Chlorophyll b absorbs mostly blue and yellow light. They both also absorb light of other wavelengths with less intensity. However, none of them absorbs green, so the leaf looks green because that light is reflected to our eyes instead of being absorbed by the leaf.
Since there are no other strong pigments present in leaves, that is the whole story. Chlorophyll molecules have a ring shape at one end — called a porphyrin — with a magnesium ion in the center. If you boil a leaf in water, this magnesium ion gets replaced by a hydrogen ion — i. A slight change of the molecular structure leads to a change of the optical behavior.
Furthermore, chlorophyll a and b only differ in a substituent of the porphyrin, for chlorophyll a it is a methyl group -CH 3 and for chlorophyll b it is an aldehyde group -CHO in the C7 position, but it is sufficient to significantly alter the absorption spectrum of the molecule. Molecular structures of chlorophyll a left and b right. They only differ in a substituent of the porphyrin ring. Photosynthetic chlorophyll pigments are not alone in the leaf cells; they are usually in a protein pocket.
It is this interaction with the surrounding microenvironment what fine-tunes chlorophylls to cover as much of the visible spectrum as possible. To know how much this microenvironment affects the visible color, we first need to know what the true color of chlorophyll is. This is very important if we are ever going to understand how photosynthesis works and if we want to use this knowledge to build truly efficient photovoltaic devices.
This is not an easy question to answer, though. You might think that it is as simple as preparing a solution of chlorophyll and use a spectrometer to get the answer. This has already been done with different solvents. Soils vary from tropical red soils with low organic matter to brown soils with high organic matter Song et al. Detailed geographical information of the region is presented in Table S1. The field survey was conducted between July and August of The geographical details, plant species composition, and community structure were determined for each plot.
All common species were identified in each plot, including trees, shrubs, and herbs. Subsequently, we collected more than 30 pieces of leaves from each species, of which four pieces were randomly selected to cut up and determine chlorophyll content from each plant species within the plot.
We chose mature and healthy trees, and collected fully expanded, sun-exposed leaves from four individuals of each plant species, which were considered as four repetitions Zhao et al. Overall, the leaves of plant species were collected. Soil samples 0—10 cm depth were randomly collected from 30 to 50 points using a soil sampler 6-cm diameter in each plot and were combined to form one composite sample in each plot Tian et al.
Fresh leaves were cleaned to remove soil and other contaminants, and 0. The Chl content Chl a and Chl b of the filtered solution was measured using the classical spectrophotometric method with a spectrophotometer Pharma Spec, UV, Shimadzu, Japan Mackinney, ; Li et al. According to the Lambert-Beer law, the relationship between concentration and optical density is:.
Soil pH was determined using a pH meter Mettler Toledo Delta , Switzerland by using a slurry of soil and distilled water 1: 2. Eight hundred and twenty-three species of plants were used to construct a phylogenetic tree at the species and family levels. At the family level, according to the evolution of angiosperm families provided by molecular and fossil dating data, we identified the evolution of angiosperm families.
Plant species were divided by plant functional groups PFGs, trees, shrubs, and herbs , growth forms coniferous or broadleaved tree , and leaf types evergreen or deciduous tree.
We used regression analyses to explore latitudinal patterns of Chl. To analyze the factors influencing Chl, we calculated Spearman's rank correlation coefficients for plant species across sites, PFGs, and leaf types. We then used redundancy analysis RDA to analyze the relative influences of climate, soil, and the interspecific variation of species. We tested the significance of this phylogenetic signal by comparing the actual system to a null model without phylogenetic structure.
All analyses were conducted using the software SPSS All figures were produced in Sigma Plot Chl a ranged from 0. Figure 1. The black lines across the boxes are median values and red points represent the means.
Numbers in brackets represent the number of sample species in the specific site. Table 1. Table 2. Table 3.
To quantify the variance of different groups sites, life forms , interpretation ratios were calculated within groups and among groups. Similar results were obtained for different plants with respect to life forms and leaf types Figures 2B—D. Figure 2. Partitioning the total spatial variance of leaf chlorophyll content in the nine forests A , different life forms B , different growth forms C , and different leaf shapes D. Figure 3. Latitudinal trends of leaf chlorophyll traits in the forests of China.
