Topping Inhibited Potassium Uptake via Regulating Potassium Flux and Channel Gene Expression in Tobacco

Potassium (K+) is mainly absorbed by plants from the soil and is primarily transported within the plant through the xylem. Topping has been reported to cause efflux and loss of K+ in plants; however, its effect on the real-time flow rate and genotypes with varying K+ accumulation ability is still unknown. Therefore, we carried out a pot experiment containing sand culture using two tobacco cultivars EY1 (high K+ accumulating) and Y87 (low K+ accumulating). The results demonstrated the change of K+ flow direction from influx to efflux in the roots of both cultivars due to topping. The percentage ratio of K+ efflux to influx was estimated to be 18.8% in EY-1 and 157.0% in Y87, respectively. We noticed a decline in indole acetic acid (IAA) content due to topping, which activated the expression of K+ efflux channel gene NTORK1 and inhibited the expression of K+ influx channel genes NKT1 and NtKC1. Furthermore, K+ loss from the roots increased due to topping, which led to decreased K+ concentration in entire tobacco plant. Topping had a more serious impact on the K+ efflux rate and K+ loss in Y87. IAA application after topping, in turn, decreased the K+ loss in both the cultivars. We conclude that topping caused a decrease in IAA concentration and K+ losses in tobacco leaves, and these losses can be mitigated by the exogenous application of IAA.