Shaded Shoots Research Articles

Redeployment of Shoots into Better-Lit Positions within the Crowns of Saplings of Five Species with Different Growth Patterns

Research Highlights: We demonstrate the first quantitative evidence that the shoot shedding of fast-growing species growing in a high-light environment is part of the process of shoot redeployment into better-lit outer parts of the crown. Background and Objectives: Light foraging by redeploying organs from shaded regions of a tree crown into better-lit regions is considered to apply to both leaves and shoots. To date, however, this hypothesis has never been tested for shoots. Materials and Methods: We investigated the shoot dynamics of saplings of five deciduous woody species. We included fast-growing and slow-growing species (Alnus sieboldiana Matsum., Castanea crenata Siebold & Zucc., Betula ermanii Cham., Acer distylum Siebold & Zucc., and Fagus crenata Blume). Results: Shoots in the shaded regions of the crowns of the fast-growing trees showed higher mortality rates than those at better-lit positions. Because of the selective shedding of the shaded shoots, at the end of the growth period the light environment experienced by the shoots that survived until the following spring was similar to that at the early stage of the same growth period. By contrast, the slow-growing trees displayed slow and determinate growth, with a very low mortality rate of shoots at all positions in the crown. Conclusions: The rapid shoot turnover of the fast-growing species resulted in the redeployment of shoots into better-lit positions within the tree crown in a manner similar to the redeployment of leaves.

Norway spruce needle size and cross section shape variability induced by irradiance on a macro- and microscale and CO2 concentration

Needle cross-section parameters differ according to needle orientation on a shoot corresponding to irradiance microgradient. Irradiance is a stronger morphogenic factor determining needle size and shape than CO2 concentration. We investigated the effects of irradiance on macroscale and microscale, elevated CO2 concentration [CO2], and their interaction on Norway spruce (Picea abies L. Karst.) needles. The irradiance macroscale was represented by sun and shade shoots from two vertical positions in a crown and the irradiance microscale corresponded to spatial orientation of individual needles on a shoot (upper, side, and lower needles relative to the shoot axis). Determination of needle cross section shape using generalized Procrustes analysis and principal component analysis provided a novel approach for evaluating needle morphometry. As expected, shade needles on the irradiance macrogradient were flatter and had less cross-sectional area and smaller volume than did sun needles. The irradiance microgradient was detected within both sun and shade shoots, being steeper in sun shoots than shade shoots. On the microscale, the irradiance gradient induced changes in needle size and cross section shape according to needles’ orientation on a shoot. Due to a more favourable light environment the traits of the upper needles within both sun and shade shoots resembled more the sun needle traits. The sun needle volume was significantly larger in the case of elevated [CO2] as compared to ambient [CO2]. Irradiance was a stronger morphogenic factor determining needle size and cross section shape compared to CO2 concentration. We demonstrated that generalized Procrustes analysis can be a very powerful tool in ecophysiological studies for evaluating small-scale, subtle leaf shape changes on an intraspecific level caused by environmental factors.

Fluxes of biogenic volatile organic compounds above temperate Norway spruce forest of the Czech Republic

Norway spruce (Picea abies), the most representative forest species in central and northern Europe, has previously been described as a monoterpene emitter. However, past studies have shown variable emission rates. In order to understand emissions at the ecosystem scale, a technique utilizing proton-transfer-reaction-time-of-flight (PTR-TOF) mass spectrometry coupled with eddy covariance was applied to determine fluxes of volatile organic compounds and CO2 above a mountainous Norway spruce forest in the Czech Republic during an intensive field campaign in summer. In addition, an Inverse Lagrangian Transport Model was applied to derive fluxes of various monoterpenes using concentrations measured along a vertical canopy profile by wet effluent diffusion denuder. The Model of Emissions of Gases and Aerosols from Nature (MEGAN) was applied using basal emission factors for sun and shade shoots to predict diurnal fluxes and annual monoterpene emission sums for 5 years. The forest showed itself to be a monoterpene emitter up to 2.03nmolm−2s−1. Isoprene and 2-methyl-3-buten-2-ol (MBO) reached maximum levels during central hours of the day of 1.6nmolm−2s−1, whereas daily average maximum fluxes were 1.29nmolm−2s−1 and 0.77nmolm−2s−1 for monoterpenes and sum of isoprene and MBO, respectively. The relationship between monoterpene and CO2 fluxes revealed that as much as 47% of variance in MT emission is predictable from the actual gross primary production of a spruce ecosystem. Modelled MT fluxes agreed with the measured fluxes in terms of diurnal pattern, particularly when basal emission factors 2.72nmolm−2s−1 and 0.55nmolm−2s−1 for sun and shade shoots, respectively, were used. Nevertheless, strong seasonal variability in MT emission was observed. Our results contribute to better understanding emissions of biogenic volatile organic compounds in central Europe.