Significant phylogenetic signals were observed for Chl a within life forms, communities, and across the whole transect, except for shrubs and conifer trees Table 4. The K -values were approximately equal to 0. Significant phylogenetic signals were observed for Chl b within life forms and communities, and across the whole transect, except for shrubs and conifer trees. K -values were approximately equal to 0. Table 4. Strength of the phylogenetic signal in chlorophyll traits for different growth forms.
Table 5. Strength of the phylogenetic signal in chlorophyll traits for each of the nine forests. Large variation in Chl was observed among the plant species in the natural forest communities. Across all plants, there were significant differences of Chl among different species, life forms, and communities.
Although interspecific differences in Chl a and Chl b were very big, there should be a strong linear relationship between Chl a and Chl b Li et al. This phenomenon might be explained by a large amount of Chl in the forest communities being redundant, some of the plant chlorophyll is not involved in the photosynthetic reaction. Therefore, chlorophyll content is not necessarily linked to this reaction, even though temperature and water are the important factors in the synthesis of chlorophyll.
Alternatively, high interspecific variation might have led to a weak latitudinal pattern in our study, with the highest coefficient of variation of Chl reaching 0. Yet, interspecific species variation could not be explained by environmental differences between the scale of the study and the observed weak latitudinal patterns.
Furthermore, chlorophyll content might be affected by community structure, because the light shading effect on vertical structure might change Chl. With the development of the molecular clock theory molecular evolution speed constancy and fossil dating data, researchers found that phylogenic history play a decisive role for some plant traits Comas et al. Some plant traits i. In contrast, in our study, although Chl in the overall and different life forms had significant phylogenetic signals, the phylogenetic signal K values were close to zero.
Previous studies have demonstrated that if a plant trait has a significant phylogenetic signal, it could be considered as a conservative trait. Traits also perform more similarly when the genetic relationship of different species is closer Felsenstein, , and vice versa. Therefore, our results showed that Chl is almost not influenced by phylogeny in forests at a large scale. Plants have a much higher ability to dissipate the light energy absorbed by the LHCII antenna as heat. This could be the one of the major reasons to protect the core antenna from strong solar radiation.
Since light-use efficiency is an important component of biomass production, several leaf photosynthesis models have been proposed that consider the light absorption profile based on the optimal use of PAR photons in the terrestrial environment. Most discussions around this have focused on the efficient use of incident PAR photons in photosynthesis. However, the relationships between the spectral characteristics of incident radiation from the sun and the energy balance of chloroplasts and pigment characteristics, and the ways in which these affect leaf physiological conditions are also crucially important Kume The waveband of the green region of the spectrum — nm is identical to that of strong, directional solar irradiance at midday under a clear sky Figs.
Consequently, changes in the light-harvesting system may have contributed greatly to the evolution of terrestrial green plants, which are fine-tuned to reduce excess energy absorption rather than to absorb PAR photons efficiently.
The photochemical reaction center and core antennae of terrestrial plants only include Chl a , which has low solar radiation absorptivity, with the peripheral antenna complex containing Chl b and carotenoids being arranged around this. It is well known that light is the most limiting resource for plant growth and that competition between plants affects their various responses to environmental changes Anten ; Givnish ; van Loon et al. Thus, the efficient use of PAR under cloudy or shaded conditions may be important.
These spectral differences between PAR dir and PAR diff ensure that diffuse solar radiation, which has much less tendency to cause canopy photosynthetic saturation, is used more effectively by plant canopies than direct solar radiation. Thus, our findings suggest that the absorption spectrum of LHCII enables the efficient use of PAR diff and cloudy-day radiation, and that diffuse and direct radiation trigger different responses in canopy photosynthesis.
The changeability of the LHC antenna size, which is reflected in changes in spectral absorption, has a major effect on the distribution of plants as it allows flexibility in PAR use efficiency and avoidance of the strong heat produced by PAR dir e. Notably, the effects of spectral differences between PAR dir and PAR diff are negligible for whole-leaf absorption properties.
Kume has demonstrated that the absorption spectra of the intact leaves of terrestrial plants function as a gray body. The photon absorption of the whole leaf is efficiently regulated by photosynthetic pigments through a combination of pigment density distribution and leaf anatomical structures. The spectral characteristics of absorbers are important factors for the energy regulation of chloroplasts and smaller-scale energy processes.
Our findings indicate that the absorbance spectra of photosynthetic pigments and the photosystems and antenna proteins they construct significantly align with the spectra of PAR dir and PAR diff to enable the safe and efficient use of solar radiation on land.
