Water-deficit treatment followed by re-watering stimulates seminal root growth associated with hormone balance and photosynthesis in wheat (Triticum aestivum L.) seedlings

Elucidating the growth responses of roots to water status will reveal physiological mechanisms underlying drought tolerance and water conservation in plants. Hydroponic experiments were conducted using two winter wheat cultivars, Wangshuibai (drought-sensitive) and Luohan 7 (drought-tolerant), and a water deficit was induced using a 20 % (m/v) aqueous solution of polyethylene glycol 6000 (-0.6 MPa). The lack of water significantly reduced the plant dry weight, leaf area, total root length (TRL) and surface area in seminal (SRs) and nodal roots (NRs), but the effects were less pronounced in Luohan 7 than in Wangshuibai. After re-watering, leaf area, TRL and surface area of Luohan 7 increased significantly, as compared to the controls, due to rapid compensatory growth of SRs, while those of Wangshuibai were still significantly reduced. Under water-deficit conditions, the concentrations of indole-3-acetic acid (IAA) and cytokinin (CTK) and their ratio (IAA/CTK) in SRs and NRs of both cultivars were significantly lower than those of controls, but increased after re-watering. However, Luohan 7 showed significantly increases in IAA/CTK of SRs as compared to the control. Net photosynthetic rate was much lower during water deficit in both cultivars, but it was enhanced significantly after re-watering, especially for Luohan 7. Moreover, sucrose content was significantly increased in leaves while reduced in roots under water-deficit conditions. After re-watering, sucrose content in leaves of both cultivars and in roots of Wangshuibai was severely reduced, while the values in roots of Luohan 7 were significantly increased as compared to the control. These results indicate that the drought-tolerant cultivar has a greater ability to maintain plant growth under water deficit and greater compensatory growth in SRs associated with higher IAA/CTK and photosynthetic products supply after re-watering.