Ultraviolet action spectrum for anthocyanin formation in broom sorghum first internodes

An action spectrum for anthocyanin formation in dark-grown broom sorghum (Sorghum bicolor Moench, cv Acme Broomcorn and cv Sekishokuzairai Fukuyama Broomcorn) seedlings was determined over the wavelength range from 260 to 735 nanometers. The action peaks were at 290, 650, 385, and 480 nanometers in descending order of height. The action of the 290-nanometer peak was not affected by subsequently given far red light, whereas those of the other three action peaks were nullified completely. The nullification of the 385-nanometer peak action by far red light was reversible. When an irradiation at these action peaks was followed by a phytochrome-saturating fluence of red light irradiation, the action of the 290-nanometer peak remained, whereas that of the 385-nanometer peak as well as those of the 650- and 480-nanometer peaks was masked by the action of the second irradiation. These findings suggested that the 290- and 385-nanometer action peaks involved different photoreceptors, the latter being phytochrome. The blue light-absorbing photoreceptor as reported to be a prerequisite for phytochrome action in milo sorghum was not found to exist in the broom sorghums.

The action spectrum deprived of the involvement of phytochrome was determined in the ultraviolet region by irradiating with far red light following monochromatic ultraviolet light. The spectrum had a single intense peak at 290 nanometers and no action at all at wavelengths longer than 350 nanometers.

     

Acclimation of photosynthesis to high irradiance in rice: gene expression and interactions with leaf development

Rice (Oryza sativa L.) has been used to study the long-term responses of photosynthesis to high irradiance focusing on the composition of the photosynthetic apparatus and leaf morphology. Typical sun/shade differences in chloroplast composition are seen in the fifth leaf following growth in high irradiance compared with low irradiance (1000 and 200 micromol m(-2) s(-1), respectively): higher light-saturated rates of photosynthesis (P(max)), higher amounts of Rubisco protein, and a lower chlorophyll a:b ratio. In addition, leaves were thicker under high light compared with low light. However, responses appear more complex when leaf developmental stage is considered. Using a system of transferring plants from low to high light in the laboratory responses that occur before and after full leaf extension have been studied. Acclimation of photosynthesis is limited by leaf age: the transfer to high light, post-leaf extension, is characterized by alterations in chlorophyll a:b but not in Rubisco protein, which may be limited by leaf morphology. Microarray analysis of gene expression was carried out on plants that were transferred to high light post-leaf extension. A down-regulation of light-harvesting genes was seen. No change in the expression level of Rubisco genes was observed. Up-regulation of genes involved in photoprotection was observed. It was also shown that high-light leaf morphology is established prior to formation of the zone of cellular elongation and division. The endogenous and environmental factors which establish the characteristics of high light acclimation may be important for attaining high rates of assimilation in leaves and crop canopies, and the fifth leaf in rice provides a convenient model system for the determination of the mechanisms involved.     

Photosynthesis, Dark Respiration, and Growth of Rumex patientia L. Exposed to Ultraviolet Irradiance (288 to 315 Nanometers) Simulating a Reduced Atmospheric Ozone Column

Net photosynthesis, dark respiration, and growth of Rumex patientia L. exposed to a ultraviolet irradiance (288-315 nanometers) simulating a 0.18 atm·cm stratospheric ozone column were determined. The ultraviolet irradiance corresponding to this 38% ozone decrease from normal was shown to be an effective inhibitor of photosynthesis and leaf growth. The repressive action on photosynthesis accumulated through time whereas leaf growth was retarded only during the initial few days of exposure. Small increases in dark respiration rates occurred but did not continue to increase with longer exposure periods. A reduction in total plant dry weight and leaf area of approximately 50% occurred after 22 days of treatment, whereas chlorophyll concentrations remained unaltered.     

Evidence for Involvement of Phytochrome in Tumor Development on Plants

The regulation of nonpathogenic tumorous growths on tomato plants by red and far-red radiation was studied using leaf discs floated on water and irradiated from beneath. It was found that red light (600-700 nanometers) was required for the induction of tumors on tomato (Lycopersicon hirsutum Humb. & Bonpl. Plant Introduction LA 1625), while both blue (400-500 nanometers) and green (500-600 nanometers) light had little effect on tumor development. Detailed studies with red light demonstrated that tumor development increased with increasing photon flux and duration, though duration was the more significant factor. It was observed that tumor development could be prevented by the addition of far-red irradiance to red irradiance or by providing far-red irradiance immediately following red irradiance. The effectiveness of red and farred irradiance in the regulation of tumor development indicates phytochrome involvement in this response. These findings should provide additional insight into the multiplicity of physiological factors regulating the development of nonpathogenic tumorous growths in plants.     

