Abstract: 【Objectives】 Potassium (K) is essential for oilseed rape (Brassica napus L.), insufficient supply will decline the leaf photosynthetic capacity, as well as seed yield. The present study was conducted to compare the response of different leaves (namely long-stalk leaf, short-stalk leaf and no-stalk leaf) to K deficiency at stem elongation stage under field conditions. Additionally, we attempted to find out the dominant limitations and reveal the underlying mechanisms due to K deficiency which restrained the leaf photosynthesis, and to provide theoretical basis for the reasonable application of potash fertilizer on the purpose of improving seed yield. 【Methods】 A field experiment with no K supply (-K) and normal K supply (+K) was conducted using two rapeseed cultivars (Huayouza No.9 and Zhongshuang No.9). The plant dry matter biomass, leaf dry matter content, leaf area, leaf K concentration and chlorophyll content were concurrently determined. The leaf gas exchange parameters, including net photosynthesis (Pn) and stomatal conductance (gs), and leaf chlorophyll fluorescence parameters, including maximum quantum yield of PSⅡ(Fv/Fm) and effective quantum efficiency of PSⅡ (PSⅡ), the changes of Ci and stomatal limitations (Ls) were detected.【Results】1) Potassium deficiency leaded to a significant reduction of seed yield, in Huayouza No.9 the decrease was 13.9% and in Zhongshuang No.9 was 27.2%. 2) The growth of Zhongshuang No.9 was significantly suppressed under insufficient potassium supply, with 12.4% and 19.2% decreases of plant dry matter content and leaf dry matter content in long-stalk leaf, respectively, while those in Huayouza No.9 were unaffected. 3) Potassium deficiency mainly affected the photosynthetic physiological characteristics of long-stalk leaf, but not on those of short and no-stalk leaves. Under K deficient, the K content, chlorophyll content and Pn of long-stalk leaf were sharply declined. The dominant factors involved in Pn decline were different between the two cultivars. Potassium deficiency led to sharply decline of Ci but 16.6% increase in Ls in Huayouza No.9, indicating that Pn was primarily affected by stomatal limitations. Conversely, K deficiency led to the dramatic increase in Ci and 14.0% of decrease in Ls in Zhongshuang No.9, which suggested that Pn was primarily limited by non-stomatal limitations. 4) Moreover, K deficiency caused sharp decline in the leaf Fv/Fm, PSⅡ, qP and ETR of Zhongshuang No.9, but not in those of Huayouza No.9, indicating that the PSⅡreaction center was suffered of damage. Meanwhile, the obvious increment of NPQ hinted that non-stomatal limitations induced the decline of Pn. 【Conclusions】 The decline of photosynthetic efficiency in the long-stalk leaf of rapseed is exacerbated under K deficiency, which resultes in the decrease of seed yield. The Zhongshuang No.9 was more sensitive to K-starvation than Huayouza No.9, which is associated with its tolerance to K deficiency.