JXB Advance Access originally published online on November 8, 2004
Journal of Experimental Botany 2005 56(409):191-203; doi:10.1093/jxb/eri019
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RESEARCH PAPER |
Internal axial light conduction in the stems and roots of herbaceous plants
1Photodynamics Research Center, RIKEN (The Institute of Physical and Chemical Research), Sendai 980-0845, Japan
2Faculty of Science and Engineering, Ishinomaki-Senshu University, Ishinomaki 986-8580, Japan
* Present address and to whom correspondence should be sent: Department of Viticulture and Enology, College of Agricultural and Environmental Sciences, University of California, Davis, CA 95616, USA. Fax: +1 530 752 0384. E-mail: qiasun{at}ucdavis.edu
Received 27 February 2004; Accepted 25 August 2004
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Abstract |
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In order to reveal any roles played by stems and roots of herbaceous plants in responding to the surrounding light environment, the optical properties of the stem and root tissues of 18 herbaceous species were investigated. It was found that light was able to penetrate through to the interior of the stem and was then conducted towards the roots. Light conduction was carried out within the internodes and across the nodes of the stem, and then in the roots from the tap root to lateral roots. Light conduction in both the stem and root occurred in the vascular tissue, usually with fibres and vessels serving as the most efficient axial light conductors. The pith and cortex in many cases were also involved in axial light conduction. Investigation of the spectral properties of the conducted light made it clear that only the spectral region between 710 nm and 940 nm (i.e. far-red and near infra-red light) was the most efficiently conducted in both the stem and the root. It was also found that there were light gradients in the axial direction of the stem or root, and the light intensity generally exhibited a linear attenuation in accord with the distance of conduction. These results revealed that tissues of the stem and root are bathed in an internal light environment enriched in far-red light, which may be involved in phytochrome-mediated metabolic activities. Thus, it appears that light signals from above-ground directly contribute to the regulation of the growth and development of underground roots via an internal light-conducting system from the stem to the roots.
Key words: Axial light conduction, far-red light, herbaceous dicotyledons, light gradients, monocotyledons, optical properties of stem and root tissues, photomorphogenesis, phytochromes
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Introduction |
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The light-related metabolic activities of plant tissues are not directly dependent upon the external light environment of plants but rather upon an internal one, in which tissues and cells are actually bathed. The construction of the internal light environment is derived from diverse modifications by the plant tissues themselves of the incident light (Walter-Shea and Norman, 1991
; Vogelmann, 1993, 1994). A detailed investigation of the optical properties of plant tissues is therefore necessary to understand the characteristics of the internal light environment and, furthermore, to understand its exact, functional significance.
Investigations of the influences of plant tissues on incident light have so far been restricted to leaves and seedlings (especially etiolated seedlings). In leaves, the epidermis (Poulson and Vogelmann, 1990; Kolb et al., 2001), palisade parenchyma (McClendon and Fukshansky, 1990; Fukshansky et al., 1992; Vogelmann and Han, 2000), spongy parenchyma (Terashima and Saeki, 1983; Cui et al., 1991), and sclerenchyma (Karabourniotis et al., 1994, 2000) have attracted the most attention to date. The chief roles of the leaf tissues in modifying the light passing through them are to contribute to the formation of an intensity-attenuated, diffuse, and green and far-red rich internal light environment, which is believed to have adaptive significance for leaf photosynthesis (Bone et al., 1985; Fukshansky et al., 1991; Myers et al., 1994). In the etiolated seedlings, investigations of the optical properties of tissues have focused on cotyledons (Seyfried and Schäfer, 1983; Turunen et al., 1999), mesocotyls (Mandoli and Briggs, 1982; Kunzelmann and Schäfer, 1985), and coleoptiles (Vogelmann and Haupt, 1985; Kunzelmann et al., 1988). Light intensity gradients across both the transverse and axial directions of the seedling organs (Seyfried and Fukshansky, 1983; Parks and Poff, 1985; Vogelmann and Haupt, 1985) have been revealed, and are believed to be involved in the observed phototropic responses of seedlings (Piening and Poff, 1988; Iino, 1990) and in regulating the elongation of the coleoptiles and mesocotyls (Mandoli and Briggs, 1982, 1984a, b; Parks and Poff, 1986), respectively. These studies have shown that the tissues of both leaves and seedling can change the incident light in quality, quantity, and direction of conduction to form a characteristic internal light environment, which is of crucial physiological significance for light-related metabolic activities.