Shoot-level terpenoids emission in Norway spruce (Picea abies) under natural field and manipulated laboratory conditions

Norway spruce (Picea abies) is a strong emitter of biogenic volatile organic compounds (BVOCs). In the present study we investigated how shoot canopy position and high levels of stressors such as high temperature and ozone concentration, affect BVOC emission rates by means of in-situ and ex-situ experimental measurements.Therefore, BVOC emission from current-year spruce shoots was investigated under field and manipulated (temperature, ozone) laboratory conditions. Emitted BVOCs were sampled on desorption tubes, coupled with gas-exchange measurements of CO2 assimilation rate and stomatal conductance, and detected by gas chromatography coupled with mass spectrometry.Total BVOC emission rates from sun shoots under standard conditions were higher than those from shade shoots, although this was significant only in July, on the contrary, only α-pinene and γ-terpinene emission rates showed significant differences between sun and shade acclimated shoots in August. Limonene, α-pinene, β-pinene, and myrcene were identified as the most abundant BVOCs in both campaigns with emission rates above 0.2 nmol m−2 s−1. Ex-situ measurements revealed a significantly higher total BVOC emissions under high temperature level (40 °C) by ca. 175% as compared with standard temperature (30 °C), while a short-term fumigation of acute O3 concentration (200 ppb) had no effect on BVOC emissions and its spectrum.These findings might have a relevance considering the role of these compounds in protecting against oxidative stress and their possible stimulation in particular stressful conditions.Implication of such results into emission models may contribute to a more accurate estimation of BVOC emissions for Central European mountain regions dominated by Norway spruce forests and their rate under predicted climate change.

The independence of clonal shoot’s growth from light availability supports moso bamboo invasion of closed-canopy forest

Forests with closed canopy are generally thought to resist invasion by other species because of the low light conditions under the canopy. However, moso bamboo (Phyllostachys edulis) is capable of invading adjacent intact forests, including Chinese fir (Cunninghamia lanceolata) forest, broad-leaved forest, and mixed broadleaf-conifer forest. How does moso bamboo manage to expand and grow under the deep shade of forest canopies? To test this question and explore the light conditions in which bamboo shoots survive, we experimented with shading treatments under field conditions. The levels of shading included total shading of young bamboo shoots, shading with light only to the tip of the shoots (tip light), and no shading (full light). Treatments were applied to shoots developing in pure stands of moso bamboo, and shoots that were invading Chinese fir stands. Shoot survivorship, height, and diameter at breast height (DBH) were measured weekly. Total shading of bamboo shoots led to mortality of all shoots, compared with >90% survival for shaded shoots with tip light and for unshaded shoots. Bamboo shoots elongated over a 60day period, reaching heights of more than 10m. Height growth was 8–13% higher for shaded shoots with tip light than for unshaded shoots, with no significant effect of stand type. The elongation of bamboo shoots is largely independent of the light environment that would influence current photosynthesis of the shoot, indicating that shoot elongation likely depends on supplies of carbohydrates from the clonal root system. The deep tolerance of shade that results from clonal of new shoots appears to be a key factor allowing its invasion of intact forest stands.

