Chlorophyll and light gradients in sun and shade leaves of Spinacia oleracea

Abstract. Light gradients were measured and correlated with chlorophyll concentration and anatomy of leaves in spinach (Spinacia oleracea L.). Light gradients were measured at 450, 550 and 680 nm within thin (455 μm) and thick (630 μm) leaves of spinach grown under sun and shade conditions. The light gradients were relatively steep in both types of leaves and 90% of the light at 450 and 680 nm was absorbed by the initial 140 μm of the palisade. In general, blue light was depleted faster than red light which, in turn was depleted faster than green light. Light penetrated further into the thicker palisade of sun leaves in comparison to the shade leaves. The distance that blue light at 450 nm travelled before it became 90% depleted was 120 μm in sun leaves versus 76 μm in shade leaves. Red light at 680 nm and green light at 550 nm travelled further but the trends were similar to that measured at 450nm. The steeper light gradients within the palisade-of shade leaves were caused by increased scattering of light within the intercellular air spaces and/or cells which were less compact than those in sun leaves. The decline in the amount of light within the leaf appeared to be balanced by a gradient in chlorophyll concentration measured in paradermal sections. Progressing from the adaxial epidermis, chlorophyll content increased through the palisade and then declined through the spongy mesophyll. Chlorophyll content was similar in the palisade of both sun and shade leaves. Chloroplast distribution within both sun and shade leaves was relatively uniform so that the chlorophyll gradient appeared to be caused by greater amounts of chlorophyll within chloroplasts located deeper within the leaf. These results indicate that the anatomy of the palisade may be of special importance for controlling the penetration of photo-synthetically active radiation into the leaf. Changing the structural characteristics of individual palisade cells or their arrangement may be an adaptation that maximizes the absorption of light in leaves with varying mesophyll thickness due to different ambient light regimes.     

Coupled response of stomatal and mesophyll conductance to light enhances photosynthesis of shade leaves under sunflecks

Light gradients within tree canopies play a major role in the distribution of plant resources that define the photosynthetic capacity of sun and shade leaves. However, the biochemical and diffusional constraints on gas exchange in sun and shade leaves in response to light remain poorly quantified, but critical for predicting canopy carbon and water exchange. To investigate the CO₂ diffusion pathway of sun and shade leaves, leaf gas exchange was coupled with concurrent measurements of carbon isotope discrimination to measure net leaf photosynthesis (A_n), stomatal conductance (g_s) and mesophyll conductance (g_m) in Eucalyptus tereticornis trees grown in climate controlled whole‐tree chambers. Compared to sun leaves, shade leaves had lower A_n, g_m, leaf nitrogen and photosynthetic capacity (A_max) but g_s was similar. When light intensity was temporarily increased for shade leaves to match that of sun leaves, both g_s and g_m increased, and A_n increased to values greater than sun leaves. We show that dynamic physiological responses of shade leaves to altered light environments have implications for up‐scaling leaf level measurements and predicting whole canopy carbon gain. Despite exhibiting reduced photosynthetic capacity, the rapid up‐regulation of g_m with increased light enables shade leaves to respond quickly to sunflecks.     

Variability in leaf optical properties among 26 species from a broad range of habitats

Leaves from 26 species with growth forms from annual herbs to trees were collected from open, intermediate, and shaded understory habitats in Mississippi and Kansas, USA. Leaf optical properties including reflectance, transmittance, and absorptance in visible and near infrared (NIR) wavelengths were measured along with leaf thickness and specific leaf mass (SLM). These leaf properties and internal light scattering have been reported to vary with light availability in studies that have focused on a limited number of species. Our objective was to determine whether these patterns in leaf optics and light availability were consistent when a greater number of species were evaluated. Leaf thickness and SLM varied by tenfold among species sampled, but within-habitat variance was high. Although there was a strong trend toward thicker leaves in open habitats, only SLM was significantly greater in open vs. understory habitats. In contrast, leaf optical properties were strikingly similar among habitats. Reflectance and reflectance/transmittance in the NIR were used to estimate internal light scattering and there were strong relationships (r1 > 0.65) between these optical properties and leaf thickness. We concluded that leaf thickness, which did not vary consistently among habitats, was the best predictor of NIR reflectance and internal light scattering. However, because carbon allocation to leaves was lower in understory species (low SLM) yet gross optical properties were similar among all habitats, the energy investment by shade leaves required to achieve optical equivalence with sun leaves was lower. Differences in leaf longevity and growth form within a habitat may help explain the lack of consistent patterns in leaf optics as the number of species sampled increases.     

