Sufficient potassium improves inorganic phosphate-limited photosynthesis in Brassica napus by enhancing metabolic P fractions and Rubisco activity.

Crop photosynthesis (A) and productivity are often limited by a combination of nutrient stresses, such that changes in the availability of one nutrient may affect that of another nutrient, in turn influencing A. In this study, we examined the synergistic effects of phosphorus (P) and potassium (K) on leaf A in a nutrient amendment experiment, in which P and K were added individually or in combination to Brassica napus grown under P and K co-limitation. The data revealed that P addition gradually removed the dominant limiting factor (i.e., P) and improved leaf A. Strikingly, the addition of K synergistically improved gross P uptake, mainly by boosting plant growth, and compensated the physiological demand for P by prioritising investment in metabolic P pools (P-containing metabolites and inorganic phosphate, Pi). The enlarged pool of metabolically active P was partially associated with upregulation of Pi regeneration through release from triose phosphates rather than replacement of P-containing lipids. This process mitigated P restrictions on A by maintaining the ATP/NADPH and NADPH/NADP + ratios and increasing the content and activity of Rubisco. Our findings demonstrate that sufficient K increased Pi-limited A by enhancing metabolic P fractions and Rubisco activity. Thus, ionic synergism may be exploited to mitigate nutrient-limiting factors to improve crop productivity.