Growth response of threeFestuca rubra clones to light quality and arbuscular mycorrhiza

The influence of the arbuscular mycorrhizal fungus—Glomus etunicatum and changes in light quality (decrease of red/far-red ratio) on the growth of threeFestuca rubra clones, ecotypes originating from a mountain grassland region, was studied in a growth chamber experiment. Inoculation with the arbuscular mycorrhizal fungus and low red/far-red ratio decreased both the number of tillers and the biomass of treated plants. Significant interactions between the treatments were found and most of the growth characteristics were reduced further when both treatments were applied simultaneously. Inoculation with the arbuscular mycorrhizal fungus also resulted in reduced maximum height of tillers, whereas low red/far-red ratio caused the maximum height of tillers to increase. Differences in plasticity were found for the threeFestuca rubra clones. Response to one treatment was strongly modified by the other treatment. This indicates that the arbuscular mycorrhizal fungus and red/far-red ratio can differentially influence the growth ofFestuca rubra clones and thus modify their relative competitive abilities which can consequently have implications for the coexistence mechanisms within plant population, thereby potentially influencing plant canopy and community structure.     

Within population genetic differentiation in traits affecting clonal growth: Festuca rubra in a mountain grassland

Festuca rubra , a clonal grass of mountain grasslands, possesses a considerable variability in traits related to spatial spreading (rhizome production, length and branching; tussock architecture). Since these traits highly influence the success of the species in a spatially heterogeneous system of grasslands, a combined field and growth chamber approach was adopted to determine the within-population variation in these parameters. Clones were sampled in a mountain grassland (The Krkono?e Mts., Czech Republic); the environment (mean neighbour density) of individual clones varied highly. Before the clones were collected, shoot demography and tussock architecture within these clones were recorded in the field for four seasons. Their clone identity was determined using DNA RAPD. Vegetatively propagated plants from these clones were cultivated in a common garden experiment to demonstrate variation in tussock growth and architecture. Their response to change in red/far red light ratio was determined in the growth chamber. Highly significant variation among clones was found in almost all parameters. In the common garden, the clones differed in tussock growth (mean tillering rate), architecture (mean shoot angle, mean tussock density) and proportion of flowering shoots. In the growth chamber, both the aboveground parameters and parameters of the rhizome system were strongly affected by red/far red ratio; among-clone variation was also almost always significant. The genotype × environment interaction was significant for tillering rate and rhizome architecture. The structure of the rhizome system (which is the major component of clonal spread in space) is a complex result of several components whose inter-clone variations differ: (i) genetically determined mean rhizome system size, (ii) overall plasticity in rhizome system size (with no significant genetic variation in plasticity), and (iii) genetically determined plasticity in rhizome architecture. Because of the variation in plasticity in rhizome architecture, some c lones seemed to possess the ability to exploit a favourable habitat patch by producing short branches when there; whereas the remaining clones appeared to possess only a simple escape mechanism from unfavourable patches. Environmental variation in the light levels in the studied grassland is fine grained; horizontal growth rates of F. rubra are sufficient to make genets experience different patches in their lifetime. The high variation in both genotype means and plasticities is likely to be due to selection early in genet life in an environment which is heterogeneous at a fine scale.     

Tillering responses to red:far-red light ratio during different phenological stages in Eragrostis curvula

Tillering responses to light quality in different phenological stages of a perennial warm-season grass Eragrostis curvula were investigated in controlled environments. In vegetative plants, the tillering rate was greater (P<0.01) in the high (1.1–1.3) than in the low (0.59–0.70) red:far-red ratio (R/FR) light regime. Tillering rates were higher in the low R/FR treatment when the plants in the high R/FR regime reached the reproductive stage, while the plants in the low R/FR regime remained vegetative. After the reproductive tillers were removed by defoliation, more tillers were produced in the defoliated plants grown in the high R/FR regime. When the plants in both light treatments entered the reproductive stage, the tillering rate under the two light regimes became similar, suggesting a significant interaction between tillering and inflorescence development. The more advanced inflorescence development in the high R/FR regime may have reduced assimilate availability to tiller growth and overshadowed the effect of high R/FR on tillering. Both tillering and inflorescence development appeared to be controlled by R/FR ratio. The higher rate of aerial tiller production on the reproductive culms during the post-anthesis period in the high R/FR regime suggested that high R/FR ratio stimulated not only basal tillering, but also aerial tillering.     

