Sun Plants Research Articles

Physiological responses to drought and shade in two contrasting Picea asperata populations

The responses of photosynthetic gas exchange, chlorophyll fluorescence, activities of antioxidant enzymes and lipid membrane peroxidation of two contrasting Picea asperata Mast. populations to 30% of full sunlight (shade) and full sunlight (sun) were investigated under well‐watered and drought conditions. Two contrasting populations were from the wet and dry climate regions in China, respectively. For both populations tested, drought resulted in lower needle relative water content (RWC), CO2 assimilation rate (A), stomatal conductance (gs) and effective PSII quantum yield (Y), and higher non‐photochemical quenching (qN), superoxide dismutase (SOD), ascorbate peroxidase (APX) activities as well as malondialdehyde (MDA) levels and electrolyte leakage in sun plants, whereas these changes were not significant in shade plants. For the wet climate population, shade plants showed higher chlorophyll contents (Chla, Chlb and Chla + b) than sun plants under both well‐watered and drought conditions. Our study results implied that shade, applied together with drought, ameliorated the detrimental effects of drought. On the other hand, compared with the wet climate population, the dry climate population was more tolerant to drought in the sun treatment, as indicated by less decreases in A and mass‐based leaf nitrogen content (Nmass), more responsive stomata, greater capacity for non‐radiative dissipation of excitation energy as heat (analysed by qN), and higher level of antioxidant enzyme activities as well as lower MDA content and electrolyte leakage. These results demonstrated that the different physiological strategies were employed by the P. asperata populations from contrasting climate regions when the plants were exposed to drought and shade.

Stromal Over-reduction by High-light Stress as Measured by Decreases in P700 Oxidation by Far-red Light and its Physiological Relevance

The oxidation level of P700 induced by far-red light (DeltaA(FR)) in briefly dark-treated leaves of some sun plants decreased during the daytime and recovered at night. The dark recovery of decreased DeltaA(FR) proceeded slowly, with a half-time of about 5 h. We propose that stromal over-reduction induced by sunlight was the direct cause of the depression of DeltaA(FR). The depression of DeltaA(FR) found during the daytime was reproduced by controlled illumination with saturating light of fully dark-treated leaves. Simultaneous measurement of P700 redox and chlorophyll fluorescence showed that the depression of DeltaA(FR) was associated with dark reduction of the plastoquinone pool, which represented cyclic electron transport activity. The decrease of DeltaA(FR) in the light-stressed chloroplasts was partly reversed by treatment with 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, an inhibitor of electron transport at the cytochrome b6/f complex, and the subsequent addition of methyl viologen, an efficient electron acceptor from photosystem I (PSI), stimulated further recovery, showing that both cyclic electron flow around PSI and the charge recombination within PSI were responsible for the light-induced depression of DeltaA(FR). The dark level of blue-green fluorescence, an indicator of NAD(P)H concentration, from intact chloroplasts was increased by high-light stress, suggesting that NADPH accumulated in stroma as a result of the high-light treatment. Possible effects on photosynthetic activity of over-reduction and its physiological relevance are discussed.

Ecophysiology of selected tree species in different plant communities at the periphery of the Atlantic Forest of SE—Brazil III. Three legume trees in a semi-deciduous dry forest