LEAF OPTICAL PROPERTIES OF RAINFOREST SUN AND EXTREME SHADE PLANTS

The optical properties of the leaves of twelve tropical sun species and thirteen tropical extreme shade species were examined with an integrating sphere attached to a spectroradiometer. Measurements of diffuse reflectance and transmittance allowed calculations of absorptance, 350–1,100 nm. Although some shade species absorbed higher percentages of quantum flux densities for photosynthesis (400–700 nm, PPFD) than the mean for the sun species, the sun and shade species as groups were not significantly different from each other: 90.2, S.D. 3.6% for shade species and 88.6, S.D. 2.4% for the sun species. The groups of species did not differ in total absorptance of energy 350–1,100 nm. Furthermore, the sun and shade species were identical in their shift of absorptance at wavelengths between 650 and 750 nm. The anthocyanic coloration of the leaf undersurfaces of two species polymorphic for this characteristic (Trionela hirsuta and Ischnosciphon pruinosus) is correlated with increased absorptance at the upper end of the action spectrum of photosynthesis. Although sun and shade species have similar optical properties, the energy investment (as documented by dry wt per unit area of leaf surface) is much less for the shade species.     

Functional consequences of stenophylly for leaf productivity: comparison of the anatomy and physiology of a rheophyte, Farfugium japonicum var. luchuence, and a related non-rheophyte, F. japonicum (Asteraceae)

We investigated the anatomical and physiological characteristics of stenophyllous leaves of a rheophyte, Farfugium japonicum var. luchuence, and sun and shade leaves of a non-rheophyte, F. japonicum, comparing three different populations from coastal, forest floor, and riparian habitats. Light adaptation resulted in smaller leaves, and riparian adaptation resulted in narrower leaves (stenophylly). The light-saturated rate of photosynthesis (P _max) per unit leaf area corresponded to the light availability of the habitat. Irrespective of leaf size, the P _max per unit leaf mass was similar for sun and shade leaves. However, the P _max per mass of stenophyllous leaves was significantly lower than that of sun and shade leaves. This was because the number and size of mesophyll cells were greater than that required for intercellular CO₂ diffusion, which resulted in a larger leaf mass per unit leaf area. Higher cell density increases contact between mesophyll cells and enhances leaf toughness. Stenophyllous leaves of the rheophyte are frequently exposed to a strong water flow when the water level rises, suggesting a mechanical constraint caused by physical stress.     

The functional significance of palisade tissue: penetration of directional versus diffuse light

Light gradients were measured in leaves that had different types of anatomical development of the mesophyll but similar pigment content. Leaves of the legume, Thermopsis montana, had columnar palisade and spongy mesophyll whereas leaves of the monocot, Smilacina stellata, had spongy mesophyll only. Light gradients were measured at 550 nm in both types of leaves when they were irradiated with collimated or diffuse light. When irradiated with collimated light, light gradients were steeper in leaves with spongy mesophyll in comparison to those that had palisade tissue. On the other hand, light gradients were similar between both leaf types when they were irradiated with diffuse light. Thus, columnar palisade cells facilitated the penetration of collimated light over diffuse light. These results suggest that palisade tissue may help distribute light more uniformly to chloroplasts within the leaf. Moreover, the functional significance of palisade tissue may be related to the amount of collimated light within the natural environment.     