Chl b tends to absorb scattered solar radiation complementary to Chl a and so its incorporation into the peripheral antennae increases the absorption capacity of plants for this type of radiation. By contrast, Chl b is excluded from the core antennae to avoid the absorption of strong, direct solar radiation. Thus, it appears that the effect of the absorption and scattering of solar radiation in the atmosphere on the spectral absorption of photosynthetic organisms was a primary driver of the selection of photosynthetic pigments and the evolution of photosystems.
This research adds to the growing body of evidence that suggests that terrestrial green plants are fine-tuned to the spectral and temporal dynamics of incident solar radiation.
However, further field observations and analyses are required to better understand the spectral adaptation of terrestrial organisms. Agric For Meteorol —— Article Google Scholar.
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J Phycol — Givnish TJ Adaptation to sun and shade: A whole-plant perspective. Aust J Plant Physiol — Biochim Biophys Acta — Science Review of Earth organisms. Astrobiology — It is interesting to note that the retention motif in all LHCs that contain Chl b is followed by a Trp residue, which may be involved in synthesis of Chl b. A converse mutagenesis approach would provide a rigorous test of the hypothesis.
A stable complex should be achieved with only Chl a, in a Chl b -less plant or by in vitro reconstitution, when weak ligands in LHCPs are replaced with stronger Lewis bases.
Increased strength of the engineered coordination bonds with Chl a should compensate for the lack of Chl b. In particular, a stable complex should accumulate after Gln, Glu, Asn and Gln in Lhcb1 are replaced with His. A stronger ligand could also be introduced in the position of Gly78, which seems to be the weakest ligand in the complex. Substitution of these amino acids in the sequence of Lhcb1, a major LHCP that can not be detected in Chl b -less plants [ 5 , 8 ], would be expected to restore accumulation of the protein with only Chl a.
This experiment provides a positive in vivo selection for validation of the hypothesis, in contrast to the dramatic decrease in accumulation of the proteins when ligands are removed by substitution with non-ligand amino acids [ 42 ]. Furthermore, whereas stable complexes can be achieved by reconstitution with wild-type Lhcb1 and only Chl b but not only Chl a [ 37 , 43 ], the hypothesis predicts that stable complexes can be reconstituted with the mutant protein containing these substitutions and Chl a.
An extensive amount of evidence in the literature supports the hypothesis presented in this article on the role of Chl b. It should be noted, however, that several LHCPs accumulate in chloroplasts in the absence of Chl b [ 5 , 8 ], perhaps because they integrate more easily into membranes, which implies that other features of the proteins are involved.
The work already done has established that several LHCPs are imported into the chloroplast at a substantial rate only when sufficient Chl b is available and they accumulate initially in the envelope membrane. Results from in vivo experiments have shown that interaction of Chl b with the first membrane-spanning region, including the retention motif, is critical for progression of import of these proteins.
The initial steps in assembly also require the abundant xanthophyll lutein [ 26 ], which has not been the focus of this article. The availability of Chl b thus strongly regulates import of LHCPs as well as assembly and eventual accumulation of light-harvesting complexes.
The resulting dramatic enhancement in the efficiency of light capture for photosynthesis apparently provided a strong evolutionary pressure for development of the ability of photosynthetic organisms to synthesize Chl b or Chl c [ 44 ]. The structure of LHCs has been extensively studied and linkage of the complexes to reaction centers, physically and functionally, is well understood. Further understanding of LHC assembly requires a better knowledge of the characteristics of the reaction catalyzed by Chl ide a oxidase and whether Chl b is restricted to these complexes because LHCP serves as a specific effector of the oxidation of Chl ide a or whether the protein simply provides binding sites for Chl b and prevents its conversion back to Chl a [ 45 ].
The latter appears less likely as a specific effect, because similar ligands should occur in other proteins. In particular, the early-light induced proteins are homologous to LHCPs but bind little if any Chl b [ 46 ]. The mechanism of Chl b synthesis, an oxidation of the methyl group at position 7 [ 41 ], will be an area of active research in the future, now that the gene for Chl ide a oxidase has been identified [ 47 , 48 ].
Moreover, it is not known whether a pool of free Chl b exists in a local environment in chloroplast membranes that is mimicked by the amount of Chl b in reconstitution experiments. Attempts to understand assembly of the complex in vivo will provide ample opportunity for additional experimental work.
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