Modelling competitive ability of Neotropical savanna grasses: Simulation of shading and drought impacts on biomass production

In this paper, we develop a technique to model the spatial distribution of shoots along vertical and horizontal dimensions of a plant community. We use it to simulate the growth of a tropical savanna near the city of Barinas, Venezuela, to explore the responses of the peak biomass of a plant community to a range of 10-50% reduction of rainfall. We selected three dominant grass species: Elyonurus adustus, Leptocoryphium lanatum, and Andropogon semiberbis in a 4×7m study plot. We estimate parameters values from data measured in the field. The number of shoots for each plant is obtained according to soil water availability and distributed vertically by 10cm levels using a transition matrix. Convolution allows calculation of leaf area index for each cell and vertical level, which is then used to calculate light attenuation and thus the proportion of shaded shoots in each cell and level. With this information, maximum evapotranspiration is determined to calculate soil moisture using daily rain time series. Biomass is calculated for all species based on shoot biomass measured in the field and fire is simulated by removing a fraction of the shoots segments of all species. Modeled biomass fits reasonably well to field data.The model predicts significant differences in the response of each species to drought. A. semiberbis was the most drought resistant of the three species, whereas L. lanatum was the least, and E. adustus was intermediate, in agreement with observations in the literature. Our model suggests that drought resistance increases with the biomass/transpiration ratio for the species considered.

Variation in light-intercepting area and photosynthetic rate of sun and shade shoots of two Picea species in relation to the angle of incoming light

We investigated the effects of sun- and shade-shoot architecture on the photosynthetic rates of two Picea species by applying light from various angles in the laboratory. Compared with sun shoots, shade shoots were characterized by lower mass allocation per light-intercepting area, less leaf mass per shoot mass, less mutual shading among leaves and more efficient allocation of chlorophyll to photosynthesis. The shoot silhouette to total leaf area ratio (STAR(ϕ)) decreased with increasing shoot inclination angle (ϕ, the shoot axis angle relative to the projection plane) and was consistently higher for the shade shoots. Morphological and physiological characteristics of the shade shoots resulted in maximum rates of net photosynthesis at ϕ = 0° (P(max,0)) similar to that of the sun shoots when expressed on a leaf mass, total leaf area and chlorophyll basis. When the angle of incoming light was varied, P(max,ϕ) per total leaf area (P(max,ϕ )/A(T)) of the shade shoots increased linearly with increasing STAR(ϕ), while P(max,ϕ) per shoot silhouette area did not change. In contrast, the response of the sun shoots was non-linear, and an optimum angle of incoming light was determined. Our results suggest that shade-shoot morphology is adaptive for utilizing diffuse light incoming from various angles, while sun-shoot morphology is adaptive for avoiding the negative effects of strong direct radiation and for enhancing light diffusion into the canopy. We propose that the angle of incoming light should be taken into account when estimating photosynthetic rates of sun shoots of conifer trees in the field.

Size-dependent variability of leaf and shoot hydraulic conductance in silver birch

Variation in leaf and shoot hydraulic conductance was examined on detached shoots of silver birch (Betula pendula Roth), cut from the lower third (shade leaves) and upper third of the crown (sun leaves) of large trees growing in a natural temperate forest stand. Hydraulic conductances of whole shoots (K S), leaf blades (K lb), petioles (K P) and branches (i.e. leafless stem; K B) were determined by water perfusion using a high-pressure flow meter in quasi-steady state mode. The shoots were exposed to irradiance of photosynthetic photon flux density of 200–250 μmol m−2 s−1, using different light sources. K lb depended significantly (P < 0.001) on light quality, canopy position and leaf blade area (A L). K lb increased from crown base to tree top, in parallel with vertical patterns of A L. However, the analysis of data on shade and sun leaves separately revealed an opposite trend: the bigger the A L the higher K lb. Leaf anatomical study of birch saplings revealed that this trend is attributable to enhanced vascular development with increasing leaf area. Hydraulic traits (K S, K B, K lb) of sun shoots were well co-ordinated and more strongly correlated with characteristics of shoot size than those of shade shoots, reflecting their greater evaporative load and need for stricter adjustment of hydraulic capacity with shoot size. K S increased with increasing xylem cross-sectional area to leaf area ratio (Huber value; P < 0.01), suggesting a preferential investment in water-conducting tissue (sapwood) relative to transpiring tissue (leaves), and most likely contributing to the functional stability of the hydraulic system, essential for fast-growing pioneer species.

