A field experiment was established to study sweet potato growth, starch dynamic accumulation, key enzymes and gene transcription in the sucrose-to-starch conversion and their relationships under six K₂O rates using Ningzishu 1 (sensitive to low-K) and Xushu 32 (tolerant to low-K). The results indicated that K application significantly improved the biomass accumulation of plant and storage root, although treatments at high levels of K, i.e., 300–375 kg K₂O ha⁻¹, significantly decreased plant biomass and storage root yield. Compared with the no-K treatment, K application enhanced the biomass accumulation of plant and storage root by 3–47% and 13–45%, respectively, through promoting the biomass accumulation rate. Additionally, K application also enhanced the photosynthetic capacity of sweet potato. In this study, low stomatal conductance and net photosynthetic rate (Pn) accompanied with decreased intercellular CO₂ concentration were observed in the no-K treatment at 35 DAT, indicating that Pn was reduced mainly due to stomatal limitation; at 55 DAT, reduced Pn in the no-K treatment was caused by non-stomatal factors. Compared with the no-K treatment, the content of sucrose, amylose and amylopectin decreased by 9–34%, 9–23% and 6–19%, respectively, but starch accumulation increased by 11–21% under K supply. The activities of sucrose synthetase (SuSy), adenosine-diphosphate-glucose pyrophosphorylase (AGPase), starch synthase (SSS) and the transcription of Susy, AGP, SSS34 and SSS67 were enhanced by K application and had positive relationships with starch accumulation. Therefore, K application promoted starch accumulation and storage root yield through regulating the activities and genes transcription of SuSy, AGPase and SSS in the sucrose-to-starch conversion.

Plants can adapt to the spatial heterogeneity of soil nutrients by changing the morphology and architecture of the root system. Here, we explored the role of auxin in the response of sweetpotato roots to potassium (K⁺) deficiency stress. Two sweetpotato cultivars, Xushu 32 (low-K-tolerant) and Ningzishu 1 (low-K-sensitive), were cultured in low K⁺ (0.1 mmol L⁻¹, LK) and normal K⁺ (10 mmol L⁻¹, CK) nutrient solutions. Compared with CK, LK reduced the dry mass, K⁺ content, and K⁺ accumulation in the two cultivars, but the losses of Xushu 32 were smaller than those of Ningzishu 1. LK also affected root growth, mainly impairing the length, surface area, forks number, and crossings number. However, Xushu 32 had significantly higher lateral root length, density, and surface area than Ningzishu 1, closely related to the roots’ higher indole-3-acetic acid (IAA) content. According to the qPCR results, Xushu 32 synthesized more IAA (via IbYUC8 and IbTAR2) in leaves but transported and accumulated in roots through polar transport (via IbPIN1, IbPIN3, and IbAUX1). It was also associated with the upregulation of auxin signaling pathway genes (IbIAA4 and IbIAA8) in roots. These results imply that IAA participates in the formation of lateral roots and the change in root architecture during the tolerance to low K⁺ stress of sweetpotato, thus improving the absorption of K⁺ and the formation of biomass.

Background: Sweetpotato is a typical ‘‘potassium (K⁺) favoring’’ food crop, which root differentiation process needs a large supply of potassium fertilizer and determine the final root yield. To further understand the regulatory network of the response to low potassium stress, here we analyze physiological and biochemical characteristics, and investigated root transcriptional changes in two sweetpotato genotypes, namely, - K tolerant “Xu32” and - K susceptible“NZ1”. Result: We found Xu32 had the higher capability of K⁺ absorption than NZ1 with better growth performance, higher net photosynthetic rate and higher chlorophyll contents under low potassium stress, and identified 889 differentially expressed genes (DEGs) in Xu32, 634 DEGs in NZ1, 256 common DEGs in both Xu32 and NZ1. The Gene Ontology (GO) term in molecular function enrichment analysis revealed that the DEGs under low K⁺ stress are predominately involved in catalytic activity, binding, transporter activity and antioxidant activity. Moreover, the more numbers of identified DEGs in Xu32 than that in NZ1 responded to K⁺-deficiency belong to the process of photosynthesis, carbohydrate metabolism, ion transport, hormone signaling, stress-related and antioxidant system may result in different ability to K⁺-deficiency tolerance. The unique genes in Xu32 may make a great contribution to enhance low K⁺ tolerance, and provide useful information for the molecular regulation mechanism of K⁺-deficiency tolerance in sweetpotato. Conclusions: The common and distinct expression pattern between the two sweetpotato genotypes illuminate a complex mechanism response to low potassium exist in sweetpotato. The study provides some candidate genes, which can be used in sweetpotato breeding program for improving low potassium stress tolerance.

Potassium (K ⁺ ) is an essential nutrient element for the growth and development of sweetpotato [ Ipomoea batatas (L.) Lam.]. To investigate growth and physiological responses to K ⁺ deficiency during early growth stage of sweetpotato, two representative cultivars with different tolerance to K ⁺ deficiency were chosen. The seedlings of ‘Xushu 32’ (tolerance to K ⁺ deficiency) and ‘Ningzishu 1’ (sensitive to K ⁺ deficiency) were cultured in three different K ⁺ concentrations (K0: 0 mmol·L ⁻¹ K ⁺ ; K1: 5 mmol·L ⁻¹ K ⁺ ; and K2: 20 mmol·L ⁻¹ K ⁺ , the control) of nutrient solution. Results showed that the extreme K ⁺ deficiency (K0) significantly reduced the total dry weight, leaf number, root length, and chlorophyll content (CCI) compared with K2. However, the growth traits of ‘Xushu 32’ were less suppressed than those of ‘Ningzishu 1’. The net photosynthetic rate ( P _n ), stomatal conductance ( g _S ), and transpiration rate ( T _r ) of ‘Ningzishu 1’ were significantly decreased in K0 and K1 (low K ⁺ ), whereas ‘Xushu 32’ showed no significant change in K1 treatment. Increasing minimal fluorensence ( F ₀ ) of ‘Ningzishu 1’ comes with decreased maximum quantum efficiency of photosystem II (PSII) photochemistry ( F _v / F _m ) and photochemical quenching ( q _P ) at K0 treatment. However, all the chlorophyll fluorescence parameters in ‘Xushu 32’ were nonsignificantly changed by K ⁺ deficiency (K0 and K1). These results suggest that ‘Xushu 32’ could maintain a better growth state to adapt to K ⁺ deficiency stress, which may be mainly because of a lighter affected photosynthesis and a less damaged PSII reaction center.