Morphological responses of Datura ferox L. seedlings to the presence of neighbours

Summary We studied the effects of density on the dynamics of seedling growth and canopy microclimate within experimental stands composed of Datura ferox L. seedlings grown in individual pots. Interception of photosynthetically active radiation (PAR) by seedlings was evaluated either indirectly, by measuring leaf area, proportion of leaf area shaded by neighbouring individuals and laminar orientation with respect to sunlight, or directly, by measuring PAR at individual leaves at their natural angle of display. An integrating cylinder, with a geometry approximating that of a stem, was used within the canopies to measure the red:far-red (R:FR) ratio of the light flux from all compass points parallel to the soil surface. Seedlings responded rapidly (i.e. 1–2 weeks) to increased density by producing longer internodes and partitioning more dry matter to stems relative to leaves. These responses were observed before either PAR interception of growth rate were reduced by the presence of neighbours. Conversely, morphogenetic adjustment was preceded by a consistent effect of plant density on the R:FR ratio of the light received by the integrating cylinder. Air and soil temperature were not affected by density in these experiments. Differences in wind velocity within the canopy associated with plant density were avoided by the experimental procedure. The results support the idea that the drop in R:FR ratio of the light flux parallel to the ground — e.g. reflected sunlight — is an early signal that allows rapid adjustment of plant form to changes in canopy structure.     

Contrasting shade avoidance responses in two perennial grasses: a field investigation in simulated sparse and dense canopies

We designed an experiment with potted plants grown outdoors to investigate the expression of shade avoidance in simulated sparse and dense canopies by two perennial grasses known to express contrasting responses to low red:far-red ratios (R:FR). Plants were grown in canopy microenvironments designed to lower the R:FR by reflection of horizontally propagated FR from neighbors and by direct attenuation of R by filters located above plants. Two specific hypotheses were tested: (1) Paspalum dilatatum will express greater shade avoidance than Schizachyrium scoparium to low R:FR in both sparse and dense canopies, and (2) low R:FR will produce greater expressions of shade avoidance in sparse than in dense canopies in both species. P. dilatatum was more responsive to low R:FR than S. scoparium in both the sparse and dense canopies and lower ramet number plant^–1 was the only common shade avoidance response between species in sparse canopies. P. dilatatum also showed significant reductions in juvenile ramet initiation, juvenile ramet mass, total shoot mass, and shoot:root ratios in sparse canopies, but only juvenile ramet initiation was reduced in dense canopies. The suppression of juvenile ramet initiation in the dense canopy was at least partially modulated by the vertically propagated R:FR because a similar reduction in PFD and horizontally propagated R:FR showed 42% greater juvenile ramet initiation in the respective control. S. scoparium only showed a significant reduction in ramet number plant^–1 and a significant increase in blade length in sparse canopies, but no significant responses occurred in dense canopies. Consequently, neither hypothesis was rejected. Variable shade avoidance responses between species and canopy densities indicate that both interspecific variation and various proportions of vertically and horizontally propagated low R:FR can influence the expression of shade avoidance responses of perennial grasses in field settings.     

Modelling photo-modulated internode elongation in growing glasshouse cucumber canopies

? Growing glasshouse plant canopies are exposed to natural fluctuations in light quantity, and the dynamically changing canopy architecture induces local variations in light quality. This modelling study aimed to analyse the importance of both light signals for an accurate prediction of individual internode lengths. ? We conceptualized two model approaches for estimating final internode lengths (FILs). The first one is only photosynthetically active radiation (PAR)-sensitive and ignores canopy architecture, whereas the second approach uses a functional-structural growth model for considering variations in both PAR and red : far-red (R : FR) ratio (L-Cucumber). Internode lengths measured in three experiments were used for model parameterization and evaluation. ? The overall trends for the simulated FILs using the exclusively PAR-sensitive model approach were already in line with the measured FILs, but they underestimated FILs at higher ranks. L-Cucumber provided considerably better FIL predictions under various light conditions and canopy architectures. ? Both light signals are needed for an accurate estimation of the FILs, and only L-Cucumber is able to consider R : FR signals from the growing canopy. Yet this study highlights the significance of the PAR signal for predicting FILs as neighbour effects increase, which indicates a potential role of photosynthate signalling in internode elongation.     

