Shade avoidance in Trifolium repens: costs and benefits of plasticity in petiole length and leaf size

We tested whether the degree of shade-induced plasticity in petiole length and leaf area is related to the mean trait value expressed under high-light conditions, and to what extent trait values expressed under high-light and shaded conditions affect plant performance. Thirty-four Trifolium repens genotypes were used with a wide range of petiole lengths and leaf areas. Plants were subjected to a high-light environment and two shading regimes: homogeneous shading and a vertical light gradient. Absolute petiole elongation in response to both shading treatments and absolute leaf area expansion in response to homogeneous shading were independent of the trait values expressed in high light. Consequently, relative plasticity was higher for genotypes with lower high-light trait values. Plasticity was associated with enhanced plant performance in a vertical light gradient but not in homogeneously shaded conditions. We also found costs associated with the ability to express plasticity. Our results suggest that selection can act separately on trait values expressed under high-light conditions and on the degree of plasticity.     

藤本植物Potentilla reptans叶对风和遮阴的响应

生长在一个密集植物群中的植株由于相互遮蔽而不可能对风的影响做出反应,因为这样的环境条件(有限的光资源)对由风导致的矮小表型植株的生长是不利的。为弄清在密集植物群体中生长的植株对风的响应,利用藤本植物Potentilla reptans的10种基因型做实验材料,在温室条件下(光照强度为日光照的50%,红光/远红光=1.2)模拟冠层遮阴(相当于15%的日光照,红光/远红光=0.3),研究了藤本植物叶对风的响应。结果表明,Potentilla reptans的10种基因型植株在冠层遮阴下(低的红光/远红光)都表现出典型的避阴生长响应:较少的叶(叶生物量少),长而细但硬度系数高(higher Young's modulus)的叶柄;而受风影响的植株,无论遮阴或不遮阴,其植株的叶相对较多,叶柄短、粗且柔韧性强(lower Young's modulus),说明Potentilla reptans叶对风的响应并未因遮阴而被压抑,其可塑性变化不过是对复杂生境做出的一种生长权衡:尽可能增强抗风能力(矮壮)和获取最大光能(足够高而避免被遮光),即保证在存活下去的前提下获取最大的生长效率。     

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.     

Plasticity to simulated shade is associated with altitude in structured populations of Arabidopsis thaliana

Plants compete for photosynthesis light and induce a shade avoidance syndrome (SAS) that confers an important advantage in asymmetric competition for light at high canopy densities. Shade plasticity was studied in a greenhouse experiment cultivating Arabidopsis thaliana plants from 15 populations spread across an altitudinal gradient in the northeast area of Spain that contain a high genetic variation into a reduced geographical range. Plants were exposed to sunlight or simulated shade to identify the range of shade plasticity. Fourteen vegetative, flowering and reproductive traits were measured throughout the life cycle. Shade plasticity in flowering time and dry mass was significantly associated with the altitude of population origin. Plants from coastal populations showed higher shade plasticity indexes than those from mountains. The altitudinal variation in flowering leaf plasticity adjusted negatively with average and minimum temperatures, whereas dry mass plasticity was better explained by negative regressions with the average, maximum and minimum temperatures, and by a positive regression with average precipitation of the population origin. The lack of an altitudinal gradient for the widest number of traits suggests that shade light could be a driver explaining the distribution pattern of individuals in smaller geographical scales than those explored here.     

Directed overexpression of PHYA locally suppresses stem elongation and leaf senescence responses to far-red radiation