Photosynthetic symmetry of sun and shade leaves of different orientations

Summary The photosynthetic responses to light of leaves irradiated on the adaxial or abaxial surfaces, were measured for plants with contrasting leaf orientations. For vertical-leaf species of open habitats (Eryngium yuccifolium and Silphium terebinthinaceum), photosynthetic rates were identical when irradiated on either surface. However, for horizontal-leaf species of open habitats (Ambrosia trifida and Solidago canadensis), light-saturated rates of photosynthesis for adaxial irradiation were 19 to 37% higher than rates for abaxial irradiation. Leaves of understory plants (Asarum canadense and Hydrophyllum canadense) were functionally symmetrical although they had horizontal orientation. Photosynthetic rates were measured at saturating CO₂, thus differences in the response to incident irradiance presumably resulted from complex interactions of light and leaf optical properties rather than from stomatal effects. Differences in absorptance (400–700 nm) among leaf surfaces were evident for horizontal-leaf species but the primary determinant of functional symmetry was leaf anatomy. Functionally symmetrical leaves had upper and lower palisade layers of equal thickness (vertical leaves of open habitats) or were composed primarily of a single layer of photosynthetic cells (horizontal leaves of understory habitats). Photosynthetic symmetry of vertical-leaf species may be an adaptation to maximize daily integrated carbon gain and water-use efficiency, whereas asymmetry of horizontal-leaf species may be an adaptation to maximize daily integrated carbon gain and photosynthetic nutrient-use efficiency.     

Changes in leaf optical properties associated with light-dependent chloroplast movements

We surveyed 24 plant species to examine how leaf anatomy influenced chloroplast movement and how the optical properties of leaves change with chloroplast position. All species examined exhibited light-dependent chloroplast movements but the associated changes in leaf absorptance varied considerably in magnitude. Chloroplast movement-dependent changes in leaf absorptance were greatest in shade species, in which absorptance changes of >10% were observed between high- and low-light treatments. Using the Kubelka-Munk theory, we found that changes in the absorption (k) and chlorophyll a absorption efficiency (k*) associated with chloroplast movement correlated with cell diameter, such that the narrower, more columnar cells found in sun leaves restricted the ability of chloroplasts to move. The broader, more spherical cells of shade leaves allowed greater chloroplast rearrangements and in low-light conditions allowed efficient light capture. Across the species tested, light-dependent chloroplast movements modulated leaf optical properties and light absorption efficiency by manipulating the package (sieve or flattening) effect but not the detour (path lengthening) effect.     

High-light effects on CO2 fixation gradients across leaves

Chlorophyll fluorescence and internal patterns of ¹⁴CO₂ fixation were measured in sun and shade leaves of spinach after treatment with various light intensities. When sun leaves were irradiated with 2000μmol m^?2 s^?1 for 2h, F_V/F_M decreased by about 15%, but ¹⁴CO₂ fixation was unaffected, whereas shade leaves exhibited a 21% decrease in F_v/F_M and a 25% decrease in ¹⁴CO₂ fixation. Irradiation of sun and shade leaves with 4000μmol m^?1 for 4 h decreased F_V/F_M by 30% in sun leaves and 40% in shade leaves, while total ¹⁴CO₂ fixation decreased by 41% in sun leaves and 55% in shade leaves. After light treatment, gradients of CO₂ fixation across leaves were determined by measuring ¹⁴CO₂ fixed in paradermal leaf sections after a 10s pulse of ¹⁴CO₂. Gradients of ¹⁴CO₂ fixation in control sun and shade leaves were identified when expressed on a relative basis and normalized for leaf depth. Treatment of leaves with 2000 μmol PAR m^?2 s^?1 for 2h did not after patterns of carbon fixation across sun leaves, but slightly altered the pattern in shade leaves. In contrast, treatment of sun and shade leaves with 4000μmol m^?2 s^?1 for 4h decreased carbon fixation more in the palisade mesophyll cells than in the spongy mesophyll cells of sun and shade leaves, and fixation in medial tissue of shade leaves was dramatically decreased compared to the adaxial and abaxial tissue. The interaction between leaf anatomy and biochemical parameters involved in tolerance to photoinhibition in spinach is discussed.     

