Root growth is closely related to carbon import and hence, to light conditions for the shoot. Carbon gain in roots is realized predominantly by import from the shoot via the phloem, while the major loss of root carbon occurs via respiration associated with growth and ion uptake (Farrar & Jones 2000). A number of studies have investigated differences in root growth between plants acclimated to low- or high-light environments (e.g. Webb 1976; Vincent & Gregory 1989; Aguirrezabal, Deleens & Tardieu 1994) or between plants growing with variable external sucrose supply (Street & McGregor 1952; Freixes et al. 2002). It has been proposed that root length is related to cumulative intercepted radiation (Aresta & Fukai 1984; Vincent & Gregory 1989). Likewise, relations between growth in primary and secondary roots (Bingham & Stevenson 1993) as well as between root and shoot growth (Thaler & Pages 1996) have also been extensively studied in steady-state experiments. A key factor directly connecting the irradiation of the shoot and the elongation of root tips is the local hexose concentration, which correlates very well with growth rates of individual roots of a given species (Scheible et al. 1997; Freixes et al. 2002). An increase in the sugar content of root tissue promotes growth of primary and secondary roots without affecting branching patterns or overall root architecture (Bingham, Blackwood & Stevenson 1997), in contrast to increases of other growth substrates such as NO3- (Zhang et al. 1999) or other mineral nutrients (Forde & Lorenzo 2001), which can lead to marked alterations of root architecture via localized effects on growth (e.g. PO43−; see Watt & Evans 1999).
While a large amount of data on the reaction of overall root growth to different steady-state light conditions is available, much less is known about the differential reactions within the growth zone to changing light intensities that are common in natural environments. Muller, Stosser & Tardieu (1998) showed that in maize the length of the growth zone decreased with decreasing light intensity, in much the same way as with decreasing water availability (Sharp, Hsiao & Silk 1988). The immediate effect of short-term changes in light intensities on the amplitude and distribution of relative elemental growth rates (REGR) within the root growth zone has, however, hardly been explored. Recent studies using high-resolution, automated optical growth monitoring methods have shown that alterations in root growth can take place within less than an hour in reaction to changes in temperature or nutrient availability (Walter et al. 2002; Van der Weele et al. 2003; Walter, Feil & Schurr 2003). The question arises whether a change in light environment of the shoot can also induce such fast reactions of root growth. Comparisons of maize root growth in decreasing light intensities indicate that a reduction in root elongation rate takes about 4 d (Muller et al. 1998).
If sucrose export from the shoot is among the signals linking shoot light interception and root growth, transgenic plants with reduced sucrose content (Chen et al. 2005) could reveal the degree to which this process is regulated via sucrose. Response kinetics of REGR distribution to shoot light conditions may also give some information about mechanisms that are involved in the regulation of this process. Sugar import into the root growth zone can regulate a vast number of enzymes involved in carbon metabolism (Koch 1996; Ho et al. 2001) or the cell cycle (Riou-Khamlichi et al. 2000). Genes regulating root growth have been recently characterized in Arabidopsis (Birnbaum et al. 2003) and maize (Bassani, Neumann & Gepstein 2004).
In this study, the hypothesis was tested whether a sudden increase in light intensity leads to a rapid increase of root growth. Hence, the effect of light intensity on plant growth, root expansion and sugar concentration both in steady-state and changing light environments was investigated.