Dormancy In Rice Research Articles

Effects of growth substances on rice seedlings grown from seeds irradiated with gamma rays

Studies were made on the modifications of biological effects caused by ionizing radiations by post-treatment with growth substances, i.e., gibberellic acid (GA 3), indole acetic acid (IAA) and indole butyric acid (IBA). Dormant rice seeds (moisture content 13%) variety IR8 were exposed to gamma ray doses of 10, 20, 30 and 40 kR. The irradiated and non-irradiated seeds were soaked for 24 hr in 10 ppm of either GA 3, IAA, IBA or distilled water at 29°C Gamma rays induced a remarkable decrease in plant height. However, reversal of the radiation effect was obtained by applying GA 3, IAA or IBA. The magnitude of the reversal effect decreased with increasing doses of gamma rays. The lengths of the coleoptile and the first leaf were markedly decreased by the radiation treatment. The application of IAA and IBA produced no significant reversal effect in either case but GA 3 showed a slight reversal for both parameters. Seminal root length was inhibited by radiation. Furthermore, IAA or IBA alone showed some inhibiting effect on seminal root length, while GA 3 did not produce any effect. The total number of crown roots was not affected by 10 kR, but was decreased at higher doses. On the other hand, all doses increased the number of “stunted roots”. GA 3 failed to show any effect on the number of total crown as well as on “stunted roots”. IAA and IBA increased the total number of crown roots up to 20 kR but not at higher doses. The number of “stunted roots” showed a further increase when treated with IAA and IBA.

Mechanisms of Dormancy in Rice Seeds : II. New Growth Inhibitors, Momilactone-A and-B Isolated from the Hulls of Rice Seeds

In order to isolate growth and germination inhibitors from the hulls of dormant rice seeds, the seeds of a Japonica cultivar 'Koshihikari' and an Indica one 'Surjamukhi', which have strong dormancy, were used. The rice hull (ca 200kg) was soaked in 80% methanol for about 2 months. The methanol extracts were concentrated to an aqueous phase by the flash evaporator at below 40°C. The aqueous phase was extracted with chloroform and the active fraction was moved in chloroform. The active chloroform fraction was successively fractionated by organic solvents : n-hexane, benzene, ether, ethylene chloride, and ethyl acetate. The biological activity of each extract was checked by lettuce germination test. The active fraction checked was further separated according to solubility in dilute acid or base. The neutral fraction of benzene and ether extracts showed a strong inhibitory activity. The active fraction was used for further chromatographic study. By the silica gel thin layer chromatography with a developing solvent, benzene : ethyl acetate (10 : 1), two active compounds were crystallized and were identified as momilactone-A (C20H2603) and -B (C20H26O4) The momilactones inhibited the seed germination and the seedling growth of rice and lettuce at a concentration of 10-3M. Especially, these substances remarkably inhibited the root growth of rice seedling at 10-5-10-3M. Momilactone-B had stronger inhibition than momilactone-A. The inhibitory activity of momilactone-B was similar to that of a typical growth regulator, abscisic acid. 'Koshihikari' contained more momilactone-A than momilactone-B, while 'Surjamukhi, which has stronger dormancy than 'Koshihikari', held more mornilactone-B than momi-lactone-A. It was supposed that the content of momilactone-A and -B in rice seeds might vary with the rice cultivar. The inhibitory activity of momilactones was partially reduced by GA3 or IAA. In considering the breaking of dormacy in rice seeds, it may be neces-sary to check the interaction between momilactones and GA or IAA which has an important role during rice seed germination.

Grain dormancy, peroxidase activity and oxygen uptake in Oryza sativa

In developing grains of rice ( Oryza sativa L.) of the dormant variety H4, peroxidase activity decreased sharply about a week before grain maturity without any change in grain dormancy and oxygen uptake of intact grain. During storage or after-ripening of mature dormant intact grains of four varieties (H4, H6, Mayang Ebos and Seraup 27) at 25–30°, the critical range in peroxidase activity was 1·0–1·4 μmol purpurogallin/hr/grain above which rice grains were almost completely dormant and below which the grains were almost completely nondormant. The oxygen uptake of intact H4 grain tended to decrease during the loss of dormancy. The decrease in both the peroxidase activity and oxygen uptake could be attributed mainly to the lower activities of the hull. Dehulling of developing and mature H4 grains reduced dormancy and increased the oxygen uptake of the grain. Thus, reduction by the hull of the level of oxygen available to the dehulled grain (embryo) was mainly responsible for grain dormancy in rice.