Tillering Responses of Lolium multiflorum Plants to Changes of Red/Far-Red Ratio Typical of Sparse Canopies

Casal, J. J., Sáchez, R. A. and Deregibus, V. A. 1987.Tillering responses of Lolium multiflorum plants to changesof red/far-red ratio typical of sparse canopies.—J. exp.Bot. 38: 1432–1439. Plants of Lolium multiflorum were grown either in growth roomsunder radiation sources providing equivalent PAR but differentR/FR ratios between 0·46 and 2·36, or under sunlightsupplemented with a series of small fluence rates of FR throughoutthe photoperiod. In both kinds of experiments a reduction oftillering was found along with small depressions of R/FR ratiobelow the values typical of sunlight. In this range both thesquare-root-transformed rate of tillering and the site-fillingrate were linearly related to the estimated phytochrome photo-equilibriaand the slope was very steep under conditions that allowed hightillering rates. The addition of low fluence rates of red lightat the base of the shoots raised tillering of plants exposedto low R/FR ratios to the level of plants grown under high R/FRratios in spite of the ratios received by the rest of the plant.Moreover, irradiation of plant bases with intermediate-low R/FRratios reduced tillering indicating that this would be one siteof perception. These results show a high sensitivity to changesof R/FR ratio typical of different degrees of shade in sparsecanopies. This suggests that by monitoring these changes Loliummultiflorum plants would be able to perceive an anticipatedsignal of impending competition in growing canopies.     

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.     

Photocontrol of germination of cucumis anguria l

Germination ofCucumis anguria was inhibited by white, blue (B), and far-red (FR) irradiation and promoted by darkness and red (R) irradiation. The effect of white light was greater when supplied after rather than before the dark period. Darkness was more effective in reversing the effect of FR than FR in reversing the effect of darkness. FR was also more effective than B. When darkness followed B pretreatments, final germination percentage was higher than with FR pretreatment. R fully reversed the inhibitory effect of FR.     

Red : far-red light ratio and UV-B radiation: their effects on leaf phenolics and growth of silver birch seedlings

The natural variation in quantity and quality of light modifies plant morphology, growth rate and concentration of biochemicals. The aim of two growth‐room experiments was to study the combined effects of red (R) and far‐red (FR) light and ultraviolet‐B (UV‐B) radiation on the concentrations of leaf phenolics and growth and morphology of silver birch (Betula pendula Roth) seedlings. Analysis by high‐performance liquid chromatography showed that the leaves exposed to supplemental FR relative to R contained higher concentrations of total chlorogenic acids and a cinnamic acid derivative than the leaves treated with supplemental R relative to FR. In contrast, concentration of a flavonoid, quercetin 3‐galactoside, was higher in the R + UV‐B leaves than in the FR + UV‐B leaves. The UV‐B induced production of kaempferols, chlorogenic acids and most quercetins were not modified by the R : FR ratio. Growth measurements showed that the leaf petioles and stems of FR seedlings were clearly longer than those of R seedlings, but leaf area was reduced by UV‐B radiation. Results of these experiments show that exposure of silver birch seedlings to supplemental FR compared to R leads to fast elongation growth and accumulation of phenolic acids in the leaves.     

