Growing Characteristics of Rice Seedlings of Over-Optimum Age for Mechanical Transplanting

The rice seedlings grown for a long time and not suitable for mechanical transplanting are called asseedlings of over-optimum age. In this study, Yangjing 9538 was planted in seedling raising plates with different treatments of seedbed soil fertilizing levels. In the stage of 28 days after sowing, the above ground parts of seedlings could still maintain a few growth, seedling dry weight and the dry weight perunit of seedling height still had a little increment. But leaf color (SPAD) descended quickly. The roots grew stagnantly, root length and number of roots stopped increasing, the activity of root and root/shoot ratio descended quickly, and obviously. In the stage of 36 days after sowing, the seedling height increased quickly none the less, the dry weight per unit of seedling height descended significantly, seedling leaf color kept in a low level. The activity of root was low and the root system growth activity was in the lowest level. The seedling sugar metabolism was sensitive to the stage when the seedlings were at over-optimum age, while nitrogen metabolism was blunt, so the ratio of seedling sugar to nitrogen in the stage of 36 days after sowing descended significantly. The characteristics of seedling growth in the stage when the seedlings were at over-optimum age affected significantly by different fertilization levels. Under high level of fertilization, the basal part width of shoot wasn't changes obviously, but seedling height increased quickly, so the dry weight per unit of seedling height descended quickly;the roots grew weakly than these under low level of fertilization, root number and root length were less, the root system growth activity descended quickly, root/shoot ratio descended obviously. The seedbed soil fertilization level affected carbon and nitrogen metabolisms obviously, nutrition decreased largely under high level of fertilization. The above results showed that, rice seedling growth in the stage of 28 days especially 36 days after sowing was decided by the growing condition in a very great degree. The rice seedling in the low fertilization level can maintain higher sugar to nitrogen ratio in the stage when the seedling were at over-optimum age, and keep better nourishment for seedling growth.