Effects of drought on the xylem formation process and growth of <i>Betula platyphylla</i> and <i>Fraxinus mandshurica</i>.

We investigated the effects of drought on xylem formation of two-year-old potted <i>Betula platyphylla</i> (diffuse-porous wood) and <i>Fraxinus mandshurica</i> (ring-porous wood) seedlings in Northeast China. With normal water treatment as the control (75% field water capacity), we examined the effects of mild drought (50% field water capacity) and severe drought (30% field water capacity) on xylem anatomical characteristics, stem hormone concentrations, and root enzyme activities. The results showed that mild and severe drought significantly reduced the height of <i>B. platyphylla</i> by 23.5% and 27.0%, respectively, but had no significant effect on the height and basal diameter of <i>F. mandshurica</i>. Severe drought significantly decreased the vessel area of <i>B. platyphylla</i> by 31.0% and 33.5% in the mid-term (2 months of water control) and late-term (4 months of water control) treatment stages, respectively, while significantly increased the vessel density by 63.8% and 47.8%, respectively. In contrast, the vessel area of <i>F. mandshurica</i> change little, but the vessel density significantly increased by 66.1% and 110.5%, respectively. Mild and severe drought significantly increased the superoxide dismutase activity in <i>B. platyphylla</i> (45.4% and 35.3%) and <i>F. mandshurica</i> (26.2% and 33.1%), but had no significant effect on catalase activity. Under mild and severe droughts, the activity of peroxidase did not change in <i>B. platyphylla</i> but significantly decreased by 32.0% and 25.0% in <i>F. mandshurica</i>, respectively. Drought significantly decreased the indole-3-acetic acid concentration in stems of both species. The gibberellin concentration in <i>B. platyphylla</i> significantly decreased by 14.9% in the early stage of drought and then stabilized, while in <i>F. mandshurica</i>, it initially increased and then decreased under mild drought but showed no significant change under severe drought. During drought, the abscisic acid concentration in <i>B. platyphylla</i> significantly decreased by 4.3% to 14.1%, while it significantly increased by 7.4% to 16.5% in <i>F. mandshurica</i>, respectively. In conclusion, drought influences xylem formation and growth in broad-leaved trees by regulating hormone concentrations (such as indole-3-acetic acid) and antioxidant enzyme activities (such as superoxide dismutase, peroxidase and catalase), with the effects being modulated by tree species properties and the intensity and duration of drought.