ROOT-GROWTH RESPONSES TO LEAD IN YOUNG MAIZE SEEDLINGS

This work was undertaken to follow the appearance and development of s ymptoms of lead toxicity in growing roots of seedlings. The effects of lead nitrate (10(-2)-10(5) M) were studied on the roots of maize (Zea mays) seedlings, cvs. Diamant and Sterling. The roots were grown on f ilter paper either on glass in trays or in large Petri dishes. The fol lowing characteristics of root growth were studied: seed germination, length of primary and seminal roots, number of seminal and lateral roo ts, length of branching zone, length of meristem and fully-elongated c ells and the number of fully-elongated cells along the daily length in crement. 10(-2) M lead nitrate exerted a clear toxic effect on root el ongation just after radicle emergence; its influence on shoot growth w as weak. However 10(-2) M Pb solution did not affect either radicle em ergence itself or seminal root emergence, which can be explained by th e impermeability of seed testa to lead salt. The inhibitory effect of 10(-3) M lead nitrate appeared a day later and was not as toxic: the g rowth of primary and seminal roots proceeded at lower rate due to a pa rtial inhibition of cell division and cell elongation in them. 10(-3) M lead nitrate modified the root system morphology: it exerted no effe ct on the emergence of lateral roots and their number, but induced a m ore compact distribution of lateral roots along a shorter branching zo ne due to a reduced length of mature cells in the primary root. As a r esult of the more prominent inhibition of primary root growth, a short er branching zone with more compactly located lateral roots occupied a position much closer to the root tip than in roots grown without the influence of lead.