Ectopic overexpression of an oat PHYA cDNA in tobacco under the cauliflower mosaic virus 35S promoter results in plants with reduced morphological responses to far-red radiation (FR). We have tested the hypothesis that it is possible to molecularly ‘mask’ steins and leaves to FR-induced elongation and senescence responses by targeting the over-expression of PHYA with appropriate promoters. Oat PHYA was expressed in tobacco (Nicotiana tabacum L. cv Xanthi) under the 35S and two Arabidopsis promoters: UBQ1 and CAB. The internodes of wild type, UBQ:PHYA, and CAB:PHYA plants, which exhibited little or no oat PHYA overexpression, responded to localized FR treatments with a marked increase in elongation. In contrast, 35S:PHYA plants, which overexpressed PHYA to high levels in all parts of the shoot, did not respond to FR treatments directed to their stems. Leaf senescence responses to FR were remarkably localized, and sensitivity to FR was also inversely correlated with the local PHYA expression level. Thus, chlorophyll content, specific leaf weight, and nitrate reductase activity in leaf spots treated with FR were highly reduced in wild type and UBQ:PHYA plants, but not in the CAB:PHYA and 35S:PHYA counterparts. Our results suggest that it may be feasible to obtain transgenic crop plants in which certain organs or tissues are made ‘blind’ to phytochrome-perceived signals of canopy density, but whose general photomorphogenic competence is not greatly disturbed.     

An effect of shade on the boron requirement for leaf blade elongation in black gram (Vigna mungo L. Hepper)

The present experiment was undertaken to examine whether shading affects the critical boron (B) concentration for leaf blade elongation in black gram. Six days after germination (D6), black gram seedlings were transferred to 8 pairs of pots containing basal nutrient solution: one pot of each pair contained 1 M H₃BO₃ and the other 10 M H₃BO₃. On D10, one day after the emergence of the first trifoliolate leaf blade (TF₁) and one day before the emergence of TF₂, four pairs of pots were shaded, decreasing the light intensity they received in the glasshouse to about 35% of full sunlight compared with 70% received by the unshaded plants. The response to B supply of dry matter (DM) and elongation rate (LBER) of TF₂ were less pronounced in shaded than in unshaded plants. Critical B concentrations for LBER in TF₂ of black gram were 10 mg B kg^-1DM in shaded and 15 mg B unshaded plants, suggesting that shading may have decreased the B requirement for LBER. Thus the present results suggest that light may need to be considered when setting critical values for the diagnosis of B deficiency in black gram.     

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.     

Height convergence in response to neighbour growth: genotypic differences in the stoloniferous plant Potentilla reptans

Using a new experimental set up, the way in which height growth of stoloniferous plants is adjusted to that of their neighbours, as well as differences between genotypes in their ability to keep up with neighbour height growth were tested. Five Potentilla reptans genotypes inherently differing in petiole length were subjected to three experimental light gradients, involving light intensity and red : far-red ratio. Each plant was placed in a vertically adjustable cylinder of green foil, and the treatments differed in the speed of cylinder height increase and final height. Total weight of plants decreased from the 'Slow' to the 'Fast' treatment, while petiole length increased. Leaves reaching the top of the cylinder stopped petiole elongation, resulting in similar final heights for all genotypes in the 'Slow' treatment. In the 'Fast' treatment only the fastest-growing genotype maintained its position in the top of the cylinder and genotypes differed strongly in final height within the cylinders. Plants adjust their height growth to that of the surrounding vegetation, leading to height convergence in short light gradients that slowly increase. These adjustments and genotypic differences in ability to keep up with fast-growing neighbours can influence the outcome of competition for light.     

The relationship between leaf growth and ABA accumulation in the grass leaf elongation zone

Detached barley (Hordeum vulgare L.) shoots, maintained at different air temperatures and VPDs, were fed ABA via the sub-crown internode in a leaf elongation assay. Analysis of variance of leaf elongation rate (LER) showed significant effects of temperature (T), fed [ABA] and the interaction T × [ABA]. However, the interaction became non-significant when LER was modelled against the [ABA] of the elongation zone, [EZ-ABA] When detached barley shoots were fed sap from droughted maize (Zea mays L.) plants, sap [ABA] could not explain the growth inhibitory activity. Measurement of [EZ-ABA] accounted for this ‘unexplained’ growth inhibition. The detached shoot experiments indicated that [EZ-ABA], and not xylem sap [ABA], was an appropriate explanatory variable to measure in droughted plants. However, ABA accumulation in the elongation zone could not explain a 35% growth reduction in intact droughted plants; thus we considered an interaction of water status and ABA. Using a coleoptile growth assay, we applied mild osmotic stresses (ψ=0 to ?0.06 MPa) and 10^?4 mol m^?3 ABA. Individually, these treatments did not inhibit growth. However, osmotic stress and ABA applied together significantly reduced growth. This interaction may be an important mechanism in explaining leaf growth inhibition of droughted plants.     

