Comparison of root traits among different maize subgroups. Differences in (a) AREA, (b) WIDTH_MED, (c) ANG_TOP, (d) SKL_WIDTH, (e) AVG_DEN, (f) WIDTH_MAX, (g) ANG_BTM and (h) RTP_COUNT among TST, NSS, SS and Mixed subgroups of maize inbred lines. Letters above the plots indicate significant differences between different subgroup according to a one-way ANOVA and Duncan’s multiple comparisons. Four subgroups: Stiff stalk (SS), Non-stiff stalk (NSS), Tropical/subtropical (TST), and Mixed group (Mixed). Different lowercase letters represent significant differences among four maize subgroups (p < 0.05).