Polar Transport of 45Ca2$ across the Elongation Zone of Gravistimulated Roots

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Abstract
The movement of calcium across the elongation zone of gravistimulated primary roots of maize (Zea mays L.) was measured using 45Ca2$. Radioactive calcium was applied to one side of the elongation zone about 4 mm back from the root tip and the distribution of radioactivity across the root in the region of application was determined using scintillation spectrometry. The movement of 45Ca2$ across the elongation zone was non-polar in vertically oriented roots. In gravistimulated roots the movement of label was polarized with about twice as much label moving from top to bottom as from bottom to top. A variety of treatments which interfere with gravitropism was found to eliminate the polar movement of 45Ca2$ across the elongation zone. In maize cultivars which require light for gravitropic competency, dark grown roots exhibited neither gravitropism nor polar movement of 45Ca2$ across the elongation zone. Upon illumination the roots developed both gravitropic competency and gravity-induced polar movement of 45Ca2$ across the elongation zone. Similarly, roots of light-grown seedlings lost both gravitropic competency and 45Ca2$ transport polarity upon transfer to the dark. The results indicate a close correlation between calcium movement and gravitropism in primary roots of maize.