THE GENETIC ARCHITECTURE OF PLASTICITY TO DENSITY IN IMPATIENS CAPENSIS

Plant responses to crowding may be mediated by resource availability and/or by a specific environmental cue, the ratio of red:far red wavelengths (R:FR) perceived by phytochrome. This study examined the contribution of phytochrome-mediated photomorphogenesis to genetic variation in plastic responses to density in the annual plant Impatiens capensis. Inbred lines derived from open and woodland populations were grown under low density high density, and high density with selective removal of FR wavelengths to block phytochrome-mediated perception of neighbor proximity. Genetic variation in plasticity to density and to the R:FR cue was detected for several traits Plants grown at high density displayed increased internode elongation; decreased branch, flower, and node production; increased menstem dormancy; and decreased leaf area and specific leaf weight compared to plants grown at low density. Stem elongation responses to density were suppressed when phytochrome perception was blocked at high density. For these phytochrome-mediated traits, a genotype's plasticity to density was strongly correlated with its response to R:FR. Phytochrome-mediated traits were tightly correlated with one another, regardless of the density environment. However, the responses to density of meristem allocation to branching and leaf traits were less strongly phytochrome-mediated. These traits differed in patterns of plasticity, and their genetic correlations often differed across environments. In particular, genetic trade-offs involving meristem allocation to branching were expressed only at low density. The observed density dependence of phenotypic and genetic correlations implies that indirect selection and the potential for correlated response to selection will depend upon the competitive environment. Thus the differential sensitivity of characters to the R:FR cue can influence the evolution of integrated plastic responses to density.     

Seasonal changes in Sphagnum peatland testate amoeba communities along a hydrological gradient

Testate amoebae are an abundant and functionally important group of protists in peatlands, but little is known about the seasonal patterns of their communities. We investigated the relationships between testate amoeba diversity and community structure and water table depth and light conditions (shading vs. insolation) in a Sphagnum peatland in Northern Poland (Linje mire) in spring and summer 2010. We monitored the water table at five sites across the peatland and collected Sphagnum samples in lawn and hummock micro-sites around each piezometer, in spring (3 May) and mid-summer (6 August) 2010. Water table differed significantly between micro-sites and seasons (Kruskal–Wallis test, p = 0.001). The community structure of testate amoebae differed significantly between spring and summer in both hummock and lawn micro-sites. We recorded a small, but significant drop in Shannon diversity, between spring and summer (1.76 vs. 1.72). Strongest correlations were found between testate amoeba communities and water table lowering and light conditions. The relative abundance of mixotrophic species Hyalosphenia papilio, Archerella flavum and of Euglypha ciliata was higher in the summer.     

Glasshouse evaluation of the growth ofAlnus rubra andAlnus glutinosa on peat and acid brown earth soils when inoculated with four sources ofFrankia

The effects of soil type (an acid peat and 2 acid brown earths) andFrankia source (3 spore-positive crushed nodule inocula and spore-negative crushed nodules containing the singleFrankia ArI5) on nodulation, N content and growth ofAlnus glutinosa andA. rubra were determined in a glasshouse pot experiment of two years duration. Plants on all soils required additional P for growth. Growth of both species was very poor on peat withA. glutinosa superior toA. rubra. The former species was also superior toA. rubra on an acid brown earth with low pH and low P content. Some plant-inoculum combinations were of notable effectivity on particular soils but soil type was the major source of variation in plant weight. Inoculation with crushed nodules containingFrankia ArI5 only gave poor infection of the host plant, suggesting that inoculation with locally-collected crushed nodules can be a preferred alternative to inoculation withFrankia isolates of untested effectivity. Evidence of adaptation ofFrankia to particular soils was obtained. Thus, while the growth of all strains was stimulated by mineral soil extracts, inhibitory effects of peat extracts were more apparent with isolates from nodules from mineral soils than from peat, suggesting that survival ofFrankia on peat may be improved by strain selection.     