On the opportunity cost of the photosynthate invested in stem elongation reactions mediated by phytochrome

Summary Seedlings of shade-intolerant species react to alterations of the light climate caused by their neighbors with morphological changes that may influence the pattern of resource acquisition and utilization at the whole-canopy level. One such change, the increased stem elongation rate that is triggered by low red (R, 660 nm) to far-red (FR, 730 nm) ratios (R:FR) in dense canopies, might reduce the amount of assimilates available for leaf area expansion or root growth, and in that way affect resource capture by the canopy. We have tested this hypothesis by comparing the growth of both isolated individuals and canopies of the weed Amaranthus quitensis under conditions differing only in the spectral distribution of the incident light. When canopies received the full spectrum of sunlight, the stems were a large proportion (40–57%) of total biomass. Filtering the FR waveband (and hence raising the R:FR ratio to eliminate the neighbors' proximity-signal) resulted in shorter canopies with lighter stems. However, the growth of leaves and roots was not promoted by this treatment, indicating that the opportunity cost of the assimilates invested in the stems was nil or very small. Filtering the FR had no effect on biomass accumulation when plants were grown as isolated individuals. The higher growth of the canopics under full spectrum could be due to a higher light interception or to a higher efficiency of light conversion into biomass. The first possibility is weakened by the observation that filtering the FR had no effect on the dynamics of soil covering by the crops. The second is indirectly strengthened by results of an experiment with isolated plants showing that stem elongation, stem growth, and total plant biomass can be increased by reducing the flux of R light received by the stems without affecting the light climate of the leaves. Further work is needed to distinguish between these two possibilities; whatever the cause, our results show that the elongation responses to decreased R:FR may lead to a net increase in canopy productivity, and do not necessarily have a negative impact on the growth of resource-harvesting organs.     

Molecular analysis of natural leaf senescence in Arabidopsis thaliana

Using artificial canopies, several authors have shown that horizontally propagated and overall propagated radiation beneath the canopy differ substantially in spectral distribution in the red (R) and far red (FR) wavelengths. Given the lack of information about light quality under real crop canopies, the R:FR ratio of vertical and horizontal radiation beneath field-grown maize, soybean and wheat was monitored until leaf area index (LAI) reached 4, 2.5 and 6.9, respectively.
A Li-Cor 1800 spectroradiometer with a remote cosine receptor fitted with a quartz fibre-optic light-guide was used. To isolate radiation coming from a given direction, a black coated tube was fitted to the cosine receptor. The viewing angle was 15°. In open conditions, the values of R:FR from the upper hemisphere were between 1.07 and 1.20. For vertically and horizontally-propagated light, average values were 1.22 and 0.75 respectively.
Beneath the canopy, both R:FR and photosynthetic photon flux density (PPFD) from the entire upper hemisphere decreased in relation to LAI and crop height. R:FR of the horizontal component were found to be generally much lower than the vertical, which decreased significantly only in the later measurements.
The lowest R:FR values were recorded under wheat and soybean canopies. Even the very low LAIs present at early development stages were enough to cause a sharp decrease of R:FR in the horizontal fluxes. Referring to the entire upper hemisphere, PPFD transmittance and R:FR as a percentage of the external references appeared well correlated.     

Keeping up with the neighbours: phytochrome sensing and other signalling mechanisms

Plants ‘forage’ for light in plant canopies using a variety of photosensory systems. Far-red radiation (FR) reflected by neighbours is an early signal of competition that elicits anticipatory shade-avoidance responses. In Arabidopsis and cucumber, perception of reflected FR requires phytochrome B. Horizontal blue (B) light gradients also guide plant shoots to canopy gaps in patchy vegetation, and these B light signals are perceived by specific photoreceptors. When plants are shaded by neighbours they undergo extensive reprogramming of their morphological development. Although phytochromes and B light receptors are certainly involved in these responses to shading, other sensory systems probably play important roles in the field. Recent studies of plant–plant signalling are unveiling a paradigm of sensory diversity and sophistication, which has important implications for understanding the functioning of plant populations and communities.     

