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Notes: Abstract The ‘end-of-day’ phytochrome control of internode growth was characterized in Sinapis alba, seedlings previously grown under continuous white light for 13 d. The transition from white light to darkness caused a reduction in internode extension rate with a lag of less than 10 min. Following this, extension rate remained almost constant for at least 48 h. i.e. ‘re-etiolation’ was not noticed. The phytochorme controlling the growth processes was stable in the Pfr form. The growth rate of plants receiving a red light pulse, and the growth promotion caused by a far-red light pulse, increased with increasing fluence rate of the previous white light treatment. In far-red treated plants a first growth rate acceleration peaked at 20–30 min after the end of white light, followed by a transient deceleration which led to a growth rate minimum at 40–60 min, followed by a final growth rate recovery yielding a more-or-less steady elevated rate. Pulses establishing different Pfr/P modified the extent, but not the early kinetics, of the growth response. The relative promotion of growth caused by low Pfr/P was limited by darkness as follows: (a), The growth promotion caused by far-red directed to the internode alone was transient. (b), The promotion caused by a reduction of Pfr/P in the whole shoot persisted in darkness for at least 48 h and also persisted if, after a 3–9 h dark period, the plants were returned to continuous white light. In darkness, however, the magnitude of this growth rate promotion decreased with time, particularly when the previous white light fluence rate was low, or the pulse preceding darkness provided the lowest Pfr/P. (c), When compared over the same period in darkness, growth rate was higher in those seedlings in which Pfr/P was reduced during the continuous white light pretreatment than in those ones in which the Pfr/P was only reduced immediately before darkness. It is proposed that in the natural environment, red/far-red signals could be more effective when provided during daytime than at the end of the photoperiod, as both the background growth rate and the relative promotion caused by low Pfr/P are reduced by darkness.

Notes: The role of phytochrome A in the control of hypocotyl growth under continuous red light (Rc) was investigated using phyA and phyB mutants of Arabidopsis thaliana, which lack phytochrome A (phyA) or phytochrome B (phyB), respectively, and transgenic seedlings of Nicotiana tabacum overexpressing Avena phyA, compared to the corresponding wild type (WT). In WT seedlings of A. thaliana, hypocotyl growth inhibition showed a biphasic response to the fluence rate of Rc, with a brake at 10−2μmol m−2 s−1. At equal total fluence rate, hourly pulses of red light caused slightly more inhibition than Rc. The response to very low fluences of continuous or pulsed red light was absent in the phyA and phyA phyB mutants and present in the phyB mutant. The second part of the response was steeper in the phyA mutant than in the WT but was absent in the phyB mutant. In WT tobacco the response to Rc was biphasic. Overexpression of Avena phyA enhanced the response only at very low fluence rates of Rc (〈 10−2μmol m−2 s−1). In both species, the effect of hourly pulses of far-red light was similar to the maximum inhibition observed in the first phase of the response to Rc. Using reciprocity failure (i.e. higher inhibition under continuous than pulsed light) as the operational criterion, a ‘true’ high-irradiance reaction occurred under continuous far-red light but not under Rc or red plus far-red light mixtures. Native and overexpressed phyA are proposed to mediate very low fluence responses under Rc. In WT A. thaliana, this effect is counteracted by a negative action of phyA on phyB-mediated low-fluence responses.

Notes: The roles of phytochrome A (phyA), phytochrome B (phyB) and a putative blue-light (BL) photoreceptor (HY4) in the control of hypocotyl growth by natural radiation were investigated using phyA, phyB and hy4 mutants of Arabidopsis thaliana. Full sunlight inhibited hypocotyl growth to a larger extent in wild-type (WT) than in phyA, phyB and, particularly, hy4 seedlings. In WT seedlings, hypocotyl growth was promoted by selectively lowering BL irradiance, lowering red-light (R) plus far-red-light (FR) irradiance or lowering the R/FR ratio (which was achieved either by increasing FR or by reducing R). The effects of lowering BL were reduced in hy4 and exaggerated in phyA seedlings. The effects of lowering R+FR were reduced in phyA and exaggerated in hy4 seedlings. Neither phyB nor hy4 mutants responded to low R/FR ratios. Neighbouring plants reflecting FR without shading caused subtle reductions of the R/FR ratio. This signal promoted hypocotyl growth in WT but not in phyA, phyB or hy4 seedlings. Intermediate canopy shade produced similar effects in all genotypes. Under deep shade, de-etiolation was severely impaired in phyA seedlings, which died prematurely. Thus, the FR ‘high-irradiance reaction’ mediated by phyA could be important for seedling survival under dense canopies.

