Biophysical parameters as informative tools for elucidating the causes of root growth retardation under stressful conditions

The root growth rate in barley (Hordeum vulgare L.) seedlings was measured in parallel with temporal changes in longitudinal (δl) and transverse (δD/D) cell-wall extensibilities and membrane hydraulic conductivity (L p) in the root extension zone. The root growth rate and biophysical parameters examined were sensitive to UV-B irradiation of shoots or roots and to excessive content of ammonium, glutamate, or nickel in the nutrient medium. The root responses to the above treatments were compared with the effects of abscisic acid, salicylate, hydrogen peroxide, diethylstilbestrol, α-naphthyl acetate, oryzalin, and ionomycin. The progressive reduction of root growth under the action of various stressors was accompanied by typical temporal patterns of the growth zone parameters: the δl extensibility declined monotonically, while δD/D and L p changed nonmonotonically, exhibiting the reversion from the initial decrease to the eventual increase above the control values. The decline of δl indicated that the root growth suppression was mainly due to changes in cell-wall mechanical properties caused probably by disorganization of cortical microtubules. It was found that the decline in δD/D and L p was caused primarily by the appearance of oxidative stress, disorders in cytoplasmic H+ homeostasis in root cells, and the consequent transient activation of the plasmalemmal H+-pump. Conversely, the increase in δD/D and L p upon the abrupt retardation of root growth was presumably caused by the increase in cytoplasmic Ca2+ content, disassembling of cortical microtubules, and by partial inhibition of the plasmalemmal H+-pump. The reversion of δD/D and L p changes upon progressive reduction of root growth can be used as an indicator to distinguish moderate and severe stress conditions in the root growth zone. Furthermore, this reversion indicates the increasing disbalance in the homeostasis of reactive oxygen species, cytosolic Ca2+, and cytosolic H+ upon severe stress.