On the Formation and Development of Abscission Layer in Rice Plants, Oryza sativa L.

Abstract

Variation of grain shedding among rice varieties may be attributed to the morphological and physiological characteristics of abscission region formed between pedicel and rachilla, i.e. no abscission layer, uncracking abscission layer or cracking abscission layer. In the present study, to clarify the varietal difference in the formation and development of abscssion layer during boot stage, change of histological peculialities in the tissue between pedicel and rachilla was observed. Varieties used were two Japonica-Indica hybrids bred in Korea (Yushin and Milyang 23) with cracking abscission layer, two Korean local varieties with uncracking abscission layer (Jojeongjo and Nengjo), and a Japanese paddy rice variety without abscission layer (Akibare). The results obtained were summarized as follows. 1. Rapid growth of panicle and spikelet were observed from 16 to 4 days before heading (Fig. 1). At 16 days before heading, when the panicle length was 20 to 30 mm and spikelet length was 2 mm, abscission layer could be observed by elongation of cells in the pedicel and rachilla (Fig. 2-D and E). 2. At 12 days before heading, when panicle length was 50 to 80 mm and spikelet length was 3 to 4 mm, abscission layer was clearly distinguished from the sclerenchymatous cells in the pedicel with thickened cell wall. Further, the cracking abscission layer of Japonica-Indica hybrid was composed by two layers of parenchymatous cell, while that of uncracking abscission layer of Korean local rice by one layer (Fig. 2-G, H). 3. At six days before heading, when panicle length was 120 to 190 mm and spikelet length was 7 to 8 mm, cell division was observed in the abscission layer of Japonica-Indica hybrid but not in the layer of Korean local rice. In both rice plants with abscission layer, cell walls of all the sclerenchymatous cells were lignified (Fig. 2-J, K). 4. In the rice plants with abscission layer, diameter of the parenchymatous cell composing abscission layer varied from 6 to 8 μm at heading time. This result seemed to show that little elongation of the cell occured after cell division. In another variety without abscission layer, on the other hand, cell was about 16 μm in length and cell wall was lignified at heading time (Figs. 2-O and 3). 5. At the heading time, length of sclerenchymatous cells in the protrusion at the top of pedicel was 70 to 110 μm. This result showed that remarkable elongation of scleren-chymatous cell has occured during boot stage as compared with parenchymatous cell in the abscission layer or with sclerenchymatous cell in the abscission region (Fig. 4). 6. From the above results, no clear differences in the formation and development of abscission layer and its around tissue was observed between the both rice plants with cracking and uncracking abscission layers. At harvest time, however, the former had one or two layers of parenchymatous cell and the latter had one layer of parenchymatous cell in the abscission layer. Further, activity of cell division seems to be higher in rice plant with cracking abscission layer than that with uncracking one. On the other hand, in a Japanese paddy rice cultivar, no abscission layer was observed during boot stage (Fig. 2-M, N, O).