Potassium influx to roots of two sorghum genotypes grown under saline conditions

Potassium (K) uptake by plant roots is often suppressed by sodium (Na) in the growth medium, whose damage may be moderated by calcium (Ca). There is a debate if K influx could be used as an index to salinity tolerance, and the reliability of its determination by the ion-depletion method. Two sorghum [Sorghum bicolor (L.) Moench] genotypes (Hegari and NB-9040), that differ in their salt tolerance, grew for 28 days in nutrient solutions with 0, 25, 50 and 75 mm NaCl. In addition, the effect of was Ca was determined in the presence of 2 and 5 mM Ca²⁺ with plants grown in 50 mM NaCl. Depletion of K⁺ concentration (C) in the solutions was then determined over time, and K net influx (I_n) was calculated from K concentration depletion between two time steps. The procedure was repeated four times. Potassium influx data of the four reps were fitted by the least-squares procedure to the equation I_n = m₁(C - m₂)/(m₃ - C - m₂) + m₄C, where m₁ to m₄ are the calculated coefficients. For each genotype, the fitted equation indicated a decrease in K influx affinity to C with the increase in the NaCl concentration in the growth solution. Yet, this effect was obtained in the salinity-sensitive NB-9040 in lower NaCl concentration than in the tolerant Hegari. The use of the depletion procedure for uptake studies involved with interacting ions is discussed.