The photosynthetic action spectrum of the bean plant

The photosynthetic action spectrum of the bean plant leaf, Phaseolus vulgaris L. (variety Red Kidney), has been determined with a diffraction grating illuminated by a 6500-watt xenon arc. An infrared CO₂ analyzer was used to determine the gross photosynthetic rate of the terminal leaflet of the first trifoliate leaf. The rate was measured as a function of the light intensity at steps of 12.5 nanometers which approximates the length of the leaflet used. Twenty-five curves between 400 and 700 nanometers were used to establish the action spectrum. All light curves were some linear function of the incident intensity, and all were extrapolated to zero. The action spectrum shows the following features. (a) there are two peaks (i.e., at about 670 and 630 nanometers) and a shoulder between 600 and 612 nanometers in the red region where the highest rate of photosynthesis is found. Lower peaks in descending order are found in the blue (at about 437 nanometers) and the green (at about 500 nanometers) regions. (b) There are two small minima at about 650 nanometers and between 470 and 480 nanometers, and a broad minimum is found between 540 and 530 nanometers. (c) The photosynthetic rate declines rapidly above 680 nanometers, reaching the lowest value at 700 nanometers. (d) At wave lengths below the blue maximum, the rate decreases progressively to 400 nanometers.     

Reversal by Light of Ethylene-induced Inhibition of Spore Germination in the Sensitive Fern Onoclea sensibilis: An Action Spectrum

Spores of the fern Onoclea sensibilis L. normally germinate to produce two cells of unequal size. The larger cell divides to produce the familiar heart-shaped prothallus. The smaller cell elongates and differentiates into the rhizoid but normally does not divide again. Onoclea spores germinate in complete darkness. Dark germination can be completely inhibited by ethylene gas (10 microliters per liter is saturating). This inhibition can be reversed by light. Broad band colored light studies were designed to determine which area of the spectrum was most effective in overcoming ethylene inhibition. White light treatment resulted in 17% germination. Blue light treatment resulted in 1% germination. Red light treatment resulted in 15% germination. Red light, therefore, was most effective and accounted for most of the effects of white light. A detailed action spectrum was constructed using narrow band interference filters in the wavelength range from 350 to 764 nanometers. The action spectrum has only one major peak at 711 nanometers.     

Photomorphogenesis and photoassimilation in soybean and sorghum grown under broad spectrum or blue-deficient light sources

The role of blue light in plant growth and development was investigated in soybean (Glycine max [L.] Merr. cv Williams) and sorghum (Sorghum bicolor [L.] Moench. cv Rio) grown under equal photosynthetic photon fluxes (approximately 500 micromoles per square meter per second) from broad spectrum daylight fluorescent or blue-deficient, narrow-band (589 nanometers) low pressure sodium (LPS) lamps. Between 14 and 18 days after sowing, it was possible to relate adaptations in photosynthesis and leaf growth to dry matter accumulation. Soybean development under LPS light was similar in several respects to that of shaded plants, consistent with an important role for blue light photoreceptors in regulation of growth response to irradiance. Thus, soybeans from LPS conditions partitioned relatively more growth to leaves and maintained higher average leaf area ratios (mean LAR) that compensated lower net assimilation rates (mean NAR). Relative growth rates were therefore comparable to plants from daylight fluorescent lamps. Reductions in mean NAR were matched by lower rates of net photosynthesis (A) on an area basis in the major photosynthetic source (first trifoliolate) leaf. Lower A in soybean resulted from reduced leaf dry matter per unit leaf area, but lower A under LPS conditions in sorghum correlated with leaf chlorosis and reduced total nitrogen (not observed in soybean). In spite of a lower A, mean NAR was larger in sorghum from LPS conditions, resulting in significantly greater relative growth rates (mean LAR was approximately equal for both light conditions). Leaf starch accumulation rate was higher for both species and starch content at the end of the dark period was elevated two- and three-fold for sorghum and soybean, respectively, under LPS conditions. Possible relations between starch accumulation, leaf export, and plant growth in response to spectral quality were considered.     

