Geoperception in primary and lateral roots of Phaseolus vulgaris (Fabaceae). I. Structure of columella cells.

Primary roots of Phaseolus vulgaris (Fabaceae) are positively geotropic, while lateral roots are not responsive to gravity In order to elucidate the structural basis for this differential georesponse, we have performed a qualitative and quantitative analysis of the ultrastructure of columella cells of primary and lateral roots of P. vulgaris. Root systems were fixed in situ so as not to disturb the ultrastructure of the columella cells. The columellas of primary roots are more extensive than those of lateral roots. The volumes of columella cells of primary roots are approximately twice those of columella cells of lateral roots. However, columella cells of primary roots contain greater absolute volumes and numbers of all cellular components examined than do columella cells of lateral roots. Also, the relative volumes of cellular components in columella cells of primary and lateral roots are statistically indistinguishable. The endoplasmic reticulum is sparse and distributed randomly in both types of columella cells. Both types of columella cells contain numerous sedimented amyloplasts, none of which contact the cell wall or form complexes with other cellular organelles. Therefore, positive geotropism by roots must be due to a factor(s) other than the presence of sedimented amyloplasts alone. Furthermore, it is unlikely that amyloplasts and plasmodesmata form a multi-valve system that controls the movement of growth regulating substances through the root cap.