Stems and roots are important organs of plants, and are best characterized for two functions: mechanical support and substance transport (Esau, 1977; Fahn, 1990). Prior to this research, there have been few reports on the direct roles of these organs in dealing with the external light environment. There has been some research on stem photosynthesis which investigated the internal light environment of woody stems and its relationship to stem photosynthesis (Pfanz, 1999; Pfanz and Aschan, 2001, Pfanz et al., 2002). However, this study addressed the light environment mainly within bark, especially in the region directly under the epidermis or periderm where chlorenchyma cells are abundant. In the roots, any direct relations between them and the external light environment above-ground have yet to be investigated. In previous studies on the optical properties of the stems and roots of woody plants, it became clear that light can not only enter the interior of a woody stem but it is also conducted towards the roots along the axis (Sun et al., 2003, 2004). Certain elements of vascular tissue (fibres, tracheids, and vessels) are involved in axial light conduction, and the living tissues in the xylem and phloem of stems and roots are bathed in an internal light environment, comprised mostly of far-red light, which is probably correlated with the photomorphogenic processes in the stems and roots of woody plants.
The stems and roots of herbaceous plants differ greatly from those of woody plants not only in habit, pattern of development, and period of growth, but also in morphological structures and even in certain functional aspects etc. (Esau, 1977; Fahn, 1990). Little is known about the roles of herbaceous stems and roots in dealing with the surrounding light environment and the internal light milieu. The present study therefore investigated the ability of these tissues to modify incident light. The aim was to clarify the internal light-conducting paths, the variations in light intensity and spectral properties in the course of conduction, and the possible functional significance of the internal light environment in the stems and roots of herbaceous plants.
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Species investigated and sample preparation
Eighteen herbaceous species were used in the present study (Table 1). The selection for these species was based on differences in the phylogenetic groups of angiosperms (dicotyledons and monocotyledons) and the diversity of structural characteristics representative of the stems and roots of herbaceous plants. Samples of these species were collected from the Botanical Garden, Tohoku University (38°15'23'' N, 140°51'00'' E), or the national woodland near the Photodynamics Research Center, RIKEN (38°14'46'' N, 140°49'34'' E), Sendai, Japan.
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Five to twelve well-grown plants of each species were carefully dug out of the ground with a certain amount of soil around their root system, and immediately placed in a bucket with the root system immersed in water. After soil around the root system was carefully washed off with water, each plant was divided into two lengths by transversely cutting the stem with a sharp razor at a height of 5–10 cm above the ground surface. The upper stem length was used immediately for investigations of the light conduction properties of the stem. To the lower stem–root length, another transverse cut was made at the tap root (or the adventitious root in certain species), a primary or secondary lateral root, dividing the length into two parts: stem–root transition length and root length, respectively. These two lengths were trimmed to a length of 5–10 cm and then used for analysis of the internal light conduction from the stem to the root.
Observations and measurements of light conduction in stems and roots
The experimental set-up details have been described in previous work (Sun et al., 2003). In brief, each prepared stem, stem–root or root length was inserted with the downward cut end into a dark box through a hole at the top, and then sealed with black gum to maintain integrity of the box from light entering from the outside. Set directly under the cut end surface of the plant length was a microscope which was attached to a far red-sensitive monochrome CCD image sensor (Wat-902H, Watec Corp., Japan). The latter was connected to a personal computer with image processing software (Argus-20, Hamamatsu Photonics K. K., Japan) and image-acquisition and analysis software (Aquacosmos version 1.10, Hamamatsu Photonics K. K.). The incident light used in the investigation of the light-conducting tissues or cells was from a microscopic halogen light source or from solar radiation. By using the halogen light source through a light guide, unilateral illumination was presented to the protruding part of the plant length. Because sunlight reaches the stem surface at different angles according to the time of day, the angles of unilateral oblique illumination included 90°, 60°, 40°, and 20° to the plant axis, respectively. The distribution of light conducted in the tissues on the cut end surface was recorded by the CCD image sensor after enlargement through a microscopic objective lens. The images acquired were then processed by the personal computer to clarify the efficiency of light conduction in different cell types of the stem and root tissues. For further investigation, the illumination site was kept constant while 2–3 mm lengths were progressively cut back from the lower cut end. After every cut, the image from the newly made end surface was obtained for comparison of the distribution of transmitted light in the tissues in order to verify the tissue- and cell-dependent light conduction in the stem and root further. The same investigation on these plant lengths was also carried out under sunlight, instead of the halogen light source, to clarify the internal light conduction of the stem and root in the natural light environment.