Notes: Internode extension-growth responses to neighbouring plants and to red to far-red ratios (R:FR) were investigated in wild-type (WT) and aurea (au)-mutant seedlings of tomato grown under natural radiation. The genomic location of the au mutant is not known, but one of its consequences is the reduced phytochrome level. In WT seedlings, internode growth was promoted by the presence of non-shading neighbours reflecting far-red light (FR), the shade of a tall canopy, FR provided as a supplement during the photoperiod, and FR pulses either provided at the end of the day or delayed into the dark period. Supplementary FR during the photoperiod also promoted growth in herbicide-treated partially bleached WT seedlings. The au mutant showed higher background extension-growth rates, but only responded to the most severe treatments: deep shade light and very low R:FR at the end of the day, i.e. au-mutant seedlings were less sensitive than WT seedlings to R:FR signals. Wild-type seedlings were transferred from the glasshouse to a growth room and exposed to white light with two levels of phytochrome-absorbable radiation but similar phytochrome photoequilibria and radiation for photosynthesis. The plants exposed to the lowest level showed a transient increase of internode extension growth rate and a simultaneous reduction of response to FR pulses, i.e. reproduced some of the features of au-mutant seedlings. Phytochrome itself could set the degree of response to Pfr during neighbour detection.

Notes: Abstract. Spectral photon distributions, red:far-red ratios (i.e. R:FR) and phytochrome photoequilibria (i.e. Pfr/Ptotal) were measured at various distances from artificial canopies composed of mustard or tobacco plants. Measurements were compared for radiation propagated predominantly vertically downwards and radiation propagated predominantly horizontally. Reflection signals from the artificial canopies were computed and shown to consist of a depletion of radiation over the 400–690 nm wavelength range, and an enhancement of radiation over the 690–800 nm range. R:FR and Pfr/Ptotal increased gradually with distance from the canopies, with significant depressions of both parameters evident at least as far as 30 cm from the vegetation stands. It is concluded that, in principle at least, detection of spectral quality differences by phytochrome would allow not only the presence but also the proximity of neighbouring plants to be perceived. Proximity perception is proposed as an ecologically valuable mechanism through which plants may be able to gauge their anticipatory responses to incipient shading according to the challenge posed by the nearness of neighbouring plants.

Notes: Abstract. It has previously been proposed that the fundamental function of phytochrome in the natural environment is the perception of the relative proportions of red and far-red light, i.e. the red: far-red ratio. This paper re-evaluates this hypothesis, for vegetative green plants, in the light of recent findings. Essentially, three issues are considered: (a) the modulation of the response to red: far-red by fluence rate: (b) the anticipation of competition for light by perception of changes in red: far-red that precede actual shading: and (c) characteristics of phytochrome that may be important in the mechanism of photoperception (i.e. the accumulation of photoconversion intermediates, and the stability of Pfr). We conclude: (a) the red: far-red ratio provides a reliable signal of plant density, even before shading by neighbours occurs: (b) plants are able to perceive and respond to these signals, and that possible ambiguities due to low red: far-red at low solar angles may be avoided by modulation of the perception process by fluence-rate dependent mechanisms; (c) although direct experimental evidence does not yet exist, circumstantial evidence suggests that the perception of red: far-red may confer positive adaptive advantage; and (d) plants of certain species perceive and respond to fluence rate changes, mediated perhaps by a blue-light absorbing photoreceptor or by phytochrome, but that these responses do not necessarily lead to shade avoidance reactions and their ecological relevance is not fully understood.

