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Agronomy Journal 95:184-190 (2003)
© 2003 American Society of Agronomy

SEED

Photoblastism and Ecophysiology of Seed Germination in Weedy Rice

Nam-Jin Chunga and Nam-Chon Paek*,b

a Natl. Crop Exp. Station, RDA, Suwon 441-100, Korea
b School of Plant Sci., Seoul Natl. Univ., Suwon 441-744, Korea

* Corresponding author (ncpaek{at}plaza.snu.ac.kr)

Received for publication February 12, 2002.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
The germination of rice (Oryza sativa L.) seed has been known to be unaffected by light or darkness, but a photoblastic rice (PBR) whose germination was favored by light was discovered in weedy rice. The effects of light, temperature, and soil burial depth on the seed dormancy and germination of PBR were examined. At 30°C, the seeds germinated 100% under white or red (R) light and below 1% in darkness. They showed 67 and 20% of germination under continuous and a brief pulse of far-red (FR) light, respectively, and photoreversible germination under alternating irradiation of R and FR light, indicating that the induction of germination occurs through very-low-fluence response and low-fluence response of phytochromes. The prompt and delayed induction of germination by a pulse of R and FR light, respectively, following dark imbibition suggests that phytochrome B exists in dormant seeds and phytochrome A is synthesized during dark imbibition. Dark imbibition for longer than 3 d induced secondary dormancy. In darkness, the germination frequency was about 28% at 15 to 20°C and below 1% at 25 to 40°C. At 12 h-diurnal fluctuations of 20 and 10°C and 25 and 15°C in darkness, the germination frequencies were 77 and 27%, respectively. When the seeds were sown in the soil, emergence frequency decreased as burial depth increased, and 12-h diurnal fluctuation of 20 and 10°C induced more seedling emergence than constant 15°C. Conclusively, PBR seeds are capable of germinating by sensing light or proximity to the soil surface during seasonal fluctuations in diurnal temperatures.

Abbreviations: ABA, abscisic acid • DAI, days after imbibition • FR, far red (light) • GA, gibberellic acid • PBR, photoblastic rice • Pfr, far-red-light–absorbing phytochrome • phyA, phytochrome A • phyB, phytochrome B • Pr, red light–absorbing phytochrome • R, red (light)


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
SEEDS OF HIGHER PLANTS enter a stage of developmental arrest, known as primary dormancy, before completion of the maturation process (Gardner et al., 1985). For germination, quiescent or nondormant seeds require only rehydration after release from primary dormancy while dormant seeds need additional external stimuli such as light (Borthwick et al., 1952; Toole et al., 1955; Mancinelli et al., 1966; Thomas, 1992), temperature (Mancinelli et al., 1967; Yaniv and Mancinelli, 1967), and chemicals (Taylorson and Hendricks, 1976; Small and Gutterman, 1991). Among these stimuli, light-inducible seed germination, referred to as photoblastism, has an immense biological significance. Even under favorable growth conditions, photoblastism allows the buried seeds to remain dormant until they are exhumed. Light passing through a leafed canopy possesses mostly FR region of wavelength (Quail, 1994; Quail et al., 1995; Smith, 1995). Far-red light allows photoblastic seeds to sense that light energy for photosynthesis is not adequate for seedling growth, consequently preventing germination (Borthwick et al., 1952; Toole et al., 1955; Mancinelli et al., 1966; Cone and Kendrick, 1985). On the other hand, R light, abundant in direct sunlight, promotes the germination of photoblastic seeds. The photoreversibility of seed germination under alternating irradiation of R and FR light appears to exist as a survival mechanism of small-seeded plants; it induces seed germination only under the optimum light condition in which subsequent growth and development of the plants are most likely to succeed (Milberg et al., 2000). Seed germination is mediated by R and FR light–absorbing photoreceptors called phytochromes (Butler et al., 1959).

