Studes on Branching Habits in Crop Plants : 2. Effects of shading or transplanting on differentiation and growth of tillering buds in lowland rice seedlings.

Abstract

Rice seedlings, which were growing normally under suitable condition at the seedling space 5cm × 3cm, were given suddenly unfavourable condition as shading (plots A and B) or transplanting (plot C). Afterwards, the growth of each tillering bud was followed and compared with control (plot D) at each seedling age. Shading or transplanting was made at the 4th seedling age, that is when the leaf blade of the 4th leaf had just completely emerged out the leaf sheath of the 3rd leaf. According to the previous paper and observation in this study, the primordia of the 5th and 6th tillers have already been formed though as a mere "swelling", and the 4th or lower ones have developed into buds with one or more leaf primordia at this seedling age. Shading treatment was carried out by means of a layer of yoshizu (marshreed screen) to the light intensity of about 10% of natural sunlight for 2 days (plot A) or 4 days (plot B). The increment of dry weight per plant was very slight under shading or during rooting period. After removal of bad condition the dry weight increased, although it was finnally inferior to the control (fig. 1). The growth of the 2nd and 3rd tillering buds of treated plants were inferior to the control not only at the end of treatments but also after recovery of growth (for instance at the 7th age), especially those of transplanted plants were inhibited completely. In the 4th, 5th and 6th tillering buds, the growth were inferior to the control immediately after the treatment (at the 5th age), afterwards, however, grew more rapidly and became superior to the control. The differentiation of new leaf primordia, in addition, proceeded almost normally. In the case of shaded plants, it is supposed that the difference in growth response between the lower (the 2nd and 3rd) buds and the upper (the 4th, 5th and 6th) ones may not be due to the stage of these buds when were treated, but be due, on the one hand, to the delay of growth of the upper buds in control plants, and the other hand, to the sufficient supply of nutrient substances from main stem to the upper buds in treated plants. Concering the fact that these lower tillering buds failed to grow in transplaned plants, it is assumed to be due to a competition for nutrient substances between these buds and roots developing on the same node. As mentioned above, during the period of treatment, the 5th tillering bud was in the stage developing into bud from "swelling". The 4th tillering bud has one leaf (prophyll) primodium at the 4th age, and at the 5th age it has 3 leaf primodia. This tillering bud, therefore, was also in important stage developing with particular rapidity. Nevertheless, both of these buds ultimately carried out excellent growth rather than suffered inhibiting effect, though the lower buds were suffered ultimate supression. This is interesting fact from the viewpoint of dormancy and development of tillers. The mechanism is to be investigated.