Seed dormancy is an important physiological stage in the life cycle of many seed-bearing plants. In an ever-changing environment, dormancy increases survival by distributing germination over time. In general, seed dormancy is a quantitative trait that is influenced by environmental factors. Depending on the species and accession, dormancy is controlled by nuclear factors, maternal factors or both, and dormancy can be genetically the dominant or recessive form of the trait. Wild oat (Avena fatua) is a well developed system for dormancy investigations. Dormancy in wild oat is regulated by three major genes as well as environmental factors, and dormancy is conditionally the recessive form of the trait. This paper summarizes current knowledge on genetic and environmental factors, and the interaction of these factors, on seed dormancy in several cereal grain and dicotyledonous species. Some applications of this knowledge to improve crops and manage weeds are outlined.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0960258500004086

Genetic basis for seed dormancy

Seed germination tests of 18 germplasm accessions representing 16 rice species (Oryza L.) were conducted under a series of dormancy-breaking treatments. Hulled and intact seeds were exposed to (1) constant temperatures of 26 °C, 31 °C, and 36 °C or left at room temperature (28±1 °C ); and (2) alternating temperatures of 40/25 °C, 40/30 °C, and 45/30 °C (12 h/12 h), or left at room temperature. In a third treatment, intact seeds were exposed to dry heat at 50 °C for 7 or 14 d before germination of hulled and intact seeds under the optimum temperature regimes for each species determined from Treatments 1 or 2. Species that showed less than 85% germination in Treatment 3 were exposed to various chemical treatments, namely germination of intact seeds in 0.001 M potassium nitrate (KNO 3 ), or presoaking the seeds for 24 or 48 h in 0.1 or 0.2 M nitric acid (HNO 3 ), 1 M hydrogen peroxide (H 2 O 2 ), 1000 ppm gibberellic acid (GA 3 ), and a combination of 0.1 M HNO 3 and 1 M H 2 O 2 or 0.2 M HNO 3 and 1 M H 2 O 2 . Four conclusions emerge from this work: (1) removal of the seed hull is extremely effective for breaking seed dormancy; (2) species respond differently to various constant or alternating temperature regimes, and no single temperature regime is consistently effective to break seed dormancy across all Oryza species; (3) heat treatment generally promotes germination of the species that respond to certain constant or alternating temperature treatments, but no consistent differences were observed in seed germination of most species between heat treatments at 50 °C for 7 or 14 d; and (4) some species respond to certain chemical treatments effectively under the optimum temperature regimes. An appropriate combination of seed hull removal, dry heat or chemical treatments, and germination under the optimum temperature regimes for individual species provides the best results for breaking seed dormancy of rice species.

/pdf/responses-to-seed-dormancy-breaking-treatments-in-rice-1erhh4780m.pdf

Responses to seed dormancy-breaking treatments in rice species (Oryza L.)

Seed dormancy contributes to the adaptability of plants in nature and is of considerable importance in agriculture. The weedy rice (Oryza sativa L.) strains LD, SS18-2, and TKN12-2 and cultivar 'N22' were selected to investigate the inheritance of dormancy in controlled conditions. Initial investigations using intact seeds, caryopses, caryopses with pericarp/testa removed, and excised embryos demonstrated that seed dormancy was imposed by the hull in SS18-2 and TKN12-2, and by the hull and pericarp/testa in LD and N22. Seed dormancy at 0 d after harvest (DAH) was dominant with average degree of dominance (ADD) > 0.8 in the crosses with weedy strains. Dominance for duration of seed dormancy was incomplete when judged by days to 50% germination. Broad-sense heritability (h 2 b ) for seed germination was lower at 0 DAH and highest at 20 DAH in all the crosses. The weedy strain-derived F 2 populations maintained a higher h 2 b during afterripening. The effects of three and two major genes on seed germination at 20 DAH were detected in the SS18-2- and N22-derived F 2 populations, respectively. A positive ADD, a high h 2 b , and major gene effect for caryopsis germination at 0 DAH were detected only in the cross with LD. Seed or caryopsis dormancy was correlated with the characteristics awn and black hull or red pericarp colors in the SS18-2-or LD-derived F 2 populations. This research demonstrates that weedy rice provides ideal gene resources to elucidate mechanisms of dormancy and to improve resistance to preharvest sprouting.