Three legume tree species (Fabaceae) occurring abundantly in a semi-deciduous tropical dry forest of the Atlantic forest complex in southeastern Brazil were subjected to a comparative ecophysiological study at the end of the dry season/beginning of the wet season. The trees chosen were morphologically very similar: Caesalpinia echinata Lam. and Caesalpinia ferrea Mart. ex. Tul., both 10–20 m of height, of the sub-family Caesalpinioideae, and the somewhat smaller, 2–4 m tall, Machaerium obovatum Kuhlm. & Hoehne of the sub-family Faboideae. Despite their similarities with respect to their geographic distribution restricted to Brazilian dry forests, their comparable abundance in the study site and their phylogenetic proximity, the three species display distinctly different ecophysiological behaviour. Compared to the other two species, C. ferrea had the highest photosynthetic capacity (maximum apparent photosynthetic electron transport rate, ETRmax) and higher saturation light-intensity, was less subject to photoinhibition as indicated by potential quantum yield of photosystem II (F v/F m) and had the lowest bulk N content of which soluble non-protein N compounds were only 1.5%. It showed stronger sun plant characteristics. C. echinata had lower photosynthetic capacity, was under chronic photoinhibition and had high bulk N content of which 6.1% were soluble N compounds with high concentrations of proline. In addition to proline, high concentrations of sugars may serve as osmoprotectants. M. obovatum also showed lower photosynthetic capacity and was under chronic photoinhibition. Here, arginine may have a function as osmoprotectant. The ecophysiological differences between the three species are not related to local abundance. However, the observations presented highlight a contrasting behaviour of the otherwise very similar compatriot species.

The Effect of Light Regime on Source–Sink Relations in the Shade-Enduring Ajuga reptans Plant

A quantitative approach to the evaluation of source–sink relations in Ajuga reptans plants grown under the forest canopy (shade plants) and on an open plot (sun plants) was worked out in terms of growth characteristics, CO2 exchange, and carbon balance. Shade plants developed leaves with the relative and specific areas twice exceeding those of sun plants. Sun plants assimilated more carbon, using a significant part of it for the development of numerous runners. During a day, shade and sun plants produced 0.03 and 0.67 g of substrate, respectively. At the same time, forest (shade) plants spent 48% of assimilates for the respiration, in comparison with plants from the open plot that spent almost 70% of assimilates for respiration as they were greater in size. It was concluded, that light controls source–sink relations, which is a way of realization of the life strategy and a coordination mechanism of functional integrity of the plant organism. Light not only controls photosynthesis (source activity) but morphophysiological characteristics of plants with their hierarchical structure of sinks too.

Are bryophytes shade plants? Photosynthetic light responses and proportions of chlorophyll a, chlorophyll b and total carotenoids.

Data are presented from 39 species of mosses and 16 liverworts for ratios of chlorophylls and total carotenoids, and light saturation of photosynthetic electron flow or photosynthetic CO2 uptake, in relation to the postulate that bryophyte cells in general show shade-plant characteristics. Pigment concentrations were measured by spectrophotometer in 80 % acetone extracts. Light-saturation curves were constructed by (modulated) chlorophyll florescence and for some species by infra-red gas analysis. The pigment measurements were widely variable but broadly in line with the findings of previous authors. Median values (mosses/liverworts) were: total chlorophyll, 1.64/3.76 mg g(-1); chlorophyll a : b, 2.29/1.99; chlorophylls : carotenoids, 4.74/6.75). The PPFD values at 95 % saturation (estimated from fitted curves) also ranged widely, but were almost all <1000 micromol m(-2) s(-1); the median for mosses was 583 and for liverworts 214 micromol m(-2) s(-1). The two highest PPFD95% values were from Polytrichum species with lamella systems forming a ventilated photosynthetic tissue. Total chlorophyll, chlorophyll a : b and chlorophylls : carotenoids all correlated significantly with PPFD95%. Bryophytes include but are not inherently shade plants. Light-saturation levels for species of open sun-exposed habitats are lower than for vascular sun plants and are probably limited by CO2 diffusion into unistratose leaves; this limit can only be exceeded by bryophytes with ventilated photosynthetic tissues which provide increased area for CO2 uptake.

Plasticity, instability and canalization: is the phenotypic variation in seedlings of sclerophyll oaks consistent with the environmental unpredictability of Mediterranean ecosystems?

