| Abstract/Contents: | "In the fourth report4) of this series, it has been found that in orchardgrass seeds harvested at various stages of development and ripening, the developmental stage of seed, seed weight, embryo length, and temperature during development and ripening were important factors affecting seedling vigour following germination. The present investigations were undertaken to elucidate the seedling vigour in Italian ryegrass and perennial ryegrass seeds harvested at various stages during development and ripening under two different temperature conditions (High=15°C to 25°C, Low=10°C to 20°C). To determine seedling vigour, the seeds were grown under natural environmental conditions during September to November, and the characters as the criteria for seedling vigour, such as first leaf length, number of tillers, number of leaves, plant height, leaf area, leaf weight, stem weight, plant top weight, leaf weight ratio, and specific leaf area, were measured at the 3rd, 5th and 7th week after emergence. The results obtained were as follows; 1. In both Italian ryegrass and perennial ryegrass, the seedling emergence was dependent on the developmental stage of seed as shown in the previous reports 1-3) of this series. Emergence and survival seedlings were observed in the seeds on and after the second developmental stage under any temperature conditions at development and ripening. 2. In both Italian ryegrass and perennial ryegrass, first leaf length showed significant difference between the seeds harvested at various stages during development and ripening under high and low temperature conditions, while leaf weight ratio and specific leaf area showed non-significant difference. The characters on growth increment, such as leaf area, leaf weight, stem weight, plant top weight, showed highly significant difference between the seeds harvested at various stages during development and ripening under high and low temperature conditions in Italian ryegrass and under low temperature condition in perennial ryegrass. No character showed significant difference between the seeds under high temperature condition in perennial ryegrass. 3. In order to define the relationships between seedling vigour and the developmental stage of seed, cluster analyses were applied to the seeds harvested at various stages during development and ripening under high and low temperature conditions in Italian ryegrass and under low temperature condition in perennial ryegrass, using the measurements of the characters showing temperature condition in perennial ryegrass, using the measurements of the characters showing significant difference. The seeds at the identical stage of development and ripening, except for the fourth stage, were tightly clustered, under any temperature conditions in Italian ryegrass and perennial ryegrass. The seedling vigour of the seed at the second developmental stage were very poor and a satisfactory seedling vigour was obtained in the seeds on and after the latter half of the third developmental stage, when the seed reached a maximum weight and a full size of embryo. 4. In identical developmental stage, the seed under low temperature condition at development and ripening produced more vigorous seedling than the seed under high temperature condition, when using plant top weight as the criterion for seedling vigour. 5. The characters showing significant difference between the seeds harvested at various stages during development and ripening, such as first leaf length, plant height, leaf area, leaf weight, stem weight and plant top weight, were positively and significantly correlated with the seed weight and embryo length within the seeds under high and low temperature conditions at development and ripening in Italian ryegrass and under low temperature condition in perennial ryegrass." |
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