The reduction in leaf area precedes that in photosynthesis under potassium deficiency: the importance of leaf anatomy

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 ( D chl‐chl ) and decreased the chloroplast surface area exposed to intercellular space ( S c / S ); conversely this induced a greater limitation imposed by the cytosol on CO 2 transport, further reducing the photosynthetic rate. Changes in S/W associated with mesophyll cell morphology occurred earlier than changes in S c / S and D chl‐chl , inducing a decrease in leaf area before photosynthetic rate reduction. Adequate K nutrition simultaneously increases photosynthetic area and rate, thus enhancing crop yield.