Abstract: Seedlings of Brassica rapa L. were treated with different concentrations of boric acid （005， 060， 120， 240 mmol·L-1） supplemented in Hoagland solution for 25 d， then photosynthetic gas exchange， chlorophyll fluorescence and the plant growth parameters were characterized from the apically expanded leaves of these seedlings at five\|leaf and one heart stage. The results showed that there was no significant difference in the photosynthetic characteristics and growth parameters between 005 and 060 mmol·L-1 of boron\|treated plants. When the concentration of boron reached 120 mmol·L-1， there were marked decrease in net photosynthetic rate （Pn）， gas conductance （Gs） and transpiration rate （Tr）， while the intercellular CO2 concentration （Ci） was not affected. When the concentration of boron reached 240 mmol·L-1， Pn， Gs and Tr were decreased more sharply, and meanwhile Ci were decreased. Chlorophyll fluorescence analysis revealed that the efficiency of excitation energy capture by open PSII reaction center （Fv′/Fm′）， the effective quantum yield of PSⅡ （ΦPSⅡ）， photochemical quenching index （qP） and electron transport rate （ETR） decreased greatly， while non\|photochemical quenching coefficient （NPQ）， excess light energy （Ex） and heat dissipation （D=1-Fv′/Fm′） increased more than those in control. When plants were treated with 120 mmol·L-1 of boron， the biomass of shoots and roots were decreased， but the root\|shoot （R/T） ratio was not affected significantly;
when plants were treated with 240 mmol·L-1 born， the biomass of shoots and roots were severely lowered， meanwhile， the R/T ratio were also significantly decreased.

Key words: Brassica rapa L., boron toxicity, photosynthetic gas exchange, chlorophyll fluorescence