Genotypic variation in photosynthetic limitation responses to K deficiency of Brassica napus is associated with potassium utilisation efficiency.

Cultivars with higher potassium utilisation efficiency (KUtE) are likely to have superior photosynthesis (A) under K deficiency. However, the inner link between KUtE and photosynthesis remains unclear. A quantitative analysis of stomatal (SL), mesophyll conductance (MCL) and biochemical (BL) limitations on A in response to K supplementation was performed on upper and lower leaves of Brassica napus L. Huayouza No. 9 (H9, high KUtE) and Zhongshuang No. 11 (Z11, low KUtE). Both cultivars showed remarkable improvement in leaf area and lower leaf photosynthesis with elevated K supply. The total photosynthetic limitation of Z11 in lower leaf was 51.9% higher than that of H9 under K deficiency. Most of limitation under K deficiency was dominated by MCL for Z11, yet it was equally controlled by MCL and SL for H9. Nevertheless, with increasing K supply, SL became the primary limitation for both cultivars. Z11 needed a 12.0% higher leaf K concentration to avoid A decline, and higher K-based thresholds for each limitation. Overall, cultivars higher in KUtE have improved inherent leaf photosynthesis and area, and present lower K-based thresholds for SL, MCL and BL.