Effects of exogenous BR and IAA on drought tolerance of Populus deltoides × P.nigra

In order to study the effects of exogenous BR and IAA on the drought resistance of poplar, the 1-year-old potted seedlings of three Populus deltoides × P. nigra clones, NE19, R270, and 107 were selected for 15 days of treatment and sprayed with 10 μmol/L brassinostevoid (BR) and 100 mg/L IAA. The results showed that BR and IAA significantly improved the net photosynthetic rate (P_n), photochemical quantum yield (F_v/F_m), chlorophyll content, leaf water-holding capacity under drought condition compared with the hormone-free. The P_n of NE19, R270 and 107 was increased by 19%, 60% and 85%, respectively in BR treatment, and 35%, 50% and 80%, respectively in IAA treatment. The F_v/F_m of BR group increased by 7.7%, 7.5% and 10.9% compared with the non-hormone group, while the IAA group increased by 7.4%, 8% and 11.9%, respectively. The results indicated that exogenous BR and IAA improved the drought tolerance of Populus deltoides × P. nigra clones. Examining the leaf discs treated with exogenous hormones treatment and it was indicated that, for all three clones, the stomatal indix decreased by 12.28%, 25.60% and 20.31%, respectively in BR treatment, and the IAA treatment decreased by 13.00%, 13.15% and 14.48%, respectively. BR and IAA treatments inhibit stomatal development to reduce water loss under drought conditions.The expressions of EPF1, EPF2, EPFL9, ERECTA, FAMA, SDD1 in stomatal growth-related genes were detected by fluorescence quantitative PCR. EPF1, EPF2, ERECTA and SDD1 which negatively control stomatal development were increased, but the expressions of FAMA and EPFL9 which positively regulate the process were decreased. In summary, phytohormone BR and IAA are involved in the process of stomatal development in Populus deltoides × P.nigra, and increase drought resistance of poplar by inhibiting stomatal development under drought conditions.