The response to gravity is correlated with the number of statoliths in Chara rhizoids

In contrast to higher plants, Chara rhizoids have single membrane-bound compartments that appear to function as statoliths. Rhizoids were generated by germinating zygotes of Chara in either soil water (SW) medium or artificial pond water (APW) medium. Differential-interference-contrast microscopy demonstrated that rhizoids form SW-grown plants typically contain 50 to 60 statoliths per cell, whereas rhizoids from APW-grown plants contain 5 to 10 statoliths per cell. Rhizoids from SW are more responsive to gravity than rhizoids from APW because (a) SW rhizoids were oriented to gravity during vertical growth, whereas APW rhizoids were relatively disoriented, and (b) curvature of SW rhizoids was 3 to 4 times greater throughout the time course of curvature. The growth rate of APW rhizoids was significantly greater than that of SW-grown rhizoids. This latter result suggests that APW rhizoids are not limited in their ability for gravitropic curvature by growth and that these rhizoids are impaired in the early stages of gravitropism (i.e. gravity perception). Plants grown in APW appeared to be healthy because of their growth rate and the vigorous cytoplasmic streaming observed in the rhizoids. This study is comparable to earlier studies of gravitropism in starch-deficient mutants of higher plants and provides support for the role of statoliths in gravity perception.