Notes: The effects of overexpression of oat phytochrome A on neighbour detection and on stem-growth responses to changes in red light (R), far-red light (FR) and blue light (B) simulating neighbours were investigated in transgenic tobacco seedlings grown under natural radiation. In wild-type (WT) seedlings, stem extension growth was promoted: (1) by lowering the R:FR by means of daytime supplementary FR, end-of-day FR, neighbours reflecting FR, or selective light filters placed around the base of the shoot to reduce R without affecting FR; and (2) by lowering phytochrome-absorbable radiation (R+FR) reaching the stem. Transgenic seedlings only responded to reductions in R:FR involving no significant changes in FR irra-diance, i.e. end-of-day FR and filters placed around the stem to reduce R. Neither daytime supplementary R nor selective filters placed around the stem to reduce B affected stem growth in any genotype. In growing canopies, WT seedlings responded to the reduction of R:FR caused by FR reflected in neighbour plants. Transgenic seedlings responded to plant density about a week later, when mutual plant shading reduced R and (to a lesser extent) FR below sunlight levels. Overexpression of phytochrome A impaired early neighbour detection.

Notes: Abstract We have tested the hypothesis that a plant may detect the presence of a neighboug42r, before being shaded by it, through the perception of the spectral composition of reflected sunlight. Within seedling canopies the red: far-red ratio (R: FR) of the light received by a sensor with a geometry approximating that of a stem was significantly reduced by selective reflection. This effect was observed before any reduction in the amount of photosynthetic light energy received by an individual seedling could be detected. Small green fences of grass, east-west orientated, altered the spectral distribution of the light on the north (sunlit) side of them. Fully illuminated seedlings of Sinapis alba grown on the north side of these green fences produced longer internodes and had a lower leaf: stem dry weight ratio than those grown in front of fences of bleached grasses. A similar redistribution of growth was elicited in seedlings of Chenopodium album, Datura ferox and S. alba growing in full sunlight by exposing plants to additional small quantities of far-red reflected by selective mirrors. These results suggest that the change in the R: FR ratio serves as an early warning signal of oncoming competition.

Notes: Single, double, triple and quadruple mutants of phyA, phyB, cry1 and cry2 were exposed to different sunlight irradiances and photoperiods to investigate the roll played by phytochrome A, phytochrome B, cryptochrome 1 and cryptochrome 2 during de-etiolation of Arabidopsis thaliana seedlings under natural radiation. Even the quadruple mutant retained some hypocotyl-growth inhibition by sunlight. Hypocotyl length was strongly affected by interactions among photoreceptors. Double phyA phyB, phyA cry1, and cry1 cry2 mutants were taller than expected from the additive action of single mutations. Some of these redundant interactions required the presence of phytochromes A and/or B. Interactions among photoreceptors resulted in a 44% reduction of the response to irradiance and a 70% reduction of the response to photoperiod. The complex network of interactions among photoreceptors is proposed to buffer de-etiolation against changes in irradiance and photoperiod, i.e light fluctuations not related to the positions of the shoot above or below soil level

Notes: Knowledge of the photoperceptive function of phytochrome A has improved substantially thanks to the availability of mutants lacking phytochrome A and transgenic plants transformed with the PHYA gene in sense or anti-sense orientation. In imbibed seeds, phytochrome A mediates very-low-fluence responses. In etiolated seedlings, phytochrome A mediates very-low-fluence responses, high-irradiance responses under continuous far-red light, responsivity amplification to phytochrome B and red-light enhancement of the phototropic response to blue light. In light-grown seedings, phytochrome A modulates the extent of response to reductions in red/far-red ratio perceived by phytochrome B, perceives daylength extensions and night interruptions affecting flowering, and perceives light treatments resetting endogenous rhythms. Under natural radiation these abilities are manifested during seed germination and seedling de-etiolation under dense canopies or extremely low light fluences, and during early neighbour detection, but other processes await experimental evaluation. Phytochrome A affects growth and development throughout the whole life cycle of angiosperms.