Experimental evidence supporting the concept of light-mediated modulation of stem hydraulic conductance

It is a well-described phenomenon that plant leaves respond to changes in light intensity and duration by adjusting leaf hydraulic efficiency, and there is current consensus that up- or down-regulation of water channels (aquaporins) in the plasma membrane of the bundle sheath and mesophyll cells play a central role in the underlying mechanisms. Recently, experimental evidence has been provided also for light-mediated changes of stem hydraulic conductance (K(stem)) in field-grown laurel plants. This effect was attributed to differences in potassium ion concentration of xylem sap as a function of light conditions. In the present article, we report evidence obtained in silver birch (Betula pendula Roth), supporting the concept of light-mediated modulation of K(stem). Both canopy position (long-term effect) and current photosynthetic photon flux density (PPFD; short-term effect) had a significant impact (P < 0.001) on K(stem) measured in shoots taken from the lower (shade shoots) and upper (sun shoots) third of the crowns of ∼25-year-old trees growing in a natural forest stand. The shade shoots responded more sensitively to light manipulation: K(stem) increased by 51% in shade shoots and 26% in sun shoots when PPFD increased from 70 to 330 μmol m⁻² s⁻¹. In 4-year-old trees growing in a dense experimental plantation, K(stem), specific conductivity of branch-wood (k(bw)) and potassium ion concentration ([K(+)]) in xylem sap varied in accordance with canopy position (P < 0.001). Both K(stem) and k(bw) increased considerably with light availability, increasing within the tree crowns from bottom to top; there was a strong relationship between mean values of K(stem) and [K(+)] in hydraulically sampled branches.

Characterization of a type II chlorophyll a/b-binding protein gene (Lhcb2*Pp1) in peach. II. mRNA abundance in developing leaves exposed to sun or shade.

Leaf development of shoots exposed to full sunlight and shoots shaded by the canopy was followed in field-grown, mature peach trees (Prunus persica (L.) Batsch, cv. Loring) during the first half of the 1995 growing season. The architecture and size of shaded shoots and sun-exposed shoots differed significantly. Total number of leaves produced on shaded shoots was significantly less than on sun-exposed shoots throughout the season, and differences in leaf number between light conditions increased as the season progressed. The overall patterns of leaf development along sun-exposed and shaded shoots were qualitatively similar. The expression pattern of the type II chlorophyll a/b-binding protein gene, Lhcb2*Pp1, determined by RNA abundance in leaves at different positions along the shoot, was also similar between the two light conditions. The major difference between sun-exposed and shaded leaves was a lower abundance of Lhcb2*Pp1 RNA in mature, shaded leaves compared with sun-exposed leaves. Although the number of fruit per shoot was significantly lower on shaded shoots than on sun-exposed shoots, the rate of fruit drop was not substantially different during the growing season, indicating that quantitative differences in leaf initiation and growth caused by differences in light exposure did not adversely affect fruit retention. However, based on comparison with a previous study of leaf development in non-fruiting trees, reproductive development slowed the rate of vegetative growth without affecting the overall pattern of leaf development along the shoots.

Leaf senescence and late-season net photosynthesis of sun and shade leaves of overstory sweetgum (Liquidambar styraciflua) grown in elevated and ambient carbon dioxide concentrations.

