Root growth can react even more strongly than leaf growth to increased shoot light perception. In another study of seedlings of Nicotiana tabacum, it was shown that a very pronounced and characteristic reaction of root growth occurs when light intensity is increased (Nagel et al. 2006): During the first three hours after increase of light intensity by a factor of five, a characteristic fluctuation of root tip growth velocity was observed that was connected to parallel decreases and increases of expansion within the meristematic zone and the zone of cell elongation. The fluctuations were caused by a superposition of a transient, hydraulic decrease of growth activity due to increased transpiration and an accelerating increase of growth activity induced by sucrose import. Experiments with tobacco plants that had a decreased activity of sucrose-phosphate phosphatase (Chen et al. 2005) showed far less pronounced growth reactions. As the total increase of root growth activity by a factor of four within four days exceeded the increase of shoot growth acceleration by far, a significant shift of root-shoot-ratio was detected as a consequence of dynamic growth alterations in response to increased light intensity (Walter and Nagel 2006). Whether a shift of root-shoot-ratio is a general consequence of increased light intensity is controversial in the literature; the reaction will probably depend strongly on the species and on interaction with other factors such as nutrient availability (Lambers and Posthumus 1980; Hodge et al. 1997). Yet, an increase of root growth activity in response to increasing light intensity has been reported in a number of studies before, using different species and observing reactions on longer time scales (Webb 1976; Vincent and Gregory 1989; Aguirrezabal et al. 1994; Bingham et al. 1997).
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