The ratio of leaf to total photosynthetic area influences shade survival and plastic response to light of green-stemmed leguminous shrub seedlings

Different plant species and organs within a plant differ in their plastic response to light. These responses influence their performance and survival in relation to the light environment, which may range from full sunlight to deep shade. Plasticity, especially with regard to physiological features, is linked to a greater capacity to exploit high light and is usually low in shade-tolerant species. Among photosynthetic organs, green stems, which represent a large fraction of the total photosynthetic area of certain species, are hypothesized to be less capable of adjustment to light than leaves, because of biomechanical and hydraulic constraints. The response to light by leaves and stems of six species of leguminous, green-stemmed shrubs from dry and high-light environments was studied by growing seedlings in three light environments: deep shade, moderate shade and sun (3, 30 and 100 % of full sunlight, respectively). Survival in deep shade ranged from 2 % in Retama sphaerocarpa to 74 % in Ulex europaeus. Survival was maximal at moderate shade in all species, ranging from 80 to 98 %. The six species differed significantly in their ratio of leaf to total photosynthetic area, which influenced their light response. Survival in deep shade increased significantly with increasing ratio of leaf to total photosynthetic area, and decreased with increasing plasticity in net photosynthesis and dark respiration. Responses to light differed between stems and leaves within each species. Mean phenotypic plasticity for the variables leaf or stem specific mass, chlorophyll content, chlorophyll a/b ratio, and carotenoid to chlorophyll ratio of leaves, was inversely related to that of stems. Although mean plasticity of stems increased with the ratio of leaf to total photosynthetic area, the mean plasticity of leaves decreased. Shrubs with green stems and a low ratio of leaf to total photosynthetic area are expected to be restricted to well-lit habitats, at least during the seedling stage, owing to their inefficient light capture and the low plasticity of their stems.     

Preferential states of longitudinal tension in the outer tissues of Taraxcum officinale (Asteraceae) peduncles

We tested Wilhelm Hofmeister's hypothesis that the outer layers of herbaceous stem tissues are held in a preferential state of longitudinal tension by more internal stem tissues that are held in a reciprocal state of compression. We measured (1) the biaxial stiffness of dandelion peduncles that were barometrically inflated with a Scholander pressure bomb, and (2) the stiffness and mechanical behavior of different layers of tissues that were surgically manipulated as longitudinal strips placed in uniaxial tension. Hofmeister's hypothesis predicts that stems will shorten and expand in girth as their volume transiently increases (due to barometric or hydrostatic inflation), that they will longitudinally rupture when excessively inflated, and that the principal stiffening agents in their outer tissues will be aligned in the longitudinal direction with respect to stem length. Our experiments confirmed these predictions: (1) the longitudinal strains observed for inflated peduncles were negative and smaller than the circumferential strains such that stems contracted in length and expanded in girth, (2) peduncles longitudinally ruptured when excessively inflated, (3) surgical experiments indicated that the epidermis was stiffer in longitudinal tension than any other immature peduncle tissue and was as stiff as any other tissue region in mature stems, and (4) microscopic analyses showed that the net orientation of cellulose microfibrils in the cell walls of the outer region of stem tissues was parallel to stem length. A strong positive correlation existed between the tensile stiffness of tissues and the net orientation of cell wall microfibrils.     

A Comparison of Plastic Responses to Competition by Invasive and Non-invasive Congeners in the Commelinaceae

