Effect of Starvation Time and Extracellular Acidification on Potassium Uptake Kinetics of Cotton (Gossypium hirsutum L.) Seedlings

A growth chamber experiment with cotton (cv. Guoxin 3) seedlings at different ages (one-, three- and five-leaf stage) was conducted hydroponically to investigate the effect of K⁺ starvation time (16 , 24h, and 48 h) and pH (6.0 and 6.8) of depletion solution on potassium (K⁺) uptake kinetic parameters via a solution depletion  technique.

The longer K⁺ starvation time and stronger extracellular acidification increased net K⁺ influx in cotton seedlings of different ages, as indicated by increased V_max and reduced K_m. However, the pattern of net influx of three- and five-leaf stage seedling was different from that of one-leaf stage seedlings. The pattern of net K influx in one-leaf stage cotton seedlings could be described by a single isotherm with only one uptake phase when K⁺ starvation time was longer (24 and 48 h), whereas with two phases (faster phase, and then slower phase) when starvation time was shorter (16 h). For those seedlings at three- and five-leaf stage seedlings, the pattern of net K influx could be described by a single isotherm with two phases irrespective of the K⁺ starvation time. Furthermore, we found that the V_max and the K_m in different phases were affected by starvation time, extracellular acidification differently. For example, the K_m in faster uptake phase was more sensitive to starvation time than that in slower uptake phase, however, more insensitive to pH of depletion solution than that in the latter. These results suggested that the K⁺ transport systems (e.g. the quantity and affinity of transporter and/or channels) in two uptake phases were different. In addition, the V_max were negatively correlated to the K⁺ concentration in cotton root, thus implying that the K⁺ status of root has a feedback regulation on transport systems.