Why were Chls a and b selected for in the terrestrial environment?
The absorption spectrum of Chl a exhibited a consistent tendency to avoid PARdir and it produces consistent negative re and rp of PARglb, while that of Chl b tended to absorb PARdiff, suggesting that Chl a can effectively avoid strong, direct solar radiation and Chl b can efficiently use diffused solar radiation. These differences are caused by slight shifts in the position and band width of absorption peaks in the blue and red regions (Fig. 1a), with the high absorption efficiency of Chl b for PARglb in the terrestrial environment being related to it having a higher Soret absorption band than Chl a and the longest Soret wavelength among the Chl pigments (c. 452 nm in diethyl ether; Mimuro et al. 2011). Such functional differences between the photosynthetic pigments appears to be quite adaptive for life in the terrestrial radiation environment and suggest that green algal progenitors were selected from the many other photosynthetic organisms living in an aquatic environment with different photosynthetic pigments (Björn et al. 2009; Kunugi et al. 2016).
Kunugi et al. (2016) suggested that the elimination of Chl b from PSI core antennae contributed greatly to the evolution of terrestrial green plants. To extend this concept, we analyzed the absorption spectra of Chls c1 and d. Chl c1 is a common form of Chl c. It is widely distributed among the secondary endosymbionts derived from red algae and is suitable for the light conditions of their marine habitats (Garrido et al. 1995). Chl c functions together with Chl a and carotenoids as light-harvesting pigments. Chl a shows only weak absorbance between 450 and 650 nm, while Chl b or c show increased absorbance within this range at both the long- and short-wavelength ends (Kirk 2011). The re for PARdir and PARdiff of Chl c1 were similar to those of Chl b, but its rp values, especially on the cloudy day, were lower than those of Chl b (Fig. 5g, h). The rp and re for PARglb of Chl c1 lied between those of Chls a and b (Fig. 7). The peak absorbance of Chl c1 at the long-wavelength end is significantly smaller than that of Chl b (Fig. 1a); thus, the absorption of photons by Chl c1 in the long-wavelength region becomes much lower than that by Chl b. As a result, Chl c1 does not surpass Chl b as a light-harvesting pigment in the terrestrial environment, where longer wavelength photons are abundant.
Chl d is only found in a few cyanobacteria inhabiting aquatic environments (Kashiyama et al. 2008) and constitutes part of the light reaction center complex rather than merely occurring as an accessory pigment (Mielke et al. 2011). Interestingly, the rp and re values of Chl d consistently lay in between those of Chls a and b and remained relatively constant regardless of the PAR class (Figs. 5, 7). Thus, it appears that aquatic Chl d would not be well suited to the terrestrial, direct-diffuse radiation environment, as its absorption characteristics would be unsatisfactory for avoiding or gathering solar radiation.
It is noted that we used + 10 nm shifted spectral data of Chls in the current study to reflect the proteinaceous environment (Fig. 1a). Interestingly, however, this corrected dataset had a similar but rather weak correlation with the spectral solar radiation in comparison with the previous research (Kume et al. 2016).
What is the advantage of forming pigment-protein complexes?
The spectrum of incident radiation determines the effectiveness of the absorption spectra of pigments, but Chl biosynthesis and its regulation in the embryophytes depend on: plant species, developmental stage and environmental factors, such as light conditions, temperature, and the composition of ambient atmosphere. Thus, chlorophyll formation may be regulated on various levels. It is well established that the Chl a/b ratio increases in unshaded conditions (i.e., when exposure to PARdir is high) and decreases in shadier environments (i.e., when relative PARdiff is elevated). This phenomenon occurs across all magnitudes of scale, from intra-chloroplast (Anderson et al. 1988) through to the leaves (Terashima 1989) and the whole plant (Bordman 1977). Furthermore, Kume and Ino (1993) observed clear seasonal changes in the Chl a/b ratio in the leaves of evergreen, broadleaved shrubs. Chls and carotenoids in the plant thylakoid membranes form pigment-protein complexes. Chl b occurs exclusively in LHCs, which function as peripheral antennae (Kunugi et al. 2016). In green plants, the antenna size of PSII is determined by the amount of LHCII (Jansson 1994; Tanaka and Tanaka 2011) and levels of LHCII are highly correlated with the accumulation of Chl b (Bailey et al. 2001; Jia et al. 2016), which is synthesized from Chl a by chlorophyllide a oxygenase (Tanaka and Tanaka 2011; Yamasato et al. 2005). When plants grow under low light intensities, Chl b synthesis is enhanced and the antenna size increases (Bailey et al. 2001). Since LHCII is the major light-harvesting complex of plants and the most abundant membrane protein, the absorption spectrum of the LHCII trimer may represent the average chloroplast absorption spectrum (Kume 2017). The absorption spectrum of LHCII is significantly different from a single Chl molecule or the core photosystems, particularly with regard to the secondary absorption peak that occurs at 472 nm with a shoulder at 653 nm (Fig. 1b).