Evidence supporting an association between phenotypic plasticity and invasiveness across a range of plant taxa is based primarily on comparisons between invasive species and native species whose potential invasiveness is typically unknown. Comparison of invasive and non-invasive exotic species would provide a better test of whether plasticity promotes invasion. Such comparisons should distinguish between adaptive and non-adaptive plasticity because they have different consequences for invasiveness. Adaptive plasticity is expected to promote the invasion of multiple habitats, but non-adaptive plasticity may reflect specialization for invading more favorable habitats only. We grew four invasive and four non-invasive species of the Commelinaceae with and without competitors and compared their putatively adaptive plasticity of three traits related to competitive ability and non-adaptive plasticity in performance. The invasive species grew significantly more than the non-invasive species only in the non-competitive environment. The invasive species had greater plasticity of performance (total biomass) in response to competition than non-invasives, but there was no consistent difference in the plasticities of the traits related to competitive ability. These results are consistent with specialization of these invasive taxa for invading the more productive non-competitive environment rather than a superior ability to invade both competitive and non-competitive environments. A comprehensive understanding of the relationship between plasticity and invasiveness will require many more comparisons of the plasticity of invasive and non-invasive taxa in a range of traits in response to a variety of environments.     

Rapid adaptive responses of rosette‐type macrophyte Vallisneria natans juveniles to varying water depths: The role of leaf trait plasticity

Rosette‐type submerged macrophytes are widely distributed across a range of water depths in shallow lakes and play a key role in maintaining ecosystem structures and functions. However, little is known about the rapid adaptive responses of such macrophytes to variations in water depth, especially at the juvenile stage. Here, we conducted a short‐term in situ mesocosm experiment, in which the juveniles of Vallisneria natans were exposed to a water depth gradient ranging from 20 to 360 cm. Twenty‐two leaf‐related traits were examined after 4 weeks of growth in a shallow lake. Most (18) traits of V. natans generally showed high plasticity in relation to water depth. Specifically, juveniles allocated more biomass to leaves and had higher specific leaf area, leaf length‐to‐width ratio, chlorophyll content, and carotenoids content in deep waters, displaying trait syndrome associated with high resource acquisition. In contrast, V. natans juveniles in shallow waters had higher leaf dry matter content, leaf soluble carbohydrate content, carotenoids per unit chlorophyll, and peroxidase activity, pertaining to resource conservation. Notably, underwater light intensity was found to be the key factor explaining the trait plasticity along the water depth gradient, and 1.30 mol photons m⁻² d⁻¹ (at 270 cm) could be the optimal irradiance level based on the total biomass of V. natans juveniles. The present study highlights the significance of leaf trait plasticity for rosette‐type macrophytes in response to variations in water depth and sheds new light on the differences between trade‐offs in deep‐ and shallow‐water areas.     

Phenotypic plasticity of stem elongation in two ecotypes of Stellaria longipes: the role of ethylene and response to wind

Using two ecotypes of Stellaria longipes an alpine form with low plasticity and a prairie form with high plasticity, we investigated whether ethylene was involved in the response to wind stress and might be important in controlling plasticity of stem elongation. Stem growth inhibition was positively correlated with concentration of ethephon application and elevation in ambient ethylene in alpine ecotypes, whereas stem growth in prairie plants was stimulated by low ethephon concentrations. When treated with high AVG, the effects were reversed: alpine plant growth was promoted and prairie plant growth was inhibited. Prairie plants exhibited a daily rhythm in ethylene evolution which increased and peaked at 1500 h, and which was absent in alpine plants. Ethylene evolution did not change significantly during the first 2 weeks of growth in alpine plants, whereas ethylene in prairie plants increased significantly during periods of rapid stem elongation. Wind treatment inhibited growth in both ecotypes, but only alpine plants showed a recovery of growth to control levels when wind stressed plants were pretreated with STS. In addition, only alpine plants showed an increase in ethylene evolution in response to wind simulation, whereas prairie plant ethylene evolution did not deviate from rhythms observed in unstressed plants. We concluded that ethylene dwarfs stems in alpine S. longipes in response to wind stress. However, low levels of ethylene may stimulate growth in prairie ecotypes and act independently of wind stress intensity. The contrasting ability to synthesize and respond to ethylene can account for part of the difference in plasticity documented between the two ecotypes.     