• Evergreen oaks from the Mediterranean basin exhibit a conservative resource-use strategy based on a reduced expression of phenotypic variation (i.e. canalization). We hypothesized that genetic variation across closely related species is more canalized than the response to environmental variation. • Seedlings of Quercus ilex and Q. coccifera, two important oak species from the Mediterranean basin that belong to the same subgenus and section, were grown in contrasted light and nutrient environments following a factorial design. Phenotypic variation was explored in a total of 75 variables including photosynthetic capacity, nutrient allocation, allometric relationships and crown architecture. • Path analysis showed that phenotypic variation was not significantly affected by differences between species but by those between and within environments, which are argued to be primarily linked to phenotypic plasticity and developmental instability, respectively. This finding is interpreted as evidence of genetic canalization across species. • The similar importance of plasticity and instability as sources of phenotypic variation and the high degree of genetic canalization are consistent with the expected role of the environmental unpredictability of Mediterranean ecosystems in shaping the developmental patterns of these two species.

Drought can be more critical in the shade than in the sun: a field study of carbon gain and photo‐inhibition in a Californian shrub during a dry El Niño year

AbstractDiurnal courses of leaf water potential (Ψl), gas exchange and chlorophyll fluorescence were measured in natural sun and shade populations of Heteromeles arbutifolia throughout the seasons of an unusually dry El Niño year in Central California. The onset of drought resulted in decreased stomatal conductance and net photosynthesis in both sun and shade plants. However, the decline in Ψl was much greater and carbon gain was much more strongly limited by the development of drought stress in the shade than in the sun. Photorespiratory energy dissipation was significantly higher in the sun than in the shade in spring and autumn, but not during the summer. Pre‐dawn photochemical efficiency (Fv/Fm) was significantly higher in the shade than in the sun during the spring but the differences disappeared during the summer and autumn. The strong irradiance in the open field site studied led to a chronic but only mild reduction in Fv/Fm, with values around 0·79. Summer sunflecks led to a sustained photo‐inhibition in shade plants, which exhibited a significant reduction in pre‐dawn Fv/Fm of 10% with the onset of drought. Photo‐inhibition became relatively more important for carbon gain in the shade than in the sun due to the low photochemical efficiency under the low light that follows sunflecks. Sun plants of H. arbutifolia exhibited a rather efficient photoprotection against strong irradiance conferred by both the architecture of the crown and the physiology of the leaves. There is evidence that El Niño events and the associated droughts have become more frequent and severe. Counter‐intuitively, the effects on plant performance of such extreme droughts could be more critical in the shade than in the sun.

Ecophysiological consequences of contrasting microenvironments on the desiccation tolerant moss Tortula ruralis.

Tortula ruralis is a homoiochlorophyllous-desiccation-tolerant (HDT) moss that retains all pigments when dehydrated and rapidly recovers physiological function upon rehydration. This moss forms extensive cover in exposed and shaded areas in the sandy semi-arid grasslands of Central Europe. We hypothesized that contrasting drying regimes between these microhabitats would affect plant N status, constraints to gas exchange and growth, as well as result in altered pigment concentrations and ratios, and photochemical light-response dynamics. Furthermore, we believed T. ruralis's HDT habit would limit its ability to acclimate to altered light environment. We found that sun plant T. ruralis had lower plant mass, as well as lower tissue N, C, total photosynthetic pigment concentrations and carbon isotope discrimination (Δ) values compared to shade plant counterparts. Carotenoid/chlorophyll ratios in sun plants were typical of high light-adapted tissue, but chlorophyll a/chlorophyll b ratios were lower, more characteristic of low light-adapted tissue. This unique combination of pigment responses was accompanied by sustained lower levels of optimal quantum efficiency of PSII (F v/F m) in sun plant T. ruralis, even during favorable diurnal conditions, and reduced engagement of energy-dependent thermal dissipation (NPQ). Reciprocal transplants of sun and shade plants showed that T. ruralis is capable of short-term adjustment to altered light level, as evidenced by increases in F v/F m, NPQ, and light-adapted PSII yield (φPSII) in transplanted sun plants, and concurrent decreases in sun-transplanted shade plants. However, the performance of transplanted sun plants remained consistently below that of undisturbed shade plants. These findings show that microenvironmental variation results in different patterns of resource acquisition in this HDT moss, and that growth in the open imparts greater desiccation tolerance, and the development of a greater standing engagement of slowly reversing photoprotective mechanisms. In contrast, prolonged activity and greater resource acquisition in shaded populations may allow T. ruralis to rapidly adjust to changes following disturbance to the plant canopy, fostering the persistence of T. ruralis in these semi-arid grasslands.

