IS THE ENDODERMAL BARRIER THE ONLY FACTOR PREVENTING THE INHIBITION OF ROOT BRANCHING BY HEAVY-METAL SALTS

The effect of cadmium and lead nitrates on lateral root development wa s studied in five-day old maize (Zea mays L.) seedlings. The salt conc entrations that were applied led to 50% inhibition of main root growth . For testing the barrier function of the endodermis, roots were bisec ted lengthwise along three cm starting from the root apex and incubate d in solutions of heavy metal salts. Control seedlings with intact roo ts were incubated in similar solutions. The heavy metals failed to inh ibit lateral root formation in both intact and bisected roots. Histoch emical investigation of Pb and Cd distribution in the root tissues by dithizone staining showed that both cadmium and lead failed to enter t he pericycle even in bisected roots and penetrated only to the first a nd second cell layers from the cut surface. Root primordia formation c ontinued thanks to the presence of Casparian strips in the endodermis or the structure of stelar cells. Thus, the heavy metals cannot penetr ate into the pericycle because of both impermeability of stelar cell w alls and the operation of the endodermis as a barrier to heavy metals.