Abstract
It has been known that the rice mesocotyl elongates in dark and wet condition. As the seedlings for mechanical transplanting, the elongation of mesocotyl is unfavourable because of the anxiety to grow in disheveled or of the risk to be injured at the transplanting. Indica rice varicties are apt to elongate more than japonica ones. So, in the present experiment, we surveyed the way to retard the mesocotyl growth in indica varieties using IBP, a fungicide for rice blast and other diseases, as a plant growth regulator. Exp. 1 The seeds of Oryza sativa L. cv. Hatishail (indica, aman type), Kele (indica, aus type) and Salak (javanica, tjereh type) were used as materials. Seeds were sterilized and placed on culture medium containing 0, 2.5, 5, 10, 20, 40, 80 or 160 ppm IBP in a test tube, and incubated in darkness for 12 days at 32°C. Length of mesocotyls of Hatishail, Salak and Kele were 113 mm, 76 mm and 66 mm rcspectively. However, the mesocotyl elongation of these varieties was markedly retarded with the increase of concentrations of IBP. Shoot length was incrcased by IBP application at 2.5-80 ppm. Exp. 2 Three varieties culturcd in southern district of China, belonging to the indica, shen type, were tested under the same conditions described as those in Exp. 1. These varieties had longer mesocotyls, but lesser extent than those in Exp. 1, and the mesocotyl growth was also depressed by IBP application followed by an increase of shoot growth. Exp. 3 Eight varieties cultured in Korra (senen indica × japonica type varieties and one japonica variety) were tested under the same conditions except that the IBP concentration was set at 0 or 20 ppm, as that of Exp. 1. Mesocotyl length of indica × japonica type varieties were varied from maximum 32.0 mm to minimum 2.1 mm whereas that of japonica type variety was 4.8 mm. IBP depressed the mesocotyl elongation and stimulated the shot growth likely to the above experiments. Exp. 4 According to a routine procedure of raising seedlings for mechanical transplanting, seeds were soaked in water or 10 ppm IBP solution for 4 days at 6°C, then allowed to germinate for 22 hours at 32°C. Germinated seed were sowm in nursery bed soil in box, and incubated in darkness at 32°C. After the plumule emergence, the boxes were transfered into a dim light chamber for 2 days at 25°C. Under thes conditions, the mesocotyl elongation of indica type varieties was less than those cultured in vitro experiments as in Exps. 1-3. However, in British Hondulas Creole the mesocotyl length was depressed as to only 1 mm by IBP, in comparison to about 6mm of water control. The inhibitory effect of IBP on mesocotyl growth was varied with variety. The present paper suggests that the mesocotyl elongation can be retarded by the IBP application in almost all indica varieties, but more experiments are needed to use IBP as a growth retardant, particularly in the practical use.