Characteristics of gas exchange and morphology of a spring ephemeral, Erythronium japonicum , in comparison with a sun plant, Glycine max

Morphological characteristics and responses of gas exchanges to light intensity were examined in a typical vernal species, Erythronium japonicum Decne (E. japonicum), grown (i) on the floor of a deciduous broad-leaved Quercus mongolica forest (one of its native habitats, the Q. mongolica stand); (ii) bare land left undisturbed for 9 years after forest clearing (the bare stand); and (iii) in a sun crop, soybean, grown for 110 days in an experimental field and for 17 days in pots, in order to evaluate the adaptability of the photosynthetic process of this vernal species to its shady native habitats. The daytime solar radiation, ai and leaf temperatures and leaf–air vapor pressure difference (VPD) were significantly higher at the bare stand than at the Q. mongolica stand. When environmental factors observed at the Q. mongolica and bare stands were reproduced in an assimilation chamber, leaf temperatures of E. japonicum plants increased markedly with increased radiation, whereas those of soybean plants differed little from the respective air temperatures. The photosynthetic and transpiration rates and stomatal conductance in the former plants placed under conditions at the Q. mongolica stand increased with radiation and reached respective steady state values at maximum radiation at the site; whereas, under the conditions at the bare stand, they also increased and reached respective steady state values, but then continuously decreased to be lower than the respective value at the Q. mongolica stand. However, both rates and the conductance in the soybean plants under both conditions increased significantly with radiation and reached much higher respective values at the respective maximum radiations. Water use efficiency for E. japonicum plants was much higher under conditions at the Q. mongolica stand than at the bare stand and was practically equal to those for soybean plants under both conditions. Water potential in the leaves of E. japonicum at maximum radiation at the bare stand was one-third that of those at the Q. mongolica stand. The potential in soybean leaves differed little between both conditions and was roughly equal to the low value in E. japonicum leaves at the bare stand conditions. The stomatal densities on upper and lower leaf surfaces and the ratio of root weight to leaf area (R : L) differed little between E. japonicum plants grown at both stands as well as between young and adult soybean plants. However, the densities on the upper and lower surfaces of E. japonicum were 25% and 60% of the respective values of both soybean plants. The ratios of R : L of the E. japonicum plants were only one-quarter that of the young and adult soybean plants. The cooperation between these morphological and gas exchange characteristics in E. japonicum plants is discussed in relation to adaptation to the environment in native habitats.

Photosynthesis, Chlorophyll Fluorescence and Within‐Canopy Distribution of Epiphytic Ferns in a Mexican Cloud Forest

Abstract: Parameters associated with the photosynthetic performance of eight common epiphytic ferns in a Mexican cloud forest were investigated in relation to the distribution of these species within the canopy. If the substantial microclimatic gradients within tropical forest canopies provide microhabitats exploited by different epiphytic species, we would expect to find correlations between distribution and physiological traits. Maximum rates of CO2 uptake (Amax) and photon flux densities at light compensation points (LCP) were in the range of shade plants (Amax = 0.6 ‐ 5.2 μmol m‐2 s‐1; LCP = 4 ‐ 6.5 μmol m‐2 s‐1), but saturation light intensities were more typical for sun plants (270 ‐ 550 μmol m‐2 s‐1). Amax and nitrogen content per unit dry weight were correlated with the distribution of the species within the canopy, but LCP, apparent quantum yield and dark respiration were not. When leaves were left to desiccate, the fluorescence yield of dark‐adapted leaves (Y0) remained high until the relative water content (RWC) had dropped below 30 to 20 %. Fluorescence after short illumination with 200 μmol m‐2 s‐1 declined when RWC dropped below 70 to 40 %. After exposure to full sunlight for 1 h, Y0 of species growing in the outer canopy (Pleopeltis mexicana and Polypodium plebeium) and a plant characteristic of the mid‐canopy (Elaphoglossum petiolatum) recovered better than in species from shadier locations (Trichomanes bucinatum, Asplenium cuspidatum, Phlebodium areolatum). With the exception of Ph. areolatum and a species growing at both exposed and shaded sites (Polypodium puberulum), Y0 recovered at least partially after a loss of 80 ‐ 96 % of saturation water, with the humidity‐loving filmy fern (T. bucinatum) showing no signs of permanent damage at all. The results suggest that tolerance or avoidance of desiccation and high light may be at least as important in controlling the distribution of the species studied as photosynthetic performance without stress.

