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 responses of the twining vine Ipomoea purpurea (Convolvulaceae) to physical support availability in sun and shade

Vines depend on external support to prevent shading by neighbouringplants. Hence, it is important to determine whether shading enhances thephenotypic responses of vines to support availability. I evaluated theconsequences of support availability (a vertical stake) on shoot and leaftraitsof the morning glory Ipomoea purpurea (Convolvulaceae)under full sunlight and extreme shade. It was hypothesised that phenotypicresponses of vines to support availability should be greater in the shade. Inaddition, to investigate possible constraints to such phenotypic responses, thecorrelations among phenotypic traits and the plasticity of such correlationswere evaluated. The phenotypic variation of the main stem length and of thenumber of branches was consistent with the hypothesis, i.e. greater responsestosupport availability in the shade. In contrast, both internode length and leafarea (two traits that showed a significant and positive correlation) decreasedin the sun and increased in the shade with support availability. Petiole lengthdecreased with support in the sun but had no response in the shade. On theotherhand, the number of significant trait correlations found in plants in the sunand supported plants was higher than those of shade and non-supported plants,respectively. Several of the correlations were significantly sensitive to theenvironment. Flowering only occurred in the sun treatment. Whereas no shoot orleaf trait was significantly correlated with flower number in supported plants,both petiole length and shoot biomass showed a significant correlation withsuchestimate of plant fitness in non-supported plants.     

Herbivory on Temperate Rainforest Seedlings in Sun and Shade: Resistance,Tolerance and Habitat Distribution

Differential herbivory and/or differential plant resistance or tolerance in sun and shade environments may influence plant distribution along the light gradient. Embothrium coccineum is one of the few light-demanding tree species in the temperate rainforest of southern South America, and seedlings are frequently attacked by insects and snails. Herbivory may contribute to the exclusion of E. coccineum from the shade if 1) herbivory pressure is greater in the shade, which in turn can result from shade plants being less resistant or from habitat preferences of herbivores, and/or 2) consequences of damage are more detrimental in the shade, i.e., shade plants are less tolerant. We tested this in a field study with naturally established seedlings in treefall gaps (sun) and forest understory (shade) in a temperate rainforest of southern Chile. Seedlings growing in the sun sustained nearly 40% more herbivore damage and displayed half of the specific leaf area than those growing in the shade. A palatability test showed that a generalist snail consumed ten times more leaf area when fed on shade leaves compared to sun leaves, i.e., plant resistance was greater in sun-grown seedlings. Herbivore abundance (total biomass) was two-fold greater in treefall gaps compared to the forest understory. Undamaged seedlings survived better and showed a slightly higher growth rate in the sun. Whereas simulated herbivory in the shade decreased seedling survival and growth by 34% and 19%, respectively, damaged and undamaged seedlings showed similar survival and growth in the sun. Leaf tissue lost to herbivores in the shade appears to be too expensive to replace under the limiting light conditions of forest understory. Following evaluations of herbivore abundance and plant resistance and tolerance in contrasting light environments, we have shown how herbivory on a light-demanding tree species may contribute to its exclusion from shade sites. Thus, in the shaded forest understory, where the seedlings of some tree species are close to their physiological tolerance limit, herbivory could play an important role in plant establishment.     

LEAF OPTICAL PROPERTIES ALONG A VERTICAL GRADIENT IN A TROPICAL RAIN FOREST CANOPY IN COSTA RICA

Leaf optical properties (400–1,100 nm) were compared for four species of rain forest trees with crowns in understory, mid-canopy, and canopy positions to test whether optical properties change with light environment. The species tested represent a spectrum of regeneration patterns ranging from shade tolerant to light demanding. Overall, leaf optical properties of the four species were similar. Differences in absorptance were small, but statistically significant among the species and positions along the canopy gradient. Species absorptance differences corresponded somewhat to shade tolerance; two of the shade species showed higher absorptance in lower light environments, while the sun species showed the reverse pattern. Specific leaf mass (leaf weight per unit area) and chlorophyll content per unit leaf weight also changed along the canopy gradient. Specific leaf mass was positively correlated and chlorophyll per unit leaf weight was negatively correlated with increasing light environment. Consequently, the efficiency of absorption, as represented by the absorptance per unit leaf weight, increased as light level decreased, largely due to changes in specific leaf mass. In contrast, efficiency of absorption per unit leaf chlorophyll was relatively constant with light environment for the two species measured for chlorophyll.     