PHENOTYPIC PLASTICITY EARLY IN LIFE CONSTRAINS DEVELOPMENTAL RESPONSES LATER

Abstract.— Plastic stem‐elongation responses to the ratio of red:far‐red (R:FR) wavelengths enable plants to match their phenotype to local competitive conditions. However, elongation responses early in the life history may occur at the cost of reduced plasticity later in the life history, because elongation influences both allocation patterns and structural integrity. A common‐garden experiment was performed to test whether seedling responses to R:FR affect biomass allocation, biomass accumulation, and subsequent plasticity to the cue. Seedlings of Abutilon theophrasti were stimulated to elongate by low R:FR treatments, and subsequent growth and plasticity was compared with nonelongated individuals. Elongated seedlings were less responsive than nonelongated ones to a second bout of low R:FR. Thus, seedling plasticity to R:FR reduces subsequent responsiveness to this cue. This negative association across life‐history stages suggests an important constraint on the evolution of plastic stem responses, because selection in A. theophrasti has previously been shown to favor increases in early elongation in combination with increased later elongation. The reduced responsiveness of elongated seedlings to R:FR appeared to result from a structural feedback mechanism, indicating that the opportunity cost of early responses may be lower in environments providing structural support.     

Overexpression of oat phytochrome A gene differentially affects stem growth responses to red/far-red ratio signals characteristic of sparse or dense canopies

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.     

Simulating the effects of localized red:far-red ratio on tillering in spring wheat (Triticum aestivum) using a three-dimensional virtual plant model

The outgrowth of tiller buds in Poaceae is influenced by the ratio of red to far-red light irradiance (R:FR). At each point in the plant canopy, R:FR is affected by light scattered by surrounding plant tissues. This paper presents a three-dimensional virtual plant modelling approach to simulate local effects of R:FR on tillering in spring wheat (Triticum aestivum). R:FR dependence of bud outgrowth was implemented in a wheat model, using three hypothetical responses of bud extension to R:FR (unit step, curvilinear and linear response). Bud break occurred when a threshold bud length was reached. Simulations were performed for three plant population densities. In accordance with experimental observations, fewer tillers per plant were simulated for higher plant population densities. The linear and curvilinear responses caused a delay in the increase in tiller number compared with experimental data. The unit step response approached experimental results best. It is suggested that a model based on relatively simple relations can be used to simulate degree of tillering. This study has shown that the virtual plant approach is a promising tool with which to address crop morphological and ecological research questions where the determining factors act at the level of the individual plant organ.     

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.     

Differential genetic variation in adaptive strategies to a common environmental signal in Arabidopsis accessions: phytochrome-mediated shade avoidance

Shade avoidance is a syndrome of plastic responses to light signals encountered in crowded plant communities and is a crucial component of competitive strategy in higher plants. The responses are mediated via signal perception by specific members of the phytochrome family of photoreceptors, which detect the relative proportions of red (R) and far‐red (FR) radiation within dense communities. We analysed two aspects of shade avoidance, the acceleration of flowering and the enhancement of elongation growth, displayed by more than 100 accessions of Arabidopsis thaliana (Heyn.) in response to FR‐proximity signals. Both traits showed wide variation between accessions, which was unrelated to the latitude of the location of original collection. Flowering acceleration is a major feature of shade avoidance in rosette plants such as Arabidopsis, and most accessions showed dramatic responses, but several were identified as being recalcitrant to the proximity signal. These accessions are likely to be informative in the analysis of quantitative variation in shade avoidance. Hypocotyl elongation, treated here as an indicator of elongation growth responses, also varied widely amongst accessions. The variations in flowering acceleration and elongation were not correlated, indicating that microevolution in the downstream pathways from signal perception has occurred separately.     

Photomorphogenetic responses to UV radiation and short-term red light in lettuce seedlings

Effects of red light (R), far-red light (FR) and UV radiation on growth and greening of lettuce seedlings (Latuca sativa L., cv. Berlinskii) have been investigated. UV-B and UV-C inhibited hypocotyl elongation and stimulated cotyledonary growth. R in combination with UV-B and UV-C partly eliminated these effects, but FR increased those and reversed the R effect. Chlorophyll accumulation was inhibited by UV-B and UV-C. In comparison with cotyledonary growth, R strengthened the UV inhibitory effect, and FR reversed this effect of R. Thus, UV and phytochrome system modify the effects of each other on hypocotyl and leaf growth in lettuce seedlings depending on the level of active phytochrome formed.     