Inheritance of Seed Dormancy in Weedy Rice

This research was carried out in the Laboratory of Seeds Technology and Analysis of the Fitotecnia Department of the Agrarian Science Center of Federal University of Espirito Santo, Alegre, Espirito Santo State, Brazil, out with the objective to evaluated the physiological seed quality to ten soybean genotypes: BRSMG - Garantia, MGBR-46 (Conquista), MGBR 94-7916, FT-104, CAC-1, MGBR 95-19125, BRSMG - 68 (Vencedora), UFV-16, BR 94-12773, Doko, from the 99/2000 crop year, submitted the stress condition through the accelerated aging test, with temperature of 42oC and relative humidity of, approximately, 100% for periods of 0, 24, 48, 72 and 96 hours, utilized the completely randomized design, with four replications of 25 seeds. The physiological quality of seeds was evaluated through by the germination and vigor test (first germination count and radicule length. The results indicate that the genotypes Doko and CAC-1 showed the best physiological seed quality. In order to the accelerated ageing periods above 48 hours determinate results with higher seed deterioration. The total germination decreased until the period of 96 hours of treatment.

/pdf/avaliacao-da-qualidade-fisiologica-de-sementes-de-soja-43ukh8b49i.pdf

Avaliação da qualidade fisiológica de sementes de soja produzidas em alegre-es 1

In this study, we have isolated a rice (Oryza sativa L.) glutamate decarboxylase (RicGAD) clone from a root cDNA library, using a partial Arabidopsis thaliana GAD gene as a probe. The rice root cDNA library was constructed with mRNA, which had been derived from the roots of rice seedlings subjected to phosphorus deprivation. Nucleotide sequence analysis indicated that the RicGAD clone was 1,712 bp long, and harbors a complete open reading frame of 505 amino acids. The 505 amino acid sequence deduced from this RicGAD clone exhibited 67.7 % and 61.9 % identity with OsGAD1 (AB056060) and OsGAD2 (AB056061) in the database, respectively. The 505 amino acid sequence also exhibited 62.9, 64.1, and 64.2 % identity to Arabidopsis GAD (U9937), Nicotiana tabacum GAD (AF020425), and Petunia hybrida GAD (L16797), respectively. The RicGAD was found to possess a highly conserved tryptophan residue, but lacks the lysine cluster at the C-proximal position, as well as other stretches of positively charged residues. The GAD sequence was expressed heterologously using the high copy number plasmid, pVUCH. Our activation analysis revealed that the maximal activation of the RicGAD occurred in the presence of both Ca(2+) and calmodulin. The GAD-encoded 56 approximately 58 kDa protein was identified via Western blot analysis, using an anti-GAD monoclonal antibody. The results of our RT-PCR analyses revealed that RicGAD is expressed predominantly in rice roots obtained from rice seedlings grown under phosphorus deprivation conditions, and in non-germinated brown rice, which is known to have a limited phosphorus bioavailability. These results indicate that RicGAD is a Ca(2+)/ calmodulin-dependent enzyme, and that RicGAD is expressed primarily under phosphate deprivation conditions.

http://www.koreascience.or.kr:80/article/JAKO200510103449243.pdf

Cloning and characterization of a rice cDNA encoding glutamate decarboxylase.

Dormancy in rice (Oryza sativa L.) seed is imposed by certain physical and chemical factors associated with its covering structures, i.e.hull and pericarp. The nature of these germination blocks, their mode ofaction, and processes regulating the release of dormancy are not fully understood. Of nine rice cultivars studied, Ching-shi 15, Stejaree 45, PTB10, and Mahsuri are weakly dormant, and Bansphul, Benaful, Kataktara, Dular, and N22 are dormant. Release of seed dormancy in rice by various treatments, oxidative processes and enzymic changes associated with dormancy, and parallelism between natural and artificially imposed dormancy patterns were examined. The influence of the hull in imposing dormancy was stronger and more prolonged than that of the pericarp. Application of GA3 was effective in inducing germination only in weakly dormant cultivars. Dormancy was completely released in all cultivars by subjecting the seeds to moist heat treatment, by removing the hull and pericarp, and by applying GA3 after dehulling. Dormant cultivars had higher O2 uptake rate and peroxidase activity and lower amylase and dehydrogenase activities than the weakly dormant ones. Hull removal substantially decreased peroxidase activity but enhanced amylase and dehydrogenase activities. Nonanoic acid (C90), a short-chain saturated fatty acid (SCSFA), when exogenously applied to non-dormant seeds imposed dormancy. Dry heat treatment or presoaking in 0.01 m KNO3 or 0.1 m H2O2 was very effective in releasing SCSFA-imposed dormancy. Amylase activity was greatly reduced by treatments with nonanoic acid (C90) or ABA. Considering earlier reports and results of the present study, it is proposed that seed dormancy in rice is regulated both by the presence of SCSFAs and ABA in the hull and the pericarp. The relative significance of these substances in cultivars of tropical and temperate origins and its implications in terms of ecogeographic adaptability are discussed.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0960258500000854

M. Dadlani

Papers

Mechanism of seed dormancy in rice