Why do species of woody seedlings change rank in relative growth rate between low and high irradiance?

It is often assumed that if a plant species has a higher relative growth rate (RGR) than another species in deep shade, it will have a lower RGR at high irradiance (Spurr & Barnes 1980; Thomas & Bazzaz 1999; Walter 1973). In other words, species change rank (crossover) between low and high irradiance. This idea was suggested chiefly by the finding that in deep shade the massbased net photosynthetic rates of the leaves of shade plants exceed those of sun plants, while at high irradiance the reverse is true (Bjdrkman & Holmgren 1963; Boardman 1977; Givnish 1988). This finding has been assumed to scale up to the level of whole-plant RGR (Shugart 1984). In the past decade, however, a contrary view concerning RGRs has emerged, the idea that if a plant grows faster than another at high irradiance it will also do so in the shade (Kitajima 1994; Poorter 1999). According to this later view, light plays a role in maintaining the mixture of forest species only through the well established trade-off between survival rate in deep shade and RGR in bright light (Kitajima 1994, 1996). Similar experimental studies of RGR responses to irradiance for woody seedlings report surprisingly different results. Here we indicate why such disparate results have been produced. We provide a simple analytical approach to understanding why crossovers should occur among particular species at particular stages of ontogeny. This approach is useful for understanding the maintenance of forest species richness, as well as for interpreting plant specialization in physiology and morphology to contrasting irradiance regimes.

Acclimation of Myrtus communis to contrasting Mediterranean light environments — effects on structure and chemical composition of foliage and plant water relations

Leaf anatomical and chemical characteristics, water relations and stomatal regulation were studied in the shrub Myrtus communis growing under two contrasting Mediterranean light environments (full light versus 30% of full light) during the spring–summer period. These studies aimed to assess plant response to the combined effects of light and water availability. Foliar morphology, anatomy and chemistry composition acclimated positively to light conditions. Leaves of sun-exposed plants were thicker (38.7%) than those of shaded plants, mainly due to increased palisade parenchyma thickness, had a higher nitrogen concentration and stomatal density than the shade ones, which maximized foliar area (>SLA) and Chl/N molar ratio to improve light interception. Chlorophyll concentration per leaf area (Chl a) was always higher in sun leaves while, as expressed on dry mass (Chl m), significant differences were only apparent in September, shade leaves presenting higher values. During the summer period Chl a and Chl m markedly declined in sun leaves and remained unchanged in shade ones. The ratio of chlorophyll a/b was not affected either by the light intensity or by the season. Shade leaves presented generally a higher concentration of soluble carbohydrates per dry mass. No significant differences in starch concentration were apparent between sun and shade leaves and a gradual depletion occurred during the water stress period. Maximum stomatal conductances correlated positively with predawn water potential. Throughout the season, sun plants always presented higher leaf conductance to water vapour and lower minimum leaf water potentials, indicating an interaction of light–environment on these water relation parameters. Stomatal closure constitutes a mechanism to cope with diurnal and seasonal water deficits, sun plants presenting a more efficient control of water losses during water deficiency period. In addition, both sun and shade plants evidenced leaf osmotic adjustment ability in response to water stress, which was greater in sun ones.

