Abstract
Pot-cultured cv. Nipponbare was used as the material. "Shoot vigor" was ranked based on the number of surviving flowers on the panicle at heading. The numbers of primordia of flowers and rachis-branches formed were estimated as the sum of the number of surviving ones at heading and that of vestiges of aborted ones. The results were as follows. (i) The lower the rank of shoot vigor, the fewer was the flower primordia formed on a panicle mainly due to the smaller numbers of the primary rachis-branches bearing secondary ones and of the secondary rachis-branches formed on a primary one as well. (ii) The lower the rank, the larger was the proportion of degenerated flower primordia to the total flower primordia formed. The major type of degeneration of flower primordia differed in accordance with shoot vigor. In vigorous shoots, it was degeneration together with degenerating secondary rachis-branches while in weak ones that with the primary rachis-branches. (iii) The lower the rank, the smaller was the proportion of surviving flowers on the secondary rachis-branches to the total at heading. (iv) The lower the rank, the larger was (number of flower primordia formed) / (leaf area at heading). However, (number of surviving flowers at heading) / (leaf area at heading) was almost constant among shoots irrespective of vigor due to the larger percentage degeneration of flower primordia on the weaker shoots.