We examined the effects of elevated CO2 concentration ([CO2]) on leaf demography, late-season photosynthesis and leaf N resorption of overstory sweetgum (Liquidambar styraciflua L.) trees in the Duke Forest Free Air CO2 Enrichment (FACE) experiment. Sun and shade leaves were subdivided into early leaves (formed in the overwintering bud) and late leaves (formed during the growing season). Overall, we found that leaf-level net photosynthetic rates were enhanced by atmospheric CO2 enrichment throughout the season until early November; however, sun leaves showed a greater response to atmospheric CO2 enrichment than shade leaves. Elevated [CO2] did not affect leaf longevity, emergence date or abscission date of sun leaves or shade leaves. Leaf number and leaf area per shoot were unaffected by CO2 treatment. A simple shoot photosynthesis model indicated that elevated [CO2] stimulated photosynthesis by 60% in sun shoots, but by only 3% in shade shoots. Whole-shoot photosynthetic rate was more than 12 times greater in sun shoots than in shade shoots. In senescent leaves, elevated [CO2] did not affect residual leaf nitrogen, and nitrogen resorption was largely unaffected by atmospheric CO2 enrichment, except for a small decrease in shade leaves. Overall, elevated [CO2] had little effect on the number of leaves per shoot at any time during the season and, therefore, did not change seasonal carbon gain by extending or shortening the growing season. Stimulation of carbon gain by atmospheric CO2 enrichment in sweetgum trees growing in the Duke Forest FACE experiment was the result of a strong stimulation of photosynthesis throughout the growing season.

Cold-induced photoinhibition and winter leaf-loss in the broad-leaved tree Aristotelia serrata (Elaeocarpaceae).

Relationships between cold-induced photoinhibition of photosynthesis and winter leaf-loss were investigated for leaves of a wineberry (Aristotelia serrata J. R. Forst. and G. Forst.) tree. Previous work identified consistent differences in leaf loss between shaded and exposed parts of wineberry trees, possibly related to the adverse effects of cold temperatures and high irradiance during winter frosts. Differences in leaf temperature and irradiance between shaded and exposed parts of a wineberry canopy were quantified, together with their relationships to decline and recovery of cold-induced photoinhibition of photosynthesis (measured by the ratio of variable to maximal chlorophyll fluorescence; Fv / Fm). These data, together with measurements of leaf loss from selected shoots, were used to test the hypothesis that leaf loss is greater from open than from shaded parts of wineberry canopies, and increases with decreases in Fv / Fm following winter frosts. Fv / Fm of leaves on exposed shoots was always significantly (P<0.001) lower than leaves on shaded shoots. The absolute difference in Fv / Fm ranged from 0.11 to 0.57. Fv / Fm declined markedly following periods of freezing leaf temperatures (Tl), but recovered after several days of non-freezing Tl. A multiple regression model, including terms for integrated irradiance (Qi), minimum Tl, Tl at the time of Fv / Fm measurements, and the value of Fv / Fm, on the previous day explained 95% of the variability in daily Fv / Fm. Leaf loss was greater from exposed shoots (0.22 leaves shoot-1 d-1) than from shaded shoots (0.07 leaves shoot-1 d-1; P<0.001) and increased following frost-induced decreases in the value of Fv / Fm. Measured rates of leaf loss from exposed shoots were significantly related to Fv / Fm measured 14d previously (r2=0.89, P<0.01), supporting our hypothesis.

Fertilization has little effect on light-interception efficiency of Picea abies shoots.

We investigated effects of nutrient availability on shoot structure and light-interception efficiency based on data from control (C) and irrigated + fertilized (IL) trees of Norway spruce (Picea abies (L.) Karst.). The sampling of 1-year-old shoots was designed to cover the variation in canopy exposure within the live crown zone, where current-year shoots were still found. Canopy openness was used as a measure of light availability at the shoot's position. Openness values for the sample shoots ranged from 0.02 to 0.77 on the IL plot, and from 0.10 to 0.96 on the C plot. Among needle dimensions, needle width increased most with canopy openness. At fixed canopy openness, needle width was larger, and the ratio of needle thickness to width was smaller in IL trees than in C trees. Specific needle area (SNA) and the ratio of shoot silhouette area to total needle area (STAR) decreased with canopy openness, so that the combined effect was a threefold decrease in the ratio of shoot silhouette area to unit dry mass (SMR = STAR x SNA) along the studied range of openness values. This means that the light-interception efficiency of shoots per unit needle dry mass was three times higher for the most shaded shoots than for sun shoots. A test of the effect of fertilization on the relationships of SNA, STAR and SMR indicated statistically significant differences in both slope and intercept for SNA and STAR, and in the intercept for SMR. However, the differences partly cancelled each other so that, at medium values of canopy openness, differences between treatments in predicted SNA, STAR and SMR were small. At 0.5 canopy openness, predicted STAR of IL shoots was 6.1% larger than STAR of C shoots, but SMR of IL shoots was 10% smaller than that of C shoots. The results suggest that light-interception efficiency per unit needle area or mass of the shoots is not greatly affected by fertilization.

