Abstract: Maize (Zea mays L.), the earliest crop for heterosis application, is highly sensitive to water stress. The root system is one of the important organs contributing to drought tolerance. However, few reports have provided quantitative data on the hybrid vigor in root development of maize, especially on that under water stress. The objective of this study was to investigate the hete- rosis of maize root growth at seedling stage and the effects of water availability on it. Hybrid maize Gaoyou 115 (220 × 1145) and its parents were subjected to three water treatments: 75%, 55%, and 35% of field water-holding capacity, considered as well watered (WW), moderate water stress (MWS), and severe water stress (SWS) respectively. The desired soil water regimes were well prepared at the beginning of the experiment, and kept by weighing. Under WW treatment, substantial midparent heterosis (MPH) and overparent hereosis (OPH) were observed for root dry weight (RDW), root length (RL), and root surface area (SA). MPH for these traits was in the range between 100% and 203%, and OPH between 99% and 189%. MWS clearly reduced the degree of heterosis compared with WW; MPH was in the range between 73% and 108%, and OPH between 55% and 59%. However, SWS arrested hybrid vigor in root growth almost completely. These results indicate that heterosis of root growth in maize seedlings varies with water availability. The maximum heterosis for root growth was obtained in RL with average 155.5% MPH and aver-age 124.2% OPH over WW and MWS. Compared with middle (0.25 mm < root diameter (DA)≤0.45 mm) and coarse (0.45 mm < DA) roots, fine (0.05 mm < DA≤0.20 mm) roots displayed the highest degree of MPH and OPH in absolute RL and SA under WW, MPH for its RL and SA was 265% and 246%, respectively, and OPH 218% and 208%, respectively. For the ratios of RL and SA to the total RL and SA respectively, they only displayed significant and slight MPH, but compared with middle and coarse roots, fine roots (with 21% and 44% of MPH for RL% and SA%, respectively) still had the highest degree of heterosis. Although we still detected significant MPH and OPH for absolute RL and SA regardless of root diameter classes (except for OPH for coarse roots) and significant MPH for the ratios of RL and SA concerning fine roots under MWS, it was obvious that MWS reduced het-erosis of RL and SA, especially for fine roots. MPH values for absolute RL and SA showed about one-fold decreases, and OPH showed more than two-fold decreases. Additionally, absolute RL and SA for fine roots do not reveal heterosis under SWS, but the ratios of RL and SA for them significantly displayed 9% and 16% of MPH, respectively. This suggests that the seedlings of hybrid maize develop more and greater proportion of fine roots than its parents (especially under water stress), which enable them to resist moderate drought better.

Key words: Maize (Zea mays L.), Seedlings, Heterosis, Root growth, Water availability