Root development in horticultural plants grown under abiotic stress conditions - a review

Roots usually suffer greater exposure to multiple abiotic stresses than shoots. Therefore, the root system can be as affected, or even more affected, than the aerial parts of a plant by such stresses. Despite this, the influence of abiotic stresses on root development has been considerably less studied than on shoots because of limited accessibility for root observations. This work reviews the recent scientific literature on root development and the performance of root systems in horticultural plants growing under abiotic stresses such as drought, waterlogging, salinity, extreme temperature, low illumination, nutrient deficiency or excess, heavy metals, elevated atmospheric CO2, and mechanical restrictions. Changes in the shoot:root ratio are often observed when plants are subjected to various stresses. Thus, a redistribution of metabolites from shoots to roots is frequently observed under drought, salt, or sub-optimal temperature stress, as well as during some nutrient deficiencies, or elevated levels of atmospheric CO2. Conversely, reductions in solar radiation or excess nutrient usually cause an increased shoot:root ratio Plants subjected to increased atmospheric CO2 concentrations, or to low-moisture regimes, may develop a more extensive root system; however, the other stresses reviewed here commonly inhibit root growth, and cause significant modifications to the architecture of the root system, often giving rise to more branched root systems with shorter roots. Colonisation by arbuscular mycorrhizal fungi can also induce changes in the root system of the host plant that may improve its resistance to several abiotic stresses. A wide variety of hormones and biochemical processes are involved in the regulation of root growth under abiotic stress. Essential regulatory functions have been attributed to abscisic acid, ethylene, reactive oxygen species, and reactive nitrogen species.