Calcium (⁴⁵Ca²⁺) uptake in GP‐10 sorghum root tips as influenced by hydrogen ion (H⁺) concentration and hours of exposure to H⁺‐ATPase inhibitors

Abstract

Twenty‐five sorghum [Sorghum bicolor (L.) Moench cv GP‐10] seed were planted in ‘white quartz flintshot’ sand and grown for four days. Root tips (1 cm), washed from the sand, were exposed to p‐chloromercuribenzene (PCMB) (1 mM) or p‐chloromercuribenzene sulfonic acid (PCMBS) (1 mM) plus ⁴⁵Ca²⁺ (0.1 μCi) at pH 6.0, 5.5, 5.0, 4.5, or 4.0. In untreated roots, ⁴⁵Ca uptake decreased as H+ concentration increased. PCMBS is membrane impermeant and inactivates exofacial sulfhydryl groups (SH). PCMB is lipid soluble and inactivates SH at the surface and inside of the root plasma membrane. Ion channel ⁴⁵Ca uptake (+ PCMBS) was greater than ⁴⁵Ca uptake in untreated roots at pH 6.0, 5.5, and 5.0, indicating ⁴⁵Ca efflux by Ca²⁺‐ATPases was maximized at pH 6.0 and decreased as H⁺ concentration increased. With PCMB treatment, ion channel ⁴⁵Ca uptake was less than the untreated ⁴⁵Ca uptake at pH 6.0 and 5.5 (2‐hr exposure). But at pH 4.5, ⁴⁵Ca uptake by ion channels (+ PCMB) was greater than ⁴⁵Ca uptake (pH 6.0) of the untreated roots. GP‐10 was selected for growth on low pH soil and for metolachlor tolerance. Therefore, genetic variation due to selection has resulted in ion uptake modes that are considerably different within the species.