Translocation and Partitioning Patterns of 14C Photoassimilate from Light- and Shade-adapted Shoots in Greenhouse-grown `Chardonnay' Grapevines (Vitis vinifera L.)

Potted `Chardonnay' grapevines (Vitis vinifera L.) with either two or three shoots were grown in a greenhouse for one month and then transferred to a phytotron room, where either one or two shoots were shaded. Twenty-four days after transfer, leaves at the fifth node of either the light-adapted or shade-adapted shoot were exposed to a 2-hour pulse of 14CO2. Both light environment and number of shade shoots on the vine had a significant effect on photosynthate partitioning within the plant following a 22-hour chase. Leaves fed with 14CO2 on a light-adapted shoot translocated 26.1% and 12.7% more radioactivity to the roots and trunk, respectively, than leaves from shade-adapted shoots. Photosynthates were exported from light-adapted leaves to shade-adapted shoots (1.3% of total 14C in plant). The number of shaded shoots and the light environment of the fed leaf had a large effect on partitioning of photosynthates among ethanol-insoluble, water-soluble, and chloroform-soluble fractions within the leaf. Recovered 14C in the water-soluble fraction of the fed leaf appeared to be affected more by number of shoots than by light environment of the fed leaf. The data suggest that there is a sink effect on initial carbon partitioning patterns in grapevine leaves. Sink strength may have a greater role than light environment. A large proportion of interior leaves versus exterior leaves may be costly with respect to the carbohydrate budget of a vine.

Acclimation of Leaf Anatomy, Photosynthetic Light Use, and Xylem Hydraulics to Light inAmborella trichopoda(Amborellaceae)

Recent phylogenetic analyses place Amborella trichopoda, a semiclimbing premontane rain forest shrub endemic to New Caledonia, as sister to all other extant flowering plants. Here we present new observations on leaf anatomy and morphology, leaf photosynthetic physiology, and xylem hydraulic function for Amborella in the field. We focus on experimental measurements of the degree of photosynthetic and xylem hydraulic acclimation to sunlight by examining Amborella plants growing in deeply shaded understories (0.7% full sunlight) compared with individuals from exposed montane ridge crests in New Caledonia. Amborella leaves exhibit few anatomical differences between sun and shade. Instead of primarily adjusting leaf anatomical characters to irradiance, A. trichopoda leaves exposed periodically to full sunlight modify their orientation, becoming folded up along the midvein, presumably to avoid increases in light interception. Chlorophyll a fluorescence measurements demonstrate that the photosynthetic apparatus of A. trichopoda is shade adapted, based on a low capacity for excess light energy dissipation and low photosynthetic electron transport rate. In addition, these characteristics exhibit limited upregulation in response to increased irradiance. In parallel, there are small or no differences in leaf area‐specific hydraulic conductivity between sun and shade shoots of A. trichopoda. Taken together, A. trichopoda appears to possess limited developmental and physiological flexibility to light flux density. The bearing of these observations, in the context of other early‐diverging clades of angiosperms identified by recent phylogenetic studies, on the physiological ecology of the first angiosperms is discussed.

Shoot structure and photosynthetic efficiency along the light gradient in a Scots pine canopy.