The spatial distribution of chloroplast water in Acer platanoides sun and shade leaves

We evaluated a new, two-dimensional (2-D) nuclear magnetic resonance (NMR) imaging technique as a method for measuring the distribution of chloroplasts in leaves. NMR images that showed the distribution of chloroplast water and of total water as a function of depth into Acer platanoides sun and shade leaves were compared with the distribution of chlorophyll in the same leaf types (as measured by fluorescence microscopy), with the cellular structure (by scanning electron microscopy), and with published information. Results showed that the volume fraction of chloroplast water was much larger in shade than in sun leaves, and that it averaged about one-third larger in the palisade than in the spongy parenchyma region of both leaf types. Chlorophyll fluorescence was more intense in shade than in sun leaves. In sun leaves, fluorescence was maximal in the palisade region near the junction with the spongy parenchyma, while in shade leaves, fluorescence was maximal in the upper part of the spongy layer. We concluded that 2-D NMR imaging reliably indicates the location of chloroplast water.     

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

We 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 CO₂ Elevated 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/C_i 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/C_i 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.     

Effect of local irradiance on CO(2) transfer conductance of mesophyll in walnut

The acclimation responses of walnut leaf photosynthesis to the irradiance microclimate were investigated by characterizing the photosynthetic properties of the leaves sampled on young trees (Juglans nigraxregia) grown in simulated sun and shade environments, and within a mature walnut tree crown (Juglans regia) in the field. In the young trees, the CO(2) compensation point in the absence of mitochondrial respiration (Gamma*), which probes the CO(2) versus O(2) specificity of Rubisco, was not significantly different in sun and shade leaves. The maximal net assimilation rates and stomatal and mesophyll conductances to CO(2) transfer were markedly lower in shade than in sun leaves. Dark respiration rates were also lower in shade leaves. However, the percentage inhibition of respiration by light during photosynthesis was similar in both sun and shade leaves. The extent of the changes in photosynthetic capacity and mesophyll conductance between sun and shade leaves under simulated conditions was similar to that observed between sun and shade leaves collected within the mature tree crown. Moreover, mesophyll conductance was strongly correlated with maximal net assimilation and the relationships were not significantly different between the two experiments, despite marked differences in leaf anatomy. These results suggest that photosynthetic capacity is a valuable parameter for modelling within-canopies variations of mesophyll conductance due to leaf acclimation to light.     

Photosynthetic responses to light in seedlings of selected Amazonian and Australian rainforest tree species

Summary Seedlings of the Caesalpinoids Hymenaea courbaril, H. parvifolia and Copaifera venezuelana, emergent trees of Amazonian rainforest canopies, and of the Araucarian conifers Agathis microstachya and A. robusta, important elements in tropical Australian rainforests, were grown at 6% (shade) and 100% full sunlight (sun) in glasshouses. All species produced more leaves in full sunlight than in shade and leaves of sun plants contained more nitrogen and less chlorophyll per unit leaf area, and had a higher specific leaf weight than leaves of shade plants. The photosynthetic response curves as a function of photon flux density for leaves of shade-grown seedlings showed lower compensation points, higher quantum yields and lower respiration rates per unit leaf area than those of sun-grown seedlings. However, except for A. robusta, photosynthetic acclimation between sun and shade was not observed; the light saturated rates of assimilation were not significantly different. Intercellular CO₂ partial pressure was similar in leaves of sun and shade-grown plants, and assimilation was limited more by intrinsic mesophyll factors than by stomata. Comparison of assimilation as a function of intercellular CO₂ partial pressure in sun- and shade-grown Agathis spp. showed a higher initial slope in leaves of sun plants, which was correlated with higher leaf nitrogen content. Assimilation was reduced at high transpiration rates and substantial photoinhibition was observed when seedlings were transferred from shade to sun. However, after transfer, newly formed leaves in A. robusta showed the same light responses as leaves of sun-grown seedlings. These observations on the limited potential for acclimation to high light in leaves of seedlings of rainforest trees are discussed in relation to regeneration following formation of gaps in the canopy.     

