Bioscience Center and School of Agricultural Sciences, Nagoya University. Chikusa-ku, Nagoya, 464-01 Japan
The effect of GA on shoot growth was quantitatively determined by a modification of the "microdrop method" of Murakami ( 1968). The seedlings were pretreated with a GA-biosynthesis inhibitor, uniconazole, to reduce the endogenous GA level and to enhance the sensitivity to exogenous GAs. By application of exogenous GA3 at 300 pmol/plant to the 4 day-old seedlings, the first leaf were restored to the normal phenotype. Under these conditions, the growth started after a 1-d lag period and ceased 2 d after application of GA3. The final length of the first leaf after GA treatment was 2.5 to 3 fold longer than that without GA treatment (Fig. 2, see increment).
To investigate the effects of GA on elongation and division of cells in dwarf rice seedlings, we anatomically examined the first leaf at 0-48 h after application of GA, to 4 day-old seedlings (Fig. 1 ). Seedlings were fixed with 4% paraformaldehyde and 0.25% glutaraldehyde in 50 mM Na-P buffer, dehydrated through tertial butyl alcohol series, embedded in Paraplast plus tissue embedding medium (Dmsion Sherwood Medical, St. Louis. Mo., USA), sectioned longitudinally at 10-12 m m with a rotary microtome, and mounted on silane-coated slides. The sections were stained to visualize nuclei by Feulgen reaction with a counterstain of Fast Green. Cell size was measured by an ocular micrometer. The percentage of dividing cells was determined for each 0.5 mm section from the base to tip of the first leaf in two parenchyma cell files, which corresponded to second and third cell files from the adaxial epidermis. For evaluating the effect of GA3 on cell elongation, first leaf length was divided into every 0.25 mm section from the base to the tip. The mean length of parenchyma cells was calculated by dividing the length of each section (0.25 mm) by the number of cells contained. At least three seedlings were measured at each sampling time. Three-dimensional histograms shown in Figs. 2A and B were constructed using a graphic software (DeltaGraph Pro ver. 3.5, DeltaPoint Inc. Monterey, CA, USA).
The average length of parenchyma cells in each 0.25 mm long section was determined along the leaf length (Fig. 2A). During the first 6 h after the application of GA,, the parenchyma cells were about 10 to 20 mm in length. Thereafter, the cells rapidly elongated until the 48 h. The final cell length except the basal and apical regions was
154 Rice Genetics Newsletter Vol. 14
Research Notes
B: Effect of exogenous GA3 on cell division in the first leaf.
Murakami, Y., 1968. A new rice seedling test for gibberellins, 'microdrop method', and its use for testing
deepwater rice. Planta 188: 362-368.
Zwar, J.A. and P.M. Chandler. 1995. a -Amylase production and leaf protein synthesis in a gibberellin-respon-