Regulation of growth and morphogenesis in phenotyplc and genotyplc dwarf peas

White light suppressed the stem growth and promoted leaf expansion. With increased irradiance the light effect was enhanced. The morphogenetic effects induced by light and by dwarf-mutation are similar. However, the nature of phenotyplc and genotyplc dwarfism is different. In the dwarf mutants the auxin level is not changed in contrast to the GA, ABA and QGC contents. Under high irradiance which depressed the stem growth, auxin and GA-levels were lowered while the content of QGC and of growth-inhibitor non-identical with ABA increased, but the level of ABA was not affected. The sensitivity of various pea forms to the light and to exogenous phytohormones (GA and IAA) is different. The plants with the shortest stems were more sensitive to light and GA. Data on the stem growth and rhizogenesis induced by light and by GA are presented. The metabolism of 2-¹⁴C-PCA (precursor of QGC) in tall and dwarf forms is different. The possible role of phytohormones and some phenolic compounds in the regulation of growth and morphogenesis of phenotyplc and genotyplc dwarf forms is discussed.     

Costs versus risks: Architectural changes with changing light quantity and quality in saplings of temperate rainforest trees of different shade tolerance

Light requirements and functional strategies of plants to cope with light heterogeneity in the field have a strong influence on community structure and dynamics. Shade intolerant plants often show a shade avoidance strategy involving a phytochrome‐mediated stem elongation in response to changes in red : far red ratio, while shade‐tolerant plants typically harvest light very efficiently. We measured plant size, stem diameter, internode and leaf lengths in randomly chosen saplings of 11 woody species differing in their shade tolerance in both a secondary forest and an old‐growth temperate evergreen rainforest in southern Chile. We also recorded the irradiance spectrum and the diffuse and direct light availabilities at each sampling point. Significant differences were found for the mean light environment of the saplings of each species, which also differed in basal stem diameter, internode length and leaf length, but not in plant height. Both plant slenderness (plant height/stem diameter) and mean internode length increased with increasing light availability, but no relationship was found between any of these two traits and red : far red ratio. The change in plant slenderness with light availability was of lesser magnitude with increasing shade tolerance of the species, while internode change with light availability increased with increasing shade tolerance of the species. Shade tolerators afford higher costs (thicker stems and plants), which render more biomechanically robust plants, and respond more to the light environment in a trait strongly influencing light interception (internode length) than shade intolerant species. By contrast, less shade‐tolerant plants afforded higher risks with a plastic response to escape from the understorey by making thinner plants that were biomechanically weaker and poorer light interceptors. Thus, species differing in their shade tolerances do differ in their plastic responses to light. Our results contribute to explain plant coexistence in heterogeneous light environments by improving our mechanistic understanding of species responses to light.     

Light-induced Ethylene Production in Sorghum

Ethylene production was induced in sections of dark-grown Sorghum vulgare L. seedlings by treatment with light in the blue and far red regions of the light spectrum. The action spectrum closely resembled the previously reported spectra for high irradiance response; thus, light-induced ethylene production is probably a high irradiance response with phytochrome as the initial photoreceptor.     

Light distribution in leaf chambers and its consequences for photosynthesis measurements

The impact of a heterogeneous distribution of actinic light within a leaf chamber for photosynthetic measurements by gas exchange on the photosynthesis-irradiance relationship was investigated. High-resolution light distributions were measured over the area of a commercially available clamp-on leaf chamber equipped with build-in red and blue LEDs, as well as over the area of a custom-made leaf chamber with external light source, using a low-cost digital camera and freely available software. The impact of the measured heterogeneity on the photosynthesis-irradiance response curve was calculated for two realistic scenarios. When the average light intensity over the leaf chamber area was estimated accurately, heterogeneity had minor effects on the photosynthesis-irradiance response curve. However, when the irradiance was measured in the chamber centre, which is common practice, and assumed to be homogeneous, for both leaf chambers the photosynthesis-irradiance response curve was subject to considerable error and led to serious underestimation of the light-limited quantum yield of photosynthesis. Additionally, mixed light sources with different heterogeneity patterns per light source, such as in the clamp-on leaf chamber, potentially increase errors due to heterogeneous physiological responses to light spectrum. High-resolution quantification of the leaf-chamber light distribution enables calculation of the correct average light intensity and already resolves the most pressing problems associated with heterogeneity. To exclude any light-distribution related errors in gas-exchange measurements a leaf chamber and actinic irradiance source design with a homogeneous light distribution is an absolute requirement.     