Anaerobic Respiration in the Dormancy of Rice Seed

We investigated the effects of high temperature treatment (40°C three weeks) on dormant rice seeds of Hadsaduri (Oryza sativa cv.) which was known as a variety of deep dormancy and results obtained are summarized as follows; 1. The seed of which dormancy was broken by high temperature treatment could germinate under both aerobic and anaerobic conditions, whereas dormant seed could not germinate under the anaerobic condition. 2. Warburg manometric techniques were used for the determination of respiratory gas exchange. In the case of non-dormant seeds, the amount of released carbon dioxide always exceeded that of oxygen uptake. Whereas the dormant seeds showed nearly the same amount of oxygen uptake and carbon dioxide release. 3. Determination of ethyl alcohol was carried out by gas chromatography. Non-dormant seeds usually produced more amount of ethyl alcohol than dormant seeds. When both dormant and non-dormant seeds were incubated under nitrogen gas at 30°C, the same tendency as above was observed. 4. When non-dormant seeds were co-treated with sodium iodoacetate, which was known as one of the inhibitors of enzyme SH group, at 5mM and 10mM concentrations, the production of ethyl alcohol was inhibited. Germination of hulled rice of non-dormant seeds was not affected up to the concentraton to 10mM under aerobic condition, but almost completely inhibited at 5mM under submerged condition. From the facts described above, it might be possible to consider that the dormant rice seed had the aerobic respiratory pathway but had not an active anaerobic respiratory pathway sufficient for germination. Effects of high temperature treatment on breaking the dormancy of rice seed may be attributed to the activation of anaerobic respiration.

Dormancy of paddy seeds in relation to different moisture contents of grains and dates of harvests

SummaryTwo dormant varieties of rice were harvested three times at intervals of 7 days after grain development and the seeds were subjected to different sunning and controlled humidity storage treatments. Germination percentages and moisture contents of the grains were recorded at intervals of seven days after harvest.It was noted that the dormancy of the dormant rice seeds was broken early when the seeds were subjected to continuous sunning from sunrise to sunset, followed by the treatment where the seeds were preserved under 0 % humidity conditions at room temperature. There was no break of dormancy of rice seeds when the seeds were preserved under 94 % humidity conditions.It was further noted that when the age of the seeds at the time of a germination test remained the same, the earlier the harvest, the earlier was the break of dormancy; and when the lapse of time after harvest remained the same at the time of a germination test, the later the harvest, the better was the germination.It is suggested that dryness of the seeds helps in the early break of dormancy and wetness retards or completely stops the dormancy-breaking reactions. On drying, the kernels of rice seeds shrink in volume more than the glume covers resulting in an air-space between the glumes and the kernels. This air-space helps air-diffusion in and out of the seeds, resulting in quicker oxidation-reaction processes or possibly embryo respiration, and consequently the dormancy is broken early. Increase in temperature accelerates such reactions. It is further suggested that the dormancy of rice seeds depends on the interactions between the ‘embryo-sensitivity to inhibitors’ and the ‘state of oxidation-reaction processes’ inside the seeds.

Effect of Heat on Breaking Seed Dormancy in Rice1

Effect of Heat on Breaking Seed Dormancy in Rice¹

The chimaeric formation in an X 1 panicle after irradiation of dormant rice seed

The purpose of this investigation was to furnish some information on the chimaeric formation of the X 1 panicle after irradiation of dormant rice seed. The number of chlorophyll mutants appearing in the progeny was scored at X 2 and X 3, and the proportion of mutants emerging in a mutated progeny was determined. The segregation ratio in X 2 progenies was significantly less than the Mendelian expectation of 0·25, but that in X 3 progenies approached the normal ratio. Analysis of segregation ratios disclosed two kinds of X 1 panicles, (1) those which were thought to be derived from a single cell, and (2) those which seemed to be formed from several cells. Frequency of the first group increased with increasing dose. It is supposed that survival of an initial group of cells until gametogenesis determines the genetic constitution of the progeny.