Calcium ion dependency of ethylene production in segments of primary roots of Zea mays

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Abstract
We investigated the effect of Ca2+ on ethylene production in 2‐cm long apical segments from primary roots of corn (Zea mays L., B73 × Missouri 17) seedlings. The seedlings were raised under different conditions of Ca2+ availability. Low‐Ca and high‐Ca seedlings were raised by soaking the grains and watering the seedlings with distilled water or 10 mM CaCl2, respectively. Segments from high‐Ca roots produced more than twice as much ethylene as segments from low‐Ca roots. Indoleacetic acid (IAA; 1 μM) enhanced ethylene production in segments from both low‐Ca and high‐Ca roots but auxin‐induced promotion of ethylene production was consistently higher in segments from high‐Ca roots. Addition of I‐aminocyclopropane‐I‐carboxylic acid (ACC) to root segments from low‐Ca seedlings doubled total ethylene production and the rate of production remained fairly constant during a 24 h period of monitoring. In segments from high‐Ca seedlings ACC also increased total ethylene production but most of the ethylene was produced within the first 6 h. The data suggest that Ca2+ enhances the conversion of ACC to ethylene. The terminal 2 mm of the root tip were found to be especially important to ethylene biosynthesis by apical segments and, experiments using 45Ca2+ as tracer indicated that the apical 2 mm of the root is the region of strongest Ca2+ accumulation. Other cations such as Mn2+, Mg2+, and K+ could largely substitute for Ca2+. The significance of these findings is discussed with respect to recent evidence for gravity‐induced Ca2+ redistribution and its relationship to the establishment of asymmetric growth during gravitropic curvature.