Effect of irradiance on shoot development in vitro

The growth of Disanthus cercidifolius and Rhododendron cultivars, and to a lesser extent Crataegus oxyacantha cv. Paul's Scarlet, was modified by altering either the spectral quality or the level of irradiance received by shoot cultures; which were otherwise maintained under uniform medium and plant growth regulator (PGR) conditions in vitro. When the spectrum of Philips colour 84 (white) fluorescent lamps was modified by coloured cellulose acetate filters, red light promoted shoot extension and axillary branching, whereas blue light inhibited shoot growth and reduced leaf chlorophyll content in the sensitive cultivar R. cv. Dopey. By using single or multiple layers of neutral density filters, or moving cultures nearer to the light source, irradiance from white light was varied. All cultures grew well at low levels of irradiance (c. 11 µmol m^–2 s^–1), but the growth and leaf chlorophyll content of cultures of Disanthus and Rhododendron cultivars were suppressed by increasing irradiance. In three related Rhododendron cultivars, increased irradiance promoted the development of adventitious shoots. Crataegus shoots were tolerant of a wide range of irradiances and only shoot extension was inhibited at the highest level tested; leaf chlorophyll content was unaffected. These results are discussed in terms of the differential perception of light relative to the natural habitats of these plants, and of the possible direct effect of irradiance upon PGRs in the culture system.     

Relationship between Chloroplast Development and ent-Kaurene Biosynthesis in Peas

Treatment of etiolated pea (Pisum sativum (L. cv. Alaska) seedlings with 2′-isopropyl-4′-(trimethylammonium chloride)-5′-methylphenyl piperidine-1-carboxylate (Amo-1618) prior to irradiation with white light inhibits photomorphogenesis and formation and stacking of thylakoid membranes in the chloroplasts, as well as (−)-kaur-16-ene (ent-kaurene)biosynthesis. Exogenous gibberellic acid also inhibits greening. A crudely determined action spectrum for the photoinduction of ent-kaurene biosynthesis shows two peaks, one in the blue region at 458 to 490 nanometers and another in the red region at 606 to 678 nanometers. The possible participation of phytochrome in the photoinduction of ent-kaurene biosynthesis is indicated by comparative effects of red, far red, and alternating red/far red irradiations on enhancement of enzyme activity. The activity of blue light as well as red shows a similarity of the photoinduction of ent-kaurene synthesis activity to the high irradiance responses, and indicates probable participation of a second photoreceptor. From these observations, it is concluded that photoinduction of ent-kaurene biosynthesis and chloroplast development in shoots are closely linked processes.     

Photochemical and photophosphorylation activities of chloroplasts and leaf mesostructure in Chinese cabbage plants grown under illumination with light-emitting diodes

Leaf mesostructure, photochemical activity, and chloroplast photophosphorylation (PP) in the fourth true leaf of 28-day-old Chinese cabbage (Brassica chinensis L.) plants were investigated. Plants were grown under a light source based on red (650 nm) and blue (470 nm) light-emitting diodes (LED) with red/blue photon flux ratio of 7: 1 and under illumination with high-pressure sodium lamp (HPSL) at photon flux densities of 391 ± 24 μmol/(m² s) (“normal irradiance”) and 107 ± 9 μmol/(m² s) (“low irradiance”) in photosynthetically active range. At normal irradiance, the leaf area in plants grown under HPSL was twofold higher than in LED-illuminated plants; other parameters of leaf mesostructure were little affected by spectral quality of incident light. The lowering of growth irradiance reduced the majority of leaf mesostructure parameters in plants grown under illumination with HPSL, whereas in LED-illuminated plants the lowered irradiance reduced only specific leaf weight but increased the leaf thickness and dimensions of mesophyll cells and chloroplasts. The photochemical activity of isolated chloroplasts was almost independent of growth irradiance and light spectral quality. Light quality and intensity used for plant growing had a considerable impact on PP in chloroplasts. At normal light intensity, the highest activity of noncyclic PP in chloroplasts was observed for plants grown under HPSL; at low light intensity the highest rates of PP were noted for plants grown under LED. The P/2e⁻ ratio, which characterizes the degree of PP coupling to electron transport in the chloroplast electron transport chain, showed a similar pattern. Thus, the narrow-band spectrum of the light source had little influence on leaf mesostructure and electron transport rates. However, this spectrum significantly affected the chloroplast PP activity. The PP patterns at low and normal light intensities were opposite for plants grown under LED and HPSL light sources. We suppose that growing plants under LED array at normal light intensity disturbed the chloroplast coupling system, thus preventing the effective use of light energy for ATP synthesis. At low light intensity, chloroplast PP activity was significantly higher under LED illumination, but plant growth was suppressed because of impaired adaptation to low light intensity.     