Preventing excess energy absorption in photosystems is an essential survival strategy in terrestrial environments, where the atmospheric CO2 concentration is too low to utilize incident solar radiation safely for photosynthesis and the photon flux density can fluctuate by several orders of magnitude (Kume 2017; Ruban 2015). Kume et al. (2016) found that the spectral absorbance of Chl a is strongly negatively correlated with the spectral irradiance of PARglb at noon and Kunugi et al. (2016) showed that the exclusion of Chl b from the core antennae is crucial for promoting high-light resistance. In the present study, we found that PSI and PSII cores, which do not include Chl b, showed strong negative re and rp values under PARdir, and that these values tended to be more negative than those for Chl a. However, the addition of LHCI, which includes Chl b, to PSI to form PSI-LHCI led to an increase in re, while the LHCII trimer, which has the lowest a/b ratio, showed the highest re values. These differences were mainly caused by differences in absorbance in the vicinity of the 470-nm waveband (Figs. 1b, 8). The increase in Chl b in LHCs raises the absorbance at the high SIR waveband rather than that at the high SPFD waveband.
The spectra of photosystems and LHCs are consistently adjusted to avoid the high SPFD waveband (Fig. 8a). However, the spectra of photosystems and LHCs are different because of different Chl b contents and have complementary functional relationships. Compared with PSI and PSII cores, LHCII shows higher absorbance on the short-wavelength end and a relatively lower absorbance on the long-wavelength end (Fig. 1b). The peak spectral absorbance in the high SIR waveband (< 520 nm) is high and that in the high SPFD waveband (> 670 nm) is low. Therefore, although rp values of LHCII are only slightly different from those of PSI and PSII cores, the total spectral absorbance increases with the combination of cores and LHCs.
The rp values of all pigment-protein complexes showed strong negative correlation with the exception of \({\text{PAR}}_{{{\text{diff}}}}^{{\text{P}}}\). This is the results of absorption by carotenoids in the complexes. Among carotenoids, β-carotene is almost exclusively located in PSI and PSII cores, and lutein and other carotenoids are located in LHCs (Esteban et al. 2016). These carotenoids absorb high SIR photons (400–520 nm) without attenuation in high SPFD photons (550–700 nm), and reduce the absorption of high SIR photons by Chls (Kume et al. 2016). Kume (2017) previously discussed the filtering effects of accessory pigments and has defined the surplus energy (Es) as the part of energy potentially exchanged as heat in the absorbed photon energy. The absorption spectra of carotenoids are quite effective for eliminating photons that produce high Es. Since carotenoids are functioning both in light capture and photoprotection, further studies are required to understand the functional differentiation of carotenoids in pigment-protein complexes.
Notably, LHCII is the peripheral antenna for PSII and can associate with PSI depending on light conditions (e.g., Benson et al. 2015; Grieco et al. 2015). The LHCI complexes mediate energetic interaction between “extra” LHCII and PSI core in the intact membrane (Benson et al. 2015; Grieco et al. 2015). 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.
Why do plants absorb less green light?
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 2017).
The waveband of the green region of the spectrum (500–570 nm) is identical to that of strong, directional solar irradiance at midday under a clear sky (Figs. 3a, 4i). Kume et al. (2016) showed that the spectral absorbances of photosystems PSI-LHCI and LHCII and intact leaves decrease linearly with the increased spectral irradiance of \(PAR_{{{\text{dir}}}}^{{\text{E}}}\) at noon in the high spectral irradiance waveband (450–650 nm). In the present study, the PSI and PSII cores, which do not contain Chl b, showed the lowest absorbance in the vicinity of the 460-nm waveband (Figs. 1a, 8), which contrasts with marine photosynthetic organisms that are adapted to enhance absorption efficiency in the 450–650 nm wavelength range. 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. As Ruban (2015) emphasized, the photosynthetic antenna was “reinvented” a number of times in the course of evolution and hence originates from multiple ancestors. 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. The energy state of LHCII is precisely regulated and balanced by various photochemical mechanisms (Galka et al. 2012; Ruban 2015), resulting in plants being protected from high PAR while achieving high light absorption efficiency.
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 2005; Givnish 1988; van Loon et al. 2014). Thus, the efficient use of PAR under cloudy or shaded conditions may be important. On sunny days, PARdir contributes more than 80% to the incident global PAR energy (Fig. 4m), but this decreases to less than 50% on cloudy days and almost 0% on cloudy mornings (Fig. 4n). By contrast, PARdiff remains relatively stable in terms of the amount of incident energy and λmax. These spectral differences between PARdir and PARdiff 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 PARdiff 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 PARdir (e.g., Murchie and Horton 1997). Thus, leaves that are exposed to sun and shade may be regarded as PARdir and PARdiff adapted, respectively.
Notably, the effects of spectral differences between PARdir and PARdiff are negligible for whole-leaf absorption properties. Kume (2017) 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.