The genetic and physiological aspects of germination mechanisms induced by phytochrome-mediated light signal have been intensively investigated (Toyomasu et al., 1993, 1994, 1998; Shinomura et al., 1994, 1996, 1998). Phytochrome A (phyA) and phytochrome B (phyB) play major roles among plant photoreceptors in controlling low-energy-light-induced germination and R light–FR light reversible germination (Casal and Sánchez, 1998). Phytochrome has two photoreversible conformations: R light–absorbing phytochrome (Pr) is converted into FR light–absorbing phytochrome (Pfr) by R light, and the FR light converts Pfr into Pr form. This photoconversion of phytochromes by R and FR light regulates the onset of seed germination. Light-labile phyA is synthesized de novo during dark imbibition, and light-stable phyB is present in all plant tissues (Dehesh et al., 1991; Shinomura et al., 1994, 1996; Quail et al., 1995). Seed germination is induced primarily by phyB and secondarily by phyA in Arabidopsis thaliana (L.) Heynh. (Shinomura et al., 1994, 1996). Germination responses to the fluence of a single R light are biphasic (Cone et al., 1985). The first phase is very-low-fluence response, showing a plateau that coincides with germination levels induced by a saturating FR light. The second phase is low-fluence response that is fluence rate dependent and shows the R light–FR light reversible induction of seed germination. The phyA-mediated germination occurs irreversibly through very-low-fluence response over a large spectrum. Photoreversible phyB-mediated germination is not only a function of low-fluence response, but also of the absolute Pfr concentration in photoblastic seeds (Reed et al., 1994; Botto et al., 1996; Shinomura et al., 1998).

In general, photoblastism has been known as a common characteristic of the small-seeded plant species including lettuce (Lactuca sativa L.), tobacco (Nicotiana tabacum L.), tomato (Lycopersicon esculentum Mill.), A. thaliana, umbelliferous species, and a wide range of weeds (Simpson, 1990; Milberg et al., 2000). Under favorable seedling growth conditions, large-seeded cereal crop plants usually germinate equally well in darkness or under any wavelength of light and thus are regarded as nonphotoblastic seeds (Gardner et al., 1985). Seed germination becomes less dependent on light with increasing seed mass, suggesting that light response and seed mass coevolved (Milberg et al., 2000). However, it has been considered that germination of nonphotoblastic seeds may be induced independently of light by pre-existing Pfr in dormant seeds or rapid conversion to Pfr during rehydration (Mancinelli et al., 1967; Simpson, 1990).

In many plant species having photoblastism, light-inducible germination occurs in a specific set of environmental conditions (Simpson, 1990). Species of phytochromes differ in the role in germination probably because the light-inducible mechanisms have evolved independently for adaptation to local environmental conditions (Toole, 1973). Temperature has been reported to affect the onset of most kinds of germination, including phytochrome-mediated germination (Yaniv and Mancinelli, 1967; Toole, 1973; Kristie et al., 1981; Kristie and Fielding, 1994). In most cases, temperature influences the Pfr level of total phytochrome as well as the conversion between Pfr and Pr in dry or imbibed seeds (Thompson et al., 1979; Kristie et al., 1981). The Pfr requirements for germination decreased with the increase of temperature in tomato seeds (Yaniv and Mancinelli, 1967). Phytochromes were hydrated or synthesized rapidly, but the conversion of Pfr to Pr also occurred quickly at high temperatures (Toole, 1973; Kristie and Fielding, 1994). In the effect of temperature on germination response to light, the optimum temperature for Pfr action in ‘Grand Rapids’ lettuce seeds is at 25°C (Ikuma and Thimann, 1964). At temperatures above 30°C, the germination frequency declines to almost zero, and the promotive effect of exposure to R light also disappears (Kristie and Fielding, 1994). In many species, photoblastism is also affected by other environmental conditions such as dark imbibition period (Duke, 1978), diurnal fluctuation in temperature (Ekstam and Forseby, 1999; Benvenuti et al., 2001), and burial depth in the soil (Scopel et al., 1991; Benvenuti et al., 2001).

