Growth rate and gravitropic curvature studies in roots of Zea mays L. seedlings

Using infra-red video equipment it was possible, for the first time, to study the behaviour of roots grown and manipulated in total darkness, and to monitor continuously the growth and curvature of individual roots without the use of destructive sampling techniques. The main objectives of this investigation were to rationalise the conflicting reports in the literature as to the growth rate changes, and amount of curvature, in roots, in order to obtain a clear indication of the behaviour of roots under defined environmental conditions. The straight growth rate, gravitropic curvature, and the growth rate changes on the opposite sides of a gravireacting organ, were studied in individual roots, and the behaviour of the individual roots was compared to the mean response for each particular treatment to assess the validity of the use of such data which appear in published reports of experiments using destructive sampling techniques. Of particular interest were the growth rate changes on the upper and lower sides of a gravireacting organ, with regard to testing the validity of the Cholodny-Went hypothesis, as an explanation of the mechanism of gravicurvature in Zea roots. The results of these investigations have revealed that:- a) individual roots have a characteristic growth rate which is constant over time; b) the growth rate of intact roots is reduced by as little as 10 minutes illumination, but the growth rate of decapped roots is unaffected by such treatment, thereby supporting reports of light induced production of inhibitor in the rootcap; c) white, red and blue light are capable of eliciting a reduction in growth rate; d) decapping roots in darkness reduces the growth rate, indicating the possible presence of a promoting influence in darkness; e) in both darkness and light gravitropic curvature develops after a lag phase of approximately 30 minutes; after this lag phase dark-grown, and some light-grown roots (type 1) bend to their maximum angle within 2-3 hours and then fluctuate about their final angle, which is slightly less than their maximum angle of curvature. Other roots in light (type 2) continue to bend throughout the whole of the observation period; the curvature pattern of individual roots was masked in the mean curvature and curvature was enhanced by illumination; f) gravicurvature in Zea roots (cv. Fronica) developed as a result of a disproportionate increase in the growth rate on the upper side and a simultaneous, but statistically insignificant, decrease on the lower side; the increase on the upper side being twice as great as the reduction of the lower side. This disproportionality indicated that perhaps there was not merely a simple redistribution of a fixed amount of growth regulator from one side of the root to the other. In addition to relating the growth rate changes to the observed direction and magnitude of curvature in roots under similar environmental conditions, they are discussed with reference to previous studies reported in the literature, the possible changes in growth regulator levels in the roots and the validity of the Cholodny-Went hypothesis