EFFECT OF CALCIUM SUPPLY ON ALUMINUM-INDUCED CALLOSE FORMATION, ITS DISTRIBUTION AND PERSISTENCE IN ROOTS OF SOYBEAN (GLYCINE-MAX (L) MERR)

Inhibition of root elongation and induction of callose formation are sensitive responses of soybean (Glycine max (L.) Merr.) roots to aluminium (Al). As early as 30 min after supply of 50 mu M Al in solution culture, callose increased in the very root tip. There was a decrease in callose along the root axis from the apex to the base when roots were exposed to Al for up to 8 h. Short-term exposure of roots to Al revealed that the callose concentration in root tips remained constant for several hours after transfer to Al-free nutrient solution. A substantial decomposition of callose became apparent only after 22 h. New growth of formerly Al-treated roots showed no elevated callose levels. Increasing the Ca (CaSO4) supply from 20 to 1500 mu M enhanced root elongation, especially in the presence, but also in the absence of 25 mu M Al at pH 4.5. Al-induced callose formation first increased and then decreased with increasing Ca supply. There was a close quadratic relationship between Al-induced inhibition of root elongation and callose formation for Ca supplies between 50 and 1500 mu M. Maximum inhibition of root growth and enhancement of callose formation by Al occurred between 50-100 mu M Ca. However, the lowest Ca supply of 20 mu M did not fit into this pattern: Al-induced callose was much lower than predicted, indicating a Ca requirement for callose formation. Root growth but not the callose concentration in root tips could be related to Al3+ or Ca2+ activities of the nutrient solution.