Potassium application delays Zhebeimu (Fritillaria thunbergii Miq.) leaf senescence under shade

Shade could improve the medicinal quality of Zhebeimu (Fritillaria thunbergii Miq.) bulb, but lead to decreased yield resulting from leaf senescence. Although potassium application can mitigate such yield loss by delaying leaf senescence, the underlying mechanism remains largely unknown. In this two-year field experiment, the widely cultivated variety 'Zhebei 3' was subjected to shade (around 50% shading) from the squaring stage, along with potassium application of 22.5 kg K₂O ha^-1. Shade treatment elevated the levels of senescence-inducing phytohormones [abscisic acid (ABA), salicylic acid (SA), jasmonic acid (JA), and 1-aminocyclopropanecarboxylic acid (ACC, the direct precursor of ethylene)], and reduced anti-senescence phytohormones [auxin (IAA) and cytokinin (CTK)]. It also suppressed the active oxygen scavenging system by lowering the activities of antioxidant enzymes [superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and glutathione peroxidase (GPX)] and the concentrations of non-enzymatic antioxidants [phenolics, flavonoids, flavones, and ascorbic acid (AsA)], leading to increased accumulation of O₂^- and malondialdehyde (MDA). Furthermore, shade impaired nitrogen assimilation via inhibiting the activities of nitrate reductase (NR), glutamine synthetase (GS), and glutamate synthase (GOGAT), thereby decreasing amino acid and soluble protein contents. These combined effects resulted in reduced chlorophyll contents and early leaf senescence. By contrast, potassium application increased IAA and CTK levels, enhanced activities of antioxidant enzymes (SOD, POD, CAT, and GPX) and contents of non-enzymatic antioxidant including flavonoids [(+)-gallocatechin, aromadendrin, kaempferol, quercetin, and myricetin] and flavones (kaempferol 3-sophorotrioside, astragalin, rutin, nictoflorin, quercetin 3-sophorotrioside, and isoquercitrin), and improved nitrogen assimilation. These changes collectively elevated chlorophyll contents and delayed shade-induced leaf senescence. Overall, this study broadened the understanding of the mechanism of potassium application in alleviating leaf senescence under shade.