Effects of drought on the xylem formation process and growth of Betula platyphylla and Fraxinus mandshurica.

We investigated the effects of drought on xylem formation of two-year-old potted Betula platyphylla (diffuse-porous wood) and Fraxinus mandshurica (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 B. platyphylla by 23.5% and 27.0%, respectively, but had no significant effect on the height and basal diameter of F. mandshurica. Severe drought significantly decreased the vessel area of B. platyphylla 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 F. mandshurica 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 B. platyphylla (45.4% and 35.3%) and F. mandshurica (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 B. platyphylla but significantly decreased by 32.0% and 25.0% in F. mandshurica, respectively. Drought significantly decreased the indole-3-acetic acid concentration in stems of both species. The gibberellin concentration in B. platyphylla significantly decreased by 14.9% in the early stage of drought and then stabilized, while in F. mandshurica, it initially increased and then decreased under mild drought but showed no significant change under severe drought. During drought, the abscisic acid concentration in B. platyphylla significantly decreased by 4.3% to 14.1%, while it significantly increased by 7.4% to 16.5% in F. mandshurica, 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.