We examined the effects of structural and physiological acclimation on the photosynthetic efficiency of Scots pine (Pinus sylvestris L.) shoots. We estimated daily light interception (DLI) and photosynthesis (DPHOT) of a number of sample shoots situated at different positions in the canopy. Photosynthetic efficiency (epsilon) was defined as the ratio of DPHOT to the potential daily light interception (DLI(ref)) defined as the photosynthetically active radiation (PAR) intercepted per unit area of a sphere at the shoot location. To calculate DLI(ref), DLI and DPHOT, the radiation field surrounding a shoot in the canopy was first modeled using simulated directional distributions of incoming PAR on a clear and an overcast day, and estimates of canopy gap fraction in different directions provided by hemispherical photographs. A model of shoot geometry and measured data on shoot structure and photosynthetic parameters were used to simulate the distribution of PAR irradiance on the needle surface area of the shoot. Photosynthetic efficiency (epsilon) was separated into light-interception efficiency (epsilon(I) = DLI/DLI(ref)) and conversion efficiency (epsilon(PHOT) = DPHOT/DLI). This allowed us to quantify separately the effect of structural acclimation on the efficiency of photosynthetic light capture (epsilon(l)), and the effect of physiological acclimation on conversion efficiency (epsilon(PHOT)). The value of epsilon increased from the top to the bottom of the canopy. The increase was largely explained by structural acclimation (higher epsilon(I)) of the shade shoots. The value of epsilon(PHOT) of shade foliage was similar to that of sun foliage. Given these efficiencies, the clear-day value of DPHOT for a sun shoot transferred to shade was only half that of a shade shoot at its original position. The method presented here provides a tool for quantitatively estimating the role of acclimation in total canopy photosynthesis.

Effects of in situ experimental shading on the Mediterranean seagrass Posidonia oceanica

The response of the Mediterranean seagrass Posidonia oceanica (L.) Delile to in situ light reduction was examined over a 4 mo period (May to August) in 1993 and 1995. Shoot density, leaf growth rate, shoot size, photosynthetic activity, carbohydrate reserves, epiphyte load and tissue nutrient concentrations were measured in replicated plots under the following light conditions: ambi- ent (36% of surface irradiance (SI) control plots), moderately shaded plots (16.7% SI, MS plots) and severely shaded plots (10.4% SI, SS plots). By the end of the experimental period, all shoots under SS treatments had died, while 70 to 80% had survived under MS treatment; in control plots, shoot den- sities had remained almost constant. Leaf growth rate and biomass decrease with shading were evi- dent 30 d after the beginning of the experiment; shoot density and number of leaves per shoot also fell with shading, but this was apparent only 60 to 90 d after the onset of the shading treatments. Epiphyte load was also considerably reduced by shading. Photosynthetic activity (Pmax) and dark res- piration rates (R) of control plants showed a characteristic increase from spring (May) to the end of summer (August), resulting in higher saturation (Isat) and compensation (Ic) points and lower photo- synthetic efficiencies (α ) at the end of the experimental period; however, photosynthetic parameters in shaded plants (MS treatment) first increased (as in control plots) from May to June, but decreased afterwards (from June to August), indicating photoacclimation of P. oceanica to low light conditions. Shading depressed carbohydrate storage in the rhizomes of shaded plants, in which starch concen- trations were quickly reduced to 60% of those of control plants. The leaves of shaded shoots showed a significant increase in nutrient (specially nitrogen) concentrations. This was probably due to translocation from shed leaves and also to low utilisation rates for growth. We infer that minimum light requirements of P. oceanica are between 16 and 10%, of surface irradiance, values which are consistent with light availability at the deep limits of nearby meadows. One year after shades had been removed, shoot densities remained unchanged (relative to the moment of screen removal), indi- cating both the absence of cascading effects and a very low recovery capacity of this seagrass.

Shoot growth responses to light microenvironment and correlative inhibition in tree seedlings under a forest canopy.