Leaf damage induces twining in a climbing plant

Successful climbing by vines not only prevents shading by neighbouring vegetation, but also may place the vines beyond ground herbivores. Here we tested the hypothesis that herbivory might enhance climbing in a vine species, and that such induced climbing should be greater in the shade. We assessed field herbivory in climbing and prostrate ramets of the twining vine Convolvulus arvensis. We evaluated plant climbing after mechanical damage in a glasshouse under both sun and shade conditions, and determined whether control and damaged plants differed in growth rate or photosynthetic capacity. Plants experienced greater herbivory when growing prostrate than when climbing onto companion plants, in both an open habitat and a shaded understorey. Experimental plants increased their twining rate on a stake after suffering leaf damage, in both high- and low-light conditions, and this induced climbing was not coupled to an increase in growth rate. Increased photosynthesis was associated with enhanced twining rate only in the shade. Herbivory may be an ecological factor promoting the evolution of a climbing habit in plants.     

The differential responses of lianas and vines to rainfall gradients in distribution and abundance in Qinling Mountains, China

To clarify the differential responses of lianas and vines to rainfall, we investigated the distribution and abundance of climbers along the rainfall gradient in the Qinling Mountains of China. In our study, 28 climbing species were investigated across a rainfall gradient of 500?C1,200?mm. Results indicated that most lianas and vines show restricted distribution along the rainfall gradient, which imply their adaptation to different water availabilities. A significant increase in the percentage of the total climbing species with rainfall was observed. The abundance and richness of lianas increased significantly with rainfall, whereas the vines remained constant. This difference implies that lianas have a higher sensitivity to water availability than vines. During drought, lianas may be vulnerable because of the drought-induced embolism, whereas vines are drought-resistant because of their shorter life spans and safer nutrient storage. Further research on the ecophysiological differences between lianas and vines is needed to provide a mechanistic explanation for the patterns described here.     

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.     

DOES SHADE PROLONG JUVENILE DEVELOPMENT? A MORPHOLOGICAL ANALYSIS OF LEAF SHAPE CHANGES IN CUCURBIT A ARGYROSPERMA SUBSP. SORORIA (CUCURBITACEAE)

Several studies have concluded that shade extends the juvenile phase of plant development based on the prolonged production of juvenile-looking leaves along the shoot. Until now, the alternative hypothesis that leaves produced in shade converge in shape with more juvenile leaves through plastic responses of individual leaves has not been investigated. The literature has shown that differences in shape among leaves in a heteroblastic series are manifest very early in development, often at or near inception, whereas divergence in development between sun and shade leaves does not become apparent until considerably later. This study is the first to distinguish between these alternatives by comparing the developmental morphology of young leaves of the heteroblastic plant Cucurbita argyrosperma subsp. sororia. Differences in shapes of mature leaves along the shoot in sun and shade were quantified in terms of leaf area, perimeter, and shape using truss analysis. Developmental morphology from initiation through expansion was examined for representative transition and for later (adult) leaves using scanning electron microscopy and allometry. Determinants of shape established very early in development were the same for leaves at the same position grown in sun and shade. Differences in morphology between sun and shade leaves at the same position did not arise until these leaves reached lamina lengths greater than 1,000 μm. Thus, the less-lobed, more juvenile looking leaf produced at later positions in the shade arose through later developmental responses of individual leaves to shade, rather than through a prolonged phase of juvenile development.     

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.     

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.     

