Changes in root architecture and endogenous hormone levels in two Malus rootstocks under alkali stress

High soil pH is a serious agricultural problem in northern China, where it affects apple metabolism, specifically root development. To explain the physiological responses of apple plants to alkali stress (non-stressed control, pH 6.0 ± 0.1, versus alkali stress treatment, pH 9.0 ± 0.1), we used hydroponics culturing to study the performance of two rootstocks with contrasting degrees of alkali tolerance. These trials included two alkaline salts (Na2CO3:NaHCO3; 1:1, v:v) that were intended to simulate soil conditions in northern China. For both rootstocks, values for fresh and dry weights, plant heights, root lengths, and root activity were decreased in response to stress treatment while relative electrolyte leakage in the roots was increased when compared with their respective controls. However, the amplitude of this response was much smaller in the alkali-tolerant Malus prunifolia than in the alkali-sensitive M. rockii. The effects of alkali stress on endogenous hormones also varied significantly between stress-treated rootstocks and their controls. After 15 days of alkali stress treatment, for M. prunifolia (AT), compared with the control, the contents of dihydrozeatin riboside (DHZR) and jasmonic acid (JA) were decreased while the contents of indole-3-acetic acid (IAA), abscisic acid (ABA), gibberellin (GA3) and zeatin riboside (ZR) were increased in the roots, among which the contents of ABA, ZR and JA in the roots were significantly different from their controls. As for M. rockii (AS), the contents of ABA, IAA, JA and DHZR were decreased while the content of GA3 and ZR were increased in the roots, among which ABA, IAA, JA and ZR in the roots were significantly different from the controls. However, there were significant differences in the contents of ABA and IAA in the roots between two apple rootstocks seedlings under alkali stress. Especially for the IAA levels in the roots, compared with their controls, was increased by 30.0% in M. prunifolia but decreased by 17.8% in M. rockii. After 15 days of alkali stress treatment, the root architecture of M. rockii was also more seriously influenced by alkali stress compared with M. prunifolia. Compared with their controls, the descent range of each index of the root architecture of M. rockii was almost twice as high as that of M. prunifolia. Especially for the number of root tips, the decrease amplitude of M. rockii was 17.6 times greater than that of M. prunifolia. These differences in root architecture might be explained by changes in the activity of hormones, especially for IAA.