Relationship of light intensity to leaf resin composition and yield in the tropical leguminous genera Hymenaea and Copaifera

Hymenaea and Copaifera are closely related morphologically distinct tree genera whose leaf resins appear to be composed of the same sesquiterpene hydrocarbons which occur in similar quantitative compositional patterns. The seedlings of both genera grow in varying light intensities in the understories of different lowland tropical ecosystems from desert thorn forests to equatorial evergreen rain forests; all eventually grow into high light intensity. Previous controlled environment studies of the effects of photoperiod, temperature and moisture status have shown little phenotypic plasticity in the leaf resin composition of Hymenaea seedlings. In this study, focusing on light intensity, both control chamber experiments comparing seedlings of H. courbaril var. courbaril, H. courbaril var. subsessilis, C. officinalis and C. pubiflora, grown under low and high light treatments, and field data from saplings of C. multijuga, growing under shaded and unshaded conditions, also show lack of phenotypic plasticity in resin composition. The yield of resin (mg/g leaf tissue), however, varied considerably among individuals, and increased significantly from low to high light conditions in both control chamber and field conditions. The relationship of increased synthesis is discussed with regard to photosynthetic capacity, allocation of carbon and herbivory.     

Transpiration rates of the understory annual Impatiens capensis (Balsaminaceae) in response to the autumnal changes in canopy leaf area

Leaf transpiration rates of Impatiens capensis were measured beneath a broadleaved deciduous forest canopy over successive growing seasons using a steady-state porometer. The transpiration measurements, which continued into early autumn, provided a framework for assessing whether I. capensis exhibits stomatal opening in response to the autumnal increase in available direct-beam radiation reaching the forest floor. The deciduous canopy LAI (leaf area index) decreased from a growing season maximum of 3.94 m² m⁻², while the understory I. capensis population located along a stream channel maintained LAI values ranging from 0.58 to 1.05 m² m⁻² late into the growing season. Late morning and early afternoon leaf transpiration rates during the months of June and July averaged about 8 μg cm⁻² s⁻¹, with a mean stomatal conductance of 0.5 cm s⁻¹. In August, leaf transpiration averaged almost 12 μg cm⁻² s⁻¹, with stomatal conductance exceeding 1.5 cm s⁻¹. However, beginning in early to mid-September, before canopy leaf-fall, the persistent green leaves of I. capensis exhibited a sharp decline in transpiration, possibly a result of decreasing vapor pressure deficits or non-lethal physiological damage induced by cold stress. This physiological decline offsets any advantage that could have been gained by the increased exposure to direct-beam radiation after canopy leaf-fall in mid-October. Although green leaf area and seed-bearing capsules may persist until the first frost in October or early November, there is no evidence of stomatal opening suggestive of carbon assimilation for enhanced seed development during this early autumn period. We conclude that the persistent green leaf area of I. capensis fails to exploit the increase in available direct-beam radiation in the final stage of its life cycle.     

Genetic and environmental contributions to variation in leaf mono- and sesquiterpenes of Heterotheca subaxillaris

Field and greenhouse experiments were performed to measure genetic and environmental contributions to variation in the leaf mono- and sesquiterpenes of Heterotheca subaxillaris. Under greenhouse conditions, plants from a nitratevariable habitat (abandoned peach orchard) exhibit greater phenotypic variation in response to nitrate availability than plants from a continuously nitrate-poor environment (coastal sand dune). A genetic contribution to variation in volatile terpenes was measured only in the nitrate-variable orchard habitat. These results suggest that the magnitude of both genetic variation and phenotypic plasticity in volatile terpene production can differ between populations and may increase with greater environmental heterogeniety in the resources which affect terpene metabolism.     

The role of phenotypic plasticity in responses of hunted thinhorn sheep ram horn growth to changing climate conditions

When phenotypic change occurs over time in wildlife populations, it can be difficult to determine to what degree it is because of genetic effects or phenotypic plasticity. Here, we assess phenotypic changes over time in horn length and volume of thinhorn sheep (Ovis dalli) rams from Yukon Territory, Canada. We considered 42 years of horn growth from over 50 000 growth measurements in over 8000 individuals. We found that weather explained a large proportion of the annual fluctuation in horn growth, being particularly sensitive to spring weather. Only 2.5% of variance in horn length growth could be explained by an individual effect, and thus any genetic changes over the time period could only have had a small effect on phenotypes. Our findings allow insight into the capacity for horn morphology to react to selection pressures and demonstrate the overall importance of climate in determining growth.