臭氧和气溶胶复合污染对杨树叶片光合作用的影响

由于经济的快速发展, 中国大部分地区正面临着严峻的复合型大气污染, 其中臭氧和气溶胶是两种主要污染物。已有的研究表明臭氧对叶片的氧化性伤害能够抑制光合作用, 而气溶胶可通过增加散射辐射比例或缓解高温抑制促进光合作用。但复合污染下, 臭氧和气溶胶如何共同调控叶片光合作用, 仍缺乏研究。该研究利用北京及周边地区之间的污染梯度, 选择加杨(Populus × canadensis)作为实验对象, 于2012-2013年生长季期间对叶片光合速率进行连续观测, 并同时监测臭氧浓度(AOT40)、气溶胶光学厚度(AOD)、空气温度和冠层内外光合有效辐射(PAR)等环境因子, 以期探讨大气复合污染下臭氧和气溶胶变化对植物叶片光合作用的影响及相关机制。结果表明: (1)臭氧浓度与空气温度、气溶胶浓度之间均呈显著正相关关系, 但气溶胶浓度与空气温度没有显著相关关系; (2)臭氧浓度增加显著抑制了阳生叶片的光合作用, 但气溶胶浓度上升促进了阳生叶片的光合作用; 臭氧浓度升高对阴生叶片光合作用的影响较小, 但气溶胶浓度上升促进了阴生叶片的光合作用; (3)标准化后的结果显示, 臭氧对阳生叶片光合作用的影响最大, 此时气溶胶的促进作用一定程度上补偿了臭氧浓度上升所带来的抑制效应。对于阴生叶片光合作用而言, 气溶胶则是最重要的影响因素。该研究发现复合污染下阴生叶和阳生叶光合响应不同, 这表明冠层结构可能通过影响阴生叶和阳生叶的比例, 从而对植物生长产生不同影响。该研究对理解大气复合污染如何影响光合作用提供了的机理支持, 同时也表明, 为了维持生态系统生产力及功能, 需要同时控制气溶胶和臭氧污染。     

Effects of ozone and aerosol pollution on photosynthesis of poplar leaves

由于经济的快速发展, 中国大部分地区正面临着严峻的复合型大气污染, 其中臭氧和气溶胶是两种主要污染物。已有的研究表明臭氧对叶片的氧化性伤害能够抑制光合作用, 而气溶胶可通过增加散射辐射比例或缓解高温抑制促进光合作用。但复合污染下, 臭氧和气溶胶如何共同调控叶片光合作用, 仍缺乏研究。该研究利用北京及周边地区之间的污染梯度, 选择加杨(Populus × canadensis)作为实验对象, 于2012-2013年生长季期间对叶片光合速率进行连续观测, 并同时监测臭氧浓度(AOT40)、气溶胶光学厚度(AOD)、空气温度和冠层内外光合有效辐射(PAR)等环境因子, 以期探讨大气复合污染下臭氧和气溶胶变化对植物叶片光合作用的影响及相关机制。结果表明: (1)臭氧浓度与空气温度、气溶胶浓度之间均呈显著正相关关系, 但气溶胶浓度与空气温度没有显著相关关系; (2)臭氧浓度增加显著抑制了阳生叶片的光合作用, 但气溶胶浓度上升促进了阳生叶片的光合作用; 臭氧浓度升高对阴生叶片光合作用的影响较小, 但气溶胶浓度上升促进了阴生叶片的光合作用; (3)标准化后的结果显示, 臭氧对阳生叶片光合作用的影响最大, 此时气溶胶的促进作用一定程度上补偿了臭氧浓度上升所带来的抑制效应。对于阴生叶片光合作用而言, 气溶胶则是最重要的影响因素。该研究发现复合污染下阴生叶和阳生叶光合响应不同, 这表明冠层结构可能通过影响阴生叶和阳生叶的比例, 从而对植物生长产生不同影响。该研究对理解大气复合污染如何影响光合作用提供了的机理支持, 同时也表明, 为了维持生态系统生产力及功能, 需要同时控制气溶胶和臭氧污染。     

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.     

