Potassium Influences Expression of Key Genes Involved in Sorbitol Metabolism and Its Assimilation in Pear Leaf and Fruit

Potassium (K⁺) plays a pivotal role in fruit quality improvement. Four K₂O levels of 0 (K0), 150 (K1), 300 (K2), and 450 (K3) kg ha^?1 were applied to pear (Pyrus bretschneideri Rehd) trees at different growth stages. The results showed that K increased individual fruit weight and yield, leading to a higher yield (16.7% on average) than K0. The leaf K concentration and sorbitol concentration in leaves and fruit were significantly increased by all four K₂O levels. At all stages of development, the expression of sorbitol-6-phosphate dehydrogenase (PbS6PDH1), sorbitol dehydrogenase (PbSDH4 and PbSDH14), and sorbitol transporter (PbSOT9) genes in leaves was up-regulated by K, whereas PbS6PDH3, PbSDH2, PbSDH13, and PbSOT22 were down-regulated. During the young fruit stage, the expression of PbSDH2 and PbSDH4 in fruit was up-regulated by K, whereas at maturity, it was the opposite. Meanwhile, the up-regulation of PbS6PDH3, PbSDH12, PbSDH13, PbSDH14, and PbSOT22 in fruit was promoted by K from the enlargement stage II to the maturity stage, indicating that sorbitol assimilation and transport between source (leaf) and sink (fruit) were regulated by K. In conclusion, K regulated expression of key genes involved in sorbitol metabolism in both source and sink, leading to sugar accumulation for the improvement of fruit quality.