Effects of submergence and hypoxia on the growth and anatomy of rice (Oryza sativa L.) seedlings

Aerenchyma - a specialized tissue with abundant, interconnected gas spaces - facilitates oxygen transport from shoots to roots in many emergent wetland plants. The importance of oxygen transport to root elongation and aerenchyma development in rice (Oryza sativa L.) seedlings was demonstrated in this investigation. Germinated seeds of Lemont (a commercial cultivar) and red rice (an agricultural weed) were positioned at 4.5, 2.5, or 1.0 cm below the surface of aerated or hypoxic water to control the duration of leaf submergence and the availability of oxygen. Primary root and leaf lengths were measured daily. Aerenchyma was compared among emergent and completely submerged seedlings that grew in aerated and hypoxic water. Root elongation rates were slower in hypoxic water than in aerated water. Root elongation rates increased after leaves emerged from 1.0 and 2.5 cm of overlying hypoxic water but not after leaves emerged from 4.5 cm of overlying hypoxic water. The benefits of oxygen transport to root elongation may therefore depend upon submergence depth. Measurements of transverse shoot sections taken between 2 and 3 mm above the seed revealed that 1) red rice coleoptiles grown in hypoxic water had 3 to 12 times more gas space than aerated coleoptiles and 2) the amount of gas space in primary leaves was two to five times greater in emergent seedlings that grew in hypoxic water than in aerated seedlings and completely submerged seedlings that grew in hypoxic water. Aerenchyma gas space formation occurred closer to the root tip and over a shorter distance in roots of emergent seedlings that grew in hypoxic water than in aerated roots. Roots of emergent seedlings that grew in hypoxic water also had a larger maximum amount of gas space than aerated roots. The increase in gas space between tissue ages of 10 and 45 h was similar among aerated roots and roots of emergent seedlings that grew in hypoxic water. This study is the first to report that hypoxia greatly reduced the amount of gas space and the rate of gas space formation in roots when oxygen transport was prevented by complete submergence in hypoxic water.