An X-ray induced panicle-degenerating mutant in rice

Abstract

The mutant strain called in this paper " Ieafy head" was found among the third generation lines of an Indica variety "Liew-chew" treated with X-rays. The mutant plants tended to be dwarf, and had a larger number of internodes per stem, shorter internodes, smaller number of tillers and shorter leaves than the normal ones. They produced at the top of the stems many branches with small leaves, simultaneously with the heading of normal plants ; the clump of leaves thus corresponds to a panicle. . The original plant of this mutant appeared in an X3 Iine consisting of 36 plants. Of the 36, 27 were normal and 9 were the mutant, giving a gbod fitness to the 3 : 1 ratio. However, observing the segrega-tion ratio foir two generations using the offspring of normal plants, deviations from the 3 : 1 ratio due to a reduction of mutants were found in many lines. The normal plants in those lines had a high fertility. From anatomical observations of panicle primordia, the course of development of the leafy heads was divided into four stages, preliminary, first, second and third. At the preliminary stage, the growing point changes to a shoot apex, while some difference from that of normal plants is found as soon as the boot leaf primordium develops ; the number of shoot primordia, possibly corresponding to bract leaves. is larger than that in normal plants. In the first stage, two to three branches develop in the same manner as the differentiation of first rachis primordia in normal plants. In the second stage, many leaf, buds develop from the branches, and in the third stage, several shoots are produced again at the top of the branches. No flower primordia are fcrmed. The leafy heads continually grow under the natural environments of Taichung. However, after a shortday treatment (10 hours), the plants developed normal panicles. It may be assumed that the mutant has a recessive gene whose dominant allelomorph is necessary for panicle development, and cannot produce a sufficient amount of some hormone-like substance required for flower initiation, but shortdays may increase the production of the substance.