Red rice is a weed that is highly related to cultivated rice but has greater primary dormancy; more tillers; longer culms; high susceptibility to seed shattering; pubescent leaves; and red pigmentation of pericarp, seed coat, or both (Cohn and Hughes, 1981; Diarra et al., 1985; Suh et al., 1997; Noldin et al., 1999). Like other cereal crops, light irradiation is not generally required for germination of wild, weedy, or cultivated rice seeds. Here, we report the discovery of PBR and describe the effects of environmental factors on the germination of PBR seeds. The ecological significance and the possible mechanisms of PBR seed germination in the field are discussed with relation to light, temperature, and soil burial depth.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Seed Materials and Germination Tests
Photoblastic rice was originally collected from the rice fields in South Korea, classified as the japonica-type weedy rice, and maintained in National Crop Experiment Station (NCES), Rural Development Administration (RDA), Suwon, Korea. The seeds of PBR (accession no. K019173, Natl. Cent. for Agric. Plant Genet. Resour., RDA, Suwon, Korea) and the japonica rice cultivar, Juanbyeo (control), were ripened under field conditions, harvested 45 d after heading, and air-dried until the moisture content of the seed was 13 to 15%. Sealed in plastic bags, the seeds were kept in cold storage maintained at 4°C and 40% relative humidity until use. To confirm the genetic stability of photoblastism, the germination response to light or darkness was tested using the seed produced in each of 3 yr.

Germination tests were performed in plant growth chambers after primary dormancy was broken using heat treatment at 50°C for 7 d (Naredo et al., 1988). The seeds were sterilized in 2% (v/v) sodium hypochloride solution with 0.1% (v/v) Tween 20 [polyoxyethylenesorbitan monolaurate] (Promega, Madison, WI) and rinsed with sterilized distilled water three to four times, and 100 seeds were immediately placed in the 110-mm petri plates supplied with 25 mL of distilled water or chemical solutions. Seeds having a protruding shoot after 6 to 7 d were counted as germinated.

Light Sources and Treatments
For the dark treatment, the Petri plates were wrapped with two layers of aluminum foil. Seeds were imbibed for 1 d at 30°C in darkness, and then were exposed to R, FR, or white light. Fluorescent (90 µmole m-2 s-1) (Phillips TL8 W/33), R (106 µmole m-2 s-1), and FR (62 µmole m-2 s-1) light-emitting diode (LED) lamps (Good Feeling Co., Korea) were used as light sources for seed germination. After the light treatments, the Petri plates were kept in darkness again for 6 d, and germination frequencies were measured.

Gibberellic Acid, Abscisic Acid, and Fluridone Treatments
To examine the effect of gibberellic acid [(1{alpha},2ß,4a{alpha},4bß,10ß)-2,4a,7-trihydroxy-1-methyl-8-methylenegibb-3-ene-1,10-dicarboxylic acid 1,4a-lactone] (GA) on germination, the seeds were imbibed with GA3 (Sigma, St. Louis, MO) solutions at six different concentrations and kept in darkness. To determine the effect of abscisic acid {[S-(Z,E)]-5-(1-hydroxy-2,6,6-trimethyl-4-oxo-2-cyclohexen-1-yl)-3-methyl-2,4-pentadienoic acid} (ABA) on light induction of germination, the seeds were treated with ABA (Sigma, St. Louis, MO) solutions at four different concentrations in darkness. After 1 d of dark imbibition, the ABA-treated seeds were exposed to R light for 5 min and then kept in darkness. To examine whether the biosynthesis of ABA during dark imbibition is involved in secondary dormancy, the seeds were imbibed with three different concentrations of fluridone {1-methyl-3-phenyl-5-[3-(trifluoromethyl)phenyl]-4(1H)-pyridinone} (Duchefa, Haarlem, the Netherlands) solutions and kept in darkness. The germination frequencies were measured 7 d after the chemical treatments.