Responses of carbon acquisition traits to irradiance and light quality in Mercurialis annua (Euphorbiaceae): evidence for weak integration of plastic responses

It is often suggested that traits will be integrated, either because of pleiotropy or because natural selection may favor suites of integrated traits. Plant responses to different environments can provide evidence of such integration. We grew Mercurialis annua plants in high-density stands in high irradiance, in neutral shade, and in high red to far-red (R:FR) shade, resulting in environments of high irradiance, low R:FR; low irradiance, low R:FR; and low irradiance, high R:FR. We measured gas exchange, leaf morphology, stem elongation, and biomass traits and tested the prediction that traits within each functional group would show higher trait integration, as evidenced by high correlations among traits within environments, higher correlations of trait plasticity, and lower plasticity of trait correlations. Overall, we found evidence of only moderate integration for some groups of traits. Functionally related groups of traits, or pairs of traits, could be strongly integrated by one criterion but weakly integrated by another of the criteria. Stem elongation traits, though often observed to be strongly integrated in other taxa, showed little evidence of integration. Internode traits exhibited a novel pattern of responses to low R:FR, with increased elongation of the hypocotyl, decreased elongation of the first internode, and no change in the second internode. We propose that these responses to light are more likely to be the result of natural selection than the consequence of constraints imposed by pleiotropy.     

Red Light-inhibited Mesocotyl Elongation in Maize Seedlings: II. Kinetic and Spectral Studies

Red light induces two distinct inhibition responses in mesocotyls of etiolated corn seedlings. A light dose of 10 nanoeinsteins per square centimeter is saturating for the more sensitive response, whereas doses above 1,000 nanoeinsteins per square centimeter are required to exceed the threshold sensitivity of the less sensitive one. The sensitive response can be detected within 20 minutes of the onset of illumination whereas the other response does not become apparent until more than 4 hours after the beginning of irradiation. The reciprocity law is valid for the first response, but probably not for the second. An action spectrum for the first response shows two maxima, one at 640 nanometers and the other between 660 and 670 nanometers, with a pronounced minimum near 650 nanometers. The effects both of 640 and 665 nanometers of light were reversible by far red light, but doses of far red required for full reversibility were almost three orders of magnitude greater than the doses of red required either to saturate the initial inhibition or to reverse the effect of far red light. The results suggest that corn may contain a red-absorbing pigment other than phytochrome which in some way interacts with phytochrome in the inhibition of mesocotyl elongation by red light.     

Blue light regulation of the growth of Prunus persica plants in a long term experiment: morphological and histological observations

Prunus persica plants were grown under prolonged exposure to different light treatments to determine the interaction between the blue light (BL) receptor and phytochrome and/or an independent BL response in the photoregulation of shoot and leaf development. Different light conditions were established in growth chambers by changing both the state of phytochrome and the BL photon flux density (PFD) at constant photosynthetically active radiation (PAR). Furthermore, to evaluate the independent action of the BL photoreceptor, increasing amounts of BL photons were added to the light emitted by low-pressure sodium (LPS) lamps without altering irradiance and phytochrome photoequilibrium. Applying the principle of equivalent light action, the observed blue inhibition of shoot elongation, leaf expansion and thickness were clearly related to a specific BL receptor because the state of phytochrome for each treatment was nearly identical. Increasing amounts of blue photons to light emitted from LPS lamps decreased shoot elongation, whereas leaf expansion was negatively affected only at the highest blue level, suggesting a specific fluence dependence response to BL for each organ and tissue. The BL effect was evident in reducing the thickness of all the leaf tissues except for the upper epidermis, which became thicker. This could be the result of an adaptation to protect the underlying photosynthetic apparatus. Other morphological and anatomical responses to the action of the BL receptor were greatly altered when the state of phytochrome changed in the plant tissues. Received: 9 February 1999 / Accepted: 21 July 1999     

Regulation of Growth at Steady-state Nitrogen Nutrition in Lettuce (Lactuca sativa L.): Interactive Effects of Nitrogen and Irradiance