The relationship between CAM and leaf succulence in two epiphytic vines, <Emphasis Type="Italic">Hoya carnosa</Emphasis> and <Emphasis Type="Italic">Dischidia formosana</Emphasis> (Asclepiadaceae), in a subtropical rainforest in northeastern Taiwan

Past reports of correlations between Crassulacean acid metabolism (CAM) and leaf succulence are based on multi-species comparisons. When different individuals of the same species were compared in two epiphytic CAM vines growing in a subtropical rainforest in northeastern Taiwan, the degree of CAM was not correlated with leaf thickness, a measure of succulence. Leaf chlorophyll (Chl) a and b concentrations and ratios correlated well with leaf succulence, indicating that differences in leaf succulence were likely a result of sun/shade adaptations, not photosynthetic pathway. These findings challenge the assumption that CAM-succulence correlations are causal.     

Seasonal changes in leaf antioxidant systems and xanthophyll cycle characteristics in Taxus x media growing in sun and shade environments

We examined differences between summer and winter in xanthophyll cycle-dependent energy dissipation and leaf antioxidant systems in needles of the overwintering evergreen Taxus x media cv. Tauntonii (Taunton yew) growing in both sun and shade environments in Saint Paul, Minnesota. During the winter, both sun and shade plants exhibited increases in the capacity for, and utilization of, xanthophyll cycle-dependent thermal energy dissipation. Winter needles showed decreases (sun needles) or no change (shade needles) in superoxide dismutase activity (EC 1.15.1.1), no change in ascorbate peroxidase activity (EC 1.11.1.11) and no change (sun needles) or increases (shade needles) in reduced ascorbate levels. Both sun and shade needles showed large increases in glutathione reductase activity (EC 1.6.4.2) and total glutathione levels during the winter, in addition to increases in levels of α-tocopherol. These results suggest an important photoprotective role during the winter for xanthophyll cycle-dependent energy dissipation and for the antioxidants glutathione and α-tocopherol. They suggest a less important photoprotective function of the enzyme-based water–water cycle in winter acclimation in the seasonally very cold environment of Minnesota.     

Effects of the light environment on oviposition preference and survival of a leaf-mining moth, Cameraria hamadryadella (Lepidoptera: Gracillariidae), on Quercus alba L.

Abstract.  1. Oviposition preference and the survival and mortality of larvae of Cameraria hamadryadella (Lepidoptera) on the foliage of Quercus alba L. (Fagaceae) in sun and shade were examined. To separate the effects of the environment of foliation from that of rearing, C. hamadryadella were allowed to oviposit on oak saplings that foliated either in the sun or shade. Subsequently, trees were placed into the sun or shade and larval survival was monitored.
2. Trees that foliated in the shade had leaves that were 30% longer than those that foliated in the sun. However, the density of leaf mines did not differ between trees that foliated in the sun or shade.
3. Larval survival was higher in the shade than in the sun, but no difference in survival due to the site of foliation was detected. Differences in larval survival between the sun and shade environments could not be attributed to differences in natural enemy mortality. However, 'death by other causes' was higher for larvae reared in the sun than in the shade. In sunny environments photochemical processes may reduce foliage quality, which results in lower survival of C. hamadryadella larvae.
4. Ovipositing C. hamadryadella do not appear to discriminate between foliage produced in the sun or shade. This lack of discrimination, coupled with the fact that ovipositing C. hamadryadella are active after dark, accounts for the absence of a difference in abundance between sun and shade.
5. The results presented here, combined with other published studies, argue that different patterns of insect herbivory and abundance in relation to the light level experienced by host-plant foliage might be expected for species that oviposit either in the night or in the day.     