Light‐dependent changes in biomass allocation and their importance for growth of rain forest tree species

Summary Sapling growth of six rain forest tree species was compared to evaluate whether species respond in a similar way to a natural light gradient. Saplings were measured non‐destructively; production and loss of leaves, stem and branches were analysed in detail. Sapling height growth was positively related to light environment and leaf area. No single descriptor of light environment explained sapling growth best. Direct or diffuse light could explain plant growth, depending on species. Seventeen percent of the saplings had negative relative biomass growth rates, although they occurred in fairly bright conditions. Negative growth rates were caused by leaf shedding and stem breakage. Sapling relative growth rate increased with irradiance, mainly because of an increase in net assimilation rate. On a shoot basis, shaded plants had a smaller leaf mass fraction (LMF) and a larger specific leaf area, resulting in similar leaf area ratios (LAR) to those of sun plants. This contrasts with the results of seedling studies under controlled conditions, where LMF and LAR increased with shade. Biomass partitioning to leaf growth decreased with irradiance and relative growth rate of the sapling. This leaf partitioning ratio was better correlated with RGR than with irradiance. Species differed in the effect of light‐dependent changes in specific leaf area (SLA) on growth. This underscores the importance of SLA in explaining differences in species performance in a forest environment. Nevertheless, the effect of SLA was not related to the shade tolerance of the species.

Influence of light regimes on respiration, activity of alternative respiratory pathway and carbohydrates content in mature leaves of Ajuga reptans L.

Influence of different light regimes (plants grown in shade and in the sun) on respiration, activity of alternative respiratory pathway and carbohydrates content in mature leaves of Ajuga reptans L. was investigated. All experiments were carried out under natural conditions. Sun plants had higher respiration, activity of the alternative pathway and carbohydrates content compared to the shade plants. Additional experiments were performed in order to study effects of darkening as well as evolution of respiration and carbohydrates concentrations in leaves of A. reptans during the day. Darkening led to a decline in total respiration in all types of plants investigated. Activity of the alternative pathway remained unchanged by 20 % in the leaves of the shade plants. On the other hand, darkness led to a twofold decrease in the activity of the alternative pathway in the leaves of the sun plants. Content of soluble carbohydrates in the darkness did not change in the leaves of the shade plants while significantly decreased in the leaves of the sun plants. Total respiration in the mature leaves of the shade plants studied did not change significantly during the day. However, a significant increase in the total respiration was detected in the mature leaves of the sun plants in the evening and in the night. Activity of the alternative respiratory pathway did not change significantly in the leaves of the shade plants during the day and corresponds to ca 20 % of the total respiration. The sun plants examined exhibited significantly higher activity of the alternative pathway during the period of active photosynthesis. In general, activity of the alternative pathway varied in the sun plants between 30 and 35 % of the total respiration during the day. Content of soluble carbohydrates decreased significantly during the day in the mature leaves of the shade plants, while no statistically significant changes by the sun plants could be detected. It has been suggested that increase in respiration and activity of the alternative pathway in the sun plants tested can be controlled by the content of carbohydrates in leaves. No clear proposal could be done concerning mechanisms regulating alternative pathway activity in the shade plants of A. reptans.

Consequences of CO2 and light interactions for leaf phenology, growth, and senescence in Quercus rubra

SummaryWe investigated how light and CO2 levels interact to influence growth, phenology, and the physiological processes involved in leaf senescence in red oak (Quercus rubra) seedlings. We grew plants in high and low light and in elevated and ambient CO2. At the end of three years of growth, shade plants showed greater biomass enhancement under elevated CO2 than sun plants. We attribute this difference to an increase in leaf area ratio (LAR) in shade plants relative to sun plants, as well as to an ontogenetic effect: as plants increased in size, the LAR declined concomitant with a decline in biomass enhancement under elevated CO2Elevated CO2 prolonged the carbon gain capacity of shade‐grown plants during autumnal senescence, thus increasing their functional leaf lifespan. The prolongation of carbon assimilation, however, did not account for the increased growth enhancement in shade plants under elevated CO2. Elevated CO2 did not significantly alter leaf phenology. Nitrogen concentrations in both green and senesced leaves were lower under elevated CO2 and declined more rapidly in sun leaves than in shade leaves. Similar to nitrogen concentration, the initial slope of A/Ci curves indicated that Rubisco activity declined more rapidly in sun plants than in shade plants, particularly under elevated CO2. Absolute levels of chlorophyll were affected by the interaction of CO2 and light, and chlorophyll content declined to a minimal level in sun plants sooner than in shade plants. These declines in N concentration, in the initial slope of A/Ci curves, and in chlorophyll content were consistent with declining photosynthesis, such that elevated CO2 accelerated senescence in sun plants and prolonged leaf function in shade plants. These results have implications for the carbon economy of seedlings and the regeneration of red oak under global change conditions.

