Endodermal Silicon Deposits and Their Linear Distribution in Developing Roots of Sorghum bicolor (L.) Moench.

Abstract

Seedlings of Sorghum bicolor (L.) Moench. were cultured in nutrient solution containing 100 p.p.m. SiO₂. Over periods up to 14 days, the progressive accumulation of Si deposits was recorded by means of the electron-probe microanalyser and the scanning electron microscope along the seminal root length. An acropetal, linear gradient of Si deposits developed in the endodermis, beginning at the proximal end of the root after 1 day, and subsequently extended distally until after 7 days, the total silicified endodermal zone occupied the proximal 75 per cent of the root length, a value which remained relatively constant thereafter, in spite of root extension. No Si was detected below this zone towards the apex. This result is believed to be related directly to the asynchronous gradient of cell maturation exhibited by the endodermis behind the apex, and specifically to the degree of wall development therein. Opaline silica was deposited only in the endodermis, initially on the inner tangential wall (ITW) surface after only 1 day, as spherical masses of coalesced, primary particles for which the term ‘silica aggregate’ is proposed. A thin layer of silica over the wall surface was formed as a secondary phase. The aggregates reached mature size after approximately 7 days. Conditions favourable to the inception of silica deposition are discussed including the significance of the chemical composition of the aggregates, and the importance of the degree of cellulosic thickening, as well as the surface characteristics, of the ITW.