Potassium influences the photosynthesis process in a number of ways; however,the mechanisms underlying the photosynthetic response to differences in potassium (K) supply are not well understood.The aim of this study was to investigate the effect of K nutrition on photosynthetic efficiency and mesophyll conductance (gm) in hickory seedlings (Carya cathayensis Sarg.) in a greenhouse.Hydroponic experiments were conducted using five different K concentrations (0,0.4,1.0,2.0 and 5.0 mM).After two moths,leaf dry mass per unit area (MA),leaf K concentration,chlorophyll content,and parameters related to photosynthesis were determined,and gm and chloroplast CO2 concentration (Cc) were estimated from concurrent measurements of gas exchange and chlorophyll fluorescence.Our data suggests that the leaf net CO2 assimilation rate (A) is not compromised by inadequate potassium availability when leaf potassium concentration remains above 0.7–0.8%.Furthermore,there is an approximate scaling of stomatal conductance (gs) and gm with A in hickory seedlings,and the relative limitation of photosynthesis due to gm and gs decreased with increasing K supply.However,leaf potassium concentrations less than 0.7–0.8% also appeared to limit the maximum carboxylation rate of Rubisco (Vc,max) and maximum rates of electron transport (Jmax); sensitivity analysis indicated that A was most sensitive to Vc,max and Jmax.These results indicate that the photosynthetic rate is primarily limited by the biochemical processes of photosynthesis (Vc,max and Jmax),rather than by gm and gs in K-deficient plants.Additionally,MA increased in K-deficient plants compared to non K deficient plants,and gm was closely correlated with gs and MA,which indicates that decreased gm and gs may be a consequence of leaf anatomical adaptation.