不同光照质量和刈割强度对小糠草无性繁殖特性的影响

本实验于1996年在美国德克萨斯理工大学草原,野生动物和渔类系的温室和实验室中进行,实验材料均为盆栽糠草,实验设计为在自然光照下（R／FR＝1．1）,以不同留茬高度（10,15,20cm和不刈割）作为不同刈割强度,用不同红光／远红光比例（R／FR＝1．3和0．7）处理不同刈割强度的小糠草,观测不同处理对小糠草无性繁殖特性的影响,结果表明,小糠草净增蘖数随刈割强度增加而减少,除不刈割处理外,光照质量变化（R／FR的比例）对分蘖性能影响不大,所有刈割强度下都不存在分蘖的补偿性生长效应,但幼叶伸长速度随刈割强度的增大而加快,可以认为小糠草应适度利用,强度利用或不利均不利于该草生长发育。     

Susceptibility to UV damage in Impatiens capensis (Balsaminaceae): testing for opportunity costs to shade-avoidance and population differentiation

Plastic increases in leaf secondary compounds may be an adaptive strategy that reduces the damaging effects of high-energy, ultraviolet radiation (UV). Here, we examine (1) the relationship between fitness and anthocyanin and flavonoid concentrations in experimental, UV environments, (2) the effects of UV on Impatiens capensis plants derived from woodland and clearing sites, and (3) whether susceptibility to UV damage is reduced by exposure to high ratios of red?:?far-red wavelengths (R?:?FR), which also stimulate the production of leaf compounds. Seedlings from each site were exposed to either high R?:?FR typical of sunlight or low R?:?FR characteristic of foliar shade, after which plants were moved into ambient UV or UV-removal treatments. Ultraviolet radiation stimulated the production of anthocyanins and flavonoids. However, higher anthocyanin concentrations were associated with lower biomass in the UV environment. Relative to the clearing population, reproductive output of the woodland population was more detrimentally affected by exposure to UV, despite its higher concentration of anthocyanins. Increased anthocyanin production may therefore be a stress response rather than an adaptive one. The greater tolerance of the clearing population to UV suggests that populations with an evolutionary history of UV exposure evolve mechanisms to limit damage. The R?:?FR pretreatments did not influence susceptibility to UV damage.     

Reversible phytochrome regulation influenced the severity of ozone-induced visible foliar injuries in Trifolium subterraneum L.

In Trifolium subterraneum, oxidative stress caused by ozone has been shown to result in more severe visible foliar injuries when plants were kept in dim broadband white light during the night (i.e. a long photoperiod) compared to darkness during the night (a short photoperiod). As phytochrome signalling is involved in photoperiod sensing, the effect of night-time red and far-red illumination on the ozone-induced response was studied. T. subterraneum plants were treated with ozone enriched air (70?ppb) for either 1?h for a single day or 6?h for three consecutive days. After the first ozone exposure, plants were separated into six night-time light regimes during the two subsequent nights (10?h?day, 14?h night): (1) darkness, (2) far-red light (FR), (3) a short night-break of red followed by far-red light during an otherwise dark night (R FR), (4) a short night-break of red, far-red and finally red light during an otherwise dark night (R FR R), (5) dim white light (L) and (6) red light (R). The treatments L and R resulted in significantly more severe ozone-induced visible foliar injuries relative to D and FR treatments, indicating a phytochrome-mediated response. The night-breaks resulted in a photoreversible and significantly different ozone response depending on the light quality of the last light interval (R FR or R FR R), supporting a photoreversible (between P_r and P_fr) phytochrome signalling response. Thus, in T. subterraneum, the outcome of oxidative stress due to ozone appears to depend on the photoperiod mediated by the night-time conformation of phytochrome.     

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.