Studies on untreated and urea-treated rice straw from three cultivation seasons: 3. Histological investigations by light and scanning electron microscopy

Abstract

Straw of Shan-you 63 (SY63), a middle season rice variety with a high silica content, low degradability and good response to urea treatment, was used in an attempt to detect histological reasons of its low degradability and good response to urea treatment. The morphological differences of rice blade, sheath and stem were studied, whereas blade and sheath were used for studies of the treatment effect. Five per cent and 10 per cent urea were applied for studies of the urea treatment effect. Untreated and sodium hydroxide treated samples (1.5% NaOH solution) were included as negative and positive control, respectively. Light microscope (LM) and scanning electron microscope (SEM) were used for investigation of the histology and study of images. The results indicated that silica was deposited in epidermis polymerised with cuticle waxes to become a barrier to rumen microbial attack. This cuticle wax layer seemed to be affected by urea treatment. The absence of cuticle wax layer and epidermis on the inner surface of stem could explain its better degradability compared with sheath and blade. The inner surface of sheath also lacked the cuticle wax layer but had an epidermis layer which may explain why the sheath responds more to urea treatment than does blade which has cuticle wax layers on both sides. Urea treatment affecting the cuticle wax layer seems to enhance the digestibility of parenchyma tissue through swelling of the wall of parenchyma cells and cracking of the wall of vascular tubes. The results indicated that urea treatment caused both physical and chemical changes in straw tissues and cell walls.