Morpho‐anatomical and physiological differences between sun and shade leaves in Abies alba Mill. (Pinaceae,Coniferales): a combined approach

Morphology, anatomy and physiology of sun and shade leaves of Abies alba were investigated and major differences were identified, such as sun leaves being larger, containing a hypodermis and palisade parenchyma as well as possessing more stomata, while shade leaves exhibit a distinct leaf dimorphism. The large size of sun leaves and their arrangement crowded on the upper side of a plagiotropic shoot leads to self‐shading which is explainable as protection from high solar radiation and to reduce the transpiration via the lamina. Sun leaves furthermore contain a higher xanthophyll cycle pigment amount and Non‐Photochemical Quenching (NPQ) capacity, a lower amount of chlorophyll b and a total lower chlorophyll amount per leaf, as well as an increased electron transport rate and an increased photosynthesis light saturation intensity. However, sun leaves switch on their NPQ capacity at rather low light intensities, as exemplified by several parameters newly measured for conifers. Our holistic approach extends previous findings about sun and shade leaves in conifers and demonstrates that both leaf types of A. alba show structural and physiological remarkable similarities to their respective counterparts in angiosperms, but also possess unique characteristics allowing them to cope efficiently with their environmental constraints.     

Determination of mesophyll diffusion resistance in Chamaerion angustifolium by the method of three-dimensional reconstruction of the leaf cell packing

A detailed quantitative analysis of the three-dimensional organization of the mesophyll was performed, and mesophyll diffusion resistance to CO₂ in the leaves of Chamaerion angustifolium formed under different irradiance was calculated using an original method of stereometric cellular packing. For each type of leaves (sun and shade), we determined structural components of gas exchange: the volume of mesophyll per unit leaf area (V _mes), the volume of the intercellular space in the mesophyll (V _is), the area of the total mesophyll surface (S), the area of the free mesophyll surface facing the intercellular spaces (S _mes), and the ratios of the total and the free mesophyll surfaces to its volume (S/V and S _mes/V). As compared with sun leaves, in the shade leaves of Ch. angustifolium, S and V _mes decreased twofold, tissue density was reduced twofold, and the share of the intercellular space in the mesophyll rose from 49 to 72%. In shade, the diffusion resistance of the mesophyll increased by 1.8 times because of changes in the leaf structure. At the same time, the ratio S _mes/V was found to increase by 1.4 times, which facilitated the diffusion of CO₂. In the shade leaves of Ch. angustifolium, the diffusion resistance of the intercellular air spaces was reduced twofold as a result of an increase in their share in the leaf mesophyll and simplification of their geometry. Thus, the method of three-dimensional reconstruction of sun and shade leaves of Ch. angustifolium showed a comprehensive rearrangement of the mesophyll spatial organization in shade and revealed the structural mechanisms of changes in the resistance to CO₂ diffusion within the leaf.     

Contributions of diffusional limitation, photoinhibition and photorespiration to midday depression of photosynthesis in Arisaema heterophyllum in natural high light

Diurnal changes in photosynthetic gas exchange and chlorophyll fluorescence were measured under full sunlight to reveal diffusional and non‐diffusional limitations to diurnal assimilation in leaves of Arisaema heterophyllum Blume plants grown either in a riparian forest understorey (shade leaves) or in an adjacent deforested open site (sun leaves). Midday depressions of assimilation rate (A) and leaf conductance of water vapour were remarkably deeper in shade leaves than in sun leaves. To evaluate the diffusional (i.e. stomatal and leaf internal) limitation to assimilation, we used an index [1–A/A₃₅₀], in which A₃₅₀ is A at a chloroplast CO₂ concentration of 350 μ mol mol ^{? 1}. A₃₅₀ was estimated from the electron transport rate (J_T), determined fluorometrically, and the specificity factor of Rubisco (S), determined by gas exchange techniques. In sun leaves under saturating light, the index obtained after the ‘peak’ of diurnal assimilation was 70% greater than that obtained before the ‘peak’, but in shade leaves, it was only 20% greater. The photochemical efficiency of photosystem II ( Δ F/F_m ′ ) and thus J_T was considerably lower in shade leaves than in sun leaves, especially after the ‘peak’. In shade leaves but not in sun leaves, A at a photosynthetically active photon flux density (PPFD) > 500 μ mol m ^{? 2} s ^{? 1} depended positively on J_T throughout the day. Electron flows used by the carboxylation and oxygenation (J_O) of RuBP were estimated from A and J_T. In sun leaves, the J_O/J_T ratio was significantly higher after the ‘peak’, but little difference was found in shade leaves. Photorespiratory CO₂ efflux in the absence of atmospheric CO₂ was about three times higher in sun leaves than in shade leaves. We attribute the midday depression of assimilation in sun leaves to the increased rate of photorespiration caused by stomatal closure, and that in shade leaves to severe photoinhibition. Thus, for sun leaves, increased capacities for photorespiration and non‐photochemical quenching are essential to avoid photoinhibitory damage and to tolerate high leaf temperatures and water stress under excess light. The increased Rubisco content in sun leaves, which has been recognized as raising photosynthetic assimilation capacity, also contributes to increase in the capacity for photorespiration.     

