3.1 ǀ Root growth
Growth of crown roots is inhibited under Fe toxicity through decreased cell elongation and division, and lateral root initiation is also inhibited (Yamauchi & Peng, 1995; Zhang et al., 2011). A smaller root surface area will restrict excessive Fe absorption, but at the cost of nutrient absorption. Root growth will also respond to the plant nutrient status (Section 5).
To the extent that there is a gradient of reducing conditions and Fe2+ concentration with depth through the soil, concentration of root growth at shallower depths might prevent serious Fe toxicity and allow controlled acclimation to nutritional stress (Li, Kronzucker & Shi, 2016a). Supporting this, lateral root formation on crown roots is unaffected by exposure of the crown root tip to high concentrations of Fe2+. When the root tip is exposed to Fe stress, root gravitropism may change, favouring root expansion in shallower less-toxic soil.
These effects result from direct contact of the root tip with high concentrations of Fe2+ (Li, Kronzucker & Shi, 2016b). Ethylene and reactive oxygen species (ROS) are involved in most abiotic stress responses, but it is unknown whether ethylene acts alone or in conjunction with ROS in the response of root tips to high Fe (Li et al., 2016b). The root tip is also the primary sensing site for the effect on lateral root formation.