Temperature and Soil Burial Conditions
To identify the optimum temperature range for displaying the light-sensitive germination of PBR seeds, seeds were imbibed under white light or in darkness at eight temperature regimes between 10 and 45°C. To examine the effect of diurnal temperature fluctuation on the seed germination, seeds were imbibed at 12-h diurnal temperatures of 20 and 10°C and 25 and 15°C in darkness, which corresponded to daily maximum and minimum temperatures in late April (seeding stage) and in late June (tillering stage) in southern Korea, respectively. To examine the effect of soil burial depth on the seedling emergence, dry seeds and seeds imbibed under white light for 1 d at 30°C were buried at 1-, 3-, 5-, 7-, and 10-cm depths in plastic boxes (41.0 by 24.5 by 15.0 cm3) using a silty clay loam soil. The plastic boxes were kept in phytotrons at alternate 20 and 10°C (12 h each) and at constant 15°C. The soil was kept at 80% relative humidity using distilled water through the small hole on the bottom of the box. Emergence was determined 30 d after sowing.

Statistics
Means were determined over at least three tests using three replicates containing 100 seeds each. For the evaluation of individual treatment means, all data were subjected to Duncan's multiple range test (Duncan, 1952).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Discovery of Photoblastic Rice
The 235 lines of weedy rice have collected at various locations of rice fields in South Korea based on the off-type plant, the presence of red pericarp and/or black glume, more tillers, and high susceptibility to seed shattering. These lines were classified as Oryza sativa through the analyses of cross affinity to indica and japonica rice cultivars and their F1 fertility (NCES, RDA, Suwon, Korea). When their seeds were tested for emergence at seeding 5 cm beneath the soil surface, one line showed much lower seedling emergence (<20%) than the others (>95%). When the seeds were sown on flooded soil surface, however, they emerged almost 100% (data not shown). Based on these preliminary results, the seeds were placed in petri plates and incubated under white light or in darkness at 30°C, the optimum temperature of germination in rice seeds. Under white light, the seeds began to germinate 2 d after imbibition (DAI) and showed 100% of germination at 4 DAI. The seeds kept in darkness had <1% of germination after 7 DAI (Fig. 1A). The other weedy rice lines germinated nearly 100% at 3 to 4 DAI independently of light (data not shown). The effect of light on seed germination of the weedy rice line and Juanbyeo using seed produced in two more field seasons gave the same results. We conclude that the weedy rice line (accession no. K019173) requires light irradiation for seed germination and thus is designated as PBR. Photoblastic rice is the off-type plant that displays more vigorous growth and tillers, and the seed type is short grain with red pericarp, hairy yellow hull, and easy shattering habit.



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  		Fig. 1. Light-inducible germination of photoblastic rice seeds: (A) germination modes under continuous white light ({circ}) or in darkness (•), (B) germination frequencies under a pulse of red (R) and far-red (FR) light and alternating irradiation of R and FR light, (C) germination responses to a pulse of R ({circ}) and FR (•) light after the onset of dark imbibition, and (D) germination responses to different fluence rate of white light depending on dark imbibition period. After dark imbibition for 1 (•), 3 ({circ}), 5 ({blacktriangleup}), and 10 d ({Delta}) at 30°C, the seeds were exposed to different periods of white-light irradiation and then kept in darkness for 6 d at 30°C.

 
Germination Modes of Photoblastic Rice Seeds under Red and Far-Red Light
After 1 d of dark imbibition at 30°C, PBR seeds germinated 100 and 67% under continuous R and FR light, respectively (Table 1). They germinated 10 and 90% under a pulse of FR and R light, respectively (Fig. 1B). Alternating irradiation of FR–R, FR–R–FR, FR–R–FR–R, FR–R–FR–R–FR, and FR–R–FR–R–FR–R light induced 89, 14, 91, 16, and 95% of the seed germination, respectively (Fig. 1B). These results indicate that the light-inducible germination of PBR seeds is photoreversible and mediated by phytochromes.