The same apparatus was used to investigate the spectral properties of the conducted light by replacing the microscope with the detector of a photonic multi-channel analyser (PMA-11, Hamamatsu Photonics K. K.). Spectra of the transmitted light were measured under the above-mentioned different illumination conditions of the halogen light source and sunlight. The relative ratios of transmission were obtained by dividing the spectra of light transmitted by those of the corresponding incident light across the spectrum from 400 nm to 950 nm, and are presented as logarithmic values.
Using similar methods and apparatus, certain other stem, stem–root or root lengths were taken to investigate the internal light attenuation under axial illumination of the halogen light source. For each plant length, a transverse cut was made from the lower cut end to produce progressively shorter lengths of 50 mm, 40 mm, 30 mm, 20 mm, and 10 mm. The upper cut end was illuminated axially after every cut and the spectrum of light transmitted from the lower cut end was measured. The relative ratios of transmission from 400 nm to 950 nm were then obtained and compared in the same way as mentioned above.
In order to clarify whether the light conducted in stems and roots is part of the incident light or was fluorescence of the plant tissues, experiments were conducted with monochromatic light as the incident light. Monochromatic light was obtained by inserting an interference filter (half-band width 8–12 nm) between the light guide from the halogen light source and the sample. The relative intensities were matched by adjusting the iris and/or voltage of the light source. Monochromatic incident light from 400 nm to 950 nm with an interval of 10–20 nm was applied axially to illuminate a 1–2 cm long stem or root length. The spectral properties of the transmitted light were measured and compared with those of their corresponding monochromatic incident light to verify the origin of the light transmitted by stems or roots.
All these observations and measurements of the samples were performed rapidly so as to avoid any drying effects of the cut end. Routine tissue sectioning was also made after the measurements to identify the cell types involved in the light conduction within the stem and root.
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Results |
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Stem tissues of herbaceous plants are involved in internal axial light conduction
The distribution patterns of vascular tissue and the presence or absence of secondary growth are two characteristic structural differences in the stems of herbaceous angiosperm plants (Esau, 1977
; Fahn, 1990). A vascular cylinder (Fig. 1A, B) and scattered vascular bundles (Fig. 1C) are two common distribution patterns of the vascular tissue in the stems of dicotyledons and monocotyledons, respectively. There is usually no secondary growth derived from the vascular cambium in stems with either a vascular cylinder or scattered vascular bundles (classified as ‘herbaceous stems’). However, in some species, secondary growth does occur to some extent so as to form secondary xylem and secondary phloem in stems with a vascular cylinder, especially in the later stages of development (classified as ‘woody stems’) (Fig. 1D).
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As well as the above-mentioned structural differences in the stems of the herbaceous plants investigated, it was observed that external light penetrated into the interior and was conducted axially (Figs 1, 2). This axial light conduction occurred not only in the internodes and across the nodes in the stem, but also from the stem above-ground to the roots underground (Fig. 3). This internal light conduction occurred whatever the illumination angle of the incident light.
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Different stem structures, however, exhibited differences in the efficiency of the axial conduction of light. Epidermis was a poor axial light conductor in all the stems investigated (Fig. 2B, D). Cortex and pith (in stems with a vascular cylinder), and ground tissue (in stems with scattered vascular bundles) in general conducted light axially via the lumina of their cells. This conduction was relatively efficient in many cases, especially in species that develop large axially elongated cells and which contain smaller amounts of pigments (Figs 1B, 2E). Less efficient light conduction by these same tissues was observed in the stems of other species (Figs 1A, C; 2A, F). In the cortex of certain species, several layers of collenchyma cells develop on the inner side of the epidermis, and efficient axial light conduction occurs within their thick cell walls (Fig. 2C).
The vascular tissue in the stems investigated, whatever the distribution pattern or whether there was a presence or absence of secondary growth, was always efficiently involved in axial light conduction (Fig. 1). However, the cells or tissues involved exhibited certain differences in terms of the distribution pattern and secondary growth of vascular tissue (Fig. 2). Fibres are a major component of stem vascular tissue. In the vascular cylinder without or with only weak secondary growth (herbaceous stems), phloem fibres, and vascular bundle sheath fibres in certain species as well, were the most efficient axial light conductors, but xylem fibres did not conduct light as efficiently (Fig. 2A, D). In the vascular cylinder with secondary growth (woody stems), in addition to primary phloem fibres, secondary xylem fibres and secondary phloem fibres were also efficient in conducting light axially (Figs 1D; 2G–I). In the scattered vascular bundles, vascular bundle sheath fibres with thick lateral walls were observed to be efficient axial light conductors (Figs 1C, 2F). Whatever the type of fibre, light conduction within it proceeded via the cell walls, and fibre lumina were not involved in axial light conduction (Fig. 2B, F, H, I).