Examining variation in the leaf mass per area of dominant species across two contrasting tropical gradients in light of community assembly

Understanding variation in key functional traits across gradients in high diversity systems and the ecology of community changes along gradients in these systems is crucial in light of conservation and climate change. We examined inter‐ and intraspecific variation in leaf mass per area (LMA) of sun and shade leaves along a 3330‐m elevation gradient in Peru, and in sun leaves across a forest–savanna vegetation gradient in Brazil. We also compared LMA variance ratios (T‐statistics metrics) to null models to explore internal (i.e., abiotic) and environmental filtering on community structure along the gradients. Community‐weighted LMA increased with decreasing forest cover in Brazil, likely due to increased light availability and water stress, and increased with elevation in Peru, consistent with the leaf economic spectrum strategy expected in colder, less productive environments. A very high species turnover was observed along both environmental gradients, and consequently, the first source of variation in LMA was species turnover. Variation in LMA at the genus or family levels was greater in Peru than in Brazil. Using dominant trees to examine possible filters on community assembly, we found that in Brazil, internal filtering was strongest in the forest, while environmental filtering was observed in the dry savanna. In Peru, internal filtering was observed along 80% of the gradient, perhaps due to variation in taxa or interspecific competition. Environmental filtering was observed at cloud zone edges and in lowlands, possibly due to water and nutrient availability, respectively. These results related to variation in LMA indicate that biodiversity in species rich tropical assemblages may be structured by differential niche‐based processes. In the future, specific mechanisms generating these patterns of variation in leaf functional traits across tropical environmental gradients should be explored.     

CHANGES IN NUTRIENT CONTENT OF PALMARIA PALMATA IN RESPONSE TO VARIABLE LIGHT AND UPWELLING IN NORTHERN SPAIN1

Light has been identified as one of the main factors affecting seaweed ecophysiology. We investigated the dependence of nutrient metabolism on sun and shade light conditions and whether episodes of upwelling of nutrient‐rich subsuperficial water could reduce the summer nutrient limitation driving physiological changes in Palmaria palmata (L.) Kuntze. We measured the major nutrient pools, photosynthetic pigments, and light curves, under sun and shade conditions during a summer period when one upwelling was recorded. The redundancy analysis (RDA) produced two clear groups: sun‐ and shade‐acclimated algae. Light was the major predictive factor. Sun‐acclimated algae exhibited higher carbon (C) and lower nitrogen (N) and phosphorus (P) content in association with the storage of floridoside (main C reserve) to benefit from higher irradiance (under nutrient limitation). Among N pools, N reserves (phycoerythrin, nitrate) were a lower proportion of the total N in sun‐acclimated algae, suggesting their degradation to fulfill the N demands of the cell. The orthophosphate content was also lower in sun‐acclimated algae, indicating its utilization as a nutrient reserve. In contrast, N within cell walls and membranes and chl a contributed to a similar proportion of the total N in sun‐ and shade‐acclimated algae, suggesting a response to sustain cell integrity. Transient high nutrient concentration due to the upwelling was unrelated to the nutrient content of the thallus. The storage of C as floridoside from high light exposure was shown to be the driving force for the metabolic adjustment of P. palmata at the end of summer before the onset of dormancy.     

Growth and ethylene evolution by shade and sun ecotypes of Stellaria longipes in response to varied light quality and irradiance

Plants growing in the shade receive both low light irradiance and light enriched in far red (FR) (i.e., light with a low red (R) to FR ratio). In an attempt to uncouple the R/FR ratio effects from light irradiance effects, we utilized Stellaria longipes because this species has two distinct natural population ecotypes, alpine (dwarf) and prairie (tall). The alpine population occupies the open, sun habitat. By contrast, the prairie population grows in the shade of other plants. Both 'sun' and 'shade' ecotypes responded with increased stem elongation responses under low irradiance, relative to growth under 'normal' irradiance, and this increased growth was proportionally similar. However, only the shade ecotype had increased shoot elongation in response to a low R/FR ratio. By contrast, the sun ecotype showed increased stem elongation in response to increasing R/FR ratio. Varying the R/FR ratios had no significant effect on ethylene evolution in either sun or shade ecotype. Under low irradiance, only the sun ecotype showed a significantly changed (decreased) ethylene evolution. We conclude that R/FR ratio and irradiance both regulate growth, and that irradiance can also influence ethylene evolution of the sun ecotype. By contrast, R/FR ratio and irradiance, while having profound influences on growth of the shade ecotype, do not appear to regulate these growth changes via effects on ethylene production.     

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.