RESUMEN
Under potassium (K) deficiency photosynthetic carboxylation capacities are limited, affecting the photosynthetic rate of plants. However, it is not clear how ionic K within plants regulates carboxylation capacities. Therefore, the photosynthetic rate (A), ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) characteristics, and cytoplasmic pH of Brassica napus leaves with different K levels were measured to evaluate the effects of K on the carboxylation capacity by regulating subcellular pH. The results showed that biochemical limitation dominates the decrease of A. There was a close positive correlation between A and the Rubisco maximum carboxylation rate (Vcmax), which was closer than that between A and the maximum electron transport rate. The thresholds of leaf K concentrations causing decreased A, Vcmax, and Rubisco initial activity were consistent and close to 1.0% in the hydroponic experiments and 1.2% in the field experiments. K deficiency resulted in decreased Rubisco activity, which reduced carboxylation capacity. Moreover, the Rubisco initial activities in vitro with sufficient K supply or under K deficiency all were significantly reduced when the pH was decreased. The cytoplasmic pH was kept neutral at 7.5 under sufficient K supply, and decreased as the leaf K concentration declined below the threshold. Acidified cytoplasmic environment caused by K deficiency could not maintain the pH balance of the chloroplasts, leading to decreased Rubisco initial activity and photosynthetic capacity.
