Objectives  This study analyzed the effects of Ca signal transduction on photosynthetic characteristics and K absorption of crop, to provide theoretical support for alleviating low K stress.

  Methods  Hydroponic experiments were conducted using tobacco cultivar K326 as the plant material. The K levels in the nutrient solution were set as normal and low (K⁺ 5 and 0.15 mmol/L) , and tobacco seedlings were grown 8 days under the two K conditions before subjected to treatment of adding Ca channel organic blocker (Vp), Ca channel inorganic blocker (LaCl₃), and Ca²⁺ chelator (EGTA) to inhibit the calcium signal transduction, no Ca signal inhibition were used as controls. On the fourth day of treatment, the dry and fresh weight of plants, electrical conductivity, reactive oxygen species, antioxidant enzyme activity, and leaf photosynthetic characteristics were measured, the shoot and root K content, and the relative expression levels of genes related to potassium absorption were also determined.

  Results  1) Compared to normal K level, low K stress decreased the dry weight of shoots and roots by 59.29% and 39.82%, decreased the fresh weight by 51.58% and 48.19%, respectively. Moreover, low K stress decreased all the tested photosynthetic indexes, except water vapor pressure saturation deficit, decreased the K concentration in plant shoots and roots by 39.31% and 77.05%, respectively. On the contrast, low K stress significantly increased leaf soluble protein, relative conductivity, and hydrogen peroxide content, and increased the relative transcription levels of NKT2, NtKC1, NtHAK1, and NtHAK5 genes by 2.09, 3.32, 3.23, and 12.34 times, respectively. 2) At normal K levels, Ca channel inhibitor treatments did not impact on the dry and fresh weights of shoots and roots, but at low K stress, the treatments led to significant increase of dry and fresh weights. However, at both normal and low K conditions, Ca signal inhibitors significantly increased the accumulation of relative conductivity and active oxygen in leaves, decreased the activities of antioxidant enzyme SOD, POD, and chlorophyll content, decreased the photosynthetic indexes other than water vapor pressure saturation deficit. 3) Compared to the corresponding controls at both K supply levels, Ca signaling inhibitors decreased K concentrations in plant shoots and roots, The decrease under low K stress were 25.54% and 53.36% on average, the relative expression levels of K channel genes NKT2 and NtKC1 and transporter proteins NtHAK1 and NtHAK5 in roots were decreased by 60.60%,50.62%,38.61%, and 69.79% on average, respectively. The relative transcription levels of calcium channel genes NtCNGC1 and NtCNGC11 in roots were decreased by 95.36% and 87.04% on average, respectively.

  Conclusions  Low K stress impacts the seedling growth, photosynthesis, and K uptake of tobacco plants. Exogenous calcium channel inhibitors or chelators would exaggerate the adverse impaction of low potassium stress in reducing the activities of antioxidant enzymes and photosynthesis in tobacco leaves, as well as the transcription levels of genes related to potassium absorption in roots and potassium uptake in plants.