The morphological and physiological adaptation of Lactuca sativaL. (‘Vegas’) to different irradiance levels andrates of nitrogen supply was analysed in such a way that effectsof irradiance were clearly distinguished from the effects ofnitrogen. Lettuce was grown in a glasshouse in aerated nutrientsolutions containing all necessary nutrients except nitrogen.Nitrogen was supplied in excess and at limiting rates in relationto plant growth to provide steady state nutrition. Shading theplants created the low irradiance level. The effects of nitrogensupply and irradiance on growth showed a marked interaction.Dry matter production decreased strongly with decreasing nitrogensupply at high irradiance, but decreased only slightly at lowirradiance. Nitrogen had no effect on radiation use efficiencyexcept for the lowest nitrogen treatment at high irradiance.The effect of nitrogen on growth was mainly mediated by itseffect on leaf area development and hence on light interception.Decreases in leaf area were due to decreases in specific leafarea and dry matter partitioning towards the leaves, while thedecrease in specific leaf area was the result of an increasein leaf dry matter percentage at low nitrogen supply. Dry matterand nitrogen partitioning, and nitrate concentration were closelyrelated to plant nitrogen concentration. Irradiance did notaffect these relationships. Irradiance influenced partitioningonly indirectly by affecting plant nitrogen concentration. Thedemand for organic nitrogen per unit leaf area was lower atlower irradiance. Organic nitrogen per unit leaf area appearedto be adjusted to the irradiance level, independently of thenitrogen supply, suggesting priority of nitrogen usage in photosynthesis.Copyright 2000 Annals of Botany Company Lactuca sativa L., lettuce, growth, irradiance, leaf area, nitrogen, radiation use efficiency, partitioning     

Photosynthetic acclimation of plants to growth irradiance: the relative importance of specific leaf area and nitrogen partitioning in maximizing carbon gain

Changes in specific leaf area (SLA, projected leaf area per unit leaf dry mass) and nitrogen partitioning between proteins within leaves occur during the acclimation of plants to their growth irradiance. In this paper, the relative importance of both of these changes in maximizing carbon gain is quantified. Photosynthesis, SLA and nitrogen partitioning within leaves was determined from 10 dicotyledonous C₃ species grown in photon irradiances of 200 and 1000 µmol m^?2 s^?1. Photosynthetic rate per unit leaf area measured under the growth irradiance was, on average, three times higher for high‐light‐grown plants than for those grown under low light, and two times higher when measured near light saturation. However, light‐saturated photosynthetic rate per unit leaf dry mass was unaltered by growth irradiance because low‐light plants had double the SLA. Nitrogen concentrations per unit leaf mass were constant between the two light treatments, but plants grown in low light partitioned a larger fraction of leaf nitrogen into light harvesting. Leaf absorptance was curvilinearly related to chlorophyll content and independent of SLA. Daily photosynthesis per unit leaf dry mass under low‐light conditions was much more responsive to changes in SLA than to nitrogen partitioning. Under high light, sensitivity to nitrogen partitioning increased, but changes in SLA were still more important.     

Selected Components of the Shade-Avoidance Syndrome Are Displayed in a Normal Manner in Mutants of Arabidopsis thaliana and Brassica rapa Deficient in Phytochrome B

Several growth parameters associated with the phytochrome-mediated shade avoidance syndrome have been measured in seedlings and mature plants of a wild-type and a hy3 mutant of Arabidopsis thaliana deficient in phytochrome B. Growth parameters were compared in plants grown in either white light (high red:far-red [R:FR] ratio) or white light plus added far-red (FR) light (low R:FR ratio). Wild-type Arabidopsis exhibited increased hypocotyl and petiole extension under a low, compared with a high, R:FR ratio. The hy3 mutant did not respond to low R:FR ratio by increase in hypocotyl or petiole length. Extension growth of wild-type plants was stimulated by brief end-of-day FR pulses, but similar treatment had no effect on extension growth of hy3 mutant plants. However, some responses to low R:FR ratio seen in the wild-type plants were also evident in the hy3 mutants. The number of days to bolting, the developmental stage at bolting, the leaf area, and the specific stem weight (weight per unit of length) all decreased in the wild-type and hy3 seedlings in response to low R:FR ratio. Low R:FR ratio caused a larger decrease in leaf area and specific stem weight in the mutant seedlings than in wild-type seedlings. The effects of low R:FR ratio on leaf area and specific stem weight were opposite to those of the hy3 lesion, which resulted in increased leaf area and specific stem weight in comparison with the wild type. Both leaf area and specific stem weight responses to low R:FR ratio also were unchanged in the ein mutant of Brassica rapa, known to be deficient in phytochrome B. These responses represent components of the shade-avoidance syndrome, and, consequently, the results indicate that phytochrome B cannot be solely responsible for the perception of R:FR ratio and the induction of shade-avoidance responses. The hypothesis is proposed that different phytochromes may be responsible for the regulation of extension growth and the regulation of lateral or radial expansion.