Flavonoids accumulate in leaves and glandular trichomes of Phillyrea latifolia exposed to excess solar radiation.

Experiments were conducted on Phillyrea latifolia plants grown under a dense overstorey of Pinus pinea (shade plants) or on seashore dunes (sun plants) in a coastal area of Tuscany (42° 46' N, 10° 53' E). Total integrated photon flux densities averaged 1.67 and 61.4 m mol m-2 d-1 for shade and sun sites, respectively. A leaf morphological-structural analysis, a qualitative and quantitative analysis of phenylpropanoids of leaf tissue and leaf surface, and a histochemical localization of flavonoids were conducted. The area of sun leaves reached 57% of that of shade leaves, whereas leaf angle (β), sclerophylly index (ratio of leaf d. wt:leaf area), and trichome frequency (trichome number mm-2 ) were markedly greater in leaves exposed to full solar radiation than in leaves acclimated to shade. The total thickness of sun leaves was 78% higher than that of shade leaves, mostly owing to a greater development of both palisade parenchyma and spongy mesophyll. The concentration, but not the composition, of leaf tissue phenylpropanoids varied significantly between sun and shade leaves, with a marked increase in flavonoid glycosides in sun leaves. Flavonoids occurred almost exclusively in the upper epidermal cells of shade leaves. By contrast, flavonoids largely accumulated in the upper and lower epidermis, as well as in the mesophyll tissue of leaves that were acclimated to full sunlight. Flavonoid glycosides were found exclusively in the secretory products of glandular trichomes of P. latifolia leaves exposed to high levels of light; luteolin 7-O- glucoside and quercetin 3-O-rutinoside were the major constituents. By contrast, verbascoside and an unidentified caffeic acid derivative constituted 72% of total phenylpropanoids secreted by glandular trichomes of shade leaves, whereas they were not detected in glandular trichomes of sun leaves. These findings suggest that the light-induced synthesis of flavonoids in glandular trichomes of P. latifolia probably occurs in situ and concomitantly inactivates other branch pathways of the general phenylpropanoid metabolism. This is the first report of the key role of glandular trichomes and of flavonoid glycosides in the integrated mechanisms of acclimation of P. latifolia to excess light.

Acclimation Potential to High Irradiance of Two Cultivars of Watermelon

The acclimation potential to high irradiance of two cultivars of watermelon, Reina de Corazones and Toro, calculated as the ratio of sun vs. shade activities of O2- and H2O2 scavenging enzymes and non-radiative energy dissipation, was similar. However, Reina de Corazones exhibited a higher capacity in absolute terms for photoprotection (harmless dissipation of absorbed light energy at PS 2 and ascorbate and O2- and H2O2 scavenging enzymes) suggesting a larger resistance of this cultivar to high irradiance. This could be seen as smaller decreases in fruit productivity and in lower oxidative injury as probed by malondialdehyde content in sun plants of Reina de Corazones than in Toro plants. Additionally, the results show that shading might be beneficial to both cultivars, presumably because it reduces the susceptibility of high irradiance-induced stress.

Growth, biomass allocation and photosynthesis of Rolandra fruticosa (asteraceae) in response to shade

The effects of shade on growth, biomass allocation patterns and photosynthetic response was examined for Rolandra fruticosa (L.) Kuntze, a common perennial weed shrub in cultivated pastures and agricultural areas of Brazilian Amazonia, for plants grown in full sunlight and those shaded to 30 % of full sunlight over a 34-d period. Specific leaf area and leaf area ratio were higher for shade plants during all the experimental period. Shade plants allocated significantly less biomass to root tissue than sun plants and relative growth rate was higher in sun plants. Sun leaves had significantly higher dark respiration and light saturated rates of photosynthesis than shade leaves. The apparent quantum efficiency was higher for shade leaves, while light compensation point was higher for sun leaves. These results are discussed in relation to their ecological and weed management implications.

