Potassium application alleviated Zhebeimu (Thunberg Fritillary) yield loss under shading by promoting leaf source capacity

Shading environments are favourable for Zhebeimu ( Thunberg Fritillary ) bulb medical component biosynthesis but harmful to the yield formation. Potassium application is a common practice to increase crop yield. This research aimed to determine whether potassium application could mitigate the adverse effects of shading on Zhebeimu ( Thunberg Fritillary ) bulb yield using a widely cultivated cultivar Zhebei 3. Here, natural sunlight was regarded as control, shading was administered using a sunlight net and three potassium rates of 11.2, 22.5, and 45.0 Kg K 2 O·hm −2 were set under shading and marked as S-K1, S-K2, and S-K3. Compared with control, the bulb yields were significantly reduced by 15.6 %-17.6 % under shading and retrieved to control levels under S-K2 and S-K3. Compared with shading, S-K2 significantly up-regulated the expression of genes encoding potassium transporters and channels in Zhebeimu leaves, leading to increased K + contents. Consequently, stomatal conductance ( g s ) was improved, resulting in promoted net photosynthesis ( Pn ). Regarding sucrose generation in Zhebeimu leaves, shading constrained the sucrose synthesis by down-regulating the gene expressions and activities of sucrose synthase (SuSy) and sucrose phosphate synthase (SPS). Meanwhile shading restricted the breakdown of sucrose by diminishing the gene expressions and activities of invertase (INV). Under S-K2, the sucrose synthesis and hydrolysis were restored to the control levels. Concerning sucrose outflow, shading suppressed the sucrose transport in Zhebeimu leaves by reducing gene expressions of sucrose transporters including sucrose transporter 4 ( SUT4 ) and sugars will eventually be exported transporter 2a-like ( SWEET2a-like ). Under S-K2, the sucrose efflux capacity was restored by up-regulating the expression of SUT4 and SWEET2a-like . Overall, this study revealed the mechanism of potassium application alleviating Zhebeimu yield loss due to shading by promoting leaf source capacity, and potassium application emerges as a valuable strategy for optimizing Zhebeimu cultivation under shaded conditions. AMY, amylase; F6P, fructose-6-phosphate; G1P, glucose-1-phosphate; HAK, high-affinity potassium transporter; INV, invertase; KAT, K + channel; KUP, K + uptake permease; SPS, sucrose phosphate synthase; SuSy, sucrose synthase; SUT, sucrose transporter; SWEET, sugars will eventually be exported transporter; TPC, two-pore potassium channel; UDPG, uridine-5’-diphosphoglucose. • Shading decreased Zhebeimu bulb yield by restricting leaf source capacity. • Potassium improved Zhebeimu leaf net photosynthesis rate by increasing K + intake. • Potassium promoted sucrose and starch metabolism in Zhebeimu leaf under shading. • Potassium application facilitated Zhebeimu leaf sucrose outflow under shading.