Effect of lateral far-red light supplementation on the growth and morphology of birch seedlings and its interaction with mineral nutrition

The effects of lateral far-red light (FR) and nutrient supply on the growth and nitrogen accumulation of silver birch (Betula pendula) seedlings were studied with the objective of testing the following hypotheses: (1) silver birch seedlings grow taller in response to increased FR irradiance received from the side; (2) this response is modulated by the nutritional status of the seedlings; and (3) an increase in lateral FR irradiance, and concomitant decrease in red to far-red photon ratio, affects the carbon and nitrogen economies of the seedlings. Two factorial experiments, each with two levels of mineral nutrient availability and two light treatments (background 'white' light with and without additional lateral FR simulating light reflected by neighbours in a sparse canopy) were done with small seedlings. The two experiments differed in PAR irradiance. The results of these experiments were that (1) stem elongation rate was increased by lateral FR addition, (2) there was no interaction between this effect and the nutritional status of the seedlings, and (3) neither whole plant accumulation of nitrogen nor dry weight increment was affected by lateral FR under either mineral nutrient supply regime.     

Influence of elevated CO2 on canopy development and red:far-red ratios in two-storied stands ofRicinus communis

Vertical structure of plant stands and canopies may change under conditions of elevated CO₂ due to differential responses of overstory and understory plants or plant parts. In the long term, seedling recruitment, competition, and thus population or community structure may be affected. Aside from the possible differential direct effects of elevated CO₂ on photosynthesis and growth, both the quantity and quality of the light below the overstory canopy could be indirectly affected by CO₂-induced changes in overstory leaf area index (LAI) and/or changes in overstory leaf quality. In order to explore such possible interactions, we compared canopy leaf area development, canopy light extinction and the quality of light beneath overstory leaves of two-storied monospecific stands ofRicinus communis exposed to ambient (340 μl l⁻¹) and elevated (610 μl l⁻¹) CO₂. Plants in each stand were grown in a common soil as closed “artificial ecosystems” with a ground area of 6.7 m². LAI of overstory plants in all ecosystems more than doubled during the experiment but was not different between CO₂ treatments at the end. As a consequence, extinction of photosynthetically active radiation (PAR) was also not altered. However, under elevated CO₂ the red to far-red ratio (R:FR) measured beneath overstory leaves was 10% lower than in ecosystems treated with ambient CO₂. This reduction was associated with increased thickness of palisade layers of overstory leaves and appears to be a plausible explanation for the specific enhancement of stem elongation of understory plants (without a corresponding biomass response) under elevated CO₂. CO₂ enrichment led to increased biomass of overstory plants (mainly stem biomass) but had no effect on understory biomass. The results of this study raise the possibility of an important indirect effect of elevated CO₂ at the stand-level. We suggest that, under elevated CO₂, reductions in the R:FR ratio beneath overstory canopies may affect understory plant development independently of the effects of PAR extinction.     

Far-red enrichment and photosynthetically active radiation level influence leaf senescence in field-grown sunflower

Basal leaves frequently senesce before anthesis in high population density crops. This paper evaluates the hypothesis that quantitative and qualitative changes in the light environment associated with a high leaf area index (LAI) trigger leaf senescence in sunflower ( Helianthus annuus L.) canopies. Mean leaf duration (LD, time from achievement of maximum leaf area) of leaf 8 was significantly ( P < 0.05) reduced from 51 to 19 days as crop population density was increased from 0.47 to 4.76 plants m⁻². High compared to low plant population density was associated with earlier reduction in the photosynthetically active radiation (PAR) and red/far-red ratio (R/FR) reaching the target leaf. However the changes in R/FR preceded those in PAR. When the light environment of individual leaves of isolated plants growing under field conditions was manipulated using filters and FR-reflecting mirrors, LD was positively and linearly related with the mean daily PAR (MDR) received in the FR- (no FR enrichment) treatments. FR enrichment of light reaching the abaxial surface of the leaf significantly ( P < 0.05) reduced LD by 9 days at intermediate PAR levels with respect to FR-controls, but did not affect LD at the maximum PAR used in these experiments. However, when light reaching both leaf surfaces was enriched with FR, LD (for leaves receiving maximum PAR) was 13 days shorter than that of the FR- control. These results show that basal leaf senescence in sunflower is enhanced both by a decrease in PAR and by a decrease in R/FR.     