Vessels are another main constituent of xylem. In the vascular cylinder of herbaceous stems, primary xylem vessels varied among species in the efficiency of axial light conduction, but generally did not conduct light as efficiently as phloem fibres (Fig. 2A, D). However, in the vascular cylinder of woody stems, vessels in the secondary xylem were involved in efficient axial light conduction. In the scattered vascular bundles, the larger vessels were observed to conduct light efficiently (Fig. 2F). Light conduction by vessels took place via the large lumina. Other types of stem tissues or cells (such as sieve tubes, companion cells, phloem parenchyma cells, and xylem parenchyma cells) were generally poor axial light conductors (Fig. 2A, D, F, G).
Root tissues of herbaceous plants are also involved in internal axial light conduction
The present investigation dealt with both the tap root and fibrous root systems, which together represent the characteristic root systems of herbaceous dicotyledons and monocotyledons, respectively (Esau, 1977; Fahn, 1990). The tap root system investigated usually included a tap root, primary lateral roots, and secondary lateral roots. The most obvious structural differences among these roots were in the vascular cylinder, where the tap root and thick primary lateral roots generally had undergone secondary growth to form a large amount of secondary xylem (Fig. 3A). The thin primary lateral roots tended to include less secondary structure (Fig. 3B), but the much thinner secondary lateral roots developed only primary structure (Fig. 3C). In the fibrous root system of the monocotyledonous species investigated, there was no secondary growth found in the vascular cylinder, but a large pith was often observed within it (Fig. 3D). Light was conducted from the stem into the roots, whatever the type of root system, and was conducted axially from the stem to the tap root, the tap root to the primary lateral roots, and thence to the secondary lateral roots (Figs 3, 4). Light conduction in roots took place whatever the illumination angle of the incident light.
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Different structures in roots showed differences in the efficiency of axial light conduction. The epidermis was found to be a poor axial light conductor. Cortex tissues were generally not involved in efficient light conduction in most cases (Fig. 3B, D). Pith (whether the pith tissues or pith cavity), when present, always conducted light efficiently (Figs 3A, D; 4C, E, G). The vascular cylinder always exhibited efficient axial light conduction in all types of roots investigated (Figs 3, 4).
The vascular cylinder of roots includes both phloem and xylem. The region composed of the phloem was much less efficient in the axial conduction of light than the xylem (Fig. 4C, D). Xylem tissues and cells involved in axial light conduction varied according to the xylem structure in the various types of roots (Fig. 4). In the vascular cylinder with obvious secondary growth (usually in the tap roots and thick primary lateral roots of certain species investigated), the secondary xylem was more efficiently involved in axial conduction than the primary xylem (Fig. 3A). In the secondary xylem, only fibres and vessels were efficient light conductors, while axial parenchyma cells and ray cells were generally relatively poor in the conduction of light (Fig. 4A, B). In the vascular cylinder with weak secondary growth (usually thin primary lateral roots or thick secondary lateral roots in the species investigated), the primary xylem played a major role in conducting light axially, and its fibres were always efficient light conductors, conducting light even more efficiently than its vessels in some cases (Fig. 4E, F, H, I). In the vascular cylinder without secondary growth (thin secondary lateral roots, adventitious roots, and fibrous roots), the primary xylem was involved in axial light conduction, but only fibres and vessels were efficient light conductors (Fig. 3C). As in the stems, in the roots light conduction by xylem fibres was carried out via the cell walls (Figs 3C; 4B, D, I), while in vessels and pith parenchyma cells light was conducted through the lumina (Figs 3C; 4B, F, G). In the stems or roots, the structural components involved in axial light conduction remained the same whatever the illumination angle of the incident light.
