[Effects of potassium levels on translocation of ¹³C-photoassimilates to fruit in 'Fuji' apple during fruit expanding period]

A field experiment was carried out in a six-year old 'Fuji'3/M26/Malus hupehensis Rehd. apple with the ¹³C tracer method to examine the changes of chlorophyll fluorescence parameters, photosynthetic characteristics of leaf, sugar transporter gene expression, ¹³C assimilation capability and the characteristics of translocation and distribution of ¹³C-photoassimilates to fruit under different levels of potassium addition (K₂O 0, 0.5%, 1.0%, 1.5%, 2.0%, expressed by CK, K₁, K₂, K₃, K₄, respectively). Potassium aqueous solution smear the leaves within 20 cm around the fruit at fruit enlargement stage. Compared with other treatments, K₃ treatment significantly increased Rubisco enzyme activity, net photosynthetic rate, maximal photochemical efficiency of PSII, actual photochemical efficiency of PSII, coefficient of photochemical quenching, sorbitol and sucrose content, sorbitol 6-phosphate dehydrogenase (S6PDH) and sucrose phosphate synthase (SPS) enzyme activities and ¹³C assimilation capability of leaves. Furthermore, K₃ treatment increased gene expression of sorbitol transporter MdSOT1 and MdSOT2 and sucrose transporter MdSUT4, and promoted the unloading of sugar in fruit. The ¹³C of self retention (self leaves and self branches) was the highest in CK (82.6%) and the lowest in K₃ treatment (60.5%). With increasing potassium concentration, the ¹³C absorption of fruit first increased and then decreased, which was the highest in K₃ treatment (1.31 mg·g^-1) and the lowest in CK (0.57 mg·g^-1). Our results indicated that foliage application of potassium solution improved PSII photochemical efficiency, activities of key enzymes related with carbon assimilation, synthesis ability, and outward transport ability of photosynthates in leaves, and consequently promoted the directional transportation of sugar to fruit. The amount of photoassimilates transported to fruit was the most under 1.5% K₂O treatment (K₃).