chilling requirement. J D MetzgerDormancy breakage by chilling: phytochrome, calciumand calmodulin, J D RossConifer bud dormancy and stress resistance, F J BigrasEarly development of bud dormancy in conifer seedlingsused in fall-planting programs, J E Mac DonaldNear-lethal stress and bud dormancy in woody plants,M Wismewski et ai

. BiochemistryStructural requirements of the ABA Molecule formaintenance of dormancy in excised wheat embryos,5 R Abrams, P A Rose and M K Walker-Simmons

• Changes in hormone sensitivity in relation to onset andbreaking of sunflower embryo dormancy,M le Page-Degivry et al.

• Processes at the plasma membrane and plasmalemmaATPase during dormancy, G Petel and M GendraudCarbohydrate metabolism as a physiological regulator ofseed dormancy, M E Foley

• Chemical mechanisms of breaking seed dormancy,M A Cohn

5. Molecular Biology• Molecular analysis of tunon formation in Spirodela

polyrrhiza - a model system for dormant bud induction,C C Smart

• Characterization of genes expressed when dormant seedsof cereals and wild grasses are hydrated and remaingrowth-arrested, M K Walker-Simmons andP J Goldmark

• Analysis of cDNA clones for differentially expressedgenes in dormant and nondormant wildoat(Avena fatua L ) embryos, R R Johnson et al.

• Photopenod-associated gene expression during dormancyin woody perennials, G D Coleman and T H H Chen

6. Dormancy Modelling• Population-based models describing seed dormancy

behaviour, implications for experimental design andinterpretation, K J Bradford

• An integrating model for seed dormancy cycling,characterization of reversible sensitivity,HWM Hilhorst, M P M Derkx and C M Karssen

• Modelling climatic regulation of bud dormancy,5 D Seeley

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