Far-red light is always conducted most efficiently by the stem and root of herbaceous plants
The significance of light in regulating the metabolic activities of plant tissues is related to its spectral properties. Comparing the light transmitted from the stem or root (Fig. 5B) with the incident light (Fig. 5A) can reveal the role of the stem or root tissues in dealing with the surrounding light environment as well as the spectral properties of the light conducted in the stem or root. This investigation found that both stem and root tissues exhibited a marked difference in transmission intensity at certain wavelengths within the spectral range 400–950 nm (Fig. 5D–I). The wavelengths transmitted most efficiently by the stems or roots were generally between 710 nm and 940 nm, regardless of the absence (Fig. 5D, F, G) or presence (Fig. 5E, H, I) of secondary growth. There was some additional transmission at 520–600 nm in the stems and roots of a small number of species, but it was much less than in the region 710–940 nm (Fig. 5D, E, G, H). Outside this region, there was no significant transmission in either stems or roots. It is also verified that this spectral region is most efficiently conducted in the stem and root under natural light (Fig. 5C). The spectral properties of the light conducted in the stem or root at different oblique illumination angles on the stem surface remained the same, and there was no change compared with those under axial illumination of the cut end of the stem (Fig. 5).
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There are various pigments and other light-absorbing substances in plant tissues that can absorb certain wavelengths of light and subsequently emit fluorescence at a longer wavelength (Vogelmann and Han, 2000
). To determine the effects of fluorescence, monochromatic light at wavelengths from 400 nm to 950 nm were applied, and the spectral composition of the emitted light transmitted by the stem or root was recorded. No obvious signals were detected when monochromatic light outside the most efficiently-conducting spectral region of the stem or root was applied as the incident light (Fig. 6A, C). When the incident light was within the spectral region conducted most efficiently, only the spectrum of the corresponding transmitted light (with a transmission peak and half band-width closely similar to the monochromatic incident light) was detected (Fig. 6B, D, E). In these transmission spectra (Fig. 6B, D, E), the transmission ratios among the different wavelengths generally remained consistent with those shown in Fig. 5D–I. This result indicates that the light conducted within the stem and root of herbaceous plants is directly derived from the optical and filtering properties, and that any fluorescence derived from the stem and root tissues does not significantly contribute to the light conducted internally.
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The intensity of the light conducted by stem and root tissues at each wavelength decreases linearly with axial distance of conduction
The efficiency of light conduction in a stem or root can be indicated by the slope of the light gradient in the direction of conduction. Attenuation of light was present in all the stems and roots investigated but it occurred to different extents, depending not only on the species but also on the structure measured (stem or root) (Figs 5D–I; 7). In most stems (Fig. 7A, C) or roots (Fig. 7B, D), including those with and without secondary growth, transmission intensity across the spectrum (400–950 nm) generally decreased in direct proportion to the distance of conduction. Light attenuation at specific wavelengths showed a negative linear relationship between the intensity of the light conducted and the distance of axial conduction at each wavelength over 500 nm (there was background noise to measurement taken below 500 nm because of the weak transmission intensity for a longer stem or root length in this region of the spectrum) (Fig. 8). Correlation coefficients for the regression lines of the stems and roots varied in a range, usually between –0.99 and –0.92 across the spectral region. The slope of the regression line is an indicator of the extent of light attenuation per centimetre. The slope value varied at different wavelengths within the same stem or root, between the stem and root, and among species, but generally lay between –1.421 and –0.574.
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Differences in internal stem and root light conduction between herbaceous and woody species
The present paper helps to clarify the characteristics of axial light conduction in the stems and roots of herbaceous species. These findings also identify certain differences between the structural components involved in light conduction in herbaceous and woody species. In woody species (Sun et al., 2003
, 2004), vascular tissue (consisting mainly of secondary xylem and secondary phloem) comprises the majority of the stem and root structure, and the vessels, fibres, and tracheids were the only efficient axial light conductors. Other tissues, such as the cortex and pith, consisting mainly of parenchyma cells, were generally poor light conductors. However, in herbaceous species, the light-conducting structural components of the stem and root were more diverse. In the stems and roots undergoing secondary growth, light conduction was relatively consistent with that in woody species (Figs 2G, H; 4A, B); while in the stems and roots without or prior to secondary growth, the vascular tissue involved in efficient light conduction occupied only a small portion of the structure in many of the species investigated, and light conduction via this route was usually overwhelmed by that occurring in the cortex, pith, and ground tissues (Figs 1B; 2E; 3D; 4G). Thus, in contrast to woody species, in herbaceous species the parenchyma of the cortex, pith, and ground tissues also contributed much to axial light conduction. The collenchyma in the cortex were also efficient light conductors (Fig. 2C).
