EFFECT OF INOCULATION WITH AZOSPIRILLUM-BRASILENSE STRAIN CD AND RHIZOBIUM ON THE ROOT MORPHOLOGY OF BURR MEDIC (MEDICAGO-POLYMORPHA L)

Inoculation of burr medic seedlings grown in pouches with Azospirillum brasilense strain Cd at a concentration of 10(6) cfu/ml significantly increased the number of lateral roots and the diameter of the main root but had no influence on the length of roots either in the presence or in the absence of Rhizobium. At Azospirillum concentrations exceeding 10(8) cfu/ml, root elongation was inhibited, although at a lesser extent when Azospirillum was combined with Rhizobium. Exogenous indoleacetic acid (IAA) or benzyladenine (BA) at concentrations of 10(-8) M stimulated root elongation. Concentrations exceeding 10(-6) M inhibited root elongation and reduced the length of the root elongation zone. Root hair density was not affected following inoculation with Azospirillum; however, the number of root hairs per unit of root length increased as a result of the increase in root diameter. In cross sections taken near the root tip shortly after inoculation with 10(9) cfu/ml Azospirillum, larger cortical cells were observed; their number (in cross section) did not increase, however, compared with the controls. DNA concentration in root segments formed 24 h after inoculation with 10(9) cfu/ml Azospirillum was significantly lower than in roots inoculated with 10(7) cfu/ml Azospirillum or than in controls. From this finding it was concluded that the reduction in root growth may be the result of a decreased cell division in the apical meristem of the root.