Light environment, sapling architecture, and leaf display in six rain forest tree species

Architecture and leaf display were compared in saplings of six rain forest tree species differing in shade tolerance. Saplings were selected along the whole light range encountered in a forest environment. Species differed largely in realized height and crown expansion per unit support biomass, but this could not be related to differences in shade tolerance. The results demonstrate that there exist various solutions to an effective expansion of plant height and crown area. It is argued that choice of the study species and the ontogenetic trajectory regarded determine to a large extent the outcome of interspecific comparisons. No evidence was found that pioneers were characterized by a multilayered and shade tolerants by a monolayered leaf distribution. Yet, sun plants had a similar crown area, a deeper crown, and a higher leaf area index compared to shade plants and their leaves were more evenly distributed along the stem. This suggests that differences in leaf layering are found between plants growing in different light environments, rather than between species differing in shade tolerance.

ASYMPTOTIC HEIGHT AS A PREDICTOR OF PHOTOSYNTHETIC CHARACTERISTICS IN MALAYSIAN RAIN FOREST TREES

Comparative studies of photosynthetic physiology in rain forest trees have focused on differences related to successional status, with the general finding that early-successional species tend to show physiological characteristics of sun plants, while late-successional species show shade-plant characteristics. The present study examines analogous evolutionary responses to vertical gradients in light availability, through an analysis of patterns of photosynthetic variation among late-successional tropical tree species that differ in adult stature. Larger statured tree species are expected to have higher values for light-saturated photosynthetic rate (Amax) as adults, due to the inevitable gradient in light availability through the canopy. However, we argue that larger statured species should also show a higher Amax, and other “sun-plant” characteristics, as saplings under relatively uniform low light conditions in the forest understory. This prediction follows if the potential for photosynthetic acclimation is finite, and if developmental processes that determine adult-phase physiology also affect the physiology of sapling leaves. We examined relationships between photosynthetic parameters and tree species’ stature using comparative data on 28 late-successional species at Pasoh Forest Reserve, West Malaysia. Species chosen for study represent four genera that each include taxa ranging in size from understory treelets to canopy-level trees, thus enabling “phylogenetically corrected” analyses and stronger inference that observed patterns reflect evolutionary convergence. Amax of understory saplings, as measured on a leaf area, mass, or nitrogen basis, was positively correlated with asymptotic height (Hmax) reached by mature trees of a given species. These relationships were similar in each of the four main study genera, thus supporting the hypothesis of an evolutionary response in photosynthetic characteristics to the vertical gradient in light availability through the canopy. Understory species also commonly exhibited higher leaf-level photosynthetic rates at low light levels than did canopy species within a given genus; however, such “crosses” in photosynthetic light response curves were only pronounced when photosynthesis was expressed on a leaf mass basis. Midcanopy leaves from adult trees displayed Amax(area) values similar to leaves from understory saplings of a given species, while Amax(mass) values for adult trees were lower than those of saplings. This pattern corresponded to lower values for specific leaf area in adult trees than in saplings, a difference that was systematically greater in larger statured species. In sum, a range of both adult tree and sapling physiological parameters, including photosynthetic capacity, light saturation point, and leaf nitrogen content, may be predicted as a function of asymptotic species height. Previous research on vertical gradients in photosynthetic characteristics of forest trees has focused on proximate mechanisms, such as light acclimation responses and nutrient reallocation within individual tree canopies. The present study documents evolved differences among species that also contribute to the overall pattern of photosynthetic variation within forest canopies. Our results suggest that much of the variation in leaf-level physiology among late-successional tropical trees is related to an evolved sun–shade trade-off that corresponds to differences in size among species.