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  		Table 1. Germination frequencies of photoblastic rice (PBR) seeds under continuous far-red (FR) and red (R) light at 30°C.

 
Germination under a Pulse of Red and Far-Red Light after Dark Imbibition
The dark-imbibition periods required for maximum germination of PBR seeds were different under an inductive pulse of R and FR light (Fig. 1C). When a pulse of R light was given at 3, 9, 12, 18, and 24 h after dark imbibition at 30°C, PBR seeds germinated 22, 59, 80, 87, and 90%, respectively. The R light–mediated germination maintained 90% until 72 h and then decreased rapidly. Under a pulse of FR light, however, little germination occurred until 12 h of dark imbibition, and then the germination frequency slowly increased 6% at 18 h and 10% at 24 to 30 h after dark imbibition. It increased maximally 20% when a pulse of FR light was given at 48 to 72 h after dark imbibition and decreased thereafter.

Induction of Secondary Dormancy during Dark Imbibition
The peak of germination frequencies started to decline after 3 d of dark imbibition at 30°C under a pulse of R and FR light (Fig. 1C), indicating that the prolonged dark imbibition decreases the germination response to inductive light signal. The inductive effect of a pulse of R and FR light on the seed germination disappeared completely at 10 DAI in darkness, indicating that secondary dormancy is induced. Furthermore, the germination frequency decreased under a given fluence of light as dark imbibition period increased (Fig. 1D). Thus, the greater fluence rate of light was required for breaking secondary dormancy of the seeds imbibed for the longer period in darkness.

Effects of Gibberellic Acid and Abscisic Acid on Photoblastic Rice Seed Germination
The germination frequencies in darkness increased proportionally with the increase of GA concentration at 30°C (Table 2). Abscisic acid amounts greater than 5 x 10-4 M were effective in inhibiting seed germination under a pulse of R light. The inhibition of ABA biosynthesis by fluridone was also effective, showing maximally 43% of seed germination in darkness at 30°C.


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  		Table 2. Germination frequencies of photoblastic rice (PBR) seeds under gibberellic acid (GA), abscisic acid (ABA), and fluridone at 30°C.

 
Effect of Temperature on Photoblastic Rice Seed Germination
Under continuous white light, PBR seeds germinated equally well (>98%) at a temperature range of 15 to 40°C (Table 3). In darkness, they showed 30 and 26% of germination at 15 and 20°C, respectively, while little germination occurred at 25 to 40°C. Beyond the lower and upper thresholds (<=10°C and >=45°C), they germinated <1% independently of light. Under diurnal temperature fluctuation (Table 3), the seeds germinated 77% at alternate 20 and 10°C (12 h each) and 27% at alternate 25 and 15°C (12 h each) in darkness, which were the mean temperatures of 15 and 20°C, respectively. In dark imbibition condition, a fluctuation of 10°C at alternate 20 and 10°C increased the germination frequency about 2.5 times as high as the germination at constant 15°C while no significant difference was observed between alternate 25 and 15°C and constant 20°C. This indicates that, under the same fluctuation gap of 10°C, diurnal mean temperature of 15°C plays a key role in the thermoinduction of PBR seed germination in darkness.


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  		Table 3. Effect of temperature on the germination of photoblastic rice (PBR) seeds under white light or in darkness.

 
Effect of Soil Burial Depth on Photoblastic Rice Seed Germination
Burial depth conditions in the soil greatly influenced the seedling emergence of PBR, which was also affected by temperature (Table 4). When the seeds were sown after 1 d of imbibition under white light at 30°C, they emerged nearly 100% at all burial depths, except for 10 cm at alternate 20 and 10°C and constant 15°C. When the dry seeds were sown, however, the deeper the burial depth in the soil, the fewer the number of seedlings emerging at both temperature conditions, indicating that soil-mediated dormancy is induced. The emergence frequencies at alternate 20 and 10°C, however, were about three times as high as those at constant 15°C. This suggests that the soil-mediated dormancy of buried PBR seeds is less induced under diurnal fluctuation in low temperatures than at constant low temperature.