Developmental process of sun and shade leaves in Chenopodium album L.

The authors’ previous study of Chenopodium album L. revealed that the light signal for anatomical differentiation of sun and shade leaves is sensed by mature leaves, not by developing leaves. They suggested that the two‐cell‐layered palisade tissue of the sun leaves would be formed without a change in the total palisade tissue cell number. To verify that suggestion, a detailed study was made of the developmental processes of the sun and shade leaves of C. album with respect to the division of palisade tissue cells (PCs) and the data was expressed against developmental time (leaf plastochron index, LPI). The total number of PCs per leaf did not differ between the sun and shade leaves throughout leaf development (from LPI ?1 to 10). In both sun and shade leaves, anticlinal cell division of PCs occurred most frequently from LPI ?1 to 2. In sun leaves, periclinal division of PCs occurred synchronously with anticlinal division. The constancy of the total number of PCs indicates that periclinal divisions occur at the expense of anticlinal divisions. These results support the above suggestion that two‐cell‐layered palisade tissue is formed by a change of cell division direction without a change in the total number of PCs. PCs would be able to recognize the polarity or axis that is perpendicular to the leaf plane and thereby change the direction of their cell divisions in response to the light signal from mature leaves.     

The fine structure and photosynthetic cost of structural leaf variegation

The leaves of some plants display an optical patchiness on their upper side, displaying light- and dark-green areas with high and low reflectance, respectively. In this investigation, we studied the fine structure of the corresponding sectors and we asked whether the lost reflected light entails a photosynthetic cost to these leaves. Four species, i.e. Arum italicum, Ranunculus ficaria, Cyclamen hederifolium and Cyclamen persicum were investigated. Scanning electron microscope examination revealed that epidermal cells of light-green sectors of all species are more bulgy than corresponding cells of neighboring dark-green leaf sectors. The comparative anatomical study revealed that (i) epidermis thickness of the light-green areas and the number of mesophyll cell layers does not differ from those of the adjacent dark-green leaf sectors and (ii) palisade cells of light-green sectors are slightly larger and more loosely arranged, allowing a much higher percentage of intercellular air spaces. The latter histological feature seems to provide the structural basis for the different optical properties between the two leaf sectors. Contrary to expectations, net photosynthetic rates (expressed on a leaf area basis) were similar in the light-green and the dark-green areas of the two cyclamen species. Yet, in C. persicum net photosynthesis was higher in the light-green areas, if expressed on a dry mass basis. The small size of the light-green spots in the rest of the test plants precluded CO₂ assimilation measurements, yet maximum linear photosynthetic electron transport rates displayed no differences between the two sectors in all plants. Thus, the assumption of a photosynthetic cost in the light-green areas was not confirmed. On the contrary, a higher construction cost was evident in the dark-green areas of three species, displaying a significantly higher specific leaf mass, without any photosynthetic benefit. The results on net photosynthesis were compatible with leaf optical properties and pigment levels. Thus, in spite of the considerably higher reflectance of the light-green areas and their lower (yet normal for a green leaf) chlorophyll levels, corresponding differences in absorptance were slight. In addition, dry mass-based pigment contents in dark-green areas were higher, while chlorophyll a/b (in two species) and carotenoid/chlorophyll ratios (in three species) were lower, pointing to a shade adaptation in these sectors. We conclude that in variegated leaves of this kind, dark-green areas are more costly to build and probably less photosynthetically active. We argue that the high pigment contents of dark-green areas establish steep light gradients in the corresponding mesophyll, rendering deeper chloroplast layers more shade adapted.     

