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The patterns of growth in organs curving under the influence of gravity were analyzed by time-lapse photography of cress and cucumber hypocotyls which were delimited into 1 mm zones by ion-exchange beads. Geotropic curvature resulted from changes in growth rate on both sides of the organ. Growth inhibition of varying degrees of intensity occurred in all the previously growing zones of the upper (concave) side. An absolute reduction in length due to compression frequently occurred in some zones. Also, in both species growth stimulation was observed on the lower (convex) side. The disparity in growth rate between the upper and lower surfaces varied with time, being more apparent in the subapical region in the first hour of curvature. A later promotion of growth rate on the lower surface subsequently increased the curvature of the more basal zones. Autotropic straightening occurred as a consequence of growth changes, both inhibitory and stimulatory, in the apical zones. These events indicate a polarity of response in which apical zones have precedence over basal zones. [ABSTRACT FROM AUTHOR]

A system is described whereby seedling development can be analysed in terms of growth rates of specific 1 mm regions of the hypocotyl. The technique involves time-lapse photography of marked hypocotyls in a specially designed chamber which accommodates seedlings in various orientations with respect to gravity, and under irradiation regimes differing in light quality, quantity and direction. The results of preliminary study of upward growth of etiolated or green cress seedlings in darkness or overhead white light are reported. Highest growth rates in etiolated seedlings were observed in zones in the upper one-third of the hypocotyl. In green seedlings, growth was more prominent within the subapical zones. Light further restricted growth of the median and basal zones in both types of seedling. However, in their immediate responses to the onset of irritation, green and etiolated seedlings differed markedly. In etiolated seedlings, recovery of growth at the apex was accompanied by the development of inhibition in the median-basal regions; green seedlings showed a transient inhibition of growth in the apical zone together with a strong immediate inhibition in the median-basal regions. [ABSTRACT FROM AUTHOR]

The phototropic and the geotropic sensitivities of cress hypocotyls differed in etiolated and in green seedlings. In etiolated seedlings, phototropism was poorly developed and geotropism seemed to be the dominant orientation mechanism. In green seedlings although geotropic sensitivity was slightly greater, phototropic responses were markedly enhanced, such that photo-signals could over-ride, or even reverse, geocurvature. The 'de-etiolation' light pretreatments required to bring about these changes in the photo-and geo-systems were different. The slight enhancement of geocurvature could be achieved by short-term exposure to red light and was reversed by far-red irradiation. The marked stimulation of photocurvature required extensive irradiation pretreatments. [ABSTRACT FROM AUTHOR]

Growth rates throughout an organ curving phototropically under continuous, unilateral white light were monitored by time-lapse photography of cress hypocotyls marked into 1 mm sections by two rows of ion-exchange beads. Curvature resulted from an integrated sequence of changes in growth rate on each side of organ, but the actual patterns of change and, therefore rates of curvature, differed within even this one species, depending upon the immediate pretreatment of the seedlings. Transference of seedlings from darkness to unilateral irradiation gave immediate growth inhibition of both sides of the organ. Curvature resulted from differential recovery of growth rate, being seen first on the shaded side, most prominently in the apical regions, on the lit (lower) side. On the other hand, transfer of seedlings from omnilateral to unilateral irradiation of the same intensity resulted in simultaneous growth inhibition on the irradiated side and stimulated growth on the shaded side; this growth stimulation of the shaded side was greater than occurred in totally darkened control plants. [ABSTRACT FROM AUTHOR]