To examine the mechanisms underlying crown development, I investigated the dependence of shoot behavior on light microenvironment in saplings of the evergreen broad-leaved tree species, Litsea acuminata (Bl.) Kurata, growing on a forest floor. The local light environment of individual shoots (shoot irradiance) and plants (plant irradiance, defined as the shoot irradiance of the most sunlit shoot of a plant) were analyzed as factors affecting shoot behavior. Daughter shoots that developed under partially sunlit conditions were longer and less leafy than daughter shoots developed under shaded conditions. Shoot production increased with increasing shoot irradiance. Terminal shoots receiving 5% or less of full sunlight produced 0.67 daughter shoots on average, whereas shoots receiving 10% or more of full sunlight produced 1.72 daughter shoots. In terminal shoots receiving 5% or less of full sunlight, the probability of producing no daughter shoots was about 63% when other shoots on the plant received 10% or more of full sunlight, but was < 35% where the rest of the plant was also shaded. Shoot death was observed only in shoots receiving 5% or less of full sunlight. The mortality of shaded shoots was higher in plants growing in high irradiance than in plants growing in low irradiance. The ecological significance of correlative inhibition (the enhanced mortality and reduced production of new shaded shoots in the presence of partially-sunlit shoots) is discussed.

Response of Photosynthesis to Radiation and Intercellular CO&lt;sub&gt;2&lt;/sub&gt; Concentration in Sun and Shade Shoots of Norway Spruce

Functional differentiation of assimilation activity of sun versus shade foliage was analysed in a Norway spruce monoculture stand (age 15 years). The investigated stand density (leaf area index 8.6) and crown structure led to variation in the photosynthetically active photon flux density (PPFD) within the crowns of the sampled trees. At the saturating PPFD, the maximum rate of CO2 uptake (P Nmax) of exposed shoots (E-shoots) was 1.7 times that of the shaded shoots (S-shoots). The apparent quantum yield (α) of E-shoots was 0.9 times that of the S-shoots. A lower ability to use excess energy at high PPFD in photosynthesis was observed in the S-layer. The CO2- and PPFD-saturated rate of CO2 uptake (P Nsat) of the E-shoots was 1.12 times and the carboxylation efficiency (τ) 1.6 times that of the S-shoots. The CO2-saturated rate of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) carboxylation (VCmax) and of actual electron transport (Jamax) in the S-needles amounted to 89 and 95 % of VCmax and Jamax in the E-needles. Thus, in addition to the irradiation conditions and thus limitation by low Ja, the important limitation of photosynthesis in shade needles is due to carboxylation. This limitation of photosynthesis is accompanied by lower stomatal conductance.

Shoot structure, light interception, and distribution of nitrogen in an Abies amabilis canopy.

We studied the effects of variation in shoot structure and needle morphology on the distributions of light and nitrogen within a Pacific silver fir (Abies amabilis (Dougl.) Forbes) canopy. Specifically, we investigated the role of morphological shade acclimation in the determination of resource use efficiency, which is claimed to be optimal when the distribution of nitrogen within the canopy is directly proportional to the distribution of intercepted photosynthetically active radiation (PAR). Shoots were collected from different heights in the crowns of trees representing four different size classes. A new method was developed to estimate seasonal light interceptance (SLI, intercepted PAR per unit needle area) of the shoots using a model for the directional distribution of above-canopy PAR, measurements of shoot silhouette area and canopy gap fraction in different directions. The ratio SLI/SLI(o), where the reference value SLI(o) represents the seasonal light interceptance of a spherical surface at the shoot location, was used to quantify the efficiency of light capture by a shoot. The ratio SLI/SLI(o) doubled from the top to the bottom of the canopy, mainly as a result of smaller internal shading in shade shoots than in sun shoots. Increased light-capturing efficiency of shade shoots implies that the difference in intercepted light by sun shoots versus shade shoots is much less than the decrease in available light from the upper to the lower canopy. For example, SLI of the five most sunlit shoots was only about 20 times greater than the SLI of the five most shaded shoots, whereas SLI(o) was 40 times greater for sun shoots than for shade shoots. Nitrogen content per unit needle area was about three times higher in sun needles than in shade needles. This variation, however, was not enough to produce proportionality between the amounts of nitrogen and intercepted PAR throughout the canopy.