Density-dependent regulation of ramet recruitment by the red:far-red ratio of solar radiation: a field evaluation with the bunchgrass Schizachyrium scoparium

Depressions in the red to far-red ratio (R:FR) of solar radiation arising from the selective absorption of R (600–700 nm) and scattering of FR (700–800 nm) by chlorophyll within plant canopies may function as an environmental signal directly regulating axillary bud growth and subsequent ramet recruitment in clonal plants. We tested this hypothesis in the field within a single cohort of parental ramets in established clones of the perennial bunchgrass, Schizachyrium scoparium. The R:FR was modified near leaf sheaths and axillary buds at the bases of individual ramets throughout the photoperiod without increasing photosynthetic photon flux density (PPFD) by either (1) supplementing R beneath canopies to raise the naturally low R:FR or (2) supplementing FR beneath partially defoliated canopies to suppress the natural R:FR increase following defoliation. Treatment responses were assessed by simultaneously monitoring ramet recruitment, PPFD and the R:FR beneath individual clone canopies at biweekly intervals over a 12-week period. Neither supplemental R nor FR influenced the rate or magnitude of ramet recruitment despite the occurrence of ramet recruitment in all experimental clones. In contrast, defoliation with or without supplemental FR beneath clone canopies reduced ramet recruitment 88% by the end of the experiment. The hypothesis stating that the R:FR signal directly regulates ramet recruitment is further weakened by evidence demonstrating that (1) the low R:FR-induced suppression of ramet recruitment is only one component of several architectural modifications exhibited by ramets in response to the R:FR signal (2) immature leaf blades, rather than leaf sheaths or buds, function as sites of R:FR perception on individual ramets, and (3) increases in the R:FR at clone bases following partial canopy removal are relatively transient and do not override the associated constraints on ramet recruitment resulting from defoliation. A depressed R:FR is probably of greater ecological significance as a signal of competition for light in vegetation canopies than as a density-dependent signal which directly regulates bud growth and ramet recruitment.     

Physiological regulation and functional significance of shade avoidance responses to neighbors

Plants growing in dense vegetations compete with their neighbors for resources such as water, nutrients and light. The competition for light has been particularly well studied, both for its fitness consequences as well as the adaptive behaviors that plants display to win the battle for light interception. Aboveground, plants detect their competitors through photosensory cues, notably the red:far-red light ratio (R:FR). The R:FR is a very reliable indicator of future competition as it decreases in a plant-specific manner through red light absorption for photosynthesis and is sensed with the phytochrome photoreceptors. In addition, also blue light depletion is perceived for neighbor detection. As a response to these light signals plants display a suite of phenotypic traits defined as the shade avoidance syndrome (SAS). The SAS helps to position the photosynthesizing leaves in the higher zones of a canopy where light conditions are more favorable. In this review we will discuss the physiological control mechanisms through which the photosensory signals are transduced into the adaptive phenotypic responses that make up the SAS. Using this mechanistic knowledge as a starting point, we will discuss how the SAS functions in the context of the complex multi-facetted environments, which plants usually grow in.Key words: competition, shade avoidance, hormones, cell wall, adaptive plasticity, photoreceptor, light     

Plasticity to light cues and resources in Arabidopsis thaliana: testing for adaptive value and costs