Carotenoid composition in sun and shade leaves of plants with different life forms

The carotenoid composition of sun leaves of nine species of annual crop plants (some with several varieties) was compared with sun and shade leaves of several other groups of plants, among those sun and shade leaves of several species of perennial shrubs and vines and deep-shade leaves of seven rainforest species. All sun leaves contained considerably greater amounts of the components of the xanthophyll cycle violaxanthin, antheraxanthin and zeaxanthin as well as of β-carotene than the shade leaves, as had previously been reported for a variety of other species by Thayer & Björkman (Photosynthesis Research, 1990, 23, 331–343). Therefore, high light specifically stimulated β,β-carotenoid synthesis. The sun leaves of these crop species did not contain α-carotene which was, however, present in large amounts in all shade leaves and in smaller amounts in sun leaves of three of the four species of perennial shrubs and vines. There was no difference in neoxanthin content on a chlorophyll basis between sun and shade leaves, and there was no consistent general difference in the lutein content between all sun and all shade leaves. The zeaxanthin (and antheraxanthin) content at peak irradiance and the xanthophyll cycle pool size were compared for sun leaves from the different groups of plants with different life forms and different metabolic activities. When growing in full sunlight the annual crop species and a perennial mesophyte had high rates of photosynthesis whereas the perennial shrubs and vines had relatively low photosynthesis rates. More zeaxanthin (and antheraxanthin) were accumulated at noon in full sunlight in those species with the lower photosynthesis rates. However, it was not such that those species also possessed the larger pools of violaxanthin + antheraxanthin + zeaxanthin. Instead, the xanthophyll cycle pools of sun leaves of the annual crop species and the perennial mesophyte were not smaller, and were even possibly larger, than those of sun leaves of the perennial shrubs and vines with low photosynthesis rates. This was so in spite of the fact that the crop species experienced much lesser degrees of excessive light at full sun than the shrubs and vines. Thus, many of the crop species converted only about 30–50% of their xanthophyll cycle pool to zeaxanthin at noon, whereas the shrubs and vines typically converted more than 80% of their pool into zeaxanthin. The crop species also had larger pools of β-carotene than the shrubs and vines but smaller pools of lutein than the majority of the latter species.     

Phenotypic plasticity in response to light in the coffee tree

Phenotypic plasticity to light availability was examined at the leaf level in field-grown coffee trees (Coffea arabica). This species has been traditionally considered as shade-demanding, although it performs well without shade and even out-yields shaded coffee. Specifically, we focused our attention on the morpho-anatomical plasticity, the balance between light capture and excess light energy dissipation, as well as on physiological traits associated with carbon gain. A wide natural light gradient, i.e., a diurnal intercepted photon irradiance differing by a factor of 25 between the deepest shade leaves and the more exposed leaves in the canopy, was explored. Responses of most traits to light were non-linear, revealing the classic leaf sun vs. leaf shade dichotomy (e.g., compared with sun leaves, shade leaves had a lower stomatal density, a thinner palisade mesophyll, a higher specific leaf area, an improved light capture, a lower respiration rate, a lower light compensating point and a limited capacity for photoprotection). The light-saturated rates of net photosynthesis were higher in sunlit than in shade leaves, although sun leaves were not efficient enough to use the extra light supply. However, sun leaves showed well-developed photoprotection mechanisms in comparison to shade leaves, which proved sufficient for avoiding photoinhibition. Specifically, a higher non-photochemical quenching coefficient was found in parallel to increases in: (i) zeaxanthin pools, (ii) de-epoxidation state of the xanthophyll cycle, and (iii) activities of some antioxidant enzymes. Intracanopy plasticity depended on the suite of traits considered, and was high for some physiological traits associated with photoprotection and maintenance of a positive carbon balance under low light, but low for most morpho-anatomical features. Our data largely explain the successful cultivation of the coffee tree in both exposed and shade environments, although with a poor resource-use efficiency in high light.