Kinetics and mechanisms of cowpea root adaptation to changes in solution calcium

BACKGROUND AND AIMS: Close regulation of cellular Ca in roots is required in the face of marked changes in soil solution Ca over time and space. This study’s aims were to quantify and gain insights into the ways in which roots respond to changes in solution Ca. METHODS: Root elongation rate (RER) of cowpea (Vigna unguiculata (L.) Walp.) seedlings was determined at 0.05 to 15 mM Ca for up to 24 h both without and with added K, Mg, or Na. Root tip concentrations of Ca, K, Mg, and Na were determined and binding of cations by root tips estimated by subsequent Cu sorption. RESULTS: Transfer from higher to lower Ca solutions (and with added K at high Ca) resulted in RER ≥ 2 mm h⁻¹ within minutes. This was attributed to greater cell wall relaxation through lower Ca binding aided by a decrease to pH ≤ 5.1 in solution. Transfer to higher Ca solutions, which remained at ~pH 5.6, led to an equally rapid decrease in RER to ~0.5 mm h⁻¹, an effect ascribed to greater cell wall binding of Ca. Thereafter, a gradual increase in RER to ~1.8 mm h⁻¹ occurred over 24 h, an effect likely due to reduced cell wall Ca binding as shown by decreasing Cu sorption at a rate of 0.027 mmol Cu kg⁻¹ FM h⁻¹ over 24 h. CONCLUSION: The kinetics of changes in RER and cations in root tips suggest that roots respond to changes in solution Ca through effects on cell wall relaxation of the rhizodermis and outer cortex in the elongation zone.