Plants shaded by neighbors or overhead foliage experience both a reduction in the ratio of red to far red light (R:FR), a specific cue perceived by phytochrome, and reduced photosynthetically active radiation (PAR), an essential resource. We tested the adaptive value of plasticity to crowding and to the cue and resource components of foliage shade in the annual plant Arabidopsis thaliana by exposing 36 inbred families from four natural populations to four experimental treatments: (1) high density, full sun; (2) low density, full sun; (3) low density, neutral shade; and (4) low density, low R:FR-simulated foliage shade. Genotypic selection analysis within each treatment revealed strong environmental differences in selection on plastic life-history traits. We used specific contrasts to measure plasticity to density and foliage shade, to partition responses to foliage shade into phytochrome-mediated responses to the R:FR cue and responses to PAR, and to test whether plasticity was adaptive (i.e., in the same direction as selection in each environment). Contrary to expectation, we found no evidence for adaptive plasticity to density. However, we observed both adaptive and maladaptive responses to foliage shade. In general, phytochrome-mediated plasticity to the R:FR cue of foliage shade was adaptive and counteracted maladaptive growth responses to reduced PAR. These results support the prediction that active developmental responses to environmental cues are more likely to be adaptive than are passive resource-mediated responses. Multiple regression analysis detected a few costs of adaptive plasticity and adaptive homeostasis, but such costs were infrequent and their expression depended on the environment. Thus, costs of plasticity may occasionally constrain the evolution of adaptive responses to foliage shade in Arabidopsis, but this constraint may differ among environments and is far from ubiquitous.     

Relationships between canopy complexity and germination microsites for Phalaris arundinacea L.

Microsites that prevent seed germination are critical for slowing the invasion of native plant communities by aggressive, clonal species. A suitable model for study is the clonal grass, Phalaris arundinacea, which reproduces prolifically from seed and is spreading into wetlands across temperate North America. Knowing that light conditions control its seed germination in the laboratory and that light varies with canopy complexity in a Wisconsin fen, we tested multiple attributes of microsites under spatially and temporally dynamic canopies (namely, presence/absence of a matrix species, number of species in the canopy, plus indirect effects of three soil water levels) for their control of germination in microcosms. Our 6-species canopies + the matrix of Glyceria striata had the densest cover and reduced P. arundinacea germination to 1.9%, compared to 7.3% for 1-species canopies + the matrix. After selectively removing canopy components, germination increased to 36.1% for 6-species and 33.0% for 1-species canopies. Comparing canopies with each of the six species, germination declined in relation to increasing leaf width. Given moist soil, P. arundinacea germination microsites are determined by canopy complexity, which affects light penetration, which in turn determines germination rate.     

The Shade Avoidance Syndrome in Arabidopsis: The Antagonistic Role of Phytochrome A and B Differentiates Vegetation Proximity and Canopy Shade

Light limitation caused by dense vegetation is one of the greatest threats to plant survival in natural environments. Plants detect such neighboring vegetation as a reduction in the red to far-red ratio (R:FR) of the incoming light. The low R:FR signal, perceived by phytochromes, initiates a set of responses collectively known as the shade avoidance syndrome, intended to reduce the degree of current or future shade from neighbors by overtopping such competitors or inducing flowering to ensure seed production. At the seedling stage these responses include increased hypocotyl elongation. We have systematically analyzed the Arabidopsis seedling response and the contribution of phyA and phyB to perception of decreased R:FR, at three different levels of photosynthetically active radiation. Our results show that the shade avoidance syndrome, induced by phyB deactivation, is gradually antagonized by phyA, operating through the so-called FR-High Irradiance Response, in response to high FR levels in a range that simulates plant canopy shade. The data indicate that the R:FR signal distinguishes between the presence of proximal, but non-shading, neighbors and direct foliar shade, via a intrafamily photosensory attenuation mechanism that acts to suppress excessive reversion toward skotomorphogenic development under prolonged direct vegetation shade.     

