Summary Synergistic improvement in leaf photosynthetic area and rate is essential for enhancing crop yield. However, reduction in leaf area occurs earlier than that in the photosynthetic rate under potassium (K) deficiency stress. The photosynthetic capacity and anatomical characteristics of oilseed rape (Brassica napus) leaves in different growth stages under different K levels were observed to clarify the mechanism regulating this process. Increased mesophyll cell size and palisade tissue thickness, in K‐deficient leaves triggered significant enlargement of mesophyll cell area per transverse section width (S/W), in turn inhibiting leaf expansion. However, there was only a minor difference in chloroplast morphology, likely because of K redistribution from vacuole to chloroplast. As K stress increased, decreased mesophyll surface exposed to intercellular space and chloroplast density induced longer distances between neighbouring chloroplasts (Dchl‐chl) and decreased the chloroplast surface area exposed to intercellular space (Sc/S); conversely this induced a greater limitation imposed by the cytosol on CO2 transport, further reducing the photosynthetic rate. Changes in S/W associated with mesophyll cell morphology occurred earlier than changes in Sc/S and Dchl‐chl, inducing a decrease in leaf area before photosynthetic rate reduction. Adequate K nutrition simultaneously increases photosynthetic area and rate, thus enhancing crop yield.

Combined application of nitrogen (N) and potassium (K) fertilizer could significantly enhance crop yield. Crop yield and photosynthesis are inseparable. However, the influence of N and K interaction on photosynthesis is still not fully understood. Field and hydroponic experiments were conducted to examine the effects of N and K interaction on leaf photosynthesis characteristics and to explore the mechanisms in the hydroponic experiment. CO2 conductance and carboxylation characteristic parameters of oilseed leaves were measured under different N and K supplies. Results indicated that detectable increases in leaf area, biomass and net photosynthetic rate (An) were observed under optimal N and K supply in field and hydroponic experiments. The ratio of total CO2 diffusion conductance to the maximum carboxylation rate (gtot/Vcmax) and An presented a linear‐plateau relationship. Under insufficient N, increased K contributed to the CO2 transmission capacity and improved the proportion of N used for carboxylation, promoting gtot/Vcmax. However, the low Vcmax associated with N insufficiency limited the An. High N supply obviously accelerated Vcmax, yet K deficiency led to a reduction of gtot, which restricted Vcmax. Synchronous increases in N and K supplementation ensured the appropriate ratio of N to K content in leaves, which simultaneously facilitated gtot and Vcmax and preserved a gtot/Vcmax suitable for guaranteeing CO2 transmission and carboxylation coordination; the overall effect was increased An and leaf area. These results highlight the suitable N and K nutrients to coordinate CO2 diffusion and carboxylation, thereby enhancing photosynthetic capacity and area to obtain high crop yield.

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