In this study, we used Populus tomentosa saplings as experimental material to analyze the leaf growth, photosynthesis and boron transporter (BOR) genes in response to shortterm excessive boron stress (1, 5, 10 mmol/L boric acid). The results showed that: (1) compared with the control (0.05 mmol/L boric acid, CK), 1 mmol/L boric acid treatment (BS₁) could lead to upregulation of chlorophyll fluorescence parameters and increase of reactive oxygen species in leaves, and a small amount of black necrotic spots appeared in the leaves at the base of the saplings. Under 5 mmol/L boric acid stress, the net photosynthetic rate, stomatal conductance and transpiration rate decreased, while intercellular CO₂ concentration increased. Chlorophyll fluorescence parameters and hydrogen peroxide content increased further, while ultraoxygen anion content was lower than BS₁, but still higher than CK. Necrotic spots showed up on most of the leaves. Under 10 mmol/L boric acid stress, gas exchange parameters, chlorophyll fluorescence parameters and reactive oxygen species content were similar to that of BS₅. All leaves showed necrotic spots with banded distribution that parallel to secondary veins. (2) The boron content in roots and stems changed slightly compared with CK, while in leaves it increased significantly under 5 mmol/L and 10 mmol/L boric acid stress. Boron translocation factor and bioconcentration factor maintained at a high level. (3) Several members of BOR gene family showed responses. In roots, following the increase of boron treatment concentration, the expression level of PtoBOR4 and PtoBOR8 increased firstly and then decreased. In stems, PtoBOR3 was downregulated while PtoBOR5 was upregulated. In leaves, PtoBOR7 and PtoBOR8 was upregulated. The present study showed that P. tomentosa leaves are sensitive to high boron stress in terms of chlorophyll fluorescence parameters, gas exchange parameters, reactive oxygen species, and the expression patterns of BOR genes, probably due to the strong ability to control the boron concentration in roots and the ability to transport boron rapidly from roots to aerial parts.