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  		Table 4. Effect of soil burial depth on the seedling emergence of photoblastic rice (PBR) depending on presowing light treatment and temperature.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
SUMMARY AND CONCLUSIONS
 REFERENCES
 
The germination of PBR seeds was induced under a pulse of FR light and also photoreversible under alternating irradiation of R and FR light at 30°C (Fig. 1B and 1C). This indicates that the germination of PBR seeds is mediated by phytochromes, which is a typical germination mode of small-seeded photoblastic plant species such as Grand Rapids lettuce and A. thaliana (Shinomura et al., 1994; Casal and Sánchez, 1998). After the onset of dark imbibition, the prompt and delayed germination was induced by a pulse of R and FR light, respectively (Fig. 1C), suggesting that phyB is present in dormant PBR seeds while phyA can be synthesized de novo after rehydration in darkness as has been reported in A. thaliana seeds (Shinomura et al., 1994).

A form of secondary dormancy can occur in seeds either submerged under water (Duke, 1978; Prasad et al., 1983) or imbibed in darkness at supraoptimal temperatures (Kristie et al., 1981). The induction and depth of secondary dormancy in PBR seeds correspond to the duration of dark imbibition (Fig. 1D), possibly due to the significant changes in physiological factors inside the seeds. The induction of secondary dormancy may be due to the effects of the endogenous levels of ABA and/or GA during the dark imbibition period, which prevent seed germination under an inductive fluence of R light (Table 2). More fluence rate of light may be required for decreasing the level of ABA and/or increasing the threshold level of GA. A linear relationship between fluence rate of light and period of dark imbibition (Fig. 1D) indicates that the Pfr level of phytochromes should be maintained high enough to break secondary dormancy. It appears that the fluence rate–dependent germination of PBR could be closely related to the induction of GA and/or the repression of ABA biosynthesis in vivo similar to other small photoblastic seeds (Toyomasu et al., 1993, 1994, 1998; Yamaguchi et al., 1998).

The induction of germination in PBR seeds was mainly determined by temperature in the absence of light (Table 3). Grand Rapids lettuce seeds germinated 20% in darkness and 5% under a pulse of FR light at 15 to 20°C due to the fact that a portion of pre-existing phyB was present as Pfr form (Kristie and Fielding, 1994). Low germination of lettuce seeds placed in darkness at >25°C was attributed to the rapid dark conversion of Pfr to Pr. Thus, it appears that a portion of the pre-existing phytochromes is also present as Pfr form in dormant PBR seeds to induce 26 to 30% of dark germination at 15 to 20°C. When the dormant PBR seeds are imbibed above 20°C in darkness, a portion of the pre-existing phytochromes is rapidly converted to Pr, resulting in total inhibition of germination (Fig. 1A and Table 3). Therefore, the germination of PBR seeds is inhibited only when they become rehydrated over 20°C in darkness, and the temperature-induced dormancy can be released completely by an inductive pulse of R light before secondary dormancy is induced after 3 d of dark imbibition (Fig. 1C).