The extent of interspecies variations in axial light attenuation of the stem and root is another important difference between herbaceous and woody species. In both groups, light intensity was attenuated linearly across the spectrum in the axial conduction (Figs 7, 8) (Sun et al., 2003). However, the degree of light attenuation (steepness of the slope in Fig. 8) among species differed between woody and herbaceous species. Among woody species, there was less interspecies difference in the extent of light attenuation; that is, the stem and root of different species tend to be more consistent in attenuating the conducted light (Sun et al., 2003). However, among herbaceous species, interspecies differences in axial attenuation were much greater (Figs 5D–I; 7).
The reason for these observed differences is probably related to differences between the structure and pigments of the stems and roots in woody and herbaceous species. Light attenuation is related not only to the light-conducting character of the light conductors themselves, but also to their quantity and the pigment content in both the stems and roots. The presence of a larger ratio of efficient light conductors presumably contributes to a higher efficiency of axial light conduction. In the stems or roots of woody plants, light-conducting elements (fibres, vessels or tracheids of vascular tissues) form the majority portion of the cross-sectional area, and generally display a relative uniformity of structure in different species, which explains the relative lack of interspecies differences in terms of the extent of light attenuation. However, in the stems or roots of herbaceous plants, efficient light conductors, such as fibres and vessels, are different in terms of both quantity and arrangement among these species. The contents and kinds of pigments in the parenchyma cells of pith, cortex, and ground tissue relevant to the efficiency of light conduction vary greatly with species, producing a wider range of interspecies differences in light attenuation observed in their stem and root.
Functional significance of the internal light environment of stems and roots of herbaceous plants
The present investigation has made it clear that light at wavelengths between 710 nm and 940 nm is conducted most efficiently by the stems and roots of herbaceous species (Fig. 5D–I), and that the spectral region rich in far-red light is only the composition in their stem and root in the natural light environment because of the spectral properties of sunlight (Fig. 5C). Thus, herbaceous species are similar to woody ones in terms of the spectral properties of the light conducted by the stem and root tissues (Sun et al., 2003).
In the stems and roots of herbaceous plants, the structural components involved in axial light conduction include the parenchyma cells of cortex, pith, and ground tissue, and the fibres and vessels in vascular tissue. Among these, the fibres and vessels die upon maturation. As indicated previously (Sun et al., 2003, 2004), the light conducted by fibres and vessels is not restricted to the light conductors only, but in fact leaks out to the surrounding living tissues during axial conduction. This has been verified in recent investigations of the spectral properties of specific tissues and cells, and even specific parts of a tissue or cell, by means of a fine light-guide detector (Q Sun and H Suzuki, unpublished data). Thus, tissues in the stems and roots of herbaceous plants are bathed in an internal light environment rich in far-red light.
Plant tissues can perceive light signals by intrinsic photoreceptors so as to regulate the processes of their growth and development. To date, several kinds of photoreceptors have been identified which respond to specific wavelengths: five phytochromes (PhyA–E; Smith, 2000), two cryptochromes (Cry1 and Cry2; Cashmore et al., 1999) and two phototropins (Phot1 and Phot2; Briggs et al., 2001). The phytochromes are a family of photoreceptors that respond mainly to red and far-red light (Quail, 2002). The far-red responses of the phytochromes depend on the molecular species (Whitelam and Delvin, 1997), and are also related to the energetic levels of far-red light, including high irradiance response (HIR; Hartmann, 1966; Shinomura et al., 2000; Cerdán and Chory, 2003), low fluence response (LFR; Fankhauser, 2001), and very low fluence response (VLFR; Botto et al., 1996; Shinomura et al., 1996). The present investigation indicates that the light from the surrounding environment of herbaceous plants enters the interior of the stem, and via an internal light-conducting system the far-red light is conducted axially towards the roots underground. This characteristic internal light environment is of crucial importance for the phytochrome-regulated metabolic activities of plant stems and roots. Attenuation of the axially conducted light along the root indicates that responses involving HIR, LFR, and VLFR probably happen in roots at different depths underground. Our primary investigation into Arabidopsis has verified that the presence or absence of far-red light can change the expression levels of many genes in the roots (K Sato-Nara, unpublished data). Thus, when analysing the metabolic activities of roots underground, direct influences derived from the light environment surrounding the plants above-ground should be taken into consideration.
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Acknowledgements |
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We thank Drs Kumi Sato-Nara and Fumio Takahashi for useful discussions and Professor Ian Gleadall for critical reading and comments on the manuscript.
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