Anatomical Development of Phi Thickening and the Casparian Strip in Loquat Roots

Abstract

The anatomical development of cell wall ingrowth (phi thickening) in the cortical tissue adjacent to the endodermis and Casparian strip in endodermal cells was investigated in loquat roots. Phi thickening initially appeared simultaneously approximately 10 mm from the root tip and expanded as the distance from the root tip increased. Phi thickening was observed with increased age in several layers of cortical tissue. Then, the phi thickening attained full size at approximately 30 mm from the root tip. When phi thickening began around the cortex, the initiation of Casparian strip formation appeared as a dot in the endodermis. At the next stage, the Casparian strip appeared partly in several endodermal cell walls facing the phloem sectors before eventually appearing in all endodermal cells. Phi thickening developed considerably before the completion of the Casparian strip in loquat roots. When the development of the Casparian strip finished in the endodermis, the cortical tissue with the phi thickening shed from the endodermis. Upon separation of the cortex and endodermis, the pericycle layers increased laterally to a thickness of 2 to 3 cells and accumulated auto-florescent substances in their cell walls. The Casparian strip appeared further away from root tip in young and white roots than in the old and light brown roots. When roots were sampled from trees planted under drought stress conditions, phi thickening was observed to have developed dramatically compared to normal conditions. The development of phi thickenings of cortex in loquat roots under drought conditions may be regarded as a defense mechanism against water stress.