Yield loss due to insufficient potassium fertilizer supply has been well documented; however, the information about the negative effect of potassium deficiency on crop yield caused by ecophysiological determinants is not enough. A field experiment with three K treatments (severe K deficiency treatment, K₁; moderate K deficiency treatment, K₂; and sufficient K supply treatment, K₃) was conducted to (1) assess the effects of potassium deficiency on green leaf area index (GLAI) reduction; (2) quantify the contributions of single leaf area, leaf senescence, and leaf appearance to GLAI reduction under potassium deficiency; (3) reveal the changes in the contributions of accumulated radiation interception (RIacc) and radiation-use efficiency (RUE) to above-ground biomass (AM) decrease of oilseed rape under different K supplies. GLAI was restrained due to potassium deficiency, with a reduction ranging from 10.6 to 45.4%. The reduced single leaf area and accelerated leaf senescence caused by potassium starvation accounted for 5.9–23.7% and 2.4–29.0% reduction in GLAI, but delayed leaf appearance rate contributed little. The RIacc during the seedling stage in the K₁, K₂, and K₃ treatments was 101.2, 110.7, and 120.0 MJ m⁻² , respectively, and the RUE in the K₁, K₂, and K₃ treatments was 1.03, 2.22, and 2.98 g MJ⁻¹ , respectively, which caused a 61.7% and 48.2% reduction of the final harvested AM in the K₁ and K₂ treatments compared with the K₃ treatment. When AM reduction was less than 24.8%, RIacc was the main determining factor; however, it transferred to RUE when biomass decreased more. In conclusion, GLAI decreased due to potassium starvation was mainly caused by the reduced single leaf area and accelerated leaf senescence, and the relative contribution of RIacc and RUE to AM decline was related to the degree of potassium deficiency.
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