Abstract: Plant mechanical strength is an important agronomic trait. A rice brittle mutant bc7(t) which derived from japonica variety Zhonghua 11 by radiation of ⁶⁰Co-g displayed normal phenotype similar to its wild type (WT) plants except for the fragi- lity of all plant body. To understand the mechanism of controlling plant mechanical strength, the anatomical structure and chemical features of leaf of brittle mutant bc7(t) were investigated. Anatomical analyses were carried out by means of various microscopic techniques, such as light microscopy, scanning electron microscopy and transmission electron microscopy. It was found that the cell walls of sclerenchyma cells of leaf sheath and leaf blade in the mutant were thinner than that in WT. For histochemical loca- lization of lignin, fresh freezing-cut transverse sections of leaf blade and sheath were stained with Wiesner reagents. Responding to the Wiesner reaction, the sclerenchyma cells below the epidermis, vascular bundle sheath and xylem were stained red. Though no noticeable staining difference in leaf blade between WT and mutant, the sclerenchyma cells of leaf sheath of mutant were stained slightly deeper than that of WT. Separation and purification of cell wall of leaf blade and sheath were carried out. The lignin content of cell wall was determined by thioglycollic acid method, the results revealed a slightly higher lignin content in mutant than in WT without significant difference. The cellulose content of cell wall was assayed with the anthrone reagent; the results showed that the amount of cellulose of leaf blade and sheath in mutant was significantly lower than that in WT. The test of silicon content of cell wall showed an increased content in the mutant. The energy dispersive X-ray micro-analysis attached to the FE-SEM provided the information on the distribution and content of silicon in the epidermal cells of leaf blade and sheath. The X-ray map analysis at the upper and lower epidermis of leaf blade and outer epidermis of leaf sheath showed that the content of silicon in mutant was obvious higher than that in WT. The result from silicon X-ray mapping of upper epidermis of leaf blade indicated that the distribution of silicon was concentrated in cell wall of silica cells. X-ray point analysis on the upper epidermis of leaf blade in the cell walls of silica cell, dork cell, long cell, and bulliform cell showed that the contents of silicon at these locations in mutant were all higher than these in WT. These results suggested that the reduction of cellulose might affect the formation of secondary cell walls, the thickness of cell wall and plant mechanical strength. Mutant might have a mechanism to balance the mechanical strength with an increase in silicon content.

Key words: Rice, Brittle mutant, Leaf, Cellulose, Silicon