Light signals generated by vegetation shade facilitate acclimation to low light in shade-avoider plants

When growing in search for light, plants can experience continuous or occasional shading by other plants. Plant proximity causes a decrease in the ratio of R to far-red light (low R:FR) due to the preferential absorbance of R light and reflection of FR light by photosynthetic tissues of neighboring plants. This signal is often perceived before actual shading causes a reduction in photosynthetically active radiation (low PAR). Here, we investigated how several Brassicaceae species from different habitats respond to low R:FR and low PAR in terms of elongation, photosynthesis, and photoacclimation. Shade-tolerant plants such as hairy bittercress (Cardamine hirsuta) displayed a good adaptation to low PAR but a poor or null response to low R:FR exposure. In contrast, shade-avoider species, such as Arabidopsis (Arabidopsis thaliana), showed a weak photosynthetic performance under low PAR but they strongly elongated when exposed to low R:FR. These responses could be genetically uncoupled. Most interestingly, exposure to low R:FR of shade-avoider (but not shade-tolerant) plants improved their photoacclimation to low PAR by triggering changes in photosynthesis-related gene expression, pigment accumulation, and chloroplast ultrastructure. These results indicate that low R:FR signaling unleashes molecular, metabolic, and developmental responses that allow shade-avoider plants (including most crops) to adjust their photosynthetic capacity in anticipation of eventual shading by nearby plants.

Vegetation proximity light signals inform shade-avoider plants to adjust their photosynthetic capacity in anticipation of eventual shading by nearby plants.     

Effets du rapport RC:RS sur les jeunes feuilles de trèfle blanc. Assimilation nette de CO2, activité photochimique et morphologie foliaire

Young leaves of white clover are subjected to low irradiance and low red to far-red (R:FR) ratio within canopies. The objectives were to investigate the consequences of low R:FR ratio on morphology, net CO₂ assimilation and photochemical activity of leaves developed under simulated light environment of canopy. We used far-red (FR) light emitting diodes to modify the R:FR ratio only at the developing leaf under a low irradiance. Net CO₂ assimilation rate, stomatal conductance and leaf morphology were not affected by low R:FR ratio. FR exposure slightly reduced the photochemical quantum yield of PSII but there were no consequences on electron flow through photosystem II. The carbon fixation by the leaf was therefore not modified by light quality. However, low R:FR ratio decreased the leaf chlorophyll content by 21 %. Those effects were only attributed to just unfolded leaves as they were not persistent in mature leaves and there were no consequences on plant biomass accumulation.     

Branching responses of a plagiotropic clonal herb to localised incidence of light simulating that reflected from vegetation

Plants sense the presence of neighbouring vegetation through phytochrome photoreceptors perceiving a lowered red to far-red ratio (R:FR) of light reflected from such vegetation. We hypothesised that it would be advantageous for the grassland clonal herb, Trifolium repens, to have an inhibitory branching response to perception by leaves of light reflected from neighbouring vegetation (i.e. light with lowered R:FR ratio) but have no response to interception of such light by the plagiotropic stem. We tested whether photoreception of reflected light by plagiotropic stems resulted in a different branching response to photoreception by leaves and whether leaf ontogeny influenced the response. To simulate light reflected from vegetation, FR light-emitting-diodes were used to supplement controlled environment room light so that the R:FR ratio, but not the photosynthetic photon fluence rate, of light incident at the stem or leaf of a phytomer of T. repens was lowered from 1.20 to 0.25. The plagiotropic stems were unresponsive to light simulating that reflected from vegetation. This response differs from that of stems of orthotropic species, indicating that plagiotropic stems have evolved an organ-specific photobiology. Treatment of the mature leaf with light of lowered FR ratio reduced phytomer production only of the branch in the axil of the treated leaf. Similar treatment of the immature leaf retarded, in addition, branching at basal phytomers on the same side of the primary stem axis. Thus the response to light simulating that reflected from neighbouring vegetation depended upon whether the light was incident at the stem or the leaf and on the stage of leaf development. We argue that such responses improve the performance and fitness of T. repens within grassland habitats by allowing axillary buds on plagiotropic stems to branch freely when stems are in receipt of light reflected from vegetation while leaves are in full light.