Many researchers have reported that seasonal variations in daily maximum and minimum temperatures stimulate seed germination, but the physiological mechanisms have not been clarified because the optimum ranges of diurnal temperatures and fluctuation gaps are quite different among the species. Buried seeds receive attenuated amounts of heat and light according to burial depth and transmissivity of the soil. Phytochromes play a major role in controlling the emergence of seedlings from seed planted at or below the soil surface (Simpson, 1990; Benvenuti et al., 2001). Higher germination frequency of PBR seeds occurred at a diurnal mean of 15°C (alternate 20 and 10°C) than at constant 15°C, but no significant difference was observed between a diurnal mean of 20°C (alternate 25 and 15°C) and constant 20°C in darkness. This indicates that the induction of PBR seed germination is mainly regulated by diurnal fluctuation in low temperatures (Table 3); when daily maximum temperatures are below 20°C, this is effective in stimulating the germination of PBR seeds under the same fluctuation gap of 10°C (20 and 10°C vs. 25 and 15°C).

Germination response to diurnal temperature fluctuation is generally regarded as an adaptation for soil depth sensing and fluctuation gap detection, which function as a season-sensing mechanism (Van Assche and Vanlerberghe, 1989). A correlation between diurnal temperature fluctuation and burial depth is also present in the germination of PBR seeds (Table 4). The degree of soil depth–induced dormancy increased proportionally with increased burial depths while PBR can emerge perfectly up to 7 cm below the soil surface if the seed germination is induced by light. Diurnal fluctuation in low temperatures (20 and 10°C) diminished the induction of the soil-mediated dormancy rather than constant low temperature (15°C). This may have ecological relevance for field emergence as diurnal fluctuation may be more important than light as an environmental signal in sensing burial conditions in the soil (Benvenuti et al., 2001). The acquisition of a very high sensitivity to light for germination of jimsonweed (Datura ferox) seeds depends on vegetation cover and burial depth (Botto et al., 1998). As shown in other species (Taylorson, 1972; Scopel et al., 1991; Derkx and Karssen, 1993), PBR seeds appear to possess light and temperature sensitivity during their burial to be capable of stimulating germination in the most favorable seedling growth season as an annual plant. Therefore, the release from dormancy by temperature in PBR seeds is mainly governed by burial conditions, especially the soil depth, in the field.


   SUMMARY AND CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 SUMMARY AND CONCLUSIONS
REFERENCES
 
Seed germination of cereal crops with a relatively long history of cultivation is generally nonphotoblastic in their natural growth environments (Gardner et al., 1985). Generally, in plants, large-seeded species are less dependent on light for the induction of germination (Milberg et al., 2000). Therefore, it has been regarded that light would not be an adequate cue for germination induction in large cereal seeds that are usually sown at some depth in the soil. In this paper, we describe the characteristics of PBR seed germination with regard to environmental and physiological factors such as light, phytohormone, temperature, and burial depth in the soil. Korea has a long history of rice cultivation of more than 4000 yr. Several historical records of old Korean literature describing rice culture indicate that red rice had been cultivated widely and has survived as a volunteer weed in rice fields up to date (Heu et al., 1990). It appears that a long-term alteration of genetic constitution in PBR has proceeded to adapt to the seasonal changes in daily temperatures in local environmental conditions. In the field, PBR seeds would normally germinate at daily maximum and minimum temperatures of 20 and 10°C rather than 25 and 15°C in the absence of light, which corresponds to the near-surface soil temperatures during spring period in late April and early summer period in late June, respectively, in southern Korea. However, the thermoinduction of seed germination is affected primarily by the soil-mediated dormancy depending on burial depths. When rehydrated under unfavorable seasonal temperatures or soil burial depths, the seeds need to be exhumed to germinate. The patterns of seed germination and seedling emergence appear to be closely related to tillage operations before cultivation practices that use transplanted rice in paddies, typical in Korea today. Therefore, the germination response of PBR seeds to light, seasonal changes in daily temperatures, and soil burial depth is a crucial survival strategy that has evolved.


   ACKNOWLEDGMENTS
 
This work was supported by a grant (CG1522) from Crop Functional Genomics Center, 21C Frontier R&D Project, Ministry of Science and Technology, Korea. We thank Dr. B.W. Lee, S.H. Lee, Y.S. Kang